ASM Metals HandBook Vol. 14 - Forming and Forging

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Reference cited in this section

"Upsetting," technical brochure, National Machinery Company, 1971, p 11

Cold Heading

Equipment

Standard cold headers are classified according to two characteristics:

• Whether the dies open and close to admit the work metal or are solid

• The number of strokes (blows) the machine imparts to the workpiece during each cycle

The die in a single-stroke machine has one mating punch; in a double-stroke machine, the die has two punches. The two

punches usually reciprocate so that each contacts the workpiece during a machine cycle.

Single-stroke solid-die headers are made in sizes of , , , , , , , , and 1 in. These sizes refer to the

approximate diameter of stock that can be headed. Because they are single-stroke machines, product design is limited to

less than two diameters of stock to form the head. Single-stroke extruding can also be done in this type of machine. These

machines are used to make rivets, rollers and balls for bearings, single-extruded studs, and clevis pins.

Double-stroke solid-die headers are available in the same sizes as single-stroke solid-die headers. These machines

can make short-to-medium length products (usually 8 to 16 diameters long), and they can make heads that are as large as

three times the stock diameter. These machines can be equipped for relief heading, which is a process for filling out sharp

corners on the shoulder of a workpiece, or a square under the head.

Some extruding can also be done in these machines. Because of their versatility over single stroke cold headers, double-

stroke solid-die headers are extensively used in the production of fasteners.

Single-stroke open-die headers are made for smaller-diameter parts of medium and long lengths and are limited to

heading 2 diameters of stock because of their single stroke. Extruding cannot be done in this type of machine, but small

fins or a point can be produced by pinching in the die, if desired. Similar machines are used to produce nails.

Double-stroke open-die headers are made in a wider range of sizes than single-stroke open-die headers and can

produce heads as large as three times stock diameter. They cannot be used for extrusion, but they can pinch fins on the

workpiece, when required. They will generally pinch fins or small lines under the head of the workpiece when these are

not required; if these fins or lines are objectionable, they must be removed by another operation.

Three-blow headers utilize two solid dies along with three punches and are classified as special machines. Having the

same basic design as double-stroke headers, these machines provide the additional advantage of extruding or upsetting in

the first die before double-blow heading or heading or trimming in the second die. Three-blow headers combine the

process of trapped extrusion and upsetting in one single machine to produce special fasteners having small shanks but

large heads. These headers are also ideal for making parts with stepped diameters in which the transfer of the workpiece

would be accomplished with great difficulty.

Transfer and progressive headers are solid-die machines with two or more separate stations for various steps in the

forming operation. The workpiece is automatically transferred from one station to the next. These machines can perform

one or more extrusions, can upset and extrude in one operation, or can upset and extrude in separate operations.

Maximum lengths of stock of various diameters headed in these machines range from 152 mm (6 in.) with 3.8 in.

diameter to 255 mm (10 in.) with in. diameter. These machines can produce heads of five times stock diameter or more.

Boltmaking machines are solid-die headers similar to transfer and progressive headers, but they can trim, point, and

roll threads. Boltmaking machines usually have a cut-off station, two heading stations, and one trimming station served

by the transfer mechanism. An ejector pin drives the blank through the hollow trimming die to the pointing station. The

trimming station can be used as a third heading station, or for extruding. Boltmaking machines are made for bolt

diameters , , , , , , , 1, and 1 in.

Rod headers are open-die headers having either single or double stroke. They are used for extremely long work (8 to

160 times stock diameter). The workpiece is cut to length in a separate operation in another machine and fed manually or

automatically into the rod header.

Reheaders are used when the workpiece must be annealed before heading is completed--for example, when the amount

of cold working needed would cause the work metal to fracture before heading was complete. Reheaders are made as

either open-die or solid-die machines, single or double stroke, and can be fed by hand or hopper. Punch presses are also

used for reheading.

Nut formers generally have four or five forming dies and a transfer mechanism that rotates the blank 180° between one

or two dies or all the dies. Therefore, both ends of the blank are worked, producing workpieces with close dimensions, a

fine surface finish, and improved mechanical characteristics. A small slug of metal is pierced from the center of the nut,

which amounts to 5 to 15% waste, depending on the design of the nut.

Operation. Most cold-heading machines used in high production are fed by coiled wire stock. The stock is fed into the

machine by feed rolls and passes through a stationary cutoff quill. In front of the quill is a shear-and-transfer mechanism.

When the wire passes through the quill, the end butts against a wire stop or stock gage to determine the length of the

blank to be headed. The shear actuates to cut the blank. The blank is then pushed out of the shear into the transfer, which

positions the blank in front of the heading die. The heading punch moves forward and pushes the slug into the die; at the

same time, the transfer mechanism releases the slug and moves back into position for another slug.

In the die, the slug is stopped by the ejector pin, which acts as a backstop and positions the slug with the correct amount

protruding for heading. In a single- or double-stroke header, the heading operation is completed in this die, and the ejector

pin advances to eject the finished piece. In a progressive header or a boltmaking machine, the transfer mechanism has

fingers in front of each of several dies. After each stroke, the ejector pin pushes the workpiece out of the die. The transfer

mechanism grips it and advances it to the next station. In boltmaking machines, the last station in the heading area is a

trimming station. The trimming die (which is on the punch side) is hollow, and the die ejector pin drives the trimmed

workpiece completely through the die and, by an air jet or other means, through a tube to the pointing station.

Pointers are of two types. Some have cutters that operate much like a pencil sharpener in putting a point on the workpiece

(thus producing some scrap); others have a swaging or extruding device that forms the point by cold flow of the metal.

The pointed workpiece is placed in a thread roller. A boltmaking machine has a thread roller incorporated into it. The

rolling dies are flat pieces of too] steel with a conjugate thread form on their faces. As the workpiece rolls between them,

the thread form is impressed on its shank, and it drops out of the dies at the end, often as a finished bolt.

Cold Heading

Tools

The tools used in cold heading consist principally of punches and dies. The dies can be made as one piece (solid dies) or

as two pieces (open dies), as shown in Fig. 4.

Fig. 4 Solid (one-piece) and open (two-piece) cold-heading dies.

Solid dies (also known as closed dies) consist of a cylinder of metal with a hole through the center (Fig. 4a). They are

usually preferred for the heading of complex shapes. Solid dies can be made entirely from one material, or can be made

with the center portion surrounding the hole as an insert of a different material. The choice of construction depends

largely on the length of the production run and/or complexity of the part. For extremely long runs, it is sometimes

desirable to use carbide inserts, but it may be more economical to use hardened tool steel inserts in a holder of less

expensive and softer steel.

When a solid die is made in one piece, common practice is to drill and ream the hole to within 0.076 to 0.13 mm (0.003 to

0.005 in.) of finish size before heat treatment. After heat treatment, the die is ground or honed to the desired size.

Solid dies are usually quenched from the hardening temperature by forcing the quenching medium through the hole,

making no particular attempt to quench the remainder of the die. By this means, maximum hardness is attained inside the

hole; the outer portion of the die is softer and therefore more shock resistant.

Because the work metal is not gripped in a solid die, the stock is cut to length in one station of the header, and the cut-to-

length slug is then transferred by mechanical fingers to the heading die. In the heading die, the slug butts against a

backstop as it is headed. Ordinarily, the backstop also serves as an ejector.

Open Dies (also called two-piece dies) consist of two blocks with matching grooves in their faces (Fig. 4b). When the

grooves in the blocks are put together, they match to form a die hole as in a solid die. The die blocks have as many as

eight grooves on various faces so that as one wears, the block can be turned to make use of a new groove. Because the

grooves are on the outer surface of the blocks, open-die blocks are quenched by immersion to give maximum hardness to

the grooved surfaces. Open dies are usually made from solid blocks of tool steel, because of the difficulty involved in

attempting to make the groove in an insert set in a holder. Open dies are made by machining the grooves before heat

treating, then correcting for any distortion by grinding or lapping the grooves after heat treating.

In open-die heading, the dies can be permitted to grip the workpiece, like the gripper dies in an upsetting machine. When

this is done, the backstop required in solid-die heading is not necessary. However, some provision for ejection is

frequently incorporated into open-die heading.

Design. The shape of the head to be formed in the workpiece can be sunk in a cavity in either the die or the punch or

sometimes partly in each. The decision on where to locate the cavity often depends on possible locations of the parting

line on the head. It must be possible to extract the workpiece from both the punch and the die. It is generally useful, but

not entirely necessary, to design some draft in the workpiece head for ease of ejection.

An important consideration in the design of cold-heading tools is that the part should stay in the die and not stick in the

punch. Therefore, it is particularly difficult to design tooling for midshaft upsets. Where possible, the longest part of the

shank is left in the die. There is less of a problem with open dies that use a special die-closing mechanism. Some punches

are equipped with a special synchronized ejector mechanism to ensure that the workpiece comes free.

At best, cold heading imposes severe impact stress on both punches and dies. Minor changes in tool design often register

large differences in tool life, as described in the following example.

Example 1: Improvements in Heading Tool Design That Eliminated Tool Failure.

The recessed-head screw shown in Fig. 5(a) was originally headed by the heading tool shown in Fig. 5(b). After

producing only 500 pieces, the tool broke at the nib portion ("Point of failure," Fig. 5b).

Fig. 5 Improvements in heading tool design to eliminate tool failure in the production of recessed-

head screws.

Dimensions given in inches.

The design of the heading tool was improved by adding a radius and a slight draft to the nib (Fig. 5c). The entire nib was

then highly polished. The redesigned tools produced 12,000 to 27,000 pieces before breakage occurred, but this tool life

was still unacceptable.

A final design improvement is shown at the right in Fig. 5(c). The nib was made to fit a split holder, using a slight taper to

prevent the nib insert from being pulled from the split holder as the header withdrew from the workpiece. Tools of this

design did not break and produced runs of more than 100,000 pieces before the nib was replaced because of wear.

Cold Heading

Tool Materials

The shock loads imposed on cold-heading tools must be considered in the selection of tool materials. For optimal tool life,

it is essential that both punches and dies have hard surfaces (preferably 60 HRC or higher). However, except for the

heading of hard materials, the interior portions of the tools must be softer (40 to 50 HRC, and sometimes as low as 35

HRC for larger tools), or breakage is likely.

To meet these conditions, shallow-hardening tool steel such as W1 or W2 is extensively used for punches and open dies

and for solid dies made without inserts. Inserts are commonly made from higher-alloy tool steels, such as D2 or M2, or

from tungsten carbide having a relatively high percentage of cobalt (13 to 25%).

Shock-resistant tool steel such as S1 is also used for the cold heading of tools, especially for the heading of intricate

shapes when a tool steel such as W1 has failed by cracking. The shock-resistant steels are generally lower in hardness

than preferred for maximum resistance to wear, but it is often necessary to sacrifice some wear resistance to gain

resistance to cracking.

Producing bolts that have square portions under the heads or dished heads or both can result in tool failure. Under these

conditions, a change in grade of steel for the tools is sometimes mandatory.

Cold Heading

Preparation of Work Metal

The operations required for preparing stock for cold heading may include heat treating, drawing to size, machining,

descaling, cutting to length, and lubricating.

Heat Treating. The cold-heading properties of most steels are improved by process annealing, spheroidizing, or stress

relieving. In general, process annealing is done at the steel mill on steels with low-to-medium carbon content. Additional

heat treatment is not used unless required, for at least two reasons:

• The process could cost more than any savings realized in cold heading

• Cold-headed products often depend for

their final strength on work hardening before and during the

heading process, and if reannealed before cold heading, they may lose much of their potential strength

Carbon steels (1000 series) with up to about 0.25% C are usually cold headed in the mill-annealed condition as received

from the steel supplier. If the heading is severe, they can be reannealed at some stage in the heading operations, but they

are rarely given a full anneal before cold heading. Carbon steels (1000 series) with 0.25 to 0.44% are also mill annealed.

However, because higher carbon content decreases workability, they are sometimes normalized or annealed above the

upper transformation temperature; more frequently, a spheroidizing treatment is used. Carbon steels that contain more

than 0.44% C, most modified carbon steels (1500 series), and all alloy steels are fully spheroidized. Heat-treating methods

for steels and nonferrous metals are described in Heat Treating, Volume 4 of the ASM Handbook. In practice, experience

often indicates the need for annealing or spheroidizing to prevent cracking of the work metal or to obtain acceptable tool

life or both.

Drawing to size produces stock of uniform cross section that will perform as predicted in dies that have been carefully

sized to fill out corners without flash or die breakage. Drawing to size also improves strength and hardness when these

properties are to be developed by cold work and not by subsequent heat treatment.

Turning and Grinding. Drawn wire can have defects that carry over into the finished workpiece, exaggerated in the

form of breaks and folds. Seams in the raw material that cause these defects may not be deep enough to be objectionable

in the shank or body of a bolt, but can cause cracks in the head during cold heading or subsequent heat treatment. Surface

seams and laps can be removed by turning, grinding, or shaving at the wire mill or by machining the headed product.

Descaling. Work metal that has been heat treated usually needs to be descaled before cold heading. Scale can cause lack

of definition, defects on critical surfaces, and dimensional inaccuracy of the workpiece.

Methods of descaling include abrasive blasting, water jet blasting, pickling, wire brushing, and scraping. Selection of

method depends largely on the amount of scale present and on the required quality of the surfaces on the headed

workpieces. Acid pickling is usually the least expensive method for complete removal of heavy scale (see the articles on

surface engineering of specific metals in Surface Engineering, Volume 5 of the ASM Handbook).

Cutting to Length. In a header that has a shear-type cutoff device as an integral part of the machine, cutting to length

by shearing is a part of the sequence. In applications in which cutting to length is done separately, shearing is the method

most commonly used for bars up to about 50 mm (2 in.) in diameter (see the article "Shearing of Bars and Bar Sections"

in this Volume). For larger diameters, sawing is generally used. Gas cutting and abrasive-wheel cutting are used less often

than shearing and sawing.

Lubrication. Although some of the more ductile metals can be successfully cold headed to moderate severity without a

lubricant, most metals to be cold headed are lubricated to prevent galling of the dies, sticking in the dies, and excessive

die wear. Lubricants used include lime coating, phosphate coating, stearates and oils, and plating with softer metals such

as copper, tin, or cadmium.

The ultimate in lubrication for steel to be cold headed is a coating of zinc phosphate with stearate soap--the same as used

for the cold extrusion of steel (see the article "Cold Extrusion" in this Volume). A similar treatment is often used for

aluminum. However, for workpieces produced entirely by cold heading, this treatment is seldom necessary, except for

extremely severe heading.

In the cold heading of carbon and alloy steel wire, common practice is to coat the work metal with a dry lubricant during

the last draw. The lubricants most often used are calcium stearate or aluminum stearate. First, the wire is pickled to

remove scale, dirt, and any previous coatings. It is then coated with lime, phosphate, or borax, which acts as a base

coating. Calcium or aluminum stearate is added as a dry lubricant. The lubricant sticks to the base coating and is fused by

the heat developed when the wire passes through the drawing die. For severe heading, extrusion oils are sometimes used

(often in addition to the treatments given above) in the header/former, particularly when experience has proved that oil

will improve results.

Stainless steel is usually electroplated with copper and then lubricated with oil or molybdenum disulfide. Oxalates are

sometimes used instead of the copper plating.

In the cold heading of nonferrous metals, the need for lubrication varies from metal to metal. Nickel-base alloys,

especially the high-strength alloys, require very good lubrication. These metals are usually copper plated and then given a

stearate coating. The coatings are later removed with nitric acid.

The more formable nickel-base alloys are usually also copper plated. If the heading is not severe, however, they can be

headed with a stearate coating only, which can be removed with hot water. Nitric acid cannot be used on Monel, because

the acid will attack the base metal.

Copper-base alloys have the least need for lubrication. For normal heading operations, oil or drawing compound is added

at the header. For severe heading, a stearate coating can be added during the last draw of the wire. Sulfurized oil should

not be used for cold heading of copper-base alloys unless some staining can be tolerated.

Aluminum header wire is generally coated with stearate. Aluminum needs more lubrication for cold heading than copper,

but much less than nickel.

In all cold heading, best practice is to use the simplest and the least lubricant that will provide acceptable results, for two

reasons:

•

Excessive amounts of lubricant may build up in the dies, resulting in scrapped workpieces or damaged

dies

• Rem

oval of lubricant is costly (the cost of removing lubricant usually increases in proportion to the

effectiveness of the lubricant)

Cold Heading

Complex Workpieces

Cold-headed products that have more than one upset portion need not be formed in two heading operations; many can be

made in one operation of a double-stroke header. The length of stock that may be partly upset is generally limited to five

times the diameter of the wire. The only other limitation is that the header must be able to accommodate the diameter and

length of wire required for the workpiece.

Three pieces, each with two end upsets, that were made completely in one operation in a double-stroke open-die header

are shown in Fig. 6(a). These parts were made at a rate of 80 pieces per minute. Production rate is limited only by the

speed of the machine used, not by the item being produced.

Fig. 6 Typical parts with center upsets or upsets at both ends. Dimensions given in inches.

The product becomes more expensive when the upsetting operation has to be performed twice, as in production of the 710

mm (28 in.) long axle bolt shown in Fig. 6(b). This part required two upsetting operations because the die in a standard

double-stroke cold header was not long enough to form both upsets in the machine at the same time. One or more

additional operations may be needed for workpieces that require pointing as well as a complex upset.

Center Upsetting. Most cold heading involves forming an upset at the end of a section of rod or wire. However, the

forming of upsets at some distance from the end is common practice.

The trailer-hitch-ball stud shown in Fig. 6(c) is representative of an upset performed midway between the ends of the wire

blank. This stud was upset and extruded in two strokes in a in. solid-die machine. The diameter of one end section is

smaller than that of the original wire, and the round center collar is flared out to more than 2 times the wire diameter.

The center-collar stud shown in Fig. 6(d) is another example of a center upset. Both ends of the stud were extruded below

wire size, while the center collar was expanded to more than three times the original wire diameter. This stud was formed

in three strokes in a progressive header.

Control of the volume of work metal to prevent formation of flash and to prevent excessive loads on the tools is important

in most cold-heading operations. In center upsetting, control of metal volume is usually even more important, not only to

prevent flash and tool overload but also to prevent folds. A technique used successfully in one application of center

upsetting is described in the following example.

Example 2: Production of a Complex Center Upset in Two Blows.

A blank for a bicycle-pedal bolt (Fig. 7) required sharp corners on the edges and corners of the square portion and a

complete absence of burrs or fins in the collar area. In heading, any excess pressure applied on the collar portion to fill the

corners and edges of the square resulted in flash or overfill on the collar portion. It was necessary to upset the collar

portion in one blow and to form the square in a second blow in order to fabricate this part successfully (Fig. 7). The folds

generally produced by this technique were avoided by careful control of size. By forming the collar completely during the

first blow and almost completely confining it during the second blow, the remainder of the metal was controlled so that it

could be directed into filling the square. Therefore, the pressure needed to form and fill the square was confined to this

area and not allowed to cause further upsetting in any other portion. Accurate control of the headed volume depended on

the accuracy of the cut blank and of the collar formed in the first blow.

Machine

in. boltmaking machine

Tool material M2 inserts, 62-64 HRC

Lubricant Stearate on stock

Production rate

4200 pieces per hour

(a)

Tool life 10,000-15,000 pieces

(a)

At 100% efficiency

Fig. 7 Production of a 1038 steel blank for a bicycle-pedal bolt in two blows on a cold upsetter.

Dimensions

given in inches.

Cold Heading

Economy in Cold Heading

Cold heading is an economical process because of high production rates, low labor costs, and material savings.

Production rates range from about 2000 to 50,000 pieces per hour, depending on part size. Fewer machines are needed to

meet production requirements than with other processes, resulting in reduced costs for equipment, maintenance, and floor

space. Labor costs are minimal because most operations are performed automatically, requiring labor only for setup,

supervision, and parts handling.

Material savings results from the elimination or reduction in chips produced. When cold heading is combined with other

operations, such as extrusion, trimming, and thread rolling, the savings is considerable (see the section "Combined

Heading and Extrusion" in this article). Subsequent machining or finishing of the cold-headed parts is usually not

necessary. This can be especially beneficial when relatively expensive work materials are used. The following example

describes the replacement of machining by cold heading to reduce production costs of a copper alloy nozzle component.

Example 3: Machining Replaced by Cold Heading to Save Material.

A blank for a threaded copper alloy C10200 (oxygen-free copper) nozzle component (Fig. 8) was originally produced by

machining from bar stock. A material savings of more than 50% was effected by producing the component by cold

heading rather than machining. The same shape and dimensional accuracy were produced by both methods. In both cases,

threads were rolled in a separate operation.

Fig. 8 Copper alloy C10200 noz

zle component blank that was originally machined but was switched to cold

heading to save the work metal indicated by the shaded regions. Dimensions given in inches.

Cold Heading

Dimensional Accuracy

Work can be produced to much closer tolerances in cold headers than in hot headers. Tolerances on parts produced by

single-stroke headers need to be wider than on parts given two or more blows. Rivets, often formed in single-stroke

machines, have tolerances of ±0.38 mm (±0.015 in.) except where otherwise specified. Shanks for rolled threads are

allowed only ±0.038 mm (±0.0015 in.). Small parts can usually have closer tolerances than large parts. Tolerances can

often be maintained as close as 0.025 mm (±0.001 in.), although maintenance of a tolerance this close increases product

cost; requires careful control of machines, tools, and work metal; and is unusual in practice.

The following example demonstrates tolerance capabilities and shows dimensional variations obtained in production runs

of specific cold-headed products.

Example 4: Variation in Dimensions of a Valve-Spring Retainer Produced in a

Nut Former.

The valve-spring retainer shown in Fig. 9 was produced from fine-grain aluminum-killed 1010 steel (No. 2 bright

annealed, cold-heading quality) in a five-station progressive nut former. To determine the capabilities of the machine and

tools for long-run production, several thousand pieces were made from three separate coils. Distribution charts were

prepared for two critical dimensions on randomly selected parts made from each coil. Results are plotted in Fig. 9. Lots 1,

2, and 3 include parts made from the three different coils. As a further test of machine and tool capabilities, the tooling

was set to a mean taper dimension for lot 1, high side for lot 2, and low side for lot 3.

Fig. 9 Variations in dimensions of 1010 steel valve spring retainers randomly selected

from three lots. Parts

were produced in a five-station nut former. Dimensions given in inches.

The accuracy that could be maintained on thickness of a flat surface is demonstrated in Fig. 9. Although specifications

permitted a total variation of 0.51 mm (0.020 in.) on seat thickness, actual spread did not exceed 0.13 mm (0.005 in.) for

parts made from the three coils. A greater total variation was experienced for the taper-depth dimension. When the tools

were set for mean, the total variation was 0.33 mm (0.013 in.) which was still within the 0.41 mm (0.016 in.) allowable

(lot 1). With tools set for high side, total variation was only 0.25 mm (0.010 in.), although one part was 0.025 mm (0.001

in.) out of the allowable range (lot 2). Optimal results were obtained on the taper dimension when tools were set for the

low side (lot 3); total spread was only 0.18 mm (0.007 in.).

Cold Heading

Surface Finish

Surfaces produced by cold heading are generally smooth and seldom need secondary operations for improving the finish.

Surface roughness, however, can vary considerably among different workpieces or among different areas of the same

workpiece, depending on:

• Surface of the wire or bar before heading

• Amount of cold working in the particular area

• Lubricant used