Metal Forming-cold Working

Fundamentals of Metal Forming

Manansala, Jeremy

Forming Processes

Forming processes tend to be complex systems consisting independent variables, dependent variables, and independent-dependent interrelations.

Manansala, Jeremy

Independent Variables

Independent variables are those aspects of the process over which the engineer has direct control, and they are generally selected or specified when setting up the process.

Manansala, Jeremy


Starting material the engineer is often free to specify the chemistry

and condition. These may also be chosen for ease in fabrication or they may be restricted by the final properties desired for the product.

Starting geometry of the workpiecethis may be dictated by previous processing or it may be selected by the engineer from a variety of available shapes. Economics often influence this decision.

Manansala, Jeremy


Tool or die geometry

this are has many aspects such as the diameter of a rolling mill roll, the die angle in wire drawing and the

cavity details when forging. Since tooling will produce and control the metal flow, success or failure of a process often depends on tool geometry.

Manansala, Jeremy


Lubrication

since lubricants also acts as coolants, thermal barriers, corrosion inhibitors, and parting compounds, their selection is an aspect of great importance. Specification includes type of lubricant amount to be applied and the method of application.

Manansala, Jeremy


Starting temperaturemany material properties vary greatly with

temperature, so its selection and control may well dictate the success or failure of an operation.

Speed of operation

since speed can directly influence the lubricant effectiveness, the forces required for deformation and the time available for heat transfer. It is obvious that its selection would be significant in a forming operation.

Manansala, Jeremy


Amount of deformation while some processes control this variable through

die design, others, such as rolling permits its selection at the discretion of the engineer.

Manansala, Jeremy

Dependent Variables

Force or power requirements

engineers cannot directly specify the force or power; they can only specify the independent variables and then experience the consequences of the selection. The ability to predict the forces or powers however

is extremely important for only by having this knowledge will the engineer be able to specify or select the equipment for the process.

Manansala, Jeremy

Dependent Variables

Material properties of the product the customer is not interested in the starting

properties but is concerned with our ability to produce the desired final shape with the desired final properties

Exit temperature

engineering properties can be altered by both the mechanical and thermal history of the material thus it is important to know and control the temperature of the material throughout the process

Manansala, Jeremy

Dependent Variables

Surface finish and precision both are characteristics of the resultant product that are dependent on the specific details of the process.

Nature of the material flow

since properties depend on deformation history, control here is vital the customer is satisfied only if the desired geometric shape is produced with the right set of companion properties and without surface or internal defects.

Manansala, Jeremy

Independent-Dependent Interrelations

Experience

this requires long time exposure to the process and is generally limited to the specific materials, equipment

and products encountered in the realm of past contact.

Experiment while possibly the least likely in error direct experiment is both time consuming and costly.

Manansala, Jeremy

Independent-Dependent Interrelations

Process modeling here one approaches the problem with a high speed computer and one or more mathematical models of

the process numerical values are provided for the various independent variables and the models are used to compute predictions for the dependent

variables

Fajardo, Jeptah Dane

Temperature Concerns

In general, an increase in temperature brings out a decrease in strength, an increase in ductility, and a decrease in the rate of strain hardening – all effects that would tend to promote ease of deformation.


Hot Working

The plastic deformation of metals above their The plastic deformation of metals above their recrystallization temperature; it is important to note, recrystallization temperature; it is important to note, however, that the recrystallization temperature varies however, that the recrystallization temperature varies greatly with different materialsgreatly with different materials

Elevated temperatures bring about a decrease in the Elevated temperatures bring about a decrease in the yield strength of a metal and an increase in ductility. yield strength of a metal and an increase in ductility. At the temperatures of hot working, recrystallization At the temperatures of hot working, recrystallization eliminates the effects of strain hardening, so there is eliminates the effects of strain hardening, so there is no significant increase in yield strength or hardness, no significant increase in yield strength or hardness, or corresponding decrease in ductility. or corresponding decrease in ductility.

In addition, the elevated temperatures promote In addition, the elevated temperatures promote diffusion that can remove or reduce chemical diffusion that can remove or reduce chemical inhomogeneities; pores can be welded shut or inhomogeneities; pores can be welded shut or reduced in size during the deformation; and the reduced in size during the deformation; and the metallurgical structure can often be altered through metallurgical structure can often be altered through recrystallization to improve the final properties.recrystallization to improve the final properties.

Structure and Property Modification by Hot Working…


Hot Working

When metals solidify, particularly in the large sections When metals solidify, particularly in the large sections that are typical cast strands, coarse structures tend to that are typical cast strands, coarse structures tend to form with a certain amount of chemical segregation. form with a certain amount of chemical segregation. The size of the grains is usually not uniform, and The size of the grains is usually not uniform, and undesirable grain shapes can be quite common, such undesirable grain shapes can be quite common, such as the columnar grains. Small gas cavities or as the columnar grains. Small gas cavities or shrinkage porosity can also form during solidification.shrinkage porosity can also form during solidification.

Temperature Variations…Temperature Variations…


Hot Working

The success or failure of a hot deformation The success or failure of a hot deformation process often depends on the ability to control the process often depends on the ability to control the temperatures with the workpiece.temperatures with the workpiece.

To minimize problems, it is desirable to keep the To minimize problems, it is desirable to keep the workpiece temperatures as uniform as possible.workpiece temperatures as uniform as possible.

Plastic deformation of metals below the Plastic deformation of metals below the recrystallization temperaturerecrystallization temperature


Cold Working

The process is usually performed at room The process is usually performed at room temperature, but mildly elevated temperatures temperature, but mildly elevated temperatures may be used to provide increased ductility and may be used to provide increased ductility and reduced strengthreduced strength

ADVANTAGES…ADVANTAGES…


Cold Working

No heating is requiredNo heating is required Better surface finish Better surface finish is obtainedis obtained

Superior dimensional Superior dimensional control is achieved, so control is achieved, so little, if any, secondary little, if any, secondary machining is requiredmachining is required

Products possess better Products possess better reproducibility and reproducibility and interchangeabilityinterchangeability

Strength, fatigue and Strength, fatigue and wear properties are wear properties are improved through improved through strain hardeningstrain hardening

Directional properties Directional properties can be impartedcan be imparted

Contamination Contamination problems are problems are minimizedminimized

DISADVANTAGES…DISADVANTAGES…


Cold Working

Higher forces are Higher forces are required to initiate and required to initiate and complete the complete the deformationdeformation

Heavier and more Heavier and more powerful equipment powerful equipment is requiredis required

Intermediate anneals Intermediate anneals may be required to may be required to compensate for the compensate for the loss of ductility that loss of ductility that accompanies strain accompanies strain hardeninghardening

Metal surfaces must be Metal surfaces must be clean and scale-freeclean and scale-free Less ductility is Less ductility is

availableavailable Imparted directional Imparted directional

properties may be properties may be detrimentaldetrimental

Undesirable residual Undesirable residual stresses may be stresses may be producedproduced

PREPARING METALS FOR COLD PREPARING METALS FOR COLD WORKING…WORKING…


Cold Working

The success or failure of a cold-working operation The success or failure of a cold-working operation depends on the quality of the starting material.depends on the quality of the starting material.

To obtain a good surface finish and maintain To obtain a good surface finish and maintain dimensional precision, the starting material must be dimensional precision, the starting material must be clean and free of oxide or scale that might cause clean and free of oxide or scale that might cause abrasion and damage to the dies or rolls.abrasion and damage to the dies or rolls.


Warm Working

Deformation produced at temperatures intermediate Deformation produced at temperatures intermediate to hot and cold workingto hot and cold working

Compared to cold working, it offers the advantages Compared to cold working, it offers the advantages of reduced loads on the tooling and equipment, of reduced loads on the tooling and equipment, increased material ductility, and a possible increased material ductility, and a possible reduction in the number of anneals due to a reduction in the number of anneals due to a reduction in the amount of strain hardening.reduction in the amount of strain hardening.

Compared to hot forming, the lower temperatures Compared to hot forming, the lower temperatures of warm working produce less scaling and of warm working produce less scaling and decarburization, and enable production of decarburization, and enable production of products with better dimensional precision and products with better dimensional precision and smoother surfaces.smoother surfaces.

The warm regime generally requires less energy The warm regime generally requires less energy than hot working due to the decreased energy in than hot working due to the decreased energy in heating the workpiece, energy saved through heating the workpiece, energy saved through higher precision and the possible elimination of higher precision and the possible elimination of postforming heat treatments.postforming heat treatments.

Tools last longer, for while they must exert 25 to Tools last longer, for while they must exert 25 to 60% higher forces, there is less thermal shock 60% higher forces, there is less thermal shock and thermal fatigue.and thermal fatigue.

Cold-working Processes

Major Cold-Working Operations…

Ocfemia, Cathrine Joy

Cold-working Processes

Bending

Squeezing

Drawing

Shearing

Presses


Squeezing Processes

Classifications of Squeezing Processes…Rolling

Swaging

Cold Forging

Extrusion

Sizing

Riveting

Staking

Coining

Peening

Burnishing

Hubbing

Thread Rolling


Squeezing Processes

ROLLING

process used in sheets, strips, bars, and rods to obtain products that have smooth surfaces and accurate dimensions; most cold-rolling is performed on four-high or cluster-type rolling mills


Squeezing Processes

SWAGING

process that reduces/increases the diameter, tapers, rods or points round bars or tubes by external hammering


Squeezing Processes

COLD FORGING

process in which slugs of material are squeezed into shaped die cavities to produce finished parts of precise shape and size.


Squeezing Processes

EXTRUSION

process which is commonly used to make collapsible tubes such as toothpaste tubes, cans usually using soft materials such as aluminum, lead, tin. Usually a small shot of solid material is placed in the die and is impacted by a ram, which causes cold flow in the material.


Squeezing Processes

SIZING

process of squeezing all or selected areas of forgings, ductile castings, or powder metallurgy products to achieve a desired thickness or precision


Squeezing Processes

RIVETING

process where a head is formed on the shrank end of a fastener to permanently join sheets or plates of material;


Squeezing Processes

STAKINGprocess of permanently joining parts together when one part protrudes through a hole in the other; a shaped punch is driven into the end of the protruding piece where a deformation is formed causing a radial expansion, mechanically locking the two pieces together


Squeezing Processes

COINING

process where metal while it is confined in a closed set of dies; used to produce coins, medals, and other

products where exact size and fine details are required, and thickness varies about a

well-defined average


Squeezing Processes

PEENING

process where the surface of the metal is blasted by shot pellets; the mechanical working of surfaces by repeated blows of impelled shot or a round-nose tool


Squeezing Processes

BURNISHING

process by which a smooth hard tools is rubbed on the metal surface and flattens the high spots by applying compressive force and plastically flowing the material


Squeezing Processes

HUBBING

process to used to form recessed cavities in various types of female tooling dies. This is often used to make plastic extrusion dies in an economical manner


Squeezing Processes

THREAD ROLLING

process is used for making external threads; in this process, a die, which is a hardened tool with the thread profile, is pressed on to a rotating workpiece

Glassl, Gerlli

Bending- the plastic deformation of metals about a linear axis with little or no

change in the surface area

- the purpose of bending is to form sheet metal along a straight line

parts manufactured by bending

center of the sheet -metal is thinned somewhat at the bend

- where the material cannot compensate by the sideways contraction of the free edges

outside: stretched inside: compressed

Glassl, Gerlli

Bending

The bending operation can be considered as a system with six components:

Product/Workpiece Equipment/ToolDeformation zone Interface EnvironmentDie

Glassl, Gerlli

Bending

Springback - the elastic recovery of the material after unloading of the tools

To compensate with the unbending action of the springback, the metal should be slightly overbent.

Glassl, Gerlli

Bending

Rotary Bending

Double Die Bending

Wiping Die Bending

Glassl, Gerlli

Bending

Classifications of Bending Processes…

Roll Forming

Roll

Seaming

Straightening

Draw and Compression

Flanging

Angle

Glassl, Gerlli

Bending

Angle Bending

Bar folder

Glassl, Gerlli

Bending

Press Brake Bending rubber

Roll Bending bending where plates, sheets and rolled shapes can be

bent to a desired curvature

Roll bending toll can bend plate up to 6 inches thick

Glassl, Gerlli

Bending

Drawing and Compression

Drawing Bending Compression Bending

- workpiece is clamped against a bending form and the entire assemble rotates to draw the workpiece across a stationary tool

- the bending form remains stationary and the pressure tool moves along the workpiece

Glassl, Gerlli

Bending

Roll Forming- involves the progressive bending of metal strip as it passes through a series of forming rolls

Any material that can be bent can be roll- formed

Glassl, Gerlli

Bending

Seaming- bending operation that can be used to join the ends of sheet metal to form containers such as cans, pails and drums

Glassl, Gerlli

Bending

Flanging-the process of rolling on sheet metal in essentially the same manner as seaming

Glassl, Gerlli

Bending

Straightening also known as flattening opposite of bending

Glassl, Gerlli

Bending

Glassl, Gerlli

BendingRoll- straightening/ Roll Leveling

-subjects the material to a series of reverse bends

-metal is bent back and forth

Stretcher Leveling-sheets are gripped mechanically

Glassl, Gerlli

Shearing

- the mechanical cutting of materials without the information of chips or the use of burning or melting

for straight cutting blades: shearingfor curved blades: blanking, piercing, notching, trimming

Aluminum Brass

Bronze Mild steel

Stainless steel

Glassl, Gerlli

Shearing

Classifications of Shearing Processes…

Blanking

Piercing

Shaving

Cutoff

NotchingTrimming

Slitting

Dinking

Glassl, Gerlli

Shearing

Slitting- shearing process used to cut rolls of sheet metal into several rolls of narrower width

used to cut a wide coil of metal into a number of narrower coils as the main coil is moved through the slitter.

Glassl, Gerlli

Shearing

Blanking

during which a metal workpiece is removed from the primary metal strip or sheet when it is punched.

punch-out: workpiece

remaining strip: scrap

Glassl, Gerlli

Shearing

Piercing

punch-out: scrapremaining strip: workpiece

Glassl, Gerlli

Shearing

Lancing- forms either a line cut (slit) or an actual hole

Perforating- piercing a large number of closely spaced holes

Glassl, Gerlli

Shearing

Notching-same as piercing- edge of the strip or black forms part of the punch-out perimeter Nibbling- produces a series

of overlapping slits/notches

Glassl, Gerlli

Shearing

Shaving

- finishing operation in which a small amount of metal is sheared away from the edge of an already blanked part

- can be used to produce a smoother edge

Glassl, Gerlli

Shearing

Trimming

Glassl, Gerlli

Shearing

- punch and die operation used to separate a stamping or other product from a strip or stock

Cutoff

Glassl, Gerlli

Shearing

Dinking

-used to blank shapes from low-strength materials such as rubber, fiber and cloth

Ligon, Jennilyn

Drawing and Sheet Metal Forming

COLD DRAWING• for sheet metal, it refers to the forming of parts where plastic form occurs over a curved axis

• for wire, rod, or tubing, it refers to the process of reducing the cross section of the material by pulling it through a die

A. Rod, Bar and Tube Drawing

Rod or bar drawing

Die

Rod

If the product cannot be bent or coiled conveniently, straight-pull draw benches are employed on finite-length stock

Ligon, Jennilyn


Tube Drawing

• can be used to produce high quality tubing where the product requires the smooth surfaces, thin walls, accurate dimension and added strength.

Mandrel

Die

Tube

Die

Die

Floating plug

Ligon, Jennilyn


B. Wire Drawing

• involves smaller diameter material

Direction of travel

Starting stock in coil form

Lubrication box

Die

Draw block

Carbide die

Steel casing

Back relief

Land

Approach angle

Entrance angle

Back radius

Ligon, Jennilyn


C. Spinning

• a cold-forming operation in which a rotating disk of sheet metal is shaped over a male form or mandrel

Form attached to the Headstock spindle

Final shape

Steps in spinning

Original blank of sheet metal

Follower held in tailstock

Ligon, Jennilyn


D. Shear Forming

Shear Forming or Flow Turning• modification of the spinning process in which each element of the blank maintains its distance from the axis of rotation• forms cones, hemisphere and similar shapes

Ligon, Jennilyn


α

tc

tb

Feed Roller

Blank

Mandrel

Direct Process

tc will var with th e angle of the particuilar region according to to the relationship: tc = tb sinα

Blank

FeedFinished part

Reverse Process

Ligon, Jennilyn


E. Stretch Forming (Stretch-wrap forming)

• a sheet of metal is gripped by two or more sets of jaws that stretch it and wrap it up around a single form block

Ligon, Jennilyn


F. Sheet Metal DrawingShell Drawing

• earliest use was the manufacture of artillery shells and cartridge casesShallow Drawing

• the process considered when the depth of the product is less than its diameter

Deep Drawing• the process considered when the depth is greater than its diameter

Shrink Forming

•The circumference decreases as the blank diameter is reduced from d1 to the cup wall diameter d’1 since the volume of material must remain constant, the decrease in circumferential dimension must be compensated by an increase in another dimension, such as thickness

d1

d’1

Stretch Forming• the diameter and the circumference increases, with a corresposponding decrease in thickness that can be the cause of subsequent service failures such as premature failure in corrosive environment

d2

d’2

Ligon, Jennilyn


F. Forming with rubber Tooling or Fluid PressureGuerin Process

• based on the phenomenon that rubber of the proper consistency, when totally confined, acts as a fluid and transmits pressure uniformly in all directions

Rubber Ram

BlankForming Block

High-pressure Flexible-die Process (Hydroforming or Flexiforming)

• the rubber pad is replaced by a flexible rubber diaphram backed by controlled hydraulic pressures

Ligon, Jennilyn


G. Drawing on a Drop Hammer

• when small quantities of shallow-drawn parts are required, they can often be made most economically through the use of Low-melting point metal dies and a drop hammer• dies can be directly cast eliminating the expense of machining operations, and when no longer needed, can be cast into other shapes

Wood shims

Work(a)

Die

(b) (c)

Ligon, Jennilyn


H. High-Energy-Rate Forming

• process of forming metals through the application of large amounts of energy in a very short time interval

1. Ironing> process of thinning the walls of a drawn cylinderby passing it between a punch and die whose separation is less than the original wall thickness

2. Embossing> presswork process in which raised lettering or other designs are impressed in sheet materials

Ligon, Jennilyn


PunchTube

Die

Embossing Die

work

3. Supeplastic Sheet Foerming> makes possible the economical production of large, complex-shaped products with compound curves

>major limitation is low forming rate that is required to maintain superplastic behavior

Ligon, Jennilyn


I. Designs and aid for shet Metal FormingStrain analysis, Forming limit diagram

•Quick and economical way of evaluating the severity of deformation in a formed part

Major strain (percent)

Minor strain (percent)

020406080

100120140

60 40 20 0 20 40 60 80

•Major and minor strain=positive >deformation is stretching >sheet metal decrease in thickness•Major strain=positive;minor strain=negative >deformation is drawing >thickness may increase, decrease or stay depending on the magnitudes of the 2 strain

Ligon, Jennilyn

PressesA. Classification of Presses

Mechanical Drive

• provides faster motion and more positive control of displacement

Hydraulic Presses

• produce motion as a result of piston movement, and longer or variable-length strokrs can be programmed within the limitations of the cylinder

Manually operated Presses

•Generally used for very light work such as shearing small sheets

Kick pressesCrank>used for most piercing and blanking operations and for drawing

Eccentric or cam drives> Used when only a short ram stroke is required

Knuckle-joint drive> provides a very high mechanical advantage along with fast action

Toggle mechanism> Used principally in drawing presses to actuate the blank holder

Screw-type drives> offer great mechanical advantage coupled witj action that resembles drop hammer

Screw presses> popular in forging industry

Single-slide

Multiple slide

Ligon, Jennilyn

PressesB. Types of Press Frame

> Imposes limitations on the size and type of work that can be accomodated, How that work is fed and unloaded, and the overall stiffness of the machine

Arch Gap Straight Sided

• seldom used today except with screw drives for coining operations

• frames are in the shape of letter C•Most verastile and commonly preffered presses•Provide unobstructive access to the dies from 3 directions and permit large workpiece to be fed into the press

• have frames that consist of a crown, 2 uprights, a base or bed , and one or more moving slide

Ligon, Jennilyn

Presses

C. Press Feeding Device>increases productivity >operators safety is an advantage

When continuous strip is used it can be fed automatically by double-roll feeds mounted on the side of the press

Documents

Metal Forming-cold Working