Presses and Equipment for SheetMetal Dies

7/30/2019 Presses and Equipment for SheetMetal Dies

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Presses and Equipment for Sheet Metal dies

POWER PRESS TYPES

The types of power presses available for metal-cutting and forming

operations are varied, the selection depending upon the type of operation.

Not all types of presses will be described because of space limitations. The

basic types of presses and press mechanisms will be described to give the

beginner the necessary background for designing press tooling.

Presses are classified by (1) type of frame, (2) source of

power, (3) method of actuation of slides, (4) number of slides incorporated,

and (5) intended use. Most presses are not classified by only category one but

several. For example, a straight-side press may be mechanically or

hydraulically driven and may be either single or double acting.

Classification by frame type: The frame of a press is fabricated by

casting or by welding heavy steel plates. Cast frames are quite stable and

rigid but expensive. Cast frame construction also has the advantage of placing

a mass of material where it is needed most. Welded frames are generally

less expensive and are more resistant to shock loading because of the greater

toughness of steel plate.

The general classification by frame includes the gap frame and the

straight side. The gap frame is cut back below the ram to form the shape of a

letter C. This allows feeding a strip from the side. Some gap-frame presses

have an open back to permit strip feeding from front to back or ejection of

finished parts out the back. Gap-frame presses are manufactured with solid

frames fixed in a vertical or inclined position. Others are manufactured with a

separate frame mounted in a base, which allows the frame to be inclined at

an angle in three different positions.

The reason for inclining the press is to allow parts to fall through the

open back by gravity. The three-position inclinable press is frequently

referred to as an open-back inclinable (OBI) press (see Fig. 3-1). Solid gap-

frame presses are obtainable in higher tonnages than inclinable ones because

of the rigid base and solid construction.


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The OBI press is the most common press in use today. It ranges from

a small 1-ton bench press to floor presses rated up to 150 tons. Its main use

is for blanking and piercing operations on relatively small work pieces,

although bending, forming, and drawing operations can also be done.

Fig, 3-2 shows the major components of an OBI press, as follows:

1) A rectangular bed, the stationary and usually horizontal part of the

press, serving as a table to which a holster plate is mounted.

2) A bolster plate, consisting of a flat steel plate from 50 mm. to 125

mm. thick, secured to the press for locating and supporting the die

assembly.

3) The ram, sometimes called the slide, which reciprocates within the

press frame and to which the punch or upper-die assembly is

fastened.

4) A knockout, consisting of a crossbar through a slot in the ram that

contacts a pin in the die to eject the work piece.


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5) The flywheel, which absorbs energy from the motor continuously and

delivers its stored energy to the work piece intermittently, making it

possible to use a smaller motor.

6) The pitman, consisting of a connecting rod to convey motion andpressure from the main shaft or eccentric to the press slide.

Fig. 3-3Single action straight side eccentric shaft mechanicalpress.

The straight slide press incorporates a slide or ram, which travels up

and down between two straight sides or housing and commonly used for large

and heavy work. The size of the press is limited to some extent because

reduce the working area. However the frame construction does permit large

bed areas and longer strokes. The drive mechanism is generally located

above the bed, The straight slide press incorporates a slide or ram, which


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travels up and down between two straight sides or housing and commonly

used for large and heavy work. The size of the press is limited to some extent

because reduce the working area. However the frame construction does

permit large bed areas and longer strokes. The drive mechanism is generally

located above the bed, although under drive presses may be obtained with

the drive mechanism located below the bed. Straight side presses are

classified as single, two or four point suspension, depending upon the number

of connection between the slide and the main drive shaft. Fig, 3-3 shows a

typical straight slide press.

Classification by source of power: The great majority of presses

receive their power mechanically or hydraulically. A few manually operated

presses are hand operated through levers or screws, but they are hardly

suited for high production.

Mechanical presses use a flywheel driven system to obtain ram

movement. The heavy flywheel absorbs energy from the motor

continuously and delivers its stored energy to the work piece

intermittently. The motor returns the flywheel to operating speed between

strokes. The permission slowdown of the flywheel during the work period is

about 7 to 10 percent in nongeared presses and 10 to 20 percent in

geared presses. The flywheel is attached directly to the main shaft of the

press (non geared), or, it is connected to the main shaft by a gear train.

Nongear drives are used on presses of low tonnage and short strokes. The

number of strokes per minute on nongear drives is usually quite high.

Gear driven presses transmit the energy of the flywheel through a single or

double reduction gear. The single reduction gear drive is suited for heavier

blanking operations or shallow drawing. The double-gear drive is used on large,

heavy presses where it is necessary to move large amounts of mass at slower

speeds. The double reduction greatly reduces the strokes per minute without

reducing the flywheel speed.

Basic types of mechanical press drives

a) Nongeared


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b) Single geared

c) Single reduction gear

d)Double reduction

Fig. 3-5Typical double action hydraulic presswith a Die cushion

Hydraulic presses have a large cylinder and piston, coupled to a hydraulic pump.

The piston and press ram are one unit. The tonnage capacity depends upon the

cross-sectional area of the piston (or pistons) and the pressure developed by the

pump. The cylinder is double acting in order to move the ram in either direction.

The advantage of a hydraulic press is that it can exert its full tonnage at any

position of the ram stroke. In addition, the stroke can be varied to any length

within the limits of the hydraulic-cylinder travel. The speed and pressure are also


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constant throughout the entire stroke Fig. 3-5 shows a typical hydraulically driven

press.

Classification by method of slide actuation: The flywheel of a press drives the

main shaft, which in turn changes the rotary motion of the flywheel into the linear

motion of the slide or ram. This is generally accomplished by incorporating

crankpins or eccentrics into the main drive shaft, as shown in Fig, 3-4. The

number of points of suspension of the slide determines the number of throws or

eccentrics on the main shaft. Points of suspension are places where pressure is

transmitted by connection to the slide. Press connections, called connecting rods

or pitman, are usually adjustable in length so that the shut height of the press can

be varied.

The most common driving device is the crankshaft, although many

newer presses use the eccentric for ram movement. The main advantage

of the eccentric is that it offers more surface area for bearing support for

the pitman and main disadvantage is its limitations on the length of

stroke. Therefore, presses having longer strokes generally utilize the

crankshaft.

In addition to eccentrics and crankpins, cams, toggles, rack andpinions, screws, and knuckles actuate slides. Space does not permit

discussion of these mechanisms in this text. Information may be obtained

by referring to the various die-design and press handbook.

3.1.4 Classification by the number of slides incorporated: The

number of slides incorporated in a single press is called the action, i.e. the

number of rams or slides on the press. Thus a single-action press has one

slide. A double-action press has two slides, an inner and an outer slide

(see Fig. 3-5). This type of press is generally used for drawing operations

during which the outer slide carries the blank holder and the inner slide

carries the punch. The outer, or blank-holder, slide, which usually has a

shorter stroke than the inner, or punch-holder slide, dwells to hold the

blank while the inner slide continues to descend, carrying the draw punch

to perform the drawing operation.

A triple-action press is the same as a double-action with the

addition of a third ram, located in the press bed, which moves upward in

the bed soon after the other two rams descend. All three-slide movements


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are properly synchronized for triple-action drawing, redrawing, and

forming.

GENERAL PRESS INFORMATION

The tool designer must know certain fundamentals of press

operation before he can successfully design press tooling.

Press tonnage: The tonnage of a press is the force that the press ram is

able to exert safely. Press slides exert forces greater than the rated

tonnage because of the built-in safety factor, but this is not a license to

overload.

The tonnage of hydraulic presses is the piston area multiplied by

the oil pressure in the cylinder. Changing the oil pressure varies the

tonnage. The tonnage of mechanical presses is determined by the size of

the bearings for the crankshaft or eccentric and is approximately equal to

the shear strength of the crankshaft metal multiplied by the area of the

crankshaft bearings. The tonnage of a mechanical press is always given

when the slide is near the bottom of its stroke because it is greatest at this

point.

Stroke: The stroke of a press is the reciprocating motion of a press

slide, usually specified as the number of inches between terminal points of

the motion. The stroke is constant on a mechanical press but adjustable on

a hydraulic press.

Shutheight: The shut height of a press is the distance from the

top of the bed to the bottom of the slide with the stroke down and the

adjustment up. The thickness of the bolster plate must always be taken

into consideration when determining the maximum die height. The shut

height of the die must be equal to or less than the shut height of the

press. The shut height of a press is always given with the adjustment up.

Lowering the adjustment of the slide may decrease the opening of the

press from the shut height down, but it does not increase the shut height.

Thus the shut height of a die must not be greater than the shut height of

the press. It may be less, because lowering the adjustment can reduce the

die opening in the press.

Die space:Die space is the area available for mounting dies in the press.


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Introduction to Die Cutting Structure

THE FUNDAMENTALS OF DIE-CUTTING OPERATIONS

While there are many die-cutting operations, some of which are very

complex, they can all be reduced to the following simple fundamentals.

Fig. 4-1 Drop-through Blanking Die Fig. 4-2 Piercing Die Assembly

Plain blanking: Fig. 4-1 shows a simple operation of this type. The

material used is called the stockand is generally a ferrous or nonferrous

strip. During the working stroke the punch goes through the material, and

on the return stroke the material is lifted with the punch and is removed

by the stripper plate. The stop pin is a gage for the operator. In practice,

he feeds the stock by hand and locates the holes to be punched as shown.

The part that is removed from the strip is always the work piece (blank) in

a blanking operation.

4.1.2 Piercing: This operation consists of simple hole punching. It differs

from blanking in that the punching (or material cut from stock) is the scrap and

the strip is the work piece. Piercing is nearly always accompanied by a blanking

operation before, after, or at the same time. Fig. 4-2shows a typical piercing

die assembly.

4.1.3 Lancing: This is a combined bending and cutting operation along a

line in the work material. No metal is cut free during a lancing operation.


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The punch is designed to cut on two or three sides and bend along the

fourth side. Fig. 4-3 and 4-4 show the principle of the lancing operation.

Fig. 4-3 lancing action Fig. 4-4 Strip lanced for free metal for

forming

4.1.4 Cutting off and parting: A cutoffoperation separates the work

material along a straight line in a single-line cut (Fig. 4-5). When the

operation separates the work material along a straight line cut in a double-

line cut, it is known asparting (Fig. 4-6) . Cutting off to separate the work

piece from the scrap strip. Cutting off and parting usually occur in the final

stages of a progressive die. Cutting off is also used to chop up the scrap

strip skeleton as it leaves the die. This makes the scrap much easier to

handle. Fig. 4-7 shows the basic principles of cutting off and parting.


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Fig. 4-5 Cutoff action Fig. 4-7 Layout for making blanks by

Cutoff

4.1.5 Notching: This operation removes metal from either or both edges

of the strip. Notching serves to shape the outer contours of the workspace

in a progressive die or to remove excess metal before a drawing or

forming operation in a progressive die. The removal of excess metal allows

the metal to flow or from without interference from excess metal on the

sides. Fig. 4-8 shows a typical example of notching

Fig. 4-8 Notching Fig. 4-9 Shaving

4.1.6 Shaving: Shaving is a secondary operation, usually following

punching, in which the surface of the previously cut edge is finished smoothly

to accurate dimensions. The excess metal is removed much as a chip is

formed with a metal-cutting tool. There is very little clearance (close to zero)

between the punch and die, and only a thin section of the edge is removed

from the edge of the work piece. Fig. 4-9described the shaving operation.


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Fig. 4-10 Trimming a horizontal flange

4.1.7 Trimming: This operation removes the distorted excess metal from

drawn or formed parts and metal that has been needed in a previousoperation. It also provides a smooth edge. Fig. 4-10 shows tooling for

Trimming a horizontal flange on a drawn shell in a separate operation.

After scrap from a sufficient number of trimmed shells has accumulated,

the piece of scrap at the bottom is severed at each stroke of the press by

scrap cutter shown in this figure and falls clear.

CUTTING ACTION IN PUNCH AND DIE OPERATIONS

The cutting action that occurs in blanking or piercing is quite similarto that of chip formation ahead of a cutting tool. The punch contacts the

work material supported by the die and a pressure buildup occurs. When

the elastic limit of the work material is exceeded, the material begins to

flow plastically (plastic deformation). The punch penetrates the work

material, and the blank, or slug, is displaced into the die opening a

corresponding amount. A radius is formed on the top edge of the hole and

the bottom edge of the slug, or blank, as shown in Fig. 4-11a. The radius

is often referred to as rollover and its magnitude depends upon the

ductility of the work material. Compression of the slug material against

the walls of the die opening burnishes a portion of the edge of the blank,

as shown in Fig. 4-11b. At the same time, the plastic flow pulls the

material around the punch, causing a corresponding burnished area in the

work material. Further continuation of the punching pressure then starts

fractures at the cutting edge of the punch and die (see Fig. 4-11c). Under

ideal cutting conditions, the fractures will meet and the remaining portion

of the slug edge will be broken away. A slight tensile burr will be formed

along the top edge of the slug edge will be broken away. A slight tensile

burr will be formed along the top edge of the slug and the bottom edge of

the work material (see Fig. 4-11d).


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Fig. 4-11 Cutting action progression when blanking

and piercing metal


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Fig. 4-12 Characteristic appearance of edges of parts produced

by piercing and blanking

Fig. 4-12 Shows the characteristic appearance of the edges of parts

produced by blanking and piercing operations in detail. The edge radius

(or rollover) is produced during the initial stage of plastic deformation.

The edgeradius is more pronounced with soft materials.

The highly burnished band is the result of the materials being forced

against the walls of the punch and die and rubbing during the final stages

of plastic deformation. The sum of the edge radius depth and the

burnished depth is referred to aspenetration, i.e., the distance the punch

penetrates into the work material before fracture occurs. Penetration is

usually expressed as a percent of material thickness, and it depends upon

the properties of the work material. As the work material becomes

harder, the percent of penetration decreases. For this reason, harder

materials have less deformation and burnished area.

The remaining portion of the cut is the fractured area, or break. The

angle of the fractured area is the breakout angle. The tensile burr is

adjacent to the break. The burr side of blank or slug is toward the punch,

and the burr side of the work material is toward the die opening.

Die Clearance: Clearance is defined as the intentional space between the

punch cutting edge and die cutting edge. Clearance is always expressed as the


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amount of clearance per side. Theoretically, clearance is necessary to allow the

fractures to meet when break occurs, as shown in Fig. 4-13. The amount of

clearance depends upon the kind, thicknessand hardness of the work material.

Excessive clearance allows a large edge radius (rollover) and

excessive plastic deformation. The edges of the material tend to be drawn

or pulled in the direction of the working force, and the break is not

smooth. Large burrs are present at the break edge.

Fig. 4-13 The effect of clearance (a) too little clearance: fracture do

not meet(b) Correct clearance: fracture do meet

Insufficient cutting clearance caused the fractures to miss and

prevents a clean break, as shown in Fig. 4-13a. A partial break occurs,

and a secondary break connects the original or main fractures. This is

often referred to assecondary shear. The secondary break does not allow

separation of the material without interference, and a second burnished

ring is formed, as shown in Fig. 4-13b. The burnished ring may appear as

a slight step around the outside edge of the blank or around the inside

edge of the hole. Insufficient clearance increase pressure on the punch

and die edge and has a marked effect on die life.


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Authorities disagree on the correct amount of clearance for a given

work piece material. One reference may recommend 6 percent of stock

material thickness per side, while another may recommend 12 percent for

the same material. The difference is probably in the end result each is

striving for. The designer should consider the application of the pierced or

blanked work piece. When the purpose is only to make a hole, as in the

case of structural steel, wide clearances may be used to increase die life.

Blanked work pieces that assemble as an integral part of a mechanism

require tighter clearances. Fig. 4-14 shows various edges for

stampings with a description and use of each. Note that the

recommended clearance varies from 2 to 21 percent for mild steel.

The diameter of the blank or pierced hole is determined by

measurement of the burnished area. Since the burnished area on the

blank is produced by the walls of the die, the diameter of the blank will be

the same as the diameter of the die (disregarding a slight expansion after

the blank is pushed from the die). The same principle applies to the

diameter of the pierced hole. The burnished area in the hole is caused by

the punch; thus the diameter of the pierced hole will be the same as the

punch. Therefore, die clearance is either placed on the punch or the die,

depending upon whether the pierced hole or the blank will be the desired

work piece. If the blank is to become the work piece, the die diameter is

made to the work piece size and the punch is reduced in size an amount

equal to the die clearance. If the pierced hole is to become part of the

work piece, the punch is made to the correct hole size and the die

opening is made oversize an amount equal to the die clearance. In simple

terms, the die controls blank size and the punch controls hole size


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Angular clearance is necessary to prevent backpressure caused by

blank or slug buildup especially when the punches or die block are fragile.

Recommended angular clearance varies from to 2 per side, depending

upon the material and the shape of the work piece. Soft materials and

heavy-gage materials require greater angular clearance. Larger angular

clearance may be necessary for small and fragile punches.


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Fig. 4-15 The use of angular clearance

Stripping: The forces that cause the blank to grip inside the die

walls also cause the stock material to grip around the punch. The stock

material will rise as the press ram is raised unless some means of

stripping the stock material from the punch is provided. Fig. 4-15 shows

the two basic types of stripping device.

Fig. 4-15 Basic types of stripping devices (a) Fixed type and

(b) Spring loadedtype

The amount of pressure required to strip the stock material from the

punch varies from 5 to 20 percent of the cutting-force requirements.

This is only a rough estimate, as many variables affect stripping

pressure. For example some materials cling more than others. Thicker


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materials require more stripping force because more material is in

contact with the punch. Holes punched close to the strip edge do not

require as much stripping force because there is less backing and the

metal can give. Punches with polished sidewalls tend to strip easier

than those with rough surfaces. More force is also required to strip

punches that are close together.

Cutting forces: The force required to penetrate the stock material

with the punch is the cutting force. If the die contains more than one

punch that penetrates t he stock material simultaneously, the cutting

force for that die is the sum of the forces for each punch. Knowledge of

cutting forces is important in order to prevent overloading the press or

failure to use it to capacity.

The formula for determining cutting forces takes into account the

thickness of the stock material, the perimeter of the cut edge and the

shear strength of the stock material. The shear strength of the stock

material is the force necessary to sever 1 sq. in. of the material by

direct shearing action.

It sometimes becomes necessary to reduce cutting forces to

prevent press overloading. One method of reducing cutting forces is to

step punch lengths, as shown in Fig. 4-16. Punches or groups of punches

progressively become shorter by about one stock-material thickness. A

second method is to grind the face of the punch or die at a small shear

angle with the horizontal. This has the effect of reducing the area in shear

at any one time. Shear also reduces shock to the press and smoothes out

the cutting operation. The shear angle chosen should provide a change in

punch length of from 1 to 1 times the stock thickness. Shear that is

equal to or greater than the stock thickness is called full shear. Cutting

forces are reduced by approximately 30 percent when full shear is applied


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Methods of reducing cutting forces:

a) Stepping punches

b) Single shear on punch

c) Single shear on die

d) Double shear on punches

e) Double shear on punches

f) Convex & concave shear

Scrap Strip Trip Layout For Blanking

In designing parts to be blanked from strip material, economical stock

utilization is of high importance. The goal should be at least 75 per cent

utilization. A very simple scrap-strip layout is shown in Fig. 6-1.

SCRAP ALLOWANCE


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A scrap-strip layout having insufficient stock between the blank and the strip

edge, and between blanks, will result in a weakened strip, subject to breakage

and thereby causing misfeeds. Such troubles will cause unnecessary die

maintenance owing to partial cuts, which defect the punches, resulting in nicked

edges. The following formulas are used in calculating scrap-strip dimensions for all

strips over 0.8 mm. thick.

t = specified thickness of the material

B = 1.25 t when C is less than 64 mm

B = 1.5 t when C is 64 mm or longer

C = L + B, or lead of the die

Example: A rectangular part, to be blanked from 1.5 mm thick steel

(Manufacturers Standard) is 10 X 27 mm. If the scrap strip is developed as in Fig.

6-2, the solution is

t = 1.5 mm

B = 1.25 X 1.5 = 1.875 mm

C = 10 + 1.875 = 11.875 mm

W = 27 + 3.75 = 30.75 mm

Nearest commercial stock is 32 mm. Therefore, the distance B will equal

2.3mm. This is acceptable since it exceeds minimum requirements.


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Minimum Scrap-Strip Allowance: If the material to be blanked is 0.6

mm thick or less, the formulas above should not be used. Instead, dimension B is

to be as follows:

Strip width W Dimension B

0 - 75 mm 1.3 mm

76 150 mm 2.4 mm

150 300 mm 3.2 mm

Other Scrap-Strip Allowance Applications:

Figure 6-3, 6-4 and 6-5 illustrates special allowances for one-pass layouts:


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View D. For layouts with sharp corners of blanks adjacent, B = 1.25 t.

Fig. 6-4 Allowances for one-passlayouts.

Percentage of Stock Used: If the area of the part is divided by the area

of the scrap strip used, the result will be the percentage of stock used.


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If A = total area of strip used to produce a single blanked

part, then

A = CW (Fig. 3-35), and a = area of the part = LH.

If C = 11.5 mm and W = 32 mm then A = 11.5 X 32 = 368

mm

If L X 9.5 mm and H = 27 mm then a = 29.5 X 27 = 256.6

mm

Percentage of stock used:

a 256.5

= = 70% approx.A 368

EVOLUTION OF A BLANKING DIE

In the planning of a die, the examination of the part print immediately

determines the shape and size of both punch and die as well as the working area

of the die set.

Die Set Selection


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A commercially available standardized two-post die set with 150 mm overall

dimensions side-to-side and front-to-back allows the available 76 mm. wide stock

to be fed through it. It is large enough for mounting the blanking punch on the

upper shoe (with the die mounted on the lower shoe) for producing the blank

shown in Fig. 6-6, since the guideposts can be supplied in lengths of from 100 to

225 mm.

Since the stock, in this case was available only in a width of 76 mm the length

of the blanked portions extended across the stock left a distance between the

edges of the stock and the ends of the blank of 6 mm or twice the stock

thickness; this allowance is satisfactory for the 3.2 mm stock.

Die Block Design

By the usual rule-of thumb method previously described, die block thickness

(of tool steel) should be a minimum of 20 mm for a blanking perimeter up to 75

mm and 25 mm for a perimeter between 75 and 100 mm. For longer perimeters,

die block thickness should be 32 mm. Die blocks are seldom thinner than 22 mm

finished thickness to allow for grinding and for blind screw holes. Since the

perimeter of the blank is approximately 178 mm a die block thickness of 38 mm

was specified, including a 6 mm grinding allowance.

There should be a margin of 32 mm around the opening in the die block; its

specified size of 150 x 150 mm allows a margin of 45 mm in which four M10 cap

screws and dia. 10 mm dowels are located at the corners 20 mm from the edges

of the block.

The wall of the die opening is straight for a distance of 3.2 mm (stock

thickness); below this portion or the straight, an angular clearance of 1 allows

the blank to drop through the die block without jamming. The dimensions of the

die opening are the same as that of the blank; those of the punch are smaller by

the clearance (6 per cent of stock thickness, or 2 mm), which result in the

production of blanks to print (and die) size.

The top of the die was ground off a distance equal to stock thickness (Fig. 6-7)

with the result that shearing of the stock starts at the ends of the die and

progresses towards the center of the die, and less blanking pressure is required

than if the top of the die where flat.

Punch Design


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The shouldered punch (57 mm) long is held against a 6 mm thick hardened

steel backup plate by a punch plate 20 mm thick) which is screwed and doweled

to the upper shoe. The shut height of the die can be accommodated by a 32-ton

(JIC Standard) open-back inclinable press, leaving a shut height of 240 mm. For

the conditions of this case study, shear strength S = 42 kg/mm, blanked

perimeter length L = 178 mm approx. and thickness T = 3.2 mm.

From the equation P = SLT

The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons.

This value is well below the 32-ton capacity of the selected

press.

The shut height (Fig. 6-7) is 178 mm less the 1.6 mm

travel of the punch into the die cavity.

print immediately determines the shape and size of both punch and

die as well as the working area of the die set.

Die Sheet Selection


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A commercially available standardized two-post die set with 150 mm

overall dimensions side-to-side and front-to-back allows the available 76 mm.

wide stock to be fed through it. It is large enough for mounting the blanking

punch on the upper shoe (with the die mounted on the lower shoe) for producing

the blank shown in Fig. 6-6, since the guideposts can be supplied in lengths of

from 100 to 225 mm.

Since the stock, in this case was available only in a width of 76 mm the

length of the blanked portions extended across the stock left a distance between

the edges of the stock and the ends of the blank of 6 mm or twice the stock


Die Block Design

By the usual rule-of thumb method previously described, die block

thickness (of tool steel) should be a minimum of 20 mm for a blanking perimeter

up to 75 mm and 25 mm for a perimeter between 75 and 100 mm. For longer

perimeters, die block thickness should be 32 mm. Die blocks are seldom thinner

than 22 mm finished thickness to allow for grinding and for blind screw holes.

Since the perimeter of the blank is approximately 178 mm a die block thickness of

38 mm was specified, including a 6 mm grinding allowance.

There should be a margin of 32 mm around the opening in the die block;

its specified size of 150 x 150 mm allows a margin of 45 mm in which four M10

cap screws and dia. 10 mm dowels are located at the corners 20 mm from the

edges of the block.


thickness); below this portion or the straight, an angular clearance of 1 allows

the blank to drop through the die block without jamming. The dimensions of the

die opening are the same as that of the blank; those of the punch are smaller by

the clearance (6 per cent of stock thickness, or 2 mm), which result in the

production of blanks to print (and die) size.

The top of the die was ground off a distance equal to stock thickness (Fig.

6-7) with the result that shearing of the stock starts at the ends of the die and

progresses towards the center of the die, and less blanking pressure is required

than if the top of the die where flat.

Punch Design


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The shouldered punch (57 mm) long is held against a 6 mm thick hardened

steel backup plate by a punch plate 20 mm thick) which is screwed and doweled

to the upper shoe. The shut height of the die can be accommodated by a 32-ton

(JIC Standard) open-back inclinable press, leaving a shut height of 240 mm. For

the conditions of this case study, shear strength S = 42 kg/mm, blanked

perimeter length L = 178 mm approx. and thickness T = 3.2 mm.


The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92 tons.

This value is well below the 32-ton capacity of the selected

press.

The shut height (Fig. 6-7) is 178 mm less the 1.6 mm

travel of the punch into the die cavity.

Stripper Design

The stripper that was designed is of the fixed type with a channel or slot

having a height equal to 1.5 times stock thickness and a width of 80 mm to allow

for variations in the stock width of 75 mm. The same screws that hold the die

block to the lower shoe fasten the stripper to the top of the die block.

If, instead of 3.2 mm stock, thin (0.8 mm) stock were to be blanked, a

spring-loaded stripper would firmly hold the stock down on top of the die block

and could, to some extent, flatten out wrinkles and waves in it.


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A spring-loaded stripper should clamp the stock until the punch is

withdrawn from the stock. The pressure that strips the stock from the

punch on the upstroke is difficult to evaluate exactly. A formula frequently

used is

Ps = 2.5 x L x t kgs.

Where Ps = stripping pressure, in kgs.

L = perimeter of cut, in mm.

t = stock thickness, in mm.

Spring design is beyond the scope of this book; die spring

data are available in the catalogues of spring manufacturers.

Stock Stops

The pin stop pressed in the die block is the simplest method for stopping

the hand-fed strip. The right-hand edge of the blanked opening is pushed against

the pin before descent of the ram and the blanking of the next blank. The 4-8 mm

depth of the stripper slot allows the edge of the blanked opening to ride over the

pin and to engage the right-hand edge of every successive opening.

The design of various types of stops adapted for manual and automatic

feeding is covered in a preceding discussion.


withdrawn from the stock. The pressure that strips the stock from the punch on

the upstroke is difficult to evaluate exactly. A formula frequently used is





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Spring design is beyond the scope of this book; die spring

data are available in the catalogues of spring manufacturers.

Stock Stops

The pin stop pressed in the die block is the simplest method for stopping

the hand-fed strip. The right-hand edge of the blanked opening is pushed against

the pin before descent of the ram and the blanking of the next blank. The 4-8 mm

depth of the stripper slot allows the edge of the blanked opening to ride over the

pin and to engage the right-hand edge of every successive opening.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Figure 6-8 gives the blanked dimensions of a linkage case cover of cold

rolled steel, stock size 3.2 x 60 x 60 mm. Production is stated to be 200 parts

made at one setup, with the possibility of three or four runs per year.


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Step 1, Part Specification

1. The production is of medium class; therefore a second-class die willbe used.

2.

Tolerances required: Except for location of the slots, alldimensions are in fractions. The slot locations, though specified in

decimals, are not very close. Thus a compound die is not

necessary; a two or three-station progressive die will be adequate.

3. Type of press to be used: Available for this production are pressesof 5-ton, 8-ton, or 10-ton capacity, with a shut height of 175 or

200 mm.

4. Thickness of material: Specified as 32 mm standard cold rolledsteel.

Step 2, Scrap-Strip Development

From the production requirements, a single-row strip will

suffice. After several trials, the scrap strip shown in Fig. 6-9 was

decided upon. Owing to the closeness of the holes it was decided to

make a four-station die.

The scrap strip would be fed into the first finger stop, and the center hole would

be pierced. The strip would then be moved in to the second finger stop, and the

two holes would then be pierced. At the third stage and third finger stop, a pilot


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would locate the strip and the four corner holes would then be pierced. At the

fourth and final stage, a piloted blanking punch would blank out the finished part.

Step 3, Press Tonnage

It is now in order to determine the amount of pressure needed. Only the

actual blanking in the fourth stage need be calculated, since the work in the first

three stages will be done by stepped punches.

From Table, the shear strength S of cold rolled steel is 40 kgs/mm. The

length L of the blanked perimeter equals 60 x 4 = 240 mm. The depth of cut

(stock thickness t) equals 3.2 mm. From the equation P = S L t

P = 40 kgs./mm x 240 mm x 3.2 mm = 30,720 kgs.

Or 30.7 tons.

This tonnage is greater than can be handled by the available presses. To

lower the pressure, shear is ground on the blanking punch to reduce the needed

pressure by on third. This, ? x 30.7 = 30.7 - 10.2 = 20.5 tons. A punch press of

25-ton capacity would do, but there is reported available only a 30-ton press with

a 190 mm shut height and a 50 mm stroke. This press is selected. The bolster

plate is found to be 300 mm deep, 140 mm from centerline of ram to back edge

of bolster, and 600 mm wide. Shank diameter is 64 mm.

Step 4, Calculation of the Die

(a) The die. The perimeter of the cut equals 240 mm and therefore the

thickness of the die must be 25 nm. The width of our scarp-strip opening is 60

mm with 32 mm extra material on each side of the opening, it will be 60 mm + 64

mm = 124 mm or 130 mm width. The distance from the left side of the opening in

stage 4 to the edge of the opening in stage 1 equals 3 C + 30 + 6 = 192 + 30 + 6

= 228 mm and plus 62 mm = 290 mm or 296 mm long. Therefore the die should

be 2.5 x 130 x 296 mm long.

(b) The die plate. As a means of filling in between the die and the die shoe, a

die plate of machinery steel is used. To secure the die plate to the die shoe M12

cap screws and dowels are used. A minimum of twice the size of the cap screw for

the distance from the edge of the die to the edge of the die plate is needed, which

will equal 25 mm. Twice this distance = 50 mm and 50 mm added to the size ofthe die will result in a die plate of 25 x 180 x 346 mm. Figure 6-10 shows the die


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and die plate fitted together, and with the holes, which show the sharpening

portion and the relief portion.

Step 5, Calculation of Punches

Good practice requires 10 per cent of the metal thickness to be removed

from the basic dimension of the blanking punch. This same value is used on the

die opening, since holes are to be pierced in the blank. The clearance rule will be

applied to the die opening in Stages 1, 2, and 3, and to the punch in Stage 4 (see

fig6-11).

For Stage 4: Blank to be 60 mm square,

Stock thickness = 3.2 mm; 10% = 0.32 mm.

Punch = 60 0.32 = 59.68 mm.

Therefore the die opening will equal 60.01 to 60 mm and

the punch will equal 59.68 to 59.67 mm.

For Stage 2: Slot to be 8 mm wide by 34 mm long.

Die = 8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm.


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Punch will equal 3.99 to 8.00 mm. wide, and 33.99 to

34.00 mm. long.

The punch and the die opening will have straight sides for at least 3-

11 also shows a 3 mm shear for the die at Stage 4 and a 3 mm shear for thepunches of Stage 2, and also the stepped arrangement of the punches for all

stages

Step 6, springs

A solid stripper plate can be used for this job.

Step 7, Piloting

Figures 6-9 and 6-11 illustrate the arrangement for piloting. In this case it

is direct piloting. However, if the part did not have a center hole, and the slots

and other holes were too small, indirect piloting would have to be provided.

Step 8, Automatic Stops

Finger stops will act as stops when a new scrap strip is being inserted but,

after that, an automatic spring drop stop must be used to halt the scrap strip.

Figures 6-12 illustrate details of the completed drawing of the die.


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Die Design

WIRE METHODOFLOCATING THE CENTEROF PRESSURE:

The center of pressure of a blank contour may be located mathematically, but

it is a tedious computation. Location of the center of pressure within 12 mm

of true mathematical location is normally sufficient.A simple procedure

accurate within such limits is to bend asoft wire to the blank contour. By

balancing this frame acrossa pencil, in two coordinates, the intersection ofthe two axes of balance will locate the desired point. As an example of the

marked influence this factor may have on tool design, a rather unusual blank

is shown in Fig. 5-1. Here, the center of pressure is near one end of the

blank, and will require the indicated imbalance in the press tool design.


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Die clearance is the space between the matting members of a die set. Proper

clearance between cutting edges enables the fractures to meet and the

fractured portion of the sheared edge has a clean appearance. For optimum

finish of a cut edge, proper clearance is necessary and is a function of the

kind, thickness and temper of the work material.

At Fig. 5-2, which shows clearance C for blanks of a given size, make die to

size and punch smaller by total clearance 2C. At B, which shows clearance for

holes of given size, make punch to size and die larger by the amount of the

total clearance 2C


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The application of the clearance for holes of irregular shape is diagrammed in

Fig. 5-13; at B the hole will be of punch size, while at A the blank will be of

the same dimensions as the die.

DIE BLOCK GENERAL DESIGN:

Overall dimensions of the die block will be determined by the minimum die

wall thickness required for strength and by the space needed for mounting

screws and dowels and for mounting the stripper plate.

Wall thickness requirements for strength will depend on the thickness of the

stock to be cut. Sharp corners in the contour may lead to cracking in heat

treatment, and so require greater wall thickness at such points.

Two, and only two, dowels should be provided in each block or element thatrequires accurate and permanent positioning. They should be located as far

apart as possible for maximum locating effect, usually near diagonally

opposite corners. Two or more screws will be used, depending on the size of

the element mounted. Screws and dowels are preferably located about 1

times their diameters from the outer edges or the blanking contour.

Die block thickness (see Table 5-1) is governed by the strength necessary to

resist the cutting forces, and will depend on the type and thickness of the

material being cut. On very thin materials 13 mm. thickness should be


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sufficient but, except for temporary tools, finished thickness is seldom less

than 22 mm., which allows for blind screw holes and also builds up the tool to

a narrower range of shut height for press room convenience.

DIE BLOCK CALCULATIONS:

Method 1 (Rule of Thumb). Assuming a die block of tool steel its thickness

should be 20 mm. minimums for a blanking perimeter of 75 mm. or less 25

mm. thick for perimeters between 75 mm. and 250 mm. and 32 mm. thick for

larger perimeters. There should be a minimum of 32 mm. margins around the

opening in the die block.

The die opening should be straight for a maximum of 3 mm; the opening

should then angle out at to 1 to the side(draft). Thestraight sides

provide for sharpenings of the die; the tapered portion enables the blanks to

drop through without jamming.

To secure the die to the die plate or die shoe, the following rules provide

sound construction:

1) On die blocks up to 175 mm square, use two M10 cap screws and two

dowels of dia. 10 mm.

2) On sections up to 200 mm. square, use three cap screws and two dowels .

3) For blanking heavy stock, use cap screws and dowels of dia 12-mm.

diameter. Counterbore the cap screws 3.2 mm. deeper than usual, to

compensate for die sharpening.

Method 2. This method of calculating the proper size of the die was derivedfrom a series of tests, whereby die plates were made increasingly thinner untilbreakage became excessive. From these data the calculation of die thicknesswas divided into four steps:1) Die thickness is provisionally selected from Table 5-1. This table takesinto account the thickness of the stock and its ultimate shear strength (seeTable 4-1).

Table 5-1. DIE THICKNESS PER TON OF PRESSURE

Stock Die Stock Die


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thickness

mm.

thickness

cm.

thickness

mm.

thickness

cm. *

0.25

0.50

0.7

5

1.00

1.2

5

*

0.

118

0.236

0.

335

0.433

0.

512

1.5

1.8

2.0

2.3

2.5

0.590

0.649

0.7

08

0.748

0.7

87

* For each ton per sq. cm. of shear strength.

Table 5-2. FACTORS FOR CUTTING EDGES EXCEEDING 50-

mm.

Cutting perimeter mm. Expansion Factor

50 to 75

75 to 150150 to 300

300 to 500

1.25

1.51.75

2.0

2) The following corrections are then made:

a) The die must never be thinner than 8 mm. to 10 mm.

b)Data in Table 5-1 apply to small dies, i.e. those with acutting perimeter of 50-mm. or less. For larger dies, the

thickness listed in Table 5-1 must be multiplied by the

factors in Table 5-2.

c) Data in Tables 5-1 and 5-2 are for die members of tool-

steel, properly machined and heat-treated. If a special alloy

of steel is selected, die thickness can be decreased.

d) Dies must be adequatelysupported on a flat die plate or die

shoes. Thickness data above do not apply if the die is placed


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over a large dopening or is not adequately supported.

However, if the die is placed into a shoe, the thickness of

the member can beDecreased up to 50 percent.

e) A grinding allowance up to 0.25 to 0.5 mm must be added

to the calculated die thickness.

3) The critical distance A, Fig. 5-4, between the cutting edge and

the die border must be determined. In small dies, A equals 1.5

to 2 times the die thickness; in larger dies it is 2 to 3 times the

die thickness.

Table 5-3 MINIMUM CRITICAL AREA

Criticaldistance A must not less than 1.5 to 2 times die

thickness.

The critical area between the die hole and the die border

must be checked against minimum values in Table 3-4 and

die thickness B corrected if necessary.

4) Finally, the die thickness must be checked against the empirical

rule that the cross-sectional area A x B (Fig. 5-4) must bear a

certain minimum relationship to the impact pressure for a die

put on a flat base. In Table 5-3 impact pressure equals

thickness times the perimeter of the cut times ultimate shearing

strength. If the die height, as calculated by steps 1and 2, does


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not give sufficient area for the critical distance A (Fig. 5-4), the

die thickness must be increasedaccordingly.

With the die block size determined, the exact size of the die

opening can now be determined. Assuming a clearance of

approximately 10 percent of the metal thickness, and by the rule-of-

thumb method.

Metal thickness = 1.6 x 10% = 0.16 mm.

If the finished die opening is 25 mm. dia., then add 0.16 mm.,

giving 26.16 0.025 mm. If the blank were made according to size,

the clearance would be applied to the punch.

PUNCH DIMENSIONING:The determination of punch dimension has been generally

based on practical experience. When the diameter of a pierced round

hole equals stock thickness, the unit compressive stress on the punch

is four times the unit shear stress on the cut area of the stock, from

the formula.

The diameter of most holes are greater than stock thickness; a

value for the ratio d / tof 1.1 is recommended. The maximum

allowable length of a punch can be calculated from the formula.

This is not to say that holes having diameters less than stock

thickness cannot be successfully punched. The punchingof such holes can be

facilitated by:

1. Punch steels of high compressive strengths

2. Greater than average clearances

3. Optimum punch alignment, finish, and rigidity

4. Shear on punches or dies or both

5. Prevention of stock slippage

6. Optimum stripper design

The determination of punch dimension has been generally based on practicalexperience. When the diameter of a pierced round hole equals stockthickness, the unit compressive stress on the punch is four times the unitshear stress on the cut area of the stock, from the

METHODS OF PUNCH SUPPORT:


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Following figure presents a number of methods to support punches to meetvarious production requirements:


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STOCK STOPS:

Finger Stops:

In its simplest form, a stock stop may be a pin or small block, against which

an edge of the previously blanked opening is pushed after each stroke of the

press. With sufficient clearance in the stock channel, the stock is momentarily

lifted by its clinging to the punch, and is thus released from the stop. Figure

6-23 shows an adjustable type of solid block stop which can be moved along a


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support bar in increments up to 25 mm to allow various stock lengths to be

cut off.

A starting stop, used to position stock as it is initially fed to a die, is shown in

Fig. 5-23, view A. Mounted on the stripper plate, it incorporates a latch, which

is pushed inward by the operator until its shoulder (1) contacts the stripper

plate. The latch is held in to engage the edge of the incoming stock; the first

die operation is completed, and the latch is released.

Automatic Stops:

The starting stops shown at view B, mounted between the die

shoe and die block, upwardly actuates a stop plunger to initially

position the incoming stock. Compression springs return the manually

operated lever after the first die operation is completed.

Trigger stops incorporated pivoted latches (1, Fig. 5-24, viewsA

and B). At the rams descent, these latches are moved out of the

blanked-out stock area by actuating pins, 2. On the ascent of the ram,

springs, 3, control the lateral movement of the latch (equal to the side

relief) which rides on the surface of the advancing stock, and drops

into the blanked area to rest against the cut edge of the cut-out area.


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When feeding the stock strip from one stage to another, some method must

be used to correctly locate and stop the strip. Automatic stops (trigger stops)

register the strip at the final die station. They differ from finger stops in that

they stop the strip automatically, the operator having only to keep the strip

pushed against the stop in its travel through the die. A typical automatic stop

designs shown in figure 5-24. In this lever end is raised by the trip screw as

punch descends and cut the blank. On the return stroke end of lever drops

and lever end would drop it former position if it were not for the endwise

action o the lever, which causes the lever end to drop onto the top surface of

the stock instead of into blank space. The mounting of the finger on the pivot

is loose enough to allow for this endwise movement.

When feeding the stock strip from one stage to another, some method must

be used to correctly locate and stop the strip.


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Automatic stops (trigger stops) register the strip at the final die station. They

differ from finger stops in that they stop the strip automatically, the operator

having only to keep the strip pushed against the stop in its travel through the

die.


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Cropping:

An unusual stop that requires no moving mechanisms is known

as the French stop or Cropping as shown in fig. 5-26. It operates on

the principle of cutting a shoulder in the edge of the stock strip, which

acts as stop. A strip wider than necessary is inserted into the strip

channel until it contacts the shoulder built into back gage. The first hit

of the press performs the first station operation and at the same time

punches from the side of the strip a section of metal equal to the

length of the pitch. This operation leaves a shoulder in the side of the

strip. The strip is then advanced until the strip shoulder contacts the

shoulder on the back gage on the return stroke of the ram.


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The advantage of the cropping is its accuracy and speed of

operation. It is especially well suited to the light-gage materials that

are easily distorted when pushed or pulled against a stop pin. Its main

disadvantages are the extra cost of tooling and extra stock scrap.

PILOTS:

Since pilot breakage can result in the production of inaccurate parts and the

jamming or breaking of die elements, pilots should be made of

good tool-steel, heat-treated for maximum toughness and to

hardness of Rock-well C57 to 60.

Press-fit Pilots:

Press-fit pilots (Figs. 5-28 and 5-29, view C), which may out of the punch

holder, are not recommended for high-speed dies but are often used in low-

speed dies. For holes 20 mm in diameter or larger, the pilot may be held


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by a socket-head screw shown at B. A typical press-fit type is shown at C.

Pilots of less than 6 mm diameter may be headed and secured by a socket

setscrew, as shown at D.

Indirect Pilots:

Designs of pilots that enter previously pierced holes in the strip as shown in

Fig. 5-27. This practice provides more support under the strip. It helps to

locate the pitch and prevent distortion.

Spring-loaded pilots:

Spring-loaded pilots should be used for stock exceeding 1.5 mm thickness

sheet as shown in Fig. 5-30. This allows the pilot to retract in case of misfeed.

Tapered slug-clearance holes through the die and lower shoe should be

provided, since indirect pilots generally pierce the strip during a misfeed.

Spring-loaded pilots are not necessary on thinner material because the pilot

will pierce the strip rather than break in the event of misfeed. In this case

tapered slug-clearance holes through the die and lower shoe should be

provided.

Misfeed detector:

Fig. 5-31 shows a precision misfeed detector used in a manner similar to that

of a pilot. The detector senses out-of-register position of stock and actuates a

switch to cut off the electric power to the press.


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STRIPPERS:

Strippers are of two types, fixed or spring-operated. The primary

function of either type is to strip the work piece from a cutting or non-cutting

punch or die. A stripper that forces a part out of a die may also be called a

knockout, an inside stripper, or an ejector. Besides its primary function, a

stripper may also hold down or clamp, position, or guide the sheet, strip, or

work piece.

Fixed strippers:

The stripper is usually of the same width and length as the die block.

In the simpler dies, the stripper may be fastened with the same screws

and dowels that fasten the die block, and the screw heads will be

counter bored into the stripper. In more complex tools and with

sectional die blocks the die block screws will usually be inverted, and

the stripper fastener will be independent. Following fig. 5-32 is will

make clear picture of fixed type stripper plate.

The stripper thickness must be sufficient to withstand the force required to

strip the stock from the punch, plus whatever is required for the stock strip

channel. Except for very heavy tools or large blank areas, the thickness

required for screw head counter bores, in the range of 10 to 16 mm will be

sufficient.


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The height of the stock strip channel should be at least 1 times the stock

thickness. This height should be increased if the stock is to be lifted over a

fixed pin stop. The channel width should be the width of the stock strip, plus

adequate clearance to allow for variations in the width of the strip as cut, as

follows:

Stock thickness mm Add to strip width in mm

Up to 1 2.0

1 to 2 2.4

2 to 3 2.8

Above 3 3.2

If the stripper length has been extended on the feed end for better

stock guidance, a sheet metal plate should be fastened to the underside of the

projecting stripper to support the stock. This plate should extend slightly in

for convenience in inserting the strip. The entry edges of the channel should

be beveled for the same reason.

5.9.2 Spring-operated strippers:


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Where spring-operated strippers are used as shown in fig. 5-33, the force

required for stripping is 35000 times cut perimeter times the stock thickness.

It may be as high as 20 per cent of the blanking force, which will determine

the number and type of springs required. The highest of these values should

be used.

Selection of stripper springs:

Die springs are designed to resist fatigue failure under severe service conditions. They are

available in medium, medium-heavy and heavy-duty grades, with corresponding permissible

deflections ranging from 50 to 30 percent of free length. The number of springs for which space is

available and the total required force will determine which grade is required. The required travel

plus the preload deflection will be the total deflection, and will determine the length of spring

required to stay within allowable percentage of deflection limits. As the punch is re-sharpened,

deflections will increase, and should also be allowed for.

To retain the stripper against the necessary preload of the springs, and

to guide the stripper in its travel, a special type of shoulder screw known as a

stripper bolt is used.

Choice of the method of applying springs to stripper plates depends on

the required pressure, space limitations, shape of the die and the nature of

the work, and production requirements. The stripping pressure may be from 5

to 20 percent of the cutting pressure. The amount of pressure needed to hold

thin firmly while it being cut must also be considered when selecting springs.

With so many variable factors involved, exact results cannot be expected from

formulas, although they may be useful as guides. The Metal Handbook gives

the following formula for stripping force:

L = KA

Where

L = Stripping force

K = Stripping

constant, Kg / cm.2

A = Area of cut

surface, cm.2

Approx. values for K, as determined by experiment: 105 for sheet

metal thinner than 1.6 mm. when cut is near an edge or near a

preceding cut; 150 for other cuts in sheet metal thinner than 1.6

mm; and 210 for sheet metal more than 1.6 mm. thick.


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(stockthickness xlength of cut)

Die springs are available in various service grades with corresponding

permissible deflections ranging from 25 to 50 percent of free length. For

example, DME Spring catalogue lists four load ratings. Based upon max. (See

fig. 5-34)

50% deflection = medium pressure (MP)

37% deflection = medium high pressure (MHP)

30% deflection = high pressure (HP)

25% deflection = extra high pressure (XMP)

An important feature of these springs is that they are similar

dimensioned thus are interchangeable. The steps in selecting die

springs are as follows:

1) Estimate the stripping force required according to formula.

2) Determine the amount of space available for spring mounting.

3) Select the max. allowable number of springs, which will fit into

the available space and total required force, will determine

which grade of spring is required.

4) Determine the deflection. The required travel plus preload

deflection will be the total deflection and will determine the

length of spring required to stop within the allowable

percentage of deflection limits. Allowance should be made for

punch sharpening, which will increase deflections over a period

of time.

Select a spring from the lowest (greatest deflection) load rating series from

the table. It is important that the spring not be compressed beyond the

specific limit for the highest-pressure spring of corresponding length and


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diameter. Then if the springs prove too light for the stripping force required,

each one is replaced with the next higher rated springs. This will provide more

stripping pressure without the need changing spring pockets, support rods

and the like.

KNOCKOUTS

Since the cut blank will be retained, by friction, in the die block, some means

of ejecting on the ram upstroke must be provided. A knockout assembly

consists of a plate, a push rod and a retaining collar. The plate is a loose fit

with the die opening contour, and moves upward as the blank is cut. Attached

to the plate, usually by rivets, is a heavy pushrod, which slides in a hole in the

shank of the die set. This rod projects above the shank, and a collar retains

and limits the stroke of the assembly. Now the assembly of the ram stroke, a

knockout bar in the press will contact the pushrod and eject the blank.

It is essential that the means of retaining the knockout assembly besecure, since serious damage would otherwise occur.

In the ejection of parts positive knockouts offer the following

advantages over spring strippers where the part shape and the die selections

allow their use:


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1) Automatic part disposal; the blank, ejected near the top of the ram

stroke, can be blown to the back of the press, or the press may be

inclined and the same result obtained.

2) Lower die cost; knockouts are generally of lower cost than spring

strippers.

3) Positive action; knockouts do not stick as spring strippers

occasionally do.

4) Lower pressure requirements, since there are no heavy springs to

be compressed during the ram descent.

Fig. 5-35 shows a plain inverted compound die, is of the simplest type.

It consists of an actuating plunger, knockout plate and a stop collar

doweled to the plunger. Shedder D consists of a shouldered pin

backed by a spring, which is confined by setscrew


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Scrap Strip Trip Layout For Blanking

In designing parts to be blanked from strip material, economical stock

utilization is of high importance. The goal should be at least 75 per cent

utilization. A very simple scrap-strip layout is shown in Fig. 6-1.

SCRAP ALLOWANCE

A scrap-strip layout having insufficient stock between the blank and

the strip edge, and between blanks, will result in a weakened strip, subject to

breakage and thereby causing misfeeds. Such troubles will cause unnecessary

die maintenance owing to partial cuts, which defect the punches, resulting in

nicked edges. The following formulas are used in calculating scrap-strip

dimensions for all strips over 0.8 mm. thick.

t = specified thickness of the material

B = 1.25 t when C is less than 64 mm

B = 1.5 t when C is 64 mm or longer

C = L + B, or lead of the die


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Example: A rectangular part, to be blanked from 1.5 mm thick

steel (Manufacturers Standard) is 10 X 27 mm. If the scrap

strip is developed as in Fig. 6-2, the solution is

t = 1.5 mm

B = 1.25 X 1.5 = 1.875 mm

C = 10 + 1.875 = 11.875 mm

W = 27 + 3.75 = 30.75 mm

Nearest commercial stock is 32 mm. Therefore, the

distance B will equal 2.3mm. This is acceptable since it exceedsminimum requirements.

Minimum Scrap-Strip Allowance: If the material to be blanked is

0.6 mm thick or less, the formulas above should not be used. Instead,

dimension B is to be as follows:

Strip width W Dimension B

0 - 75 mm 1.3 mm

76 150 mm 2.4 mm


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150 300 mm 3.2 mm

Over 300 mm 4.0 mm

Other Scrap-Strip Allowance Applications:

Figure 6-3, 6-4 and 6-5 illustrates special allowances for one-

pass layouts:

View D. For layouts with sharp corners of blanks adjacent, B =

1.25 t.

Fig. 6-4 Allowances for one-passlayouts.


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Percentage of Stock Used: If the area of the part is divided by the

area of the scrap strip used, the result will be the percentage of stock used.

If A = total area of strip used to produce a single

blanked part, then

A = CW (Fig. 3-35), and a = area of the part = LH.

If C = 11.5 mm and W = 32 mm then A = 11.5 X 32 =

368 mm

If L X 9.5 mm and H = 27 mm then a = 29.5 X 27 =

256.6 mm

Percentage of stock used:

a 256.5= = 70% approx.

A 368


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EVOLUTION OF A BLANKING DIE

In the planning of a die, the examination of the part print immediately

determines the shape and size of both punch and die as well as the working

area of the die set.

Die Set Selection

A commercially available standardized two-post die set with 150 mm overall

dimensions side-to-side and front-to-back allows the available 76 mm. wide

stock to be fed through it. It is large enough for mounting the blanking punch

on the upper shoe (with the die mounted on the lower shoe) for producing the

blank shown in Fig. 6-6, since the guideposts can be supplied in lengths of

from 100 to 225 mm.

Since the stock, in this case was available only in a width of 76 mm the length

of the blanked portions extended across the stock left a distance between the

edges of the stock and the ends of the blank of 6 mm or twice the stock


Die Block Design

By the usual rule-of thumb method previously described, die blockthickness (of tool steel) should be a minimum of 20 mm for a blanking


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perimeter up to 75 mm and 25 mm for a perimeter between 75 and 100 mm.

For longer perimeters, die block thickness should be 32 mm. Die blocks are

seldom thinner than 22 mm finished thickness to allow for grinding and for

blind screw holes. Since the perimeter of the blank is approximately 178 mm

a die block thickness of 38 mm was specified, including a 6 mm grinding

allowance.

There should be a margin of 32 mm around the opening in the die

block; its specified size of 150 x 150 mm allows a margin of 45 mm in which

four M10 cap screws and dia. 10 mm dowels are located at the corners 20

mm from the edges of the block.


thickness); below this portion or the straight, an angular clearance of 1

allows the blank to drop through the die block without jamming. The

dimensions of the die opening are the same as that of the blank; those of the

punch are smaller by the clearance (6 per cent of stock thickness, or 2 mm),

which result in the production of blanks to print (and die) size.

The top of the die was ground off a distance equal to stock thickness

(Fig. 6-7) with the result that shearing of the stock starts at the ends of the

die and progresses towards the center of the die, and less blanking pressure

is required than if the top of the die where flat.

Punch Design

The shouldered punch (57 mm) long is held against a 6 mm thick

hardened steel backup plate by a punch plate 20 mm thick) which is screwed

and doweled to the upper shoe. The shut height of the die can be

accommodated by a 32-ton (JIC Standard) open-back inclinable press, leaving

a shut height of 240 mm. For the conditions of this case study, shear strength

S = 42 kg/mm, blanked perimeter length L = 178 mm approx. and thickness

T = 3.2 mm.


The pressure P = 42 kgs. X 178 mm X 3.2 = 23.92

tons.


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This value is well below the 32-ton capacity of the

selected press.

The shut height (Fig. 6-7) is 178 mm less the 1.6 mm travel of the

punch into the die cavity.

Stripper Design

The stripper that was designed is of the fixed type with a channel or

slot having a height equal to 1.5 times stock thickness and a width of 80 mm

to allow for variations in the stock width of 75 mm. The same screws that

hold the die block to the lower shoe fasten the stripper to the top of the die

block.

If, instead of 3.2 mm stock, thin (0.8 mm) stock were

to be blanked, a spring-loaded stripper would firmly hold the

stock down on top of the die block and could, to some extent,

flatten out wrinkles and waves in it.


withdrawn from the stock. The pressure that strips the stock from the

punch on the upstroke is difficult to evaluate exactly. A formula frequently

used is



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Spring design is beyond the scope of this book; die

spring data are available in the catalogues of spring

manufacturers.

Stock Stops

The pin stop pressed in the die block is the simplest method for

stopping the hand-fed strip. The right-hand edge of the blanked opening ispushed against the pin before descent of the ram and the blanking of the next

blank. The 4-8 mm depth of the stripper slot allows the edge of the blanked

opening to ride over the pin and to engage the right-hand edge of every

successive opening.

The design of various types of stops adapted for manual and automatic

feeding is covered in a preceding discussion.

EVOLUTION OF A PROGRESSIVE BLANKING DIE

Figure 6-8 gives the blanked dimensions of a linkage case cover of cold

rolled steel, stock size 3.2 x 60 x 60 mm. Production is stated to be 200 parts

made at one setup, with the possibility of three or four runs per year.


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Step 1, Part Specification

1. The production is of medium class; therefore a second-class diewill be used.

2. Tolerances required: Except for location of the slots, alldimensions are in fractions. The slot locations, though specified

in decimals, are not very close. Thus a compound die is not

necessary; a two or three-station progressive die will be

adequate.

3. Type of press to be used: Available for this production arepresses of 5-ton, 8-ton, or 10-ton capacity, with a shut height

of 175 or 200 mm.

4. Thickness of material: Specified as 32 mm standard cold rolledsteel.


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Step 2, Scrap-Strip Development

From the production requirements, a single-row strip will suffice. After

several trials, the scrap strip shown in Fig. 6-9 was decided upon. Owing to

the closeness of the holes it was decided to make a four-station die.

The scrap strip would be fed into the first finger stop, and the center hole

would be pierced. The strip would then be moved in to the second finger

stop, and the two holes would then be pierced. At the third stage and third

finger stop, a pilot would locate the strip and the four corner holes would then

be pierced. At the fourth and final stage, a piloted blanking punch would blank

out the finished part.

Step 3, Press Tonnage

It is now in order to determine the amount of pressure needed. Only

the actual blanking in the fourth stage need be calculated, since the work in

the first three stages will be done by stepped punches.

From Table, the shear strength S of cold rolled steel is 40 kgs/mm.

The length L of the blanked perimeter equals 60 x 4 = 240 mm. The depth of

cut (stock thickness t) equals 3.2 mm. From the equation P = S L t


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P = 40 kgs./mm x 240 mm x 3.2 mm = 30,720

kgs. Or 30.7 tons.

This tonnage is greater than can be handled by the available presses.

To lower the pressure, shear is ground on the blanking punch to reduce the

needed pressure by on third. This, ? x 30.7 = 30.7 - 10.2 = 20.5 tons. A

punch press of 25-ton capacity would do, but there is reported available only

a 30-ton press with a 190 mm shut height and a 50 mm stroke. This press is

selected. The bolster plate is found to be 300 mm deep, 140 mm from

centerline of ram to back edge of bolster, and 600 mm wide. Shank diameter

is 64 mm.

Step 4, Calculation of the Die

(a) The die. The perimeter of the cut equals 240 mm and therefore the

thickness of the die must be 25 nm. The width of our scarp-strip opening is 60

mm with 32 mm extra material on each side of the opening, it will be 60 mm

+ 64 mm = 124 mm or 130 mm width. The distance from the left side of the

opening in stage 4 to the edge of the opening in stage 1 equals 3 C + 30 + 6

= 192 + 30 + 6 = 228 mm and plus 62 mm = 290 mm or 296 mm long.

Therefore the die should be 2.5 x 130 x 296 mm long.

(b) The die plate. As a means of filling in between the die and the die shoe,

a die plate of machinery steel is used. To secure the die plate to the die shoe

M12 cap screws and dowels are used. A minimum of twice the size of the cap

screw for the distance from the edge of the die to the edge of the die plate is

needed, which will equal 25 mm. Twice this distance = 50 mm and 50 mm

added to the size of the die will result in a die plate of 25 x 180 x 346 mm.

Figure 6-10 shows the die and die plate fitted together, and with the holes,

which show the sharpening portion and the relief portion.


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Step 5, Calculation of Punches

Good practice requires 10 per cent of the metal thickness to be

removed from the basic dimension of the blanking punch. This same value isused on the die opening, since holes are to be pierced in the blank. The

clearance rule will be applied to the die opening in Stages 1, 2, and 3, and to

the punch in Stage 4 (see fig6-11).

For Stage 4: Blank to be 60 mm square,

Stock thickness = 3.2 mm; 10% = 0.32 mm.

Punch = 60 0.32 = 59.68 mm.

Therefore the die opening will equal 60.01 to 60 mm and

the punch will equal 59.68 to 59.67 mm.

For Stage 2: Slot to be 8 mm wide by 34 mm long.

Die = 8 + 0.32 + 8.32 mm long = 34.32 to 34.33 mm.

Punch will equal 3.99 to 8.00 mm. wide, and 33.99 to

34.00 mm. long.


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The punch and the die opening will have straight sides for at least

3-11 also shows a 3 mm shear for the die at Stage 4 and a 3 mm shear for

the punches of Stage 2, and also the stepped arrangement of the punches for

all stages

Step 6, springs

A solid stripper plate can be used for this job.

Step 7, Piloting

Figures 6-9 and 6-11 illustrate the arrangement for piloting. In this

case it is direct piloting. However, if the part did not have a center hole, and

the slots and other holes were too small, indirect piloting would have to be

provided.

Step 8, Automatic Stops

Finger stops will act as stops when a new scrap strip is being inserted

but, after that, an automatic spring drop stop must be used to halt the scrap

strip. Figures 6-12 illustrate details of the completed drawing of the die.


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Bending, Forming And Drawing Dies

BENDING DIES

Bending is the uniform straining of material, usually flat sheet or strip

metal, around a straight axis, which lies in the neutral plane and normal to

the lengthwise direction of the sheet or strip. Metal flow takes place within the

plastic range of the metal, so that the bend retains a permanent set after

removal of the applied stress. The inner surface of a bend is in compression;

the outer surface is in tension. A pure bending action does not reproduce the

exact shape of the punch and die in the metal; such a reproduction is one of

forming.

Terms used in bending are defined and illustrated in Fig. 7-1. The

neutral axis is the plane area in bent metal where all strains are zero.

Bend Radii: Minimum bend radii vary for the various metals;

generally most annealed metals can be bent to a radius equal to the thickness

of the metal without cracking or weakening.


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Bend Allowances: Since bent metal is longer after bending, its

increased length, generally of concern to the product designer, may also have

to be considered by the die designer if the length tolerance of the bent part is

critical. The length of bent metal may be calculated from the equation

A

B = 2 ( IR + Kt )

(7-1) 360

Where B = bend allowance in mm. (along neutral axis)

A = bend angle in deg.

IR = inside radius of bend in mm.

t = metal thickness in mm.

K = 0.33 when IR is less than 2t and is 0.50 when IR is more

than 2t.


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Bending Methods: Two bending methods are commonly made use of

in press tools. Metal sheet or strip, supported by a V block (Fig. 7-2A), is

forced by a wedge-shaped punch into the block. This method, termed V

bending, produces a bend having an included angle which may be acute,

obtuse, or of 90. Friction between a spring-loaded knurled pin in the vee of a

die and the part will prevent or reduce side creep of the part during its

bending.


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Edge bending (Fig. 7-2B) is cantilever loading of a beam. The bending punch,

step1, forces the metal against the supporting die, step 2 - The bend axis is

parallel to the edge of the die. The work piece is clamped to the die block by a

spring-loaded pad, step3, before the punch contacts the work piece to prevent

its movement during downward travel of the punch.

Bending Pressures: The pressure required for V bending is

K L S t

P = W

(7-2)

Where P = bending force, tons

K = die opening factor: 1.20 for a die opening of 16 times metal

thickness,

1.33 for an opening of 8 times metal thickness

L = length of part, cm.

S = ultimate tensile strength, tons per sq cm.

W = width of V or U die, cm.


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For U bending (channel bending) pressures will be approximately twice

those required for V bending; edge bending requires about one-half those

needed for V bending.

Spring back:

After bending pressure on metal is released, the elastic stresses are also

released, which causes metal movement resulting in a decrease in the bend

angle (as well as an increase in the included angle between the bent

portions). Such a metal movement, termed spring-back, varies in steel

from to 5, depending upon its hardness; phosphor bronze may springback from 10 to 15.

V-bending dies customarily compensate for spring-back with V blocks and

wedge-shaped punches having included angles somewhat less than that

required in the part. The part is bent through a greater angle than that

required but it spri

Documents

Presses and Equipment for SheetMetal Dies