1-1 Kinds of Bending 1-2 Curve of Bending Force Versus Bending Angle 1-3 Air Bending and Coining 1-4 Springback 1-4-1 Why springback occurs 1-4-2 Positive and negative springback 1-5 Bottoming 1-6 Partial bending 1-7 Coining 1-8 Arrangement of Three Types of Bending

2-1 General 2-2 Reading the bending force chart 2-2-1 Explanation of symbols 2-2-2 Minimum flange length 2-2-3 Required tonnage 2-3 Four Relationships 2-3-1 Relationship between F and V 2-3-2 Relationship between F and t 2-3-3 Relationship between F and t 2-3-4 Relationship between F and b 2-4 Calculation of Bending Tonnage 2-5 Complement of the Bending Force Chart

3-1 The Necessity of General Knowledge 3-2 Requirements for Good Tooling 3-3 Tooling Materials, Heat Treatment, and Manufacturing Process 3-3-1 Hardened tooling 3-3-2 Thermally refined tooling 3-4 Length of Tooling 3-5 Sectional Shape of Tooling 3-5-1 Punches 3-5-2 Dies 3-6 Sectionalized Type 3-6-1 Sectionalized punch 3-6-2 Sectionalized die 3-7 Holder 3-7-1 Punch holder 3-7-2 Die holder

THE ABC OF BENDING TOOLS

Three Kinds of Bending

Use of Bending Force Chart

General Information on Bending Tools

3-7-3 Die base

4-1 Process of Selecting Tooling 4-1-1 Selection of V-width 4-1-2 Tooling as a punch and die combination 4-1-3 Machine specifications and setting of tooling 4-2 Bending Pattern 4-3 V-Bending 4-3-1 90' bending tooling 4-3-2 Tooling for acute-angled bending 4-4 Radius Bend 4-4-1 Characteristics of R-bending 4-4-2 Springback in R-bending 4-4-3 Multi-breakage bending phenomenon 4-4-4 Types of R-bending 4-4-5 Summary of R-bending 4-5 Hemming 4-5-1 Type requiring tooling replacement 4-5-2 Double-deck type 4-6 Bending Sequence 4-6-1 Procedure for determining bending sequence 4-6-2 Use of tools 4-6-3 Automatic programming 4-7 Guaranteed Allowable Tonnage of Tooling

5-1 Objective of Labor Economy 5-1-1 Elimination of problem caused by spring-up 5-1-2 Reducing the number of processes 5-1-3 Increasing tooling versatility 5-1-4 Making material handling easy 5-1-5 Reducing man-hours to replace tooling 5-1-6 Bending coated workpieces 5-1-7 Elimination of welding process 5-2 Types of Labor-Saving Tooling 5-2-1 Spring-up-preventive tooling 5-2-2 Tooling that reduces number of processes 5-2-3 Versatile tooling 5-2-4 Tooling that makes material handling easy 5-2-5 Tooling that eliminates tool change 5-2-6 Tooling that produces no marring 5-2-7 Technique which eliminates welding 5-3 Points of Tooling Design 5-3-2 L-bending tooling 5-3-2 Accordion bending tooling 5-3-3 Seaming tooling

Selection of Bending Tools

Labor-Saving Bending Tools

5-3-4 Urethane tooling

6-1 Dispersion of Bending Angles 6.2 Marring 6.3 Cracking 6.4 Warping

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Problems in Sheetmetal Bending

1. THREE KINDS OF BENDING

Reference must be made to the peripheral areas of bending in order to give accurate information concerning bending tools. The peripheral area here refers to bending machines and how to use them for bending work. As in the saying "You can't see the describe bending tools independently of bending machines and bending forest for the trees," it is not practical to only work. However, there is no point in extending the area so broadly as to cover all items related to sheetmetal bending. This "The ABC of bending tools" will, therefore, deaf with the above-mentioned subjects, concentrating on the fundamentals necessary bending tools. The fundamentals will be discussed in Chapters 1 and 2. They will be for the reader to understand referred to several times throughout the book. Such being the case, the reader should bear with us a little further if he feels there is no description of bending tools. It may be difficult to understand these tools in the future without a thorough comprehension of the fundamentals. The reader will be convinced that he has not wasted time once he has read througj this book.

1-1 Kinds of Bending Many operators encounter inexplicable troubles throughout their daily bending operation. For example, the intended product was not produced-it lost its shape, it was out of the required dimensional tolerance, or it had scores and cracks in the bent part. Perplexed with incomprehensible troubles, some may have uttered that bending is difficult and beyond their comprehension. Sheet metals such as steel, stainless steel, and aluminum should bend easily when pressed with a die and punch mounted in a bending machine, as long as the specified requirements are met. This bending phenomenon is recognized as quite a natural event, due to the fact that one can actually see the sheet metal bend. Then it does not seem that sheet metal bending is complicated and difficult. However, sheet metal bending is not as simple as it appears. This chapter teaches that there are three kinds of bending. The material to be bent (hereinafter called "work") undergoes three characteristic kinds of bending in accordance with bending force application. When told there are three kinds of bending, one may wonder, "Why?" or "It can't be true." Due to the bending phenomenon appearing simple, one cannot possibly observe the work experiencing three kinds of bending while it is being bent. In fact, here is a clue to understanding the seemingly incomprehensible characteristics of sheet metal bending. Go to Top

1-2 Curve of Bending Force Versus Bending Angle As a sheet of flat work is gradually pressed with a punch and die mounted in a bending machine, the bending of the work begins according to the application of bending force. At that time, a curve is obtained by plotting the angles to which the work bends under the respective force applied. This curve shows the relationship between the bending force and the resultant bending angle (Fig. 1-1).

Three Kinds of Bending

Kinds of Bending Bottoming Bending Force Vs. Bending Angle Partial bending Air Bending and Coining Coining Springback Three Types of Bending Table of Contents

The curve, which is also called an "S curve" due to its resemblance to the letter "S," varies greatly in shape due to the type of material. Fig. 1-1 shows a curve of the cold rolled carbon steel (SPCC). The bending force or the tonnage required for bending a work length of I meter is taken along the Y-axis, and the bending angle 0 along the X-axis. Bending angle 0 refers to the angle to which the work is bent after forming (the product angle). The work placed between the punch and die does not immediately bend after a bending force has been applied and the bending force will increase to a certain level. In Fig. 1-1, this increase in the bending force appears as a vertical rise in the curve. The work is known to have been pressed with approximately 6 tons of force. The work may look bent at a glance, but restores the previous shape when the bending force applied has been relieved. This is caused by the elasticity of the material. Elasticity is the quality or tendency to go back to the previous size or shape after being pulled or pressed. As the bending force gradually increases, the bending will proceed rapidly and the bending force will reach a peak (about 10 tons) at a bend of around 130 degrees. However, the bending force begins to decrease slightly when the bend exceeds 13.0 degrees. As shown in the figure, the bending angle in this region changes remarkably with a slight difference in bending force. This is called region 1. The required tonnage increases while the bending angle decreases below 100 degrees in the bending range. 90° bending can be performed with a bending force of approximately 12.5 tons, which is 25 percent greater than in 130° bending. The bending force that bends a material to 90 degrees is called the "required tonnage" of the material. If the material continues to be pressed, it will bend at an angle 3 or 4 degrees smaller than 90 degrees, i.e., an acute angle. This is region 2. The acute-angled bend goes back to a 90° bend if a still greater force is exerted on it. The tonnage needed at this time corresponds to approximately 75 tons, which is about 6 times the required tonnage. In Fig. 1-1, the line steadily rising along the Y-axis in the 90° bend indicates a rapid increase in the bending force. The region in which the bending angle changes very little despite the rapid increase in bending force, is called region 3. The events that occur under the bending force in the three regions,1, 2 and 3 , are referred to as partial bending, bottoming, and coining respectively. These bending force events are the foregoing three kinds of bending. Further, partial bending and bottoming are both called air bending. Go to Top 1-3 Air Bending and Coining In reference to basic nature, bending can be divided into two categories: air bending and coining. Air bending is a bend where air exists between the work and the die groove. As already stated, partial bending and bottoming belong to this category. These two methods of bending will be described in detail later. Now let us compare air bending with coining in terms of economy and precision. Air bending provides the following features:

[1] Since this bending method can bend the work with relatively little force, a machine with a small capacity can be used. The cost of equipment is low, therefore, the method provides great economy.

[2] Having been influenced by springback (refer to subsection 1.4), the bending precision may not be satisfactory.

The features of coining are as follows:

[1] A machine with a large capacity must be used because this method requires a bending force which is 5 to 8 times the required tonnage in air bending. The cost of equipment is high, therefore, the method is inferior in economy.

[2] Extremely high bending precision can be obtained because springback is eliminated.

The use of air bending or coining should be decided upon according to the application and function of the product (refer to subsection1.8). Go to Top

1-4 Springback 1-4-1 Why springback occurs Let us examine springback phenomena first. Fig. 1-2 shows a springback in V-bending by which obtuse-angled, 90°, and acute angled bends are formed. In the figure, the solid lines indicate the angle during forming (Ø´), and the dotted lines indicate the angle after being formed (Ø).

Here we will consider what causes springback to occur from two points of view. One view is springback considered from the stress-strain diagram, and the other view is considered from the displacement of the molecules inside the work.

In the stress-strain diagram in Fig. 1-3, as the exerted external force at point G in the plastic* region is reduced by low degrees, the amount of strain (deformation) in the work decreases gradually. It decreases parallel with the line OA (which is in the elastic region where stress is proportional to strain). When the work is totally removed from the external force, the strain reaches point X. This indicates that the work retains a certain amount of elasticity in the plastic region. In the diagram, G'X shows the amount in which the work has returned to its former condition, and OX shows the amount of permanent deformation. To summarize the above facts, the elasticity of the work material is not eliminated even after the stress produced in the work has exceeded the yield point (The yield point is where the work gives

way to stress; the plastic region lies beyond that point.). This is a cause of springback.

Fig. 1-4 is an exaggerated illustration of the molecular displacement in flat work when it is bent at an obtuse or 90 degree angle. This figure shows that the inner side of the work is compressed and the outer side is stretched. Between these sides, there is a plane which is neither compressed nor stretched. This plane is called a neutral plane or neutral axis. When the work is bent, stresses which are opposed to each other act on the inner and outer sides of it. In general, the compressive strength of the material is far greater than its tensile strength. Exerted pressure will permanently deform the outer side of the work, but the inner side stress does not reach the yield point. Therefore, the inner side tends to go back to the former condition. Since stress is a resisting force that acts in the opposition to the exertion of the external force, a compressive stress acts outward on the inner side. This compressive stress changes into springback.

1-4-2 Positive and negative springback You might recall the product angle if you hear the word "bending angle." This is not incorrect, but to be more exact, it should be called the "bent angle." This is because the bending angle involves the angle during formation and the already formed angle. When Ø denotes the angle after formation and Ø´ the angle during formation, we have Ø-Ø´=ØA. The delta (A) is a small quantity. This ØA is the springback which is the angle of the small amount. In Fig. 1-1, as already stated, the bent angle becomes 3 or 4 degrees less than 90 degrees in region . This is a phenomenon called spring-go or spring-in, which is expressed as negative ØA if the foregoing Ø-Ø´=ØA is applied. In brief, a springback is +ØA and a spring-go or spring-in is -ØA. ØA becomes either positive or negative, depending on the bending pressure exerted. Then what are the conditions that cause a negative springback or spring-go? Let us ex- amine the process of V-bending (Fig. 1-5). (a) During V-bending, the work inserted between the punch and die changes its state from (a) to (b) and then to (c) under the bending force. The work at that time can be regarded as a continuum that contains positive factors (springback) and negative factors (spring-go). Additionally, it can be considered that (c) these positive and negative factors are not constant, but show their qualities of being positive or negative while changing in accordance to the force applied. Therefore, the (d) springback or the spring-go will occur on the work, depending on the way bending force is applied to the work. Go to Top

1-5 Bottoming The term "bottoming" comes from the verb "bottom" which means "to reach the bottom." This bending technique is also called "bottom pressing" or "bottom striking" at the production site. Bottoming, one type of air bending, is most widely used because it can accurately bend the work with relatively low tonnage.

In Fig. 1-6, t stands for the sheet thickness, V for the 'V-width (shoulder width of a V-die), and ir for the inside radius of the bend. The desirable, proper V-width varies with sheet thickness. Table 1-1 shows the relationship between the sheet thickness and the V-width in bottoming. This table shows the optimum standards in which the coefficient of the V-width becomes greater as the sheet thickness increases. However, one should note that the V-width should also be determined by taking the flange length and inside radius of the product into consideration. The method of determining the V-width will be described in a later chapter.

The inside radius in bottoming has been experimentally clarified to be approximately 1/6 of the V-width, i.e., ir=V/6. So ir can be determined by substituting the V-width obtained from Table 1-1 into this equation. For example, if the V-width is 6 times the sheet thickness, the inside radius equals the sheet thickness (ir = t). Further, if V=12t, then ir=2t. From these examples, the inside radius ir is known to vary from It to 2t with the sheet thickness. When the inside radius (ir) is equal to t (ir=t), it is called the standard ir. For example, in the case of t=l mm, bending at the standard ir can be accomplished with a die of V=6 mm. Similarly, if t=1.6 mm, then a die of V=10 mm will produce the same result. Many products are bent at the standard ir. Bending accuracy in bottoming is influenced by springback. The most widely used countermeasure is to offset springback by bending the work an additional amount equal to the springback. This is the reason tooling for 90° bending is available in such V-groove angles as 90°, 88°, 85°, and 80°. The 90° V-tooling has no allowance for springback, but in practice the product angle can be finished at 90 degrees by applying bending force while simultaneously keeping good balance between springback and spring-go. In the tooling used for bottoming, the punch tip and die V-groove must be of the same angle. This is an essential condition to obtain satisfactory precision of the product. (This subject will be described in detail later.) Go to Top

1-6 Partial bending The name "partial bending" comes from the fact that the work partially contacts the tooling in three places during bending (A, B and C in Fig. 1-7). Partial bending is typical air bending. This bending method is characterized by a wide range of bending angles which can be selected freely. For instance, in partial bending using a 30' punch and a 30' V-die, the work can be bent at any angle between 30 and 180 degrees. Partial bending is convenient, thus, it is widely used, following bottoming.

Table 1-1 Relationship between Sheet Thickness (t) and V-width

Sheet thickness 0.5-2.6 mm 3.0-8 mm 9-10 mm 12 mm or more

V-width 6t 8t 10t 12t

The V-width in partial bending should be 12 to 15 times the sheet thickness to achieve precision. Fig. 1-8 explains the reason. Bending precision refers to bending angle variation, which is theoretically related with the depth of the punch into the die.

In Fig. 1-8, Vb is the V-width used in bottoming and Vp is the V-width used in partial bending; Vp is approximately two times greater than Vb. When the depth of the punch into the die is constant, Op (the range in which the angles vary in forming with the partial bending V-width) is small in comparison to op. This means the range of bending angle variation is smaller in Vp than in Vb. During partial bending, the use of a larger V-width than that in bottoming results in good precision. However, even if the V-width is increased, partial bending remains inferior in bending precision to bottoming. For that reason, a bottoming type of tooling should be used when high precision is required. The bottoming type of tooling are the punch and die in which springback (AO) is considered to suit the required angle of the product. Go to Top

1-7 Coining You might think it is unusual that a bending method is called "coining." Coining comes from the word coin that means "metal money" and "making metal into coins." In spite of mass production, each piece of coin is made with almost no variation in shape and size. From this, the name "coining" seems to have been applied to this bending method whereby accurate bends are obtained. Coining provides two advantages: (1) very high bending precision and (2) the capability of reducing the inside radius to as small as possible. Fig. 1-9 shows the work and tooling in the final stage of coining, from which you can see the punch tip is imbedded into the work. This penetration of the punch tip, together with a high pressure produced by the punch and die V-groove, eliminates springback. This is the reason coining requires a bending force 5 to 8 times greater than bottoming.

The V-width in coining is smaller than in bottoming, preferably 5 times sheet thickness. One reason for this is to reduce the amount of the punch tip penetration into the work by decreasing the ir (you will recall ir is 1/6 of the V-width). Another reason is to increase the V-die's surface pressure by making its V-groove area smaller. If coining is performed in a V-width close to that of bottoming, the punch tip penetration will increase as much as the ir increases, requiring an additional bending pressure. Also, since the V-groove area becomes larger, it is inevitable that the surface pressure will drop. It should be noted that

these factors will act like a brake upon satisfactory coining. In coining, the angles of the punch tip and die V-groove should be made equal to the required angle of the product. For instance, in the case where a 90' bend is to be formed, a 90' punch and 90' die should be used without considering springback. As previously explained, high tonnage is needed to perform coining. The limit of the sheet thickness bendable by coining is governed by the machine's capacity; it also varies with the amount of pressure the upper beam can withstand. Accordingly, consideration should be given to this point in determining the bending limit. The tolerable pressure of an upper beam is usually guaranteed by the tonnage per unit length. Bending pressure for coining a 1.6 mm thick cold rolled carbon steel is 75 tons of bending pressure per meter and a 2 mm thick cold rolled carbon steel is approximately 115 tons per meter. So the bending thickness limit is 2 mm, because, in general, the tolerable pressure of the upper beam of the press brake is approximately 100 tons per meter. Go to Top

1-8 Arrangement of Three Types of Bending As stated in subsection 1.3, it is necessary to select the proper bending method in accordance with the application and function of the product. Do not determine the merits and demerits of the three bending methods, but use them properly in order to make the most of the advantages of each. For example, if an NC press brake and acute-angled tooling are used, partial bending will permit products having acute-angled, 90', and obtuse-angled bends to be formed continuously from the first to the last bend. The demand for higher bending accuracy is becoming greater. In the late 1960's in Japan, excellent sheet-metal working machines, such as shearing machines, punch presses, and press brakes became popular. The appearance of quality punches and " dies followed. With such background circumstances, accuracy of finishing made remarkable progress, and sheet-metal working machines superseded part of the jobs of machine tools such as the milling machine, drilling machine, and lathe. Speaking of the bending accuracy at that time, it was at an age of "±1 mm" in terms of general, nominal tolerance. With the present technical level, "±0.2 mm" is an acceptable standard of tolerance, which tends to rise as high as "±0.l mm." Such being the case, the bending accuracy today is required to be 10 times as precise as that of twenty years ago. In other words, we have entered an age in which bending accuracy must equal the accuracy achieved in coining, and in that sense, coining must be taken into consideration. As already stated, bottoming is the most widely used method of bending because it achieves relatively high precision. Due to the high economy in the cost of equipment, bottoming will continue to play a significant part in material bending. The serious problem in bottoming is the easy occurrence of springback. However, we have now accumulated so much technical knowledge on springback and can apply it to tooling designs. That is to say, countermeasures against springback have advanced so much that bottoming can be performed with ease and confidence. The present situation of the three types of bending has been outlined. Table 1-2 shows the important points from the description I comparing these three methods. Each method should be understood properly through this table and used in bending work.

Table 1-2 A Comparison of Three Types of V-Bending

Type of bending

V-width ir Bending angle

dispersion Surface precision Features

Partial bending

12t-15t

2t-2.5t ±45´

Forms surface with large

curvature radius

Range of bending angle can be selected freely.

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Bottoming 6t-12t 1t-2t ±30´ Good Good precision is obtainable by using relatively little tonnage.

Coining 5t 0.5t-0.8t ±15´ Good

Very good precision is obtainable. The tonnage required is 5-8 times that required in bottoming

t=thickness

2-1 General

A bending force chart is one of the necessary instruments in bending operations. It can easily be found as a nameplate on the press brake or in the manufacturer's catalogue and instruction manual. This chapter explains the use of the force chart for mastery purposes. The operator should understand the chart completely in order to enhance job efficiency and technical ability. It is recommended that this chapter be reread if there are any questions concerning its contents.

2-2 Reading the bending force chart Look at Table 2-1 which is the air bending force chart. The first or leftmost column shows the sheet thickness (t) of the work. The three rows at the top indicate the V-width of the die (V), minimum flange length (b), and the inside bending radius (ir) from the top. The figures shown are the required tonnage per meter of work. The following items can be read from this table:

1. The V-width of the die to be used for bending. 2. The length of the smallest, bendable flange. 3. The required tonnage per meter of work. 4. The inside radius produced.

2-2-1 Explanation of symbols Symbol (t) in the first column indicates the sheet thickness of the work, ranging from 0.5 mm to 30 mm. Symbol V in the top row indicates the width of the die V-groove, or V-width. The figures in this row are standard commercial sizes. However, the standard size of the V-width vary from manufacturer to manufacturer.

t : Sjeet thickness (mm) (tensile strength F: Bending force per meter (ton/m) i: inside bending radius (mm) b: Minimum flange length (mm) V: V-die width (MM)

The following can be read from this chart if the Sheet thickness of the material and the inside radius of the bend are determined.

1. The required force to bend one meter of the material. 2. The V-width of the tooling used in that bending.

Use of Bending Force Chart

General Reading the bending force chart Four Relationships Calculation of Bending Tonnage Complement of the bending force chart Table of Contents

3. The minimum flange length that can be bent.

Symbol b in the middle row indicates the minimum flange length. Symbol ir stands for the inside bending radius of the product. Here let us examine the bending force chart to see if ir= V/6. When V is 10 mm, ir is 1.6 mm, and when V is 32 mm, ir is 5 mm. Therefore, ir is approximately 1/6 the V-width. F indicates the "required tonnage" used to bend one meter of the work.

2-2-2 Minimum flange length As previously mentioned, if the V-width is known from the sheet thickness, the minimum flange length can be found from the bending force chart. Let us examine why the word "minimum" is used for the flange length. Until bending is completed, the work must be securely supported at the shoulder of the V-die groove. If not, as bending progresses, the work slides off the shoulder, and the bending line changes irregularly. Good bending accuracy cannot be obtained in such a bending operation and the operation itself becomes dangerous. The "minimum flange length" is necessary in order to bend the work with sufficient accuracy and safety. Fig. 2-1 shows the minimum flange length calculation. According to the right isosceles triangle ABC, the length of side b is V2-times V/2. That is, the minimum flange length can be calculated by b=(2)½( V /2).

If a 6 mm V-width die is used to bend work, b is 4 mm according to the chart. Similarly, when the V-width is 25 mm, then b becomes 17.5 mm. If the flange size in the work drawing is larger than b, then it can be assumed the V-width is correct. However, if it is less than b, the V-width is inadequate for the bending and a smaller V-width must be used.

2.2.3 Required tonnage The statement, "tensile strength: 45-50 kg/mm², is shown in Table 2-1. This 45-50 kg/mm² refers to mild steel. As already stated, the value of F (bending force) is the required tonnage per meter of work. It is also the required tonnage in bottoming. Generally, the bending force charts indicate the required tonnage needed to bend mild steel by bottoming. The following steps should be taken to find the necessary F. First, determine the V-width according to work thickness t. Next, follow the horizontal row of the necessary t to the right, and read the figure in the position where it meets the vertical row of the determined V. For example, if a V-width of 12 mm is used to bend work that is 2 mm in thickness, you will find the required tonnage F is equal to 22 tons. Likewise, when t is 3.2 mm and V is 25 mm, then F should read 27 tons. When t is 6 mm and V is 50 mm, F should be 48 tons. These F values are the required tonnage per meter of work. Go to Top

2-3 Four Relationships The four relationships refer to those between the required tonnage and V-width, sheet thickness, bending length, and tensile strength. In many cases, the bending force chart is not utilized effectively due to the operator not understanding it. Effective use of the chart is being able to make the most of these four relationships, or those between F and V, F and t, F and and F and b.

2-3-1 Relationship between F and V Table 2-2 shows the required tonnage used per meter when 1 mm and 2 mm mild steel sheets are bent in different V-widths. As shown in the chart, if the V-width is doubled, the required tonnage will be reduced to 1/2. In short, the required tonnage F is inversely proportional to the V-width V. This inverse proportion is expressed as F=k-11V.

Table 2-2

t=1.0 mm t=2.0 mm

V(mm) 6 12 V(mm) 12 25

F(ton) 11 6 F(ton) 22 11

2-3-2 Relationship between F and t The relationship between the required tonnage and the sheet thickness is apt to be misunderstood. A conversation, "The sheet is double in thickness, so the tonnage also becomes double," is often heard in the production site. This statement is incorrect. Table 2-3 shows changes in the required tonnage when the V-width was kept constant and the sheet thickness was varied. When t doubles, F becomes approximately four-fold if the V-width is the same. That is, bending force F increases in proportion to the square of sheet thickness. This is not a simple proportional relationship. For this reason, die designers and expert operators caution by saying "Bending force is affected by the square of sheet thickness." Therefore, the above conversation on the job site must be corrected. Operators should know that if sheet thickness doubles, the tonnage will become four-fold. The relationship between F and t is expressed by F=k.t².

2-3-3 Relationship between F and L LP is the bending length of work, which is proportional to F. The tonnage shown in the bending force chart are values based on the unit bending length of one meter. To obtain the tonnage for a certain length of work, convert the length into meters and multiply by the related tonnage. The relationship between F and t is given by F=k-t.

2-3-4 Relationship between F and bb F and tensile strength bb are proportional to each other, just as F and e above. Thus, if only b is known, the required tonnage to bend materials other than SS41 can be determined by using this relationship.

Table 2-4 shows the tensile strength of commonly used sheet materials. Since a variety of materials are available, the bb of the confirmed when calculating F.

The relationship between F and bb is expressed by F=k-b. Go to Top

"Table 2-3

t=1.0 mm t=2.0 mm

t(mm) 1 2 t(mm) 1.2 2.3

F(ton) 6 22 F(ton) 6 23

Table 2-4 Tensile Strength Of Various Materials

MaterialTensile strength, kg/mm²

Soft Hard

Lead 2.5-4 -

Tin 4-5 -

Aluminum(99.0%) 9.3 171

High-tension aluminum alloy Type 4 23 48

Duralumin 26 48

Zinc 15 25

2-4 Calculation of Bending Tonnage The bending tonnage serves as the basis for judgement in selecting a new machine, as well as determining whether an already-installed press brake can achieve a bending operation from the standpoint of capacity. The required bending tonnage can be calculated by the four relationships, based on the F value in the bending force chart. Examples of bending tonnage calculations and their solutions are given below. The reader should try to answer them in order to familiarize himself with the four relationships.

[Example 1] How many tons are required to bend 4 meters of a 1.5mm thick SUS304 stainless steel sheet (tensile strength: 60 kg/mm²)?

[Solution] The sought V-width is calculated as V=6XI.5 . .10 (6xl.5=9, but V=9 mm is not a standard size so V=10 mm is substituted). In this case, t=1.5 mm is not in the bending force chart, so the bending force (F=17 tons) should be read for V=to mm and t = 1. 6 mm (the closest t). The calculating formula, which takes into consideration the sheet thickness, tensile strength, and bending length, will be given as follows: F.17X(1.5/1.6)2X60/45 x 4= 80 The sought solution is 80 tons. In this calculation, raise the fractions to units.

[Example 2] How many tons are necessary to bend a 15 mm thick and 3,100 mm long sheet of rolled SS41 steel? The flange length is 120 mm.

[Solution] The sought V-width is V=t2xl5=180.

Copper 22-28 30-40

Brass (70:30) 33 53

Brass (60:40) 38 49

Phosphor bronze 40-50 50-75

Bronze 40-50 50-75

Nickel silver 35-45 55-70

Cold rolled iron sheet 32-38 -

Steel, 0.1%C 32 40

Steel, 0.2%C 40 50

Steel, 0.3%C 45 60

Steel, 0.4%C 56 72

Steel, 0.6%C 72 90

Steel, 0.8%C 90 110

Steel, 1.0%C 100 130

Silicon steel sheet 55 65

Stainless steel sheet 65-70 -

Nickel 44-50 57-63

Although a standard V-width of 160 mm or 200 mm is acceptable, a V-width of 200 mm is not suitable from the viewpoint of length b (flange length). This is because the minimum flange length for a V-width Of 200 mm is 14o mm, which is greater than the required 120 mm. Therefore, V is set at 160 mm. Also, t=15 mm is not shown in the chart, so 107 tons is obtained when the F for t=16 mm and V=160 mm is read. Thus, we will have the following formula: F= 107 x (15/16)2 x 3. 1=292 Consequently, the sought answer is 292 tons.

[Example 3] Calculate the required tonnage when bending SPCC of t-3.2 mm and =2,400 mm with a V- width of 18 mm. 6b is 32 kg/mm².

[Solution] The sought V-width is V=8x3.2=25, and V=25 mm is adequate. However, when reducing the V- width for any reason (e.g., when the flange length is short or the inside radius of the product is small), you may lower by one rank (a blank space where the tonnage is not stated). We don't recommend lowering more than one rank because satisfactory precision cannot be obtained with a V-width of 16 mm. Because F=34 tons can be read as t=3.2 mm and V=20 mm, the required tonnage will be given as follows: F= 34 x 20/18 x 32/45 x 2.4=65 Thus, the required tonnage is 65 tons.

[Exercises] 1. Determine the tonnage required to bend SPCC of t=1.6 mm and =3,000 mm. 2. How many tons are required to bend an anti-corrosive aluminum alloy sheet with t=3.0 mm, =2,500 mm, and bb=22 kg/mm² 3. Determine the tonnage required to bend SS4] of t=20 mm and e=600 mm. 4. Calculate the tonnage required to bend soft copper (6b=25 kg/mm²) of t=8 mm and =2,500 mm. 5. Find the tonnage of F when bending SUS304 (t=8 mm and l=3,000 mm) at V=63 mm. (See page 25 for the solutions.) Go to Top

2.5 Complement of the bending force chart So far, we have described in the previous subsections the necessary fundamentals of how to read the bending force chart, the four relationships, and the calculation of bending tonnage. The information already given will enable you to properly use the chart in your job site. The inquires we have received mainly concern the usage of the chart. These inquires seem to suggest the chart is not always effectively utilized. If so, that is indeed an unfortunate situation. The above fundamentals may appear complicated, but each of them can be broken down into simpler factors. They are all made up of correlative simple factors. Just start with the first step, and you will find that it is easy to master the usage of the bending force chart. Now, let us consider a certain calculating formula that is mentioned in almost all books regarding sheet bending. It is the formula,

F=(c x b x l x t²)/(V x 1000)

At a glance, you will find that this formula involves all the factors of the four relationships. The denominator V is inversely proportional to F, and the numerator b, e and t² is proportional to F. The major problem with the above formula is knowing how to determine the value of coefficient C. Usually, the value of C is somewhere between 1.00 and 2.00, varying with V/t. The smaller V/t, the larger value C becomes. When V is 8t, C is said to be 1.33. However it is the value of just one datum. Another datum (by the same researcher) shows a difference of 15 percent in the value C. Such being the case, it causes US to question this calculating formula, and the true value for C

varying with V/t. This means the calculated tonnage may turn out to be extremely inaccurate. Our experience reveals the calculation using the tonnage in the chart (as in the exercises) is more practical than the use of calculating formula. This account for the fact that we have calculated the bending tonnage according to the bending force chart , not using the calculating formula.

[Solutions] 1. F=17x32/45x3=37 2. F =24x22/45x2.5=30 3. F=125x(20/19)²x0.6=84 4. F=52x(8/7)²x25/45x2.5=95 5. F=(52(8/7)²x60/45x3=272

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3-1 The Necessity of General Knowledge

This chapter provides the necessary background information to enable the reader to understand punches and dies, the tooling used in bending. The main section of this chapter deals with general subjects, such as the requirements for good tooling, material, and the manufacturing process. Usually, operators recognize the importance of tooling knowledge, but only within the limits of their job. Some operators do not take much interest in the general subjects noted above, and some think it is unnecessary to learn the extensive process involved in tooling. However, the precision, material, and hardness of a punch and die are directly related to the precision of the product which the operator must achieve. The more they know about tooling, the better chance they will have to attain their objective. Go to Top

3-2 Requirements for Good Tooling The requirements for good tooling must be examined when judging the quality of a punch and die. Moreover, we must know how these requirements are related to our bending work. Generally speaking, the following are essential requirements for good tooling:

1. The length should enable easy installation and removal. 2. Completely heat-treated for greater strength and longer wear resistance. 3. High accuracy of finish. 4. Interchangeable regardless of the equipment.

In summary, good tooling consists of a punch and die which provide ease of handling, a long service life, high accuracy, and interchangeability. (The relationship between the length of tooling and its replacement will be described in subsection 3-4.) Go to Top

3-3 Tooling Materials, Heat Treatment, and Manufacturing Process During the bending operation, the tooling undergoes repetitive loads such as compressive loads and bending moments. Given the fact that the tooling may perform 3,000 processes a day, the frequency of repetitive load application could total 70,000 to 80,000 times per month and 800,000 to 1,000,000 times per year. The strength and wear resistance of tooling to withstand such repetitive loads can only be obtained by using a quality material and proper heat treatment. There are two methods of heat treatment: "hardening" and "thermal refining." The terms "hardened tooling" or "thermally refined tooling" comes from these methods of heat treatment. Generally, if it is called bending tooling, it refers to either hardened tooling or thermally refined tooling. There is other tooling which is made of so-called raw material which is not heat-treated. In a strict sense, however, this should not be considered tooling because the steel used for tooling is only

General Information on Bending Tools

The Necessity of General Knowledge Sectional Shape of Tooling Requirements for Good Tooling Sectionalized Type Tooling Mat'l, Heat Treatm't, & Mfg Process Holder Length of Tooling Table of Contents

effective when it is heat-treated (though the quality of material must be suitable for heat treatment).

3-3-1 Hardened Tooling

As previously stated, the purpose of heat-treating tooling is to increase its wear resistance and strength against bending force. The material commonly used in hardened tooling is alloy tool steel, though it varies with the tool manufacturer. The paragraphs that follow will describe chromium molybdenum steel (type 4) and DM. (i) Chromium molybdenum steel, type 4 (SCM4)

Chromium molybdenum steel is one of the structural alloy steels. It is a pearlitic steel containing chromium steel and a small amount of molybdenum. This steel, which is suited for tooling material, features toughness (tenacity), great wear resistance and superior hardenability by heat treatment. This tooling material is either completely or locally hardened (= overall or local hardening*') to obtain HRC43 to 48. HRC indicates the Rockwell hardness C scale. The Rockwell hardness test is used to measure the hardness of mechanical parts in many job sites because it is a simple method of measurement.

(ii) DM DM is the trade name for "Yasuki Steel" made by Hitachi Metal. This tooling material is equivalent to JIS alloy tool steel, SKT4. However, it contains more nickel for greater toughness. Like chromium molybdenum steel, DM provides excellent toughness and wear resistance, and its hardening strain (the tendency of bending, warping, or twisting after being hardened) is less than that of other material. Therefore, while SCM4 is used in general tooling, DM is utilized specifically for tooling whose sectional shapes have a greater hardening strain* 2 . Its hardness is identical to that of SCM4. Table 3-1 compares the heat treatment of tooling materials.

*1. Local hardening and overall hardening: Local hardening means heat-treating only part of the tooling such as a punch tip and a die V groove. This does not produce much hardening strain and the portion requiring grinding is small. However, sufficient strength cannot be obtained in the essential part of the tooling (for example, in the part of a punch where the maximum bending moment acts). Amada's tooling is treated by )overall hardening. *2. Sectional shapes having a large hardening strain: This refers to the sectional shapes of a gooseneck punch, straight sword punch, sash punch, and sash die. Shapes with large relief, slender shapes, and shapes with great changes in thickness are subject to develop hardening strain.

(iii) Manufacturing process

Fig. 3-1 shows the manufacturing process of hardened tooling. Hardened tooling is generally manufactured in the order shown below.

Material => Primary forming => Hardening => Correcting => Finishing

The formed workpiece, which has been hardened, is corrected prior to finishing. Then the workpiece

Table 3-1 Heat Treatment and Tooling Materials

Operation Hardening Tempering Hardness (HRC)

SCM4 850° 4.5H Oil cooled 420° 5H Air cooled 43-48

DM 860° -880° 1.5H Oil Air cooled 850° 2H Air cooled 43-48

S45C 850° 4.5H Oil cooled 680° 5H Air cooled 23-28

is finished by grinding with a grinding machine.

Finishing work can best be performed by "profile grinding." Photo 3-1 shows a 2V die being ground by profile grinding. As seen from the photo, the contour including the V-grooves (each consists of groove face, shoulder R, and groove bottom R) and surfaces are ground simultaneously when using profile grinding. The grinding wheel used in profile grinding is dressed with a diamond dresser and finished with high accuracy by a crushing roll (Fig. 3-2). The job of the crushing roll is to smooth any roughness on the grinding wheel after dressing. This enables the wheel to function property.

3-3-2 Thermally Refined Tooling (i) Purpose of thermal refining

The purpose of thermal refining is to unify the internal structure of the tooling material so that it will remain operable during use. The difference between thermal refining and hardening is that the tempering temperature in thermal refining is higher than in hardening. As indicated in Table 3-1, there is no difference between the two in hardening temperature (thermal refining may be considered a type of hardening). Thermal refining increases wear resistance and strength in its own way. Carbon steel (S45C) is used as thermally refined tooling material. (ii) Application of thermal refining

In general, thermal refining is employed in the following cases:

1. Large, hard-to-harden tooling (This is related to the size of the electric furnace.) 2. Complicated sectional shape, hard-to-grind tooling (This is often the case with

special tooling.) 3. Long, one-piece, hard-to-grind tooling (This is related to the workable length of

the grinding machine.)

The hardness of thermally refined tooling is lower than that of hardened tooling, and is somewhere between HRC23 and 28. HRC28 is generally the maximum hardness that is workable with a bit (toot used with a lathe, shaper, and planer).

(iii) Manufacturing process The manufacturing process of thermally refined tooling differs between large and small tooling. As shown below, large tooling is produced in the same process as hardened tooling.

Material => Primary forming => Refining => Correcting => Finishing

Small tooling is fabricated of already-refined material (commercially available) in the following process.

Refined material => Primary forming => finishing

The difference in the manufacturing process of large and small tooling originates from the "mass effect" of heat treatment on each type of material. Generally, when a thin steel bar or a thin steel sheet is heat-treated, the effect of heat treatment penetrates completely through it. In thick steel bars or sheets, however, the outer part can be heat-treated to the desired hardness, but the inner part is not sufficiently hardened. This is because the cooling speed of the outer part is different from that of the inner part. Thus, the inner part is less hard than the outer part. This phenomenon is the mass effect. Since large tooling requires much working, the inner part with less hardness is exposed as a result of machining. For this reason, it is preferred that the tooling be refined after primary forming. Thermally refined tooling is finished with a planer or a profiling planer. A slow machining speed and a small amount of cut-in provide a high-precision surface finish. When working a complicated curve or arc such as those found in special tooling, an accurate template (profiling plate) should be prepared. The material is profiled by using the template and profiling planer .

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3-4 Length of Tooling With regard to length, tooling used in bending can be divided into one-piece tooling and split type tooling. One-piece tooling is simple tooling made in conformance with the required bending length. Split type tooling is tooling that can be split into individual sections. The length of the tooling greatly influences the efficiency of bending work. Now, we will examine how the length of tooling influences job efficiency in connection with tooling replacement, which is frequently performed in bending. The frequency of tooling replacement in a given day is between 5 and 20 times, with 10 being the average. Tooling replacement involves a series of preliminary operations as shown below.

Carrying => Mounting => Aligning => Accuracy adjusting => Removing => Storing

Table 3-2 compares both types of tooling with respect to this series of operations. The reader will be able to easily determine which type of tooling is more efficient.

Next, let us focus on how the type of tooling influences bending accuracy. The primary factor that affects bending accuracy is the parallelism of the tooling (tooling height). A comparison of parallelism between one-piece tooling and split type tooling is shown in Fig. 3-3. The limit of the working accuracy that a planer can achieve is 0.05 mm per meter of tooling. It is 0.02 mm in grinding work. In this example, the working accuracy of the one-piece tooling is 0.1 mm for an overall length of two meters, while the split type tooling is 0.02 mm. Thus, the split type is five times more accurate than the one- piece type.

Table 3-2 A Comparison of Tooling Replacement Operations

Operation One-piece type Split type

Carrying A forklift or more than one person is needed Can be carried by one person

Mounting, Aligning Must be done by two persons while signaling. Can be done by one person

Accuracy adjusting Must be adjusted with shims after test bending. Unnecessary if alignment is correct.

Removing More than one person is needed. Can be done by one person

Storing A forklift or more than one person is needed. Can be done by one person

So far, we have seen how the length of tooling influences job efficiency and bending accuracy. From the above facts, the reader will readily discover that split type tooling is superior. Fig. 3-4 shows the length of each tooling that can be mounted on various Amada machines. The tooling is available in two lengths: L (Long size) and S (Short size). L size is 835 mm (32.87") and S size is 415 mm (16.34").

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3-5 Sectional Shape of Tooling A long shape which has an identical cross section from end to end is characteristic of tooling. Therefore, it is important for operators to remember the sectional shape and size of individual tooling for more accurate and speedy performance. It may be difficult for operators to remember them all because there are so many types. Some hints will be provided to help the reader remember.

3-5-1 Punches Fig. 3-5 shows an example of a punch sectional shape. The sectional shape of a punch can be classified by (1) tip angle, (2) relief shape, and (3) installed height. 'These should be memorized.

(i) Tip angle There are punches with a tip angle of 30°, 45°, 60°, 88°, or 90°. Punches with a 30° or 45° tip are used for acute-angled bending, and those with 60°, 88°, and 90° tips are used for 90° bending. The punch tip for 90° bending has a "punch tip R," which corresponds to the "punch tip flat width" in the 30° and 45° punches. In Fig. 3-6, (a) illustrates a 90° punch and (b) the tip of an acute-angled punch.

(ii) Relief shape Punches have a "relief" necessary for various bending. The shape of the relief is determined by the shape of the product. The relief shapes of major punches are shown in Fig. 3-7. As shown, punches can be divided into two types. One type has a relief at the rear. The other type has relieves of one or different shapes at the front and rear. Punches (a), (b), and (c) belong to the first type and (d) and (e) belong to the latter type. In Fig. 3-7, (a) is a bottoming punch for 90' bending and is the most commonly used type; (b) is a punch for acute-angled bending or air bending; (c) is a gooseneck punch; (d) is used for products having a symmetric shape and is called a straight-sword punch; and (e) is a sash punch that is widely used in the sash industry.

(iii) Installed height The height of a punch refers to its installed height (see Fig. 3-5). In the case of the radius ruler (R punch) or the coining ruler, note that the height E of the punch holder plus the ruler's height is the installed height (Fig. 3-8). There are seven different installed height: 65, 67, 70, 90, 95, 104, and 105

3-5-2 Dies Fig. 3-9 shows an example of a die sectional shape. Dies are classified by (1) the shape of the groove and (2) the number of grooves. A die with one V-groove is called a IV die and a die with two V-grooves is called a 2V die. Further, a die with three rectangular grooves is called a 3U die.

(i) IV die The IV dies having one V-groove can be divided into two types. One type is for bending work over 4mm in thickness and the other type is used for making sashes. The first type has "working relief*" on each bank of the V-groove, as shown in Fig. 3-10. The sash die does not have such relief. As indicated in Fig. 3-11(a), the sash die is installed to thg sash die 'holder and used in combination with the sash,h punch.

*The working relief reduces forming time of the die and improves the surface precision of the groove. (;generally, dies with a large V-width are provided with working relief, but small V width dies are not.

(ii) 2V die

This is a bottoming die that can bend a maximum sheet thickness of 3.2 mm. It has two different V-grooves that have different V-widths but have identical V-groove angles. The 2V die is installed to a die holder when it is used [Fig. 3-11(b)]. Like sash dies, it has no working relief in the V-groove.

(iii) 3U die

As shown in Fig. 3-12, the 3U die consists of three different rectangular grooves and one flat surface. The die is used for partial bending in combination with a 45' punch. It is directly attached to the lower beam without using a die holder.

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3-6 Sectionalized Type The sectionalized type mentioned here is different from the split type tooling we have discussed in the previous subsections. Split type tooling is a general term. The name is a classification term indicating that split tooling is distinct from one-piece type tooling. The term "sectionalized type" indicates a die or punch that is split into small, specific sizes.

3-6-1 Sectionalized Punch Photo 3-3 shows an example of a sectionalized punch. The shape of the sectionalized punch coincides with those of the L and S sizes. Accordingly, if sectionalized punches are used in a line, various combinations of sizes, including the total length of the beam can be created according to the operating requirements. This method is effective in making box-shaped products. The sectionalized sizes are 10, 15, 20, 40, 50, 100, 200, and 300 mm. There are two types of the 100 mm punch; an overhang called a horn at the left end and the other has a horn at the right end. The shape of the horn is identical among all Amada standard punches. They are ideal for box + pan forming when a return flange is involved. Amada offers a variety of sectionalized punches enabling you to purchase only those which best suit your needs.

3-6-2 Sectionalized Die The sectionalized die is attached to the die holder on the rail, as shown in Fig. 3-13. The size of H conforms to the flange height of the workpiece that can be bent with the tooling. That is, H = 10 mm indicates bending work can be performed up to a flange height of 10 mm, and similarly, H = 40 mm up to a flange height of 40 mm. The sectionalized dies which have an H size of 10 mm can be utilized in a row because they are 26 mm or the same height as the L and S sized 2V dies. With regard to a sectionalized die with the flange height of 40 mm, it is necessary to prepare several units of the same die or make a 2V die with an installed height of 56 mm as special tooling. Sectionalized dies are divided into eight lengths: 10, 15, 20, 40, 50, 100, 200, and 400 mm.

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3-7 Holder There are two methods of attaching the tooling to the machine. One type fixes the punch or die directly on the upper or lower beam. The other type locks them by using holders. When using tooling holders, attach the punch to the punch holder and the die to the die holder.

3-7-1 Punch Holder As shown in Fig. 3-8, the use of a punch holder permits the desired shape punch to be attached as necessary, just as radius rulers of different radii or coining rulers can be attached thereto.

3-7-2 Die Holder Two types of die holders are utilized according to the type of die. One type is the die holder used in the bending of sashes [Fig. 3-11(a)]. The other type is the die holder used with 2V dies [Fig. 3-11(b)]. The die holder must be selected in conformance with the shape of the product. A detailed description of die holder selection is provided in subsection 4.3.

3-7-3 Die Base The lack of bending stroke commonly occurs in cases where a IV die (type not for sashes) is used with a machine having a large open height. The die base (Fig. 3-14) is set under the IV die to increase die height. (For the open height of the machine and the die height, refer to subsection 4. 1

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4 SELECTION OF BENDING TOOLS

This chapter describes the various types of bending tools used in V-bending, R-bending, and hemming. It explains the requirements of adequate tooling selection, as well as operational problems and care required with these three types of bending. Special shapes that cannot be formed with the above tooling must be made by using special tooling. This special tooling will be discussed in chapter 5.

4-1 Process of Selecting Tooling You have probably heard that synthetic judgment is required to select tooling. "Synthetic judgment" means there is a process for properly selecting the tooling, and that the tooling selection must be made in accordance with this process. Problems that occur during operation usually result from the wrong tooling selection. The process of selecting the proper tooling involves a V-width decision. Also, the combination of a punch and die, the machine's specifications, the bending sequence and return bends, and the guaranteed pressure tolerance of the tooling must be considered. If the process is observed, the likelihood of making a mistake in selecting proper tooling is greatly reduced. Now, we will describe the selection process.

4-1-1 Selection of V-Width The V-width has already been described in subsection 1.5, "Bottoming." Furthermore, in chapter 2, "Use of Bending Force Chart," you have tried some exercises using given V-widths. As previously noted, the starting point in selecting the proper tooling is to determine the V-Width. The V-width must be determined on the basis of relationship between sheet thickness and V-width. However, the coefficient 6 of 6t or 8 of 8t in the table is only a standard. Therefore, the V-width provided by the table must be reviewed in accordance with the actual requirements of the product (such as minimum flange size: b, inside bending radius: ir that are indicated in the drawing, and required tonnage: F). Such a review frequently leads to a reduction of the V-width or a change in product design.

Selection of Bending Tools

Process of Selecton Tooling Hemming Bending Pattern Bending Sequence V-Bending Guaranteed Allowable Tonnage R-Bending (radius bending) Table of Contents

In conclusion, the V-width obtained from Table 1-1 should be considered a provisional V-width. It should be noted that the "true V-width" can only be determined when b, ir, and F are carefully examined.

4-1-2 Tooling as a punch and die combination One of the essentials of proper tooling selection is to understand that bending tooling is a combination of a punch and a die. The combination varies with sheet thickness and shape of the workpiece, and occasionally with the type of material. Table 4-1 shows typical combinations of punches and dies from which an optimum combination with regard to bending pattern, sheet thickness, and shape can be selected. Operators should make it a practice to consider punch-die combinations. Improper attention to the selection of a punch or die may be due to a lack of recognition that a bending tool is a combination.

4-1-3 Machine specifications and setting of tooling The fundamental machine specifications which require careful consideration are capacity, beam length, open height, and stroke. The manufacturer's catalog and instruction manual should be reviewed prior to determining the necessary tooling for these major elements. Fig. 4-1 illustrates the attachment of bending tooling to the machine. In the figure, F is the open height, B is the punch height, D is the die height (including the die holder height E), G is the stroke, and C is the distance from the punch tip to the die. The distance between the punch tip and the die should allow the work to be freely loaded or unloaded in the fore-and-aft direction during operation. Some work, such as sash products, must be removed sideways because of their shapes. This method of unloading, called "sideways pull-out," is to be avoided if possible because it reduces operating efficiency. From the viewpoint of operating efficiency, it is best to load and unload the work from machine. The information provided above teaches that sufficient space bgtween the punch tip and die is advantageous in terms of operational efficiency. Generally, a large punch tip-to-die distance is required when the punch and die are high or the product is bulky. However, the stroke of the machine must be considered if a low punch and die are used; otherwise, proper bending cannot be performed because of the lack of stroke. Useful formulas which compare the distance between the punch tip and die C, and stroke G are as follows:

C = F-(A+B+D)............................................ (1) C+V/2 < G.................................................... (2)

where A is the height of the distance piece. As shown from formula (2), the stroke will be sufficient if the sum of the punch tip-to-die distance and 1/2 of the V-width is less than the stroke.

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4-2 Bending Pattern Table 4-2 shows the bending patterns of a variety of products classified according to their shapes. The bending tooling is named to correspond with the bending pattern, such as V-bending tooling and R-bending tooling.

There are three patterns which can be formed with standard Commercial tooling. These patterns are V-bending, R-bending, and hemming. No other pattern can be formed without using special tooling designed for the particular shape of each product.

The following subsections will deal with V-bending, R-bending, and hemming. Go to Top

4-3 V-Bending V-bending produces a representative pattern of all the bending techniques and is the most widely used. V-bending has a wide range of applications, from simple one-stage bending of angular shapes to complicated multi-stage bending used in forming building materials and sashes. The thickness of the work ranges from very thin sheets of approximately 0.5 mm to thick sheets of 25 mm. V-bending can be divided into bottoming, partial bending, and coining. Also, as seen from the bending shape, it can be divided into 90° bending and acute-angled bending. (Obtuse-angled bending can be accomplished by using either 90° or acute-angled tooling.)

4-3-1 90° bending tooling (i) Combination of 90° bending tooling The 90' bending tooling must be selected according to the sheet thickness of the work to be bent. In Table 4-1, "Typical Combinations of Bending Tooling," Nos. I through 3 are applicable to work up to 3.2 mm thick. Nos. 5 and 6 are applicable to sheets thicker than 3.2 mm. Sheets in the first category are subdivided into two thickness ranges: one range is from 0.5 to 2 mm and the other from 2 to 3.2 mm. Use a 90° combination (punch and die) for sheets 0.5 to 2 mm, and an 88° combination for those 2 to 3.2 mm.

(ii) Graph of return bend limits Table 4-3 shows the size limits of bending shapes. "Size A" is called a step difference and "size H" a down bend. The step difference is indicated by the minimum value at a certain V-width, and the down bend by the maximum value. Sizes A and H can be obtained in the manner shown in Fig. 4-2(a) and (b). Size H is determined by the height of the die holder. In Table 4-3, (a) indicates the numerical values that apply to V-bending

using a 39 mm high die holder, while (b) indicates those applicable to V-bending using an 81.5 mm high die holder for sashes. The limit sizes include "W" and "L" in addition to A and H. The

Table 4-3 Limit Sizes of Bendable Shapes

relationship between W (expressed as "inside-inside" size) and L (expressed as "inside-outside" size) can be read from a graph of return bend limits.

Fig. 4-3 is an example of a graph of return bend limits. To read the graph, ensure the inside line of the work in the second stage of bending agrees with the reference lines of L and W; extend a line from size W along the axis L until it meets the profile of the relief part of the tooling; take a margin approximately 1 mm from that point in the directions of L and W and read the size by using the graphic scales. The dimensions shown in the drawing are usually outside measurements which include sheet thickness. If so, subtract sheet thickness from the dimensions in the drawing. There are three types of bending tool used with sizes W and L. One type is designed for bending with a narrow W. A second design allows bending with a large L. The third design can be used for both kinds of bending. The gooseneck punch, which has a large relief shape, is the type capable of bending with a large L size. Amada's standard gooseneck punch is available in two types: one for an L size of 50 mm and the other for 60 mm. The 50 mm punch is also available for use with a narrow W.

In cases where a punch with a tip angle of 88° is used, the bending angle is 88°; therefore, sizes W and L in the return bend are somewhat smaller than those in 90° bending. Size L must be carefully selected when using a gooseneck punch because it is large.

(a) Punch No. 4 or 16 with 39 mm high die holder.

V 4 6 7 8 10 12 14 16 18 20 25

t 0.7 1 1 1.2 1.6 2 2.3 2.6 3 3.2 3.2

W (minimum) 9.7 10 10.1 10.2 10.6 11 11.3 11.6 14 14.6 18.6

A (minimum) 4.9 6.5 7.1 8 9.7 11.5 13.2 14.7 15.5 18.5 21.7

H (maximum) 44.5 mm

(b) Punch No. 200 or 201 with 81.5 mm high die holder for sash.

V 6 8 10 12 14

t 1 1.2 1.6 2 2.3

W (minimum) 9 9.3 9.6 10 10.3

A (minimum) 9 9.5 10.7 12.5 13.6

H (maximum) 91 mm

(iii) Effect of punch tip radius (R) T he No. 5 or No. 6 combination in Table 4-1 should be selected when bending sheets thicker than 3.2 mm. Use No. 5 to bend sheets 4 to 10 mm thick. Use No. 6 to bend sheets 12 to 15 mm thick. The punch used in bending sheets 4 to 10 mm thick is called a "heavy punch" or "thick work punch." Its tip R is 6 mm. Further, an R punch called a radius ruler is used to bend sheets 10 to 15 mm thick. The size of R, as stated above, differs with the thickness of the work to be bent. It is usually necessary to use a punch with a large tip R when 90' bending thick sheets. As already noted, ir of the product is basically determined by the V-width, but the punch tip R also influences ir. When the punch tip R is large, the value of ir = V/6 turns out to be slightly larger, and vice versa. To say that ir is determined by the punch R is incorrect. This is evident from a close review of the relationship between the V-width and ir in the bending force chart based on measured values. Therefore, the punch should have a tip R somewhat smaller than the ir determine by the V-width. If the punch does not meet this specification, a proper inside radius cannot be obtained and cracks may develop in the outside radius of the product. Cracks cause a considerable decline in product precision. Deeply cracked products become useless rejects. This is why radius rulers of R20 and R25

are used with a IV die of V=125 mm and V=160 mm, respectively.

(iv) IV die with V-width of 200 mm Amada offers a semi-standard IV die with a V-width of 200 mm for use with a special R punch (R: 3(l mm). Fig. 4-4 illustrates the attachment of the die to the machine. This 200 mm V-width die is 100 mm long. When used, several dies are arranged in line according to the length of the workpiece. Usually, they are not installed close together but rather at intervals of 0.5t to it. For this reason, the interval can be increased for thicker work sheets. Accordingly, the number of dies required, relative to the bending length of the work can be reduced. The main feature of this die ties in the hardened rollers provided at the shoulders of the V-groove. The work is bent with the help of the rollers and therefore the bending force can be decreased. This type of tooling is capable of bending 25 mm thick sheets at maximum.

4-3-2 Tooling for acute-angled bending (i) Tooling combination for acute-angled bending Acute-angled bending ks a type of V-bending used to bend work into a final design or for use during the preliminary stage of hemming work. Nos. 9 through 12 in the table of combinations correspond to punch and die combinations for this bending. The die V-width in acute-angle bending may be determined using the same standard as for 90° bending. Amada's standard sizes are 8, 12, 18, 25, 32, 40 mm, etc. The minimum flange length for acute-angled bending is shown in 'table 4-4. It is longer than that for 90° bending because the minimum flange length is related to the side length of the V-groove.

(Dash indicates unworkable) table

(ii) Punch tip angle Two punch tip angles are used: 30° and 45°. A punch with a 30° tip is preferable when acute-angled bending is performed in the preliminary stage of hemming. A 30° tip should also be used for sheets less than 3.2 mm thick, and a 45° tip should be used for thicker sheets.

(iii) Flat width of punch tip The punch for acute-angled bending is flat at the extremity of its tip. This corresponds to the R tip of

Table 4-4 Minimum Flange Length in Acute-Angled Bending

Sheet thickness (mm) 1.6 2.0 2.3 2.6 3.0 3.2 3.5 4.0 4.5 5.0

Minimum flange length (mm)30° 10 16 24 24 24 35 35 - - -

45° 10 10 19 19 19 - - 35 35 41

the 90° bending punch. Fig. 4-5 shows the flat tips of typical acute-angled bending punches. In acute-angled bending, fissures and cracks sometimes appear on the outside R surface of a product. The width of the flat area on the punch tip greatly influences the occurrence of cracks. When bending easily cracked aluminum sheets or thick sheets of other materials, punches with large flat areas on the tip will prevent cracking.

When bending a 2 mm thick sheet of corrosive-resistant aluminum to 30' and then hemming it, cracks formed along the bend line. Our investigation revealed that the cracks developed during the 30' bending and became more severe during hemming. The 300 punch used (No. 13 combination in Table 4-1) has a tip with a flat width of 0.5 mm. The flat area was increased to a width of 3 mm, and the modified punch produced satisfactory bends without any cracks. This is a good example to show how the flat portion of the punch tip influences bending work. Go to Top

4-4 R-Bending (radius bending) 4-4-1 Characteristics of R-bending R-bending is a bending process in which the ratio of inside radius (ir) to sheet thickness (t) of the product is large. As stated in subsection 4.3, the inside radius of a product can be obtained by ordinary V-bending with 90' or acute-angled tooling. However, air bending and coining differ from R-bending because of the small value of ir/t. Generally, in R-bending where the value of ir is large, the amount of springback is also large. This factor must be taken into consideration when radius bending. Another factor that should be taken into consideration is the multi-breakage bending phenomenon that sometimes occurs in R-bending, depending on bending conditions. These two factors, namely, a large amount of springback and the occurrence of multi-breakage bends can characterize R-bending.

4-4-2 Springback in R-bending It is necessary to know the amount of springback (AO) in advance in order to properly perform R-bending. Table 4-5 shows the amount of springback of SPCC relative to the punch R. The table also shows that springback AO increases with an increase in ir/t. If you apply the requirements of t and ir provided in the drawing to this table, you will find that the springback (AO) can be determined. For example, suppose a 1 mm thick sheet of SPCC is to be bent to a shape with an inside radius of 20 mm. You can see that the springback (At)) is 9° and the punch R (rp) is 18 mm. In addition to the ratio of inside radius to sheet thickness, the tensile strength of the work and the type of tooling also greatly influence springback. Generally, the amount of springback is proportional to the tensile strength of the work. The springback values for SUS are 1.5 times the

values of Table 4-5 (i.e., for SPCC). Thus, judging from the tensile strength, it may appear that the springback of

aluminum and copper is approximately 0.5 times as large. In fact, however, the springback of these nonferrous materials is much greater than that of SPCC. This example clearly shows that there is a need to experimentally confirm the amount of springback of each material. Commonly, R-bending is performed by air bending, bottoming, or using urethane tooling, Table 4-5 tabulates the data on R-bending by air bending. If the amount of springback in air bending is taken as 1, the amount of springback in bottoming is half that of air bending, and that of urethane tooling is 2/3 as large.

[Exercise]

Calculate the springback AO when a I mm thick stainless steel sheet is bent to a shape with an inside radius of 20 mm using urethane tooling.

4-4-3 Multi-breakage bending phenomenon Multi-breakage bending is a phenomenon that occurs in R-bending. The cause of this phenomenon is as follows: In air bending, when a radius ruler and IV die are used in combination, a phenomenon occurs in which the work separates from the R-punch tip in the course of bending, as shown in Fig. 4-6. This is an advanced bending phenomenon in which the work bends in advance of the tool's motion. As bending progresses with the application of bending pressure, the advanced bending spreads left and right and consequently, a polygonal, angular R surface is formed. This is the multi-breakage bending

Table 4-5 Punch R and Springback of SPCC (Air bending)

ø: Amount of spring (degree) ir: Inside R of product (mm) rp: Punch R (mm) t: Sheet thickness (mm)

t 10 15 20 25 30 35 40 45 50 55 60 ir

0.89.3 13.5 17.6 21.4 25.0 28.8 32.0 35.0 36.1 37.9 40.0 rp

6 9 11 13 15 16 18 20 25 28 30 ø

1.09.4 13.8 18.0 21.9 25.7 29.2 32.9 36.0 38.9 41.6 43.3 rp

5 7 9 11 13 15 16 18 20 22 25 ø

1.29.6 14.0 18.2 22.5 26.3 29.9 33.8 37.5 41.1 42.0 46.0 rp

4 6 8 9 11 13 14 15 16 18 20 ø

1.69.6 14.3 18.7 23.1 27.0 31.1 35.1 38.8 42.2 45.8 49.7 rp

3.5 4.5 6 7 9 10 11 12.5 14 15 15.5 ø

2.39.7 14.4 19.0 23.5 27.8 32.3 36.4 40.5 44.4 48.3 52.0 rp

2.5 3.5 4.5 5.5 6.5 7 8 9 0 11 12 ø

2.69.8 14.5 19.1 23.6 28.2 32.5 36.7 41.0 44.7 48.9 53.0 rp

2 3 4 5 5.5 6.5 7.5 8 9.5 10 10.5 ø

3.29.8 14.6 19.3 23.9 28.5 32.9 37.3 41.5 45.8 50.1 54 rp

2 2.5 3 4 4.5 5.5 6 7 7.5 8 9 ø

phenomenon.

Although multi-breakage bending occurs only in R-bending, it does not occur every time. The larger the ratio of sheet thickness to the inside radius, the more readily it develops. Also, multi-breakage bending occurs less frequently in materials with high tensile strength and ductility. In other words, it is less likely to occur in stainless steel than in cold-rolled steel; and in SPCE than in SPCC.

In table 4-5, the bold line indicates the region in which multi-breakage bending occurs. As a judgment standard, multi-breakage bending usually occurs when the ratio of sheet thickness to inside radius is 30 or greater (in the case of SPCC). Accordingly, counter-measures must be taken if the bending conditions fall within the above region. As already stated, multi-breakage bending is caused by advanced bending of work that has separated from the punch tip. Thus, in order to prevent it, the work should be kept in contact with the punch by applying counter-pressure from the bottom. Use of a spring-loaded counter-holder type of tooling [see Fig. 4-7(c)i and an elastic body, such as a urethane pad, is an effective means of preventing the work from multi-breakage bending.

4-4-4 Types of R-bending Fig. 4-7 illustrates the types of R-bending generally used; (a) air bending type, (b) cam type, (c) counter-holder type that utilizes a 1V die, (d) bottoming type using formed tooling, and (e) the type using urethane tooling. This subsection will discuss the air bending and urethane tooling types of R-bending.

(i)Air bending Photo 4-1 shows the air bending type of R-bending being performed with a radius ruler (R punch) and 1V die. In this type of R-bending, standard 85' die can be used if the amount of springback AØ is 5° or less. In the case of SPCC with AØ=5°, ir/t is known to be approximately 10 from Table 4-5. If this is applied to I mm thick SPCC, the inside radius will be 10 mm. If AØ is greater than 5 degrees, the V-groove relief must be modified so that the work can be bent to a smaller angle. Because the actual V-width changes to the distance between points A and B (Fig. 4-8), the bent portion of the work reaches the bottom of the die. Thus the work cannot be bent to an angle absorbing AØ, or more than 5° in this example.

The V-width of the R-bending die is determined by the following formulas:

If AØ=50, V-2.2x(rp+t) If AØ=50, V-2.5x(rp+t)

As is clearly seen from Table 4-5, the punch R should be smaller than the inside radius of the product. When selecting a punch, it is common practice to select one with a standard R which is closest to the inside radius of the product. The inside radius to which the product is finished with a commercial radius ruler is calculated by the following formula. ir = rp x90/ (90-AØ) In R-bent products, the dispersion of their inside radii is questioned more frequently than their conformance with the absolute value of ir.Therefore, standard tooling should be utilized even if design modification of the product is required. If this is not possible, a new punch and die must be designed and produced. The amount of time required for delivery and the cost of the tooling must be considered beforehand.

Because the hatched portion interferes with the work in R bending, it is advisable to cut it out of the V-bending die. Fig. 4-8 Modification of standard die

(ii) Urethane tooling Recently, urethane tooling is being used more often for R-bending. The reasons why urethane tooling is suitable for R-bending are: (1) the work is not scratched because no slippage occurs between the work and die, (2) it can serve as a counter-holder, and (3) springback can be absorbed by overforming (excess bending).

No. 8 in Table 4-1 shows combinations used in R-bending. Fig. 4-9 shows the shapes and sizes of representative retainer boxes. The urethane pads, which are placed in their respective retainer box, come in five sizes: 25x25 mm, 30X25 mm, 50X50 mm, 80X30 mm, and l00x45 mm. Some operators attempt R-bending using only a urethane pad. This is because they don't understand the principle of bending when using urethane pads, and such attempts will not produce satisfactory results. The reader should remember that a urethane pad must be used with a retainer box in bending operations. When R-bending is to be performed using urethane pads, special attention should be paid to the following points: (1) the tonnage required for the R-bending, (2) the size and hardness of the pad, and (3) the bending shape. The paragraphs that follow will discuss these three points.

Table 4-6 tabulates the tonnage required for 90° R-bending under various conditions. The tonnage required in R-bending using air bending tooling is usually very small because the V-width ratio to sheet thickness is large. On the other hand, in R-bending with urethane tooling, the required tonnage increases as the product's radius increases. The reason for this is that the required tonnage is proportional to the volume of the urethane pad forced into the retainer box. Therefore, it is necessary to determine whether the machine's capacity is sufficient to achieve the R-bending intended. In a sense, one drawback of bending with a urethane pad is that it requires extremely great force. A rectangle is more suitable than a square in regards to the sectional shape of the pad used in R-bending. Table 4-7 shows applicable die pads relative to the punch R and sheet thickness. Further, the two urethanes used in R-bending differ in hardness: "Rubber Shore A 80° "and "Rubber Shore A 90°." Use of a pad with Rubber Shore A 80° can reduce the tonnage required with a Rubber Shore A 90° pad by 20 percent. One disadvantage of urethane tooling is that it is not very durable to sharp edges. If the edges of the work (after shearing) come into contact with the upper surface of the pad while force is being applied, the life of the pad will be shortened remarkably. Also, the use of a retainer box makes urethane tooling unsuitable for bending shapes with short flanges and a small step differences.

4-4-5 Summary of R-bending The air bending type of tooling best serves R-bending. In order vo eliminate scarring caused by slipping, a coated work material must be utilized. Otherwise, a urethane sheet should be spread on the IV die. A certakn degree of multi-breakage bending is unavoidable with the air bending method. Therefore, urethane tooling should be used if you don't want polygonal-bent products. In any case, you should know the advantages and disadvantages of urethane tooling in order to use it properly.

Table 4-6 Required Tonnage for 90° R-Sending (Rubber Shore A 90°)

Punch R (mm) 10 15 17.5 20 25

t (mm) (unit: ton)

0.4 13 16 18 21 26

Go to Top

4-5 Hemming Hemming comes from the word "hem" which means to turn and sew down a border or edge of cloth. It is one of the conventional bending techniques. Hemming requires great bending force. Table 4-8 shows the tonnage required for hemming. As indicated in the table, extreme force must be applied in the second stage of the work process, when the folded edge is pressed. When selecting a machine for hemming operations, special consideration should be given to ensure adequate tonnage.

A thrust load is produced during the hemming operation. The thrust load is a force acting in the fore-and-aft direction of the machine. It exerts an adverse influence on the machine body. The thrust load occurs in the crushing stage of hemming and increases proportionally with the required tonnage.

0.6 15 18 21 24 29

0.8 17 21 23 27 33

1.0 19 23 27 30 36

1.2 22 25 29 34 39

1.6 26 30 34 40 46

2.3 34 39 43 (50) (58)

2.6 37 43 (47) (55) (63)

3.2 46 (54) (60) - -

Note: SPCC - 80x30 mm urethane pad - No. 4 retainer box

Table 4-7 Die Pad Application (Rubber Shore A 90°)

Punch R (mm) 10 15 17.5 20 25 30

t (mm) (unit: mm)

0.4

0.6 Die pad 50x50

0.8 Die pad 80x30

1.0

1.2

1.6

2.3

2.6 Die pad 110x45

3.2

Note: -Use 1 rank smaller t for Rubber Shore A 80° pad. -Large pad includes the applicable rane of smaller pad.

Preventing the occurrence of a thrust load can be considered from two aspects: the type of tooling and the working method. As stated above, the characteristics of hemming are a high tonnage requirement and the occurrence of a thrust load. Hemming is performed in two stages; acute-angled bending is the first stage. Two types of tooling are used. One type requires the use of two sets of tools, one for 30' bending in the first stage and one for flattening in the second stage. The other is a double-deck type that enables hemming without tooling replacement. The type that requires tooling replacement is further divided into two types for light and heavy work according to the sheet thickness of the work.

4-5-1 Type requiring tooling replacement There are two tooling combinations for this type: a combination of No. 9 or No. 12 (both for the first stage) and No. 14 (for the second stage), and a combination of No. 10 or No. 11 (the first stage) and No. 15 (the second stage). The former can bend a maximum sheet thickness of 2 mm, and the latter can bend sheets from 2.3 to 4.5 mm in thickness.

Material: SPCC, SS41

Sheet thickness

t (mm)

Bending shape

Required tonnage C Required tonnage 2t

0.6 9 3 23 1.2

0.8 12 3 32 1.6

1.0 15 3.5 40 2.0

1.2 17 3.5 50 2.4

1.6 22 4.6 63 3.2

2.0 30 5.5 80 4.0

2.6 55 6.5 90 5.2

3.2 70 8.0 100 6.4

*4.5 105 11.3 200 9.0

Material: SUS

Sheet thickness

t (mm)

Bending shape

Required tonnage C Required tonnage 2t

0.6 15 3 35 1.2

0.8 20 3 50 1.6

1.0 25 3.5 60 2.0

1.2 26 3.5 80 2.4

The tooling discussed above is widely used because it is simple and easy. However, after the first stage, the partially finished products must be stacked near the machine and the second stage performed after tooling replacement. Therefore, use of this tooling requires an extensive working space and often stagnates production flow. In order to prevent the undesirable effects of the thrust load, hemming must be performed with a small bending force. Press the work by slowly applying a smaller force in degrees while movkng the work back and forth. Then use the proper force. The type of tooling (combination of No. 9 or No. 12 and No. 14) mentioned above requires that bending force be applied in several steps. The die used with No. 15 in Table 4-1 is a sliding-type hemming die. The top of the die slides backward to absorb the thrust load. This type of tooling permits the hemming of SPCC sheets which have a thickness of 2.3 to 4.5 mm. According to Table 4-8, heavy spacers must always be used if the required tonnage per meter exceeds 100 tons. They protect the upper beam face and the face of the punch. However, when performing a bending operation in which the required tonnage exceeds 100 tons per meter, special consideration must be taken.

4-5-2 Double-deck type This bending technique uses double-deck type No. 103 punch and Table 4-1. The tooling combination consists of the he die selection the No. 104 or No. 1046 die (see Photo 5-9). The die selection depends upon the sheet thickness of work and the hemming size. The double-deck type is applicable to a sheet thickness range of 0.6 to 1.6 mm (t=1.2 mm at maximum for SUS). Sheets having a thickness excess of this range must be formed by using the tooling described in subsection 4.5.1.

This type of tooling is advantageous because 30º bending and crushing can be achieved continuously without any extra semi-finished products. Go to Top

4-6 Bending Sequence Bending work must be performed according to a predetermined bending sequence. The bending sequence determines whether or not products are properly formed. It also makes bending work more difficult, by causing an increase in tooling types and attachments (especially backgauges). The requirements of precision and job efficiency, however, dictate that a proper bending sequence be used. Repeated practice is required to be able to quickly determine the bending sequence. As in the saying "Practice makes perfect," one can learn the various bending sequences from repeated practice, enabling one to handle more complicated bending sequences. Next, we will briefly describe the procedure for determining the bending sequence, the tools available for determining the bending sequence, and a new technique of drawing using automatic programming.

1.5 38 4.6 95 3.0

2.0 50 5.5 130 4.0

*2.5 90 6.5 180 5.0

*3.0 100 8.0 210 6.0

*4.0 140 11.3 280 8.0

* Use a heavy spacer for sheets marked with an asterisk.

4-6-1 Procedure for determining bending sequence The procedure for determining the bending sequence involves investigation of the following:

1. The bending shape required 2. The first and final stages of bending 3. Important dimensions of the product 4. Efficiency of the work handling 5. The relationship between bending angle and backgauge position 6. Examination of "pre-bending"

In general, the bending sequence is determined by processing some of the above items together.

4-6-2 Use of tools Determination of bending sequence using only brain work is inefficient and often ends in failure. Since the bending sequence is not theoretical but concrete, it can be determined best by using various tools.

The major tools include (1) a graph of return bend limits (Fig. 4-3), (2) wire (thin copper wire is preferable), (3) a profile model of the tooling (Photo 4-2), (4) a full-size drawing of the punch and die, and (5) an attachment plan of the tooling.

When determining the bending sequence, it is a common practice to investigate the factors listed above using these tools. It is possible to decide which type of tooling should be used after the bending sequence has been determined- There are three types of tooling: a standard type, a standard type requiring modification, and a special type that must be designed and manufactured.

4-6-3 Automatic programming Automatic programming is the newest technique which determines the necessary tooling. Those items described in subsections 4.6.1 are programmed. Upon inputting the required shape, a suitable type of tooling is indicated. Then the bending sequence with the designated tooling and the figure to be bent is plotted. In addition to selecting tooling and determining the bending sequence, automatic programming can output various data. Examples of data are the NC instructions (L value, D value, pullback value, and stopper position) , work handling (turning of the work), tact time (time required for bending a product), and weight calculations.

Automatic programming is very convenient but it is not always perfect and trouble free. For instance, the bending sequence determined by automatic programming is not necessarily an optimum working order [(3), (4), 1 and (5) in item 4.6.1 may be problems] because it can only be determined after many

factors have been analyzed and unified. Go to Top

4-7 Guaranteed Allowable Tonnage of Tooling The guaranteed allowable tonnage is usually indicated by the amount of tonnage that can be applied per unit length to the tooling. The tooling should be operated under a pressure less than the allowable tonnage. Table 4-9 lists the guaranteed allowable tonnage of standard tooling, Prior to operation, the operator should know the guaranteed allowable tonnage of the tooling in order to prevent accidents and insure a long service life. Applying a bending force which exceeds the allowable tonnage is dangerous for both the operator and machine because the excessive tonnage damages the tooling and sometimes breaks or bends it. Adequate bending force must be applied when bending a work-piece. This means that pressure adjustments must be made in accordance with the respective bending conditions (work material, work thickness, bending length, tooling allowable tonnage, V-width, etc.). most tooling breakage results front failure to observe this principle.

Go to Top

Table 4-9 Guaranteed Allowable Tonnage of Standard ToolingPunch Die

Punch No. ton/m Die ton/m4, 16 100 121 60

147, 148 45 123 70116 20 124 80

45, 46 50 125, 126 10050,51 45 127 70

452, 462 70 301 60150, 151 45 302, 303 70453, 463 50 304, 305, 306 100155, 156 30 13, 14, 18, 35 100108, 109 50 5, 36, 37, 38, 39 100200, 201 100 337 30103, 210 100 340 80

8 30 341 6049 20 342 40215 100 343 703 100 320, 321, 322, 323 95

230, 231 100 324, 325, 326 100220 100 12 100

311 80

314 100

104, 1046 65

Note: Apply the value for the punch or the die, whichever is smaller.

5 LABOR-SAVING BENDING TOOLS

This chapter compliments chapters 3 and 4, which outline the fundamentals of standard tooling. It describes some special tooling and discusses this tooling from the viewpoint of saving labor. Note that the title "labor-saving bending tools" is adopted for convenience and is not a general term. Three aspects of labor economization in sheet metal bending must be investigated: the machine, the peripherals, and the bending tools. More detailed investigations in this field will eventually lead to the development of a production system of compound lines. This type of investigation, however, is not a future prospect; but there is an urgent need for it.

5-1 Objective of Labor Economy How can we economize on labor in sheet metal bending work? What are the objectives of labor economy? Here we will consider labor-saving techniques. Overall, the objectives of labor economy depend on the needs which arise on the job site. Hints for labor economy are hidden in the difficulties and problems that operators encounter daily . In this subsection, the following seven items will be discussed from the viewpoint of labor economy: (1) elimination of spring-up, (2) reducing the number of processes, (3) increasing tooling versatility, (4) making material handling easy, (5) reducing man-hours to replace tooling, (6) bending coated workpieces, and (7) elimination of the need for welding.

5-1-1 Elimination of problem caused by spring-up When bending small flanges on large work (such as switchboard doors, console boxes, partition boards, and steel doors), "buckling" occurs when the work springs up. Buckling is a phenomenon in which the work bends slightly due to its own weight in a portion off the V-groove of the die.

In addition to buckling, the work springs up during bending and drops after completion of bending. This is dangerous for the operators. It takes several operators to perform this operation safety and it is a very fatiguing job. Bending with a press brake is a type of spring-up bending (spring-up also is a problem when planning automated bending). Bending is divided into spring-up bending and folding. Folding is suitable for the above examples of product bending. A folding machine is a special machine designed specifically for folding work. Fig. 5-1(a) illustrates spring-up bending and 5-1(b) folding. A sheet follower/support system (Photo 5-1) provides a means of eliminating problems caused by spring-up. There are several types of sheet follower/support systems that correspond to the weight of the work. Each permits the operator to easily and quickly produce accurate products.

Labor-Saving Bending Tools

Objective of Labor Economy Types of Labor-Saving Tool Points of Tooling Design Table of Contents

As stated above, problems caused by spring-up can be eliminated by using a folding machine or sheet follower. Also, spring-up can be reduced significantly by the use of special dies. There are two types of tooling design. One type produces no spring-up and the other type produces a slight spring-up (refer to item 5.2.1, "Spring-up-preventive tooling")

5-1-2 Reducing the number of processes The shapes of bent products are becoming more and more complicated. This is primarily due to the existence of NC press brakes which permit easy setting of the backgauge dimension and bend angle. At the same time, there is a growing tendency to take positive attitude, "Let's bend everything." This is a desirable trend because the experience gained by bending complicated shapes is expected to help eliminate or reduce the need for welding. Bending is definitely changing due to the advent of NC. Hopefully, the number of processes can be reduced to one, bending shapes that now require two or more bending processes. However, considering the prevailing NC applications, it does not seem as advantageous as before to design special tooling (For example, tooling that forms a hat shape requiring four bends in one process). In this case, adopting special hat tooling should be avoided in consideration of dimensional changes. The following are three common characteristics of special tooling:

1. Special tooling is designed to serve a specific purpose; therefore, it is not versatile enough to cope with dimensional changes. This means that new tooling must be designed and produced each time a change in product dimensions 'is desired.

2. Special tooling is expensive and delivery takes a long time in comparison with standard types.

3. The tonnage required for bending with special tooling is greater than that with standard tooling.

Accordingly, special tooling should be employed for shapes that cannot be formed by V-bending and for mass-production goods. It is not recommended for the sole purpose of reducing the number of bending processes.

5-1-3 Increasing tooling versatility Boxes and panels consisting of two short and long sides are considered identical bending patterns, even though they differ in size. The purpose of increasing tooling versatility is to cope with various product sizes by making adequate combinations of punches and dies. Whether or not tooling versatility can be increased is dependent upon product standardization (keeping the variety of sheet thickness and sizes of products to a minimum).

5-1-4 Making material handling easy Bending work involves "turning," "reversing," and "holding" of the material. During loading and unloading operations, the material is handled either manually or by some mechanical means. A loading device, unloading device, turning table, sheet follower, reversing table, and robot are utilized in handling bending work. An effective means of using tooling more efficiently is to eliminate the necessity of reversing the work. An example is shown in Fig. 5-2. Continuous wavy folds can be formed without reversing the work. This is done by changing the conditions of the punch and die. It is best suited for forming shapes with long bending lengths or hard-to-handle thin sheets.

5-1-5 Reducing man-hours to replace tooling Labor economization in bending work refers to making material handling easy and reducing man-hours required for tooling replacement. Tooling is changed 20 times a day at most and 10 times on an average. The number of man-hours for tooling replacement is expected to increase in the future. A flexible manufacturing system (FMS) must be both "adaptable" and "versatile." A versatile manufacturing system is another name for a many-type, small-quantity production system. Production efficiency in such a system depends upon how quickly and frequently the tooling can be changed daily. In this sense, the FMS or a many-type, small-quantity production system is closely related to sheet metal processing. In order to accomplish as many daily tooling replacements as possible, it is necessary to reduce to a minimum the man-hours required per change of tooling (in other words, challenging to reduce the man-hours required for each tooling replacement to zero). An automatic tool changer (ATC) and a one-touch tooling clamp serve this purpose.

The automatic tool changer can accommodate several types of punches and dies (the tool changer shown in Photo 5-2 accommodates three types of punches and four types of dies). This tool changer, when interlocked with NC, can reduce the number of man-hours for bending if the product shape is within the applicable range of the tooling. An interesting case has been reported where the production time was shortened from 9 to 2.5 minutes per panel in the bending of industrial refrigerator panels. There are two types of ATC: (1) one is designed to cope with changes in product shape and (2) the other is designed to cope with changes in length. Although type (1) prevails at present, a new tool changer which would provide both functions is desired. ATC can keep pace with the requirements of the FMS age only if it can accommodate changes in the shape and size of the product. For information, the elimination of work reversal described above can be effected by using type (1). The one-touch clamps for punches and dies help reduce the man-hours required per tooling change. The clamps are mechanisms that cope with "single job preparation." In the future, it is desired that such clamps be provided as attachments to standard machines. Deck-type tooling is designed to reduce the number of tooling changes. This type of tooling includes the double-deck type for hemming and a triple-deck type used to bend the side panels of refrigerators. Use of deck-type tooling together with NC functions (control of L-axis, D-axis, bending speed, etc.) will enhance the economy of labor and facilities.

5-1-6 Bending coated workpieces Usually, sheet metal fabrication is performed in the following sequence:

Shearing => Punching => Bending => Welding and assembling => Painting

Some changes in this sequence have been noted with the increased use of coated sheets such as vinyl-chloride steel sheets and colored iron plates. In comparison to the conventional production process, a considerable reduction in man-hours can be obtained by the shearing, punching, and bending of coated steel sheets. The difference in total man-hours is determined from the conditional difference between the case where cubic products formed by bending are painted and the case where commercially coated materials are formed. (Of course, the man-hours required for painting is zero in this case.) As stated above, there is a great advantage in painting prior to bending because the man-hours required for painting can be reduced. Another advantage is that it will most likely reduce the cost of the painting facilities.

However, the use of coated steel sheets or painting prior to bending causes the following problems: (1) The need to prevent marring (scoring, scars made by slipping, creases, etc.) and (2) Development of a manufacturing method that supersedes welding. Presently, urethane tooling and seaming tools are utilized as countermeasures against (1) and (2).

5-1-7 Elimination of welding process Welding is necessary in sheet metal fabrication. Since welding distortions occur, many man-hours are required to smooth them. A workshop campaign, to get rid of "three evils" is carried out in many factories. The "three evils" refer to (1) marking-off, (2) hammering, and (3) finishing with a sander. Needless to say, hammering and finishing with a sander is the post-process of welding. Reducing the man-hours required for welding, smoothing, and finishing can be an effective means of saving labor in sheet metal processing. The following are useful methods to achieve the above:

1. Improvement in bending accuracy 2. Using bending instead of welding as much as possible 3. Utilization of seaming and locking 4. Use of fasteners and adhesive 5. Altering product design so welding can be eliminated

Next, we will introduce a couple examples of boxes built without welding. Photo 5-3 shows a box made of I mm thick vinyl-chloride steel without welding. In this box, welding is replaced by caulking, fasteners are substituted for hinges, and a new device is used for the door stopper. Photo 5-4 is a switch box made by bending only. It becomes a complete product simply by screwing the bottom plate. Both are the direct result of designs created to eliminate the welding process.

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5-2 Types of Labor-Saving Tooling In subsection 5.1, we outlined seven methods of saving labor in sheet metal processing. We have provided customers with a great number of tools designed to fit individual needs, but they have not been satisfactory in all respects. This merely confirms that the problems involved in bending tools are complicated and extensive. The reader should remember this when reading this subsection. The following pages describe types of tooling designed and manufactured for the purpose of "labor economy."

5-2-1 Spring-up-preventive tooling This bending tool can be divided into two types: the first produces no spring-up and the other produces slight spring-up.

(i) Tooling producing no spring-up The tooling used in L-bending, D-bending, and Z-bending are of this type. Each tool can bend in one process and is usable with a single machine or in an integrated production system.

a) Tooling for L-bending There are two types of L-bending tooling: a counter-holder type and a cam type. When the counterholder type is used, the work is formed by "wiping" after being clamped. Thus, accurate, uniform-quality products can be obtained. If a punch with a relief is used as shown in Fig. 5-3, it is possible to form ] " shapes in two stages. When bending box-like shapes such as panels, the punch may be divided into any optional length. The maximum bendable sheet thickness are 2.3 mm for SPCC and 2 mm for SUS. The obtainable inside radius of the product is 1.2 to 1.5 times the sheet thickness. Replacing the shoulder piece allows limited processing of sheets of a different thickness. The counterholder prevents the work from swelling or sliding by providing counterpressure. The cam tooling in Fig. 5-4 is a folding type. This bending method features light or little marring; unlike wiping. Furthermore, bending angle precision is easily obtained because the cam permits overforming (over-bending to compensate for springback). On the other hand, in comparison to the counterholder type, the inside radius of the producv is somewhat large (about two times sheet thickness). Also, there is a tendency for the flat surface to swell. Cam tooling requires a number of manufacturing processes; therefore, the tooling cost is high. The sheet thickness which can be used with the cam type tooling are the same as with the counterholder type.

(b)Tooling for ] bending

]-shapes can be formed in two processes by using L-bending tooling (either counterholder or cam type). The ] bending tool (Fig. 5-5) is capable of forming :]-shapes in one process. The tooling contains a cam mechanism in its counterholder, so it can also perform Z-bending, depending on the usage of the cam. When forming shelf sheets, the punch is divided into suitable lengths and ears (horns) are attached at both ends to prevent interference with the flange.

The ] bending tool is capable of bending work that is held horizontal. Thus, a production line which incorporates several press brakes equipped with this tooling and material loaders and autoloaders can provide continuous unmanned operation. Also, even with a single machine, this tooling satisfactorily serves as a labor-saving device in general bending operations.

c)Tooling for Z-bending There are two types of Z-bending tools: a single-hat type and a type similar to the counterholder type with a cam mentioned in the previous paragraph. The single-hat type (Fig. 5-7) features low cost and is high in utility value. The tooling in Fig. 5-7 is designed to bend the short sides (at a fixed length) after the long sides, enabling it to handle long sides of any desired length.

(ii) Tooling producing slight spring-up

The tooling used for beading is of this type. The principal sectional shapes after processing are semicircular, triangular, quadrilateral, trapezoidal, etc. The quadrilateral shape is called a hat bend.

Beading is effective in fabricating containers, structures, and reinforcements because a flat sheet increases in strength when beaded.

There are two types of beading tools: the air bending type and the counterholder type.

Fig. 5-8 shows a counterholder (trapezoid) type tool used for forming containers, This tooling is suitable for processing a large lot of products requiring uniform precision, and also for use in a production line. The tooling is profitable though high in cost.

An automated production line for containers is illustrated in Fig. 5-9. This is an unmanned production line consisting of a feeder, loading device, press brake with backgauges, feed device, two shearing machines, and an inverting stacker.

The beading tool (Fig. 5-8) and the backgauge which determine the bead pitch, are set in the press brake. The work carried by the feed conveyor runs against the first stopper and stops. Then the beam rises under the control of the current signal passing through the work.

After the first bending stage has been completed, the beam descends, the conveyor operates, and the work moves until it strikes the second stopper. In this way, the work advances to the fifth stopper where the final stage of forming is completed. The work can be made to pass any of the five stoppers by giving instructions. Then, the work is forwarded and trimmed at both ends by shearing machines. Finally, it is turned over and stacked. Checking the pitch size completes the entire process of this line.

5-2-2 Tooling that reduces number of processes U-bending, offset bending, or beading tooling is useful in reducing the number of processes. Many of these shapes, however, can be formed by the multi-stage V-bending method. As already noted, if V-bending with NC can be used, there is no need to provide U-bending, offset bending, or beading tooling. These tools are best suited for forming (1) a large number of products requiring a uniform finish and (2) shapes that cannot utilize the multi-stage V-bending method.

(i) Tooling for U-bending U-bending is a process whereby work is bent into a U-shape in one stage. The same U-shape can be obtained in two stages by using V-bending tooling but this is not called U-bending. U-bending tooling reduces the two stages required in V-bending to one stage. It can also form a U-shape which cannot be accomplished by V-bending; for instance, a shape whose flange is long in comparison to its width. The tooling used in U-bending includes a wedge type, roller type, counterholder type, cam type, and types using urethane. The proper tooling should be selected considering (1) parallelism, (2) bottom finish, (3) friction marring, and (4) tooling cost (in relation to the number of products).

a) Wedge type (see Fig. 5-10) Wedge type tooling is characterized by the following: (1) it is widely used regardless of the number of products, (2) the bottom is almost flat or concave and the degree of parallelism is satisfactory, and (3) it is inexpensive. Therefore, it is widely used as an economical tooling.

b) Roller type (see Fig. 5-11) Of all U-bending tooling, the roller type is the simplest and lowest in cost. It features: (1) Low friction marring, (2) the bottom is almost flat (though sometimes convex or concave), (3) the degree of parallelism is good. Roller type tooling can bend a maximum sheet thickness of 6 mm.

c) Counterholder type The counterholder type (Fig. 5-12) provides a high level of flatness and parallelism. It is suitable for the mass production of products requiring uniform, high precision. The tooling cost is high but it is sufficiently profitable if the number of products is large.

d)Cam type (see Fig. 5-13) The cam type of tooling contains cams. The work can be overformed (bent excessively in

anticipation of springback) by means of these cams. Thus, post-forming parallelism is quite satisfactory. Further, the degree of flatness is relatively good. However, this tooling is comparatively expensive because its manufacturing process requires too many man-hours. If sheet thickness is 3.2 mm or greater and the machine is lacking in stroke, this cam type should be used to yield good results.

e) Urethane tooling (Photo 5-5) U-bending with urethane tooling provides results similar to those of the counterholder type of metal die. Urethane tooling will (1) prevent die adhesion, and (2)"assure flange parallelism. With the exception of bottom flatness which tends to swell, the use of urethane yields satisfactory results in respecv to (1) and (2) above. It is also advantageous because it can form products without scratches. In addition, the tooling is inexpensive.

U-bending with urethane tooling Step bending/Spring-up

(ii) Offset bending tooling Offset bending is a process whereby work is formed into a Z-Iike shape in one stage. The same shape can be obtained in two stages by using V-bending, but offset bending tooling eliminates one stage. A shape that has a step difference equal to the sheet thickness cannot be formed by V-bending. The tooling used in offset bending is roughly divided into spring-up and horizontal types. The spring-up type allows the work to spring up during bending. Three types of tooling used are: a balanced type, an unbalanced type, and a versatile type. The horizontal type, which is generally called a non-spring-up type, is used in bending offset shapes and is suitable for bending the ends of large sheets.

a) Spring-up type (Fig. 5-14) This is the most commonly used type of offset bending. It is capable of bending shapes with step differences 3 to 10 times sheet thickness. This type of bending tool can be either a balanced type or an unbalanced type (the 90° punch tip angle is not equally divided into 45° angles, as shown in Fig. 5-14). The unbalanced type tooling produces a small thrust load but is relatively expensive. b) Horizontal type (Photo 5-6) This type is best suited to bending offset shapes in which the step difference is equal to the sheet thickness; or forming where the step difference is less than 3 times the sheet thickness. Since this tooling is simpler than the spring-up type, its mechanical processing and hardening is easier and therefore less expensive.

(iii) Beading type For details concerning beading tooling, refer to (ii) "Tooling producing slight spring-up" in the description regarding spring-up-preventive tooling in item 5.2.1.

5-2-3 Versatile tooling The versatile type tooling can form products of the same pattern in different sizes using a punch and die combination. The sectionalized punch, sectionalized die and offset bending tools are examples of this type of tooling.

(i) Sectionalized punch As previously explained in item 3.6.1, the sectionalized punch is used when forming boxes with both short and long sides. It used to be common practice to use a punch cut to the required dimensions of the product. This practice has been replaced by using a combination of punches of different lengths. Sectionalized punches are designed to

handle dimensional changes by means of a sectionalized punch combination (see Photo 3-3).

(ii) Sectionalized die As shown in Photo 5-7, the die must be cut each time to conform to the inside dimensions when a workpiece having opposing flanges is bent. The sectionalized die is designed to handle the necessary dimensions by means of sectionalized die combination (refer to item 3.6.2, "Sectionalized die").

(iii) Offset bending tooling (Fig. 5-15) The versatile type tooling can produce several kinds of step differences by using a punch and die combination. The special-use type should be used if the number of products is large. If the number of products is small and includes several kinds of step differences, the versatile type will be much more effective.

5-2-4 Tooling that makes material handling easy Accordion bending tooling and the punch-die replacement type eliminate the need for reversing work during bending. The punch-die replacement type can be built into an automatic tool changer (refer to Fig. 5-2, "Elimination of work reversal").

(i) Accordion bending (Fig. 5-16) Accordion bending is a process whereby consecutive bends of various shapes, such as a hat, U-shape with R, and triangle, are formed. There are two types of accordion bending tools: the nonreversible type and the reversible type. Nonreversible bending can be accomplished with one set of tools. However, reverse bending requires separate tooling for the preliminary stage (and occasionally for a stage prior to the preliminary stage) as well as for the following stage. Many of the work materials used in accordion bending are long and narrow and it takes many man-hours to reverse the work. This can fatigue and frustrate the operator. Therefore, it is desirable to eliminate the reversing operation. Fig. 5-16 shows an example of nonreversible type tooling.

Working conditions

Product Radiating plates for console boxes

Material Galvanized iron sheet

Sheet thickness 0.3 mm

(ii) Hinged-type sectionalized punch (Photo 5-8) Flanged shapes are formed by bending the short sides in the initial stage and then the long sides. After bending the long sides, the work must be removed from the punch by moving it backwards and tilting it slightly. This requires time and reduces work efficiency. As shown in Photo 5-8, the hinged-type tooling has a hinge at both ends so they can be moved easily. This arrangement enables the flange to be released without moving the work and considerably reduces the number of stages. Dimensional changes in the long or short side can be dealt with by means of a combination of various punch lengths.

5-2-5 Tooling that eliminates tool change The combined forming tooling is capable of forming two or three kinds of bends without tooling replacement. One type with an upper and lower working position is called double-deck (two-stage) type. Another design with three working positions is called triple-deck type.

(i) Double-deck type (Photo 5-9) This type of tooling performs 30° bending with the upper stage and hemming with the lower stage. It saves working space because work need not be piled up when shifting from the first to second stage. Furthermore, it saves a considerable amount of loading and unloading time and also assures smooth production flow.

Length 915 mm

Required tonnage 20 tons

Fig. 5-17 illustrates a triple-deck type tooling with three working positions. This tooling performs four stages of bending in each position: (1) step bending, (2) hemming, (3) 90° foldback bending, and (4) simple 90° bending. Generally, the bending work shown above is performed using five machines and ten operators positioned in a line. However, use of the special tooling, coupled with a numerical-control machine, enables two operators to accomplish the same job, resulting in considerable savings of labor and equipment.

5-2-6 Tooling that produces no marring The utilization of urethane tooling is probably the only available means of eliminating marring. Unbalanced ] bending with urethane in Fig. 5-18 is an example of making the most of urethane tooling. When the appearance of the product is a prime consideration, use vinyl-coated work or bend the work with a sheet placed between the die and work. Usually, a two-step method which uses different tooling for each step is employed. The advantages of urethane tooling can be summarized as follows:

1. It absorbs thrust load caused by unbalanced bending. 2. It functions as a counterholder in R-bending and U-bending to prevent advanced

bending. 3. It can accomplish overforming which takes the place of the cam type tooling. 4. A good-looking product can be obtained because no marring and scratching

occurs during operation.

5-2-7 Technique which eliminates welding Seaming, which is similar to hemming, is a useful bending technique which eliminates the need to weld. In this process, two sheets or members are joined together without riveting or welding (welding or screw clamping may occasionally be used). Seaming is also called locking. The Pittsburgh lock used in the U.S.A. is an example of seaming (see Fig. 5-19). The tooling utilized in seaming is either a wedge or a crushing type.

The wedge type shown in Fig. 5-20 is a single type with a wedge on one side. A double type with wedges on both sides is another wedge type. Seaming requires that the work be formed into an easy-to-seam shape in the preliminary stage, as shown in Fig. 5-21.

(ii) Crushing type Fig. 5-22 shows an example of a crushing type of seaming tool. A core must be used to seam such hollow shapes.

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5-3 Points of Tooling Design In subsections 5.1 and 5.2 above, we described the objectives of labor saving and labor-saving tooling respectively. Now we will describe the main aspects of tooling. Because of limited space, we will discuss only some of the tooling described in subsection 5.2, namely, the L-bending type, accordion bending type, seaming type, and urethane tooling. For further details, the reader should refer to "Bending Technique" published by Amada Sheet Metal Working Research Association.

5-3-1 L-bending tooling The clearance rate of the work to the sheet thickness, the depth of the bend, the punch shoulder R, and the die shoulder R are major points in tooling design.

It is not advisable to initially work the punch shoulder R to the prescribed R. Failure can be avoided if it is initially worked to about R=0.5 mm and then gradually increased through trials.

The most common problems encountered with L-bending tooling are friction marring and the inability to obtain the proper bending angle of 90'. Marring is affected by the size of the die shoulder R, and the bending angle is influenced by the clearance and punch shoulder R. A frequently used method of decreasing marring is to increase the die shoulder R. Even though an obtuse angle is formed with L-bending tooling, possible solutions are to decrease either the clearance or punch shoulder R; or increase the bending force.

5-3-2 Accordion bending tooling The open height and stroke of the press brake determines whether or not nonreversible type tooling requiring no preliminary bending can be utilized. The stroke length must be greater than 2.2 times the height of the product.

The punch angle or die groove angle (angle 0 in Fig. 5-16) should be the maximum angle in the range where it absorbs springback. Also, with regard to the punch shoulder and die shoulder, the key point in the design of this tooling is to keep the edge. This is because the desired shape is obtained by producing a spring-go effect with the edge.

When forming accordion-bent products, an accurate pitch gauge is required for determining the pitch. The pitch gauge should be structurally designed to suit the shape of each product. Further, consideration should be given to combining the use of a pitch gauge and side gauge to enhance

effectiveness.

For your reference, the maximum sheet thickness this tooling is 0-5 mm (material: galvanized iron rlate). it is essential that the height and width of the product be increased in proportion to the thickness when bending sheets thicker than the"maximum limit.

5-3-3 Seaming tooling Fig. 5-22 shows the tooling used for seaming an angular copper flume. This tooling incorporates several devices and has a wide range of applications despite its being a special-use die. A total of six preliminary stages are required: two stages of 30' bending and four stages of 90° bending. Seaming is performed by crushing as shown in the above figure.

When seaming hollow shapes like this product, the following should be considered in the tooling design: (1) use of a core, (2) a structure in which the product is extracted by moving the core, and (3) ensuring smooth extraction of the product.

Next, we will briefly describe the core and die which are major components of this tooling. As shown in Fig. 5-23, the following parts are installed to the core; a hinge pin hole used to turn the core; a slide needle bearing which supports the force that lifts the dead weight of the core; a stopper for positioning; a leaf spring to hold the product against the stopper; and an ejector (Fig. 5-22) for product ejection. The stopper and ejector are both actuated by a counterspring to facilitate product extraction.

The block-shaped die contains a core stopper, a coil spring that floats the core stopper and hinge pin, and a block that forms a bearing sliding surface. The surfaces of the die and core which come into contact with the work must have a ground finish when working with easily marred products

5-3-4 Urethane tooling At first, urethane was used as a replacement for rubber to protect the work. However, as research progressed, it was discovered that urethane outperforms conventional metal dies in some of the difficult bending operations. Therefore, future bending operations cannot be discussed without considering urethane. The reason urethane is referred to as "urethane tooling" is based on this significance. The advantages and disadvantages of urethane tooling are as follows:

1. (i) Advantages of urethane tooling (1) Work preparation time is reduced because tool alignment is not required. (2) Minimal springback occurs. (3) Alterations are not necessary when the bending angle or work thickness changes. (4) It absorbs side thrust. (5) It may also be used as a stripper or spring, or to hold the work. (6) It does not scratch or mar the work. (7) Overforming can be performed.

(8) It functions as a counterholder that prevents advanced bending. (9) It possesses excellent physical properties for easier forming. (10) Special operations such as drawing, embossing and bulging can be accomplished.

2. (ii) Disadvantages of urethane tooling (1) The tonnage required is several times greater than that for metal dies. (2) It is not durable to edges; therefore, it is not suitable for products with short flanges. (3) It has a short service life in comparison to a metal die. (4) There is a limitation on bendable sheet thickness. (5) It is not as accurate as a metal die in partial bending. (6) There is a limitation on bendable work shapes. (7) Cost is somewhat high because a retainer box is required.

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PROBLEMS IN SHEETMETAL BENDING

Problems will always occur in sheetmetal bending because they are individually or intricately related to the machine, tooling, work material, welding, etc. It would take a great amount of time to describe all the problems; therefore, we will only discuss some which are directly related to the tooling.

6-1 Dispersion of Bending Angles Most of the complaints we receive concern the dispersion of bending angles. The dispersion of bending angles is classified as either "tendencious" or "non-tendencious". "Middle bulging" (a phenomenon in which the bent angle is larger in the middle than at either end) is a tendencious instance, which is greatly influenced by the quality of the machine. The dispersion of bending angles caused by variations in material quality and different lengths of work falls within this group, even though they show different tendencies.

The bending angle is greatly affected by qualitative variations in the work (variations in sheet thickness and internal structure). Since these are not tendencious, they belong to the latter non-tendencious group.

Needless to say, tooling manufacturers guarantee the bending angles of their tooling as having synthetic precision; regardless of whether the dispersion is tendencious or non-tendencious. Generally, the angular tolerance that manufacturers guarantee is +/-30', though it depends upon the kind of bending operation. Accordingly, it is desirable that precision be maintained at +/-30' at all times. This is necessary to enhance proper maintenance of the machine and tooling.

Deformation and wear of the bending tool adversely affects the maintenance of bending precision. With regard to tooling deformation, care must be taken properly to adjust the pressure setting of the machine and consideration must be given to the tonnage tolerance of the tooling. If the tooling is deformed, there is no choice other than to partially cut it or discard it. Worn tooling can be refinished by grinding, but special precautions must be observed. Go to Top

6-2 Marring The work can be marred by slippage or scraping at the tooling joints during bending. Marring is a serious problem with stainless steel, aluminum, vinyl-coated steel, and painted steel sheets which are easily scratched. Easily scratched material is usually coated with vinyl before bending; however, the vinyl coating sometimes breaks, leaving scratches on the base metal.

The shoulder R of the die V-groove is closely related to scratches made by slippage. If other working conditions (such as the size of the step difference) are ignored, scratches made by slippage become shallower as the shoulder R increases. It is recommended that dies with a large shoulder R be used.

Problems In Sheetmetal Bending

Dispersion of Bending Angle Marring Cracking Warping Table of Contents

They are available from some manufacturers.

When such a die cannot be used, it is advisable to use the die in combination with a urethane sheet. Select a urethane sheet of adequate thickness in accordance with the work sheet thickness and place it on the die. The same effect will be obtainable as with vinyl coating. If this method is applied to R-bending, it will be more efficient than use of a urethane pad because a large bending force is not required and unmarred products can be obtained. Go to Top

6-3 Cracking Fig. 6-1 shows cracks and fissures in bent products. A bad crack is called a fissure. Cracking occurs when the inside radius is too small in relation to sheet thickness. Cracking is influenced by the tensile strength of the work material, the directions in which it was rolled and bent, the condition of its cut face, etc.

The following precautions must be observed to prevent cracking.

1. Cut the work so the end face is neat and smooth. Shearing is preferable to fusion cutting and machining is preferable to shearing. Furthermore, it is advisable to bend work with the sheared side (lustrous side) facing outward.

2. Pay attention to the direction the work material is rolled. The work is apt to crack when it is bent in parallel with the direction of roll.

3. Pay attention to the punch tip R. In the case of aluminum, the punch tip R should be greater than the sheet thickness.

4. Material with great extensibility is hard to crack. 5. Use a slow bending speed.

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6-4 Warping Fig. 6-2 illustrates a phenomenon (called camber) in which the bent work warps. When a material is bent into another shape, its texture changes. It becomes thinner on the outer surface as it is pulled in the "a" direction; so part of the material flows from the longitudinal direction, as shown by "b", to fill the thinner part. As a result, the material contracts in the longitudinal direction. The material becomes thicker on the inner surface as it is compressed in the "c" direction. Thus, material flows in the "d" direction to offset the thicker part. This event occurring on the inner surface, together with the force on the outer surface, causes the bent work to warp in the longitudinal direction, as shown in Fig. 6-2. The warp is often called a saddle camber because it looks like a horse saddle.

"

This phenomenon varies with the type, quality, sheet thickness and inside radius of the work. Generally, the ratio of h to l is somewhere between 1/1000 and 511000. Warping is more severe with longer bending lengths. If the bending length is shorter, it can be corrected more easily. In the case of long products, such corrections are made by hand in most factories.

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