ASM Metals HandBook Vol. 14 - Forming and Forging

Подождите немного. Документ загружается.

Work Metals. Of the plain carbon steels, those with a carbon content of 0.20% or less are the most swageable. These

grades can be reduced up to 70% in cross-sectional area by swaging. As carbon content or alloy content is increased,

swageability is decreased. Alloying elements such as manganese, nickel, chromium, and tungsten increase work metal

strength and therefore decrease the ability of the metal to flow. Free-machining additives such as sulfur, lead, and

phosphorus, cause discontinuities in structure that result in splitting or crumbling of the work metal during swaging.

In the cold swaging of steel (at room temperature), maximum swageability is obtained when the microstructure is in the

spheroidized condition. Pearlitic, annealed microstructures are less swageable than spheroidized microstructures,

depending on the fineness of the pearlite and on the tensile strength and hardness of the steel. Fine pearlitic

microstructures, such as those found in patented music wire and spring wire, can be swaged up to 30 to 40% reduction in

area.

Figure 1 shows the relationship between hardness and carbon content for pearlitic and spheroidized microstructures and

also shows three zones of swageability, indicating that a maximum hardness of 85 HRB is preferred for carbon steels and

that swaging is impractical when hardness exceeds 102 HRB. Figure 2 shows the influence of cold reduction on the

tensile and yield strengths of several metals.

Fig. 1 Swageability of carbon steel as a function of microstructure, hardness, and carbon content.

Fig. 2

Influence of cold reduction by swaging on mechanical properties of various alloy systems. (a) Carbon

steels. (b) Copper alloys. (c) Tool steels. (d) Commercially pure titanium. (e) Heat-

resistant alloys. (f) Stainless

steels. TS, tensile strength; YS, yield strength.

Workpieces requiring reductions greater than that which can be accomplished with one swaging pass must be stress

relieved or reannealed after the first pass to restore ductility in the metal for further reduction. Stress relieving of steel by

heating to 595 to 675 °C (1100 to 1250 °F) often restores ductility, although excessive grain growth may develop when

cold working is followed by heating within this temperature range. Stress relieving is of little value under these

conditions, and it is necessary to anneal the material fully.

Rotary Swaging of Bars and Tubes

Revised by the ASM Committee on Rotary Swaging

Metal Flow During Swaging

Metal flow during rotary swaging is not confined to one direction. As shown in Fig. 3, more metal moves out of the taper

in a direction opposite to that of the feed than through the straight portion (blade). Some metal flow also occurs in the

transverse direction, but it is restricted by the oval or side clearance in the dies (see Fig. 7).

Fig. 3 Metal flow during swaging of a solid bar.

Feedback. The action of the metal moving against the direction of feed is termed feedback, and it results from slippage

of the workpiece in the die taper when it is too steep. Feedback manifests itself as a heavy endwise vibration that causes

considerable resistance to feeding of the workpiece.

Workpiece Rotation. Unless resisted, rotation is imparted as the dies close on the workpiece, and the speed of rotation

is the speed of the roller cage. If rotation is permitted, swaging takes place in only one position on the workpiece, causing

ovaling, flash, and sticking of the workpiece in the die. Resistance to rotation is manual when the swager is hand fed;

mechanical means are used with automatic feeds.

Rotary Swaging of Bars and Tubes

Revised by the ASM Committee on Rotary Swaging

Machines

Rotary swaging machines are classified as standard rotary, stationary-spindle, creeping-spindle, alternate-blow, and die-

closing types. All these machines are equipped with dies that open and close rapidly to provide the impact action that

shapes the workpiece. The five principal machine concepts for swaging are shown in Fig. 4.

Fig. 4 Principal machine concepts for rotary swaging. (a) Standard rotary swager. (b) Stationary-

spindle

swager. (c) Creeping-spindle swager. (d) Alternate-blow swager. (e) Die-closing swager.

Swagers allow the work to be fed into the taper entrance of the swaging dies. The amount of diameter reduction per pass

is limited by the design of the entrance taper of the dies or the area reduction capability of the machine. The results are

expressed in terms of diameter reduction or area reduction.

The two methods of calculating reduction (in percent) are:

A die-closing swager has dies made with side relief that is sufficient to allow the dies to come down directly onto the

work. The maximum side relief that can be used limits the reduction in diameter per swaging pass to 25%. The die-

closing swager may have a front entrance angle and can be used as a standard rotary swager. When used in this manner,

the diameter and area reduction per pass are the same as for a standard rotary swager. However, diameter reduction

should not be confused with area reduction.

Standard Rotary Swagers. The basic rotary swager (Fig. 4a) is a mechanical hammer that delivers blows (impact

swaging) at high frequency, thus changing the shape of a workpiece by metal flow. This machine is used for straight

reducing of stock diameter or for tapering round workpieces.

A standard rotary swager consists of a head that contains the swaging components and a base that supports the head and

houses the motor. A hardened and ground steel ring about 0.5 mm (0.020 in.) larger in diameter than the bore of the head

is pressed into the head so that the ring is in compression.

The spindle, centrally located within the ring, is slotted to hold the backers and dies and is mounted in a tapered-roller

bearing. Flat steel shims are placed between the dies and backers. A roll rack containing a set of rolls is located between

the press-fitted ring and the backers. A conventional impact-type backer is shown in Fig. 5. The spindle is rotated by a

motor-driven flywheel keyed to the spindle. During rotation of the spindle, the dies move outward by centrifugal force

and inward by the action of the backers striking the rolls. The number of blows (impacts) produced by the dies is 1000 to

5000 per minute, depending on the size of the swager. The impact rate is approximately equal to the number of rolls

multiplied by the speed (rpm) of the swager spindle multiplied by a correction factor of 0.6, which allows for creep of the

roll rack.

Fig. 5 Designs of four different backer cams used in rotary swaging. (a) Conventional impact-

type backer (flat

sides). (b) Squeeze-type backer with a sine curve type crown. (c) Squeeze-

type backer with large radius on

crown. (d) Backer with replaceable insert.

The amount of the die opening when the dies are in the open position--backers positioned between the rolls--can be

changed to some extent during operation by a mechanical device that restricts the amount dies and backers can move

under centrifugal force. However, the closed position of the dies--backers positioned on the rolls--cannot be changed

during operation; the swager must be stopped and shims inserted between the dies and the backers. The severity of the

blow can be varied by using shims of different thicknesses. The dies should be shimmed tight enough to obtain a

reasonable amount of interference between the backers and the rolls when the dies are in the closed position.

The amount of shimming should be sufficient to bring the die faces together, and generally 0.05 to 0.5 mm (0.002 to

0.020 in.) of preload can be added, according to the size of machine. A swager is shimmed too tightly, or has too great a

preload, when it stalls in starting while the swager hammers are off the rolls. The lightest possible shimming should be

used; overshimming increases machine maintenance. Additional shimming will not produce a smaller section size,

because section size is controlled by the size of the die cavity when the dies are in the closed position. Insufficient

shimming, however, will increase the section size and cause variation in results, particularly in dimensions and surface

conditions.

Stationary-spindle swagers are sometimes called inverted swagers, because the spindle, dies, and work remain

stationary while the head and roll rack rotate. These machines are used for swaging shapes other than round.

The reciprocating action of the dies is the same as in swagers in which the spindle is rotated and the roll rack remains

stationary. The principal components of a stationary-spindle machine are shown in Fig. 4(b).

The stationary-spindle swager consists of a base that houses the motor and supports a bearing housing containing two

tapered-roller bearings. The head, fastened to a rotating sleeve mounted in the tapered-roller bearings, is motor driven and

acts as a flywheel. The spindle is mounted and held stationary by a rear housing that is fastened to a bearing housing.

As the head rotates, the rolls pass over the backers, which in turn cause the dies to strike the workpiece in a pulsating

hammer-type action. Die opening can be controlled by the forward feed of the workpiece, although springs are sometimes

used to open the dies. The maximum outward travel of the dies in the open position is regulated by a mechanical device in

the front of the machine. Shims are used between the dies and the backers, just as they are in swagers with rotating

spindles.

Creeping-spindle swaging (Fig. 4c) employs the principles of both standard rotary and stationary-spindle swaging.

The spindle and dies are mounted on a shaft that rotates slowly inside the rapidly rotating roller cage, thus permitting

more accurately controlled reciprocation of the dies.

Alternate-blow swaging (Fig. 4d) is accomplished by recessing alternate rolls; in this configuration, when two

opposing rolls hammer the dies, the rolls 90° away do not. This eliminates fins on the workpiece.

Die-closing swagers (Fig. 4e) are used when the dies must open more than is possible in a standard rotary swager to

permit loading. Die-closing swagers are essentially of the same construction as the standard rotary swagers described

above. Both have similar components, such as dies, rolls, roll rack, inside ring, spindle, and shims.

The main difference between die-closing and standard rotary swagers is the addition of a reciprocating wedge mechanism

that forces closure of the taper-back dies, as shown in Fig. 4(e). The wedge mechanism consists of a wedge for each die

that is positioned between the die and the backer. The rotating dies open by centrifugal force and are held open by springs

or other mechanical means when the power-actuated wedge mechanism is in the back position. Wedge control of the die

opening permits the work to be placed in the machine in a predetermined position when the dies are open. Reduction per

pass is limited to 25% of the original diameter of the workpiece, and the wedge angle of the dies should not exceed 7 °.

Swaging by Squeeze Action. The impact action common to standard rotary swagers can be slowed to produce a

squeezing action by employing a backer cam. The design of the crown and the width of the backers are such that at least

one roll is always in contact with the backer. The shape of the crown can be a single curve or two radii that approximate a

sine curve. Both of these backer designs are shown in Fig. 5. Machines that use a sine curve type backer have fewer rolls

than a standard swager.

Swaging with squeeze action is used to obtain greater reduction in area than that normally produced by impact action. It is

also used to produce intricate profiles on internal surfaces with the aid of a mandrel.

Compared to impact forming with standard swagers, squeeze forming produces less noise and vibration, requires less

maintenance of rolls and backers, and can produce greater reduction and closer tolerances. Standard rotary swagers,

however, are simpler to operate and lower in cost, require less floor space, and are faster for small reductions.

Rolls and backers used for cold swaging are made from tool steel. The grade of tool steel used varies considerably,

although many rolls and backers are made from one of the shock- or wear-resistant grades (depending on application)

hardened and tempered to 55 to 58 HRC.

Almost all rolls and backers become work hardened. The degree of work hardening depends on the severity of reduction

of the swaged workpiece, the swageability of the work metal, the material used for the rolls and backers, total operating

time, and adjustment of the machine. Rolls, backers, and dies used in cold swaging are stress relieved periodically at 175

to 230 °C (350 to 450 °F) for 2 to 3 h in order to reduce the effects of work hardening and to prolong service life. The

stress-relieving temperature used must not be higher than the original tempering temperature, or softening will result. The

frequency of stress relieving depends on the severity of swaging. Under normal conditions, steel rolls and backers should

be stress relieved after every 30 h of operation. Further improvements in tooling life and overall process costs are

achieved by using replaceable inserts in the working area of the backers as shown in Fig. 5(d). These inserts can be

carbide, and they have contoured forms that improve tool life and precision and reduce noise during swaging.

Stress relieving is usually not required for rolls and backers used for hot swaging, because some stress relief occurs each

time heat transfers from the hot workpiece to the rolls and backers. These components are also less susceptible to work

hardening than rolls and backers in cold swaging, because less force is required to form the part by hot swaging.

The rolls and roll rack of a four-die machine are subject to about 1 times as much wear as those in a two-die machine;

therefore, they must be replaced more often. Other components, such as the spindle and cap, liner plates, backers, and

dies, have about the same rate of wear in both types of machines; however, replacement cost of these components is lower

for a two-die machine.

The number of rolls in a four-die machine must be divisible by four, so that they can be placed at 90° spacing. Therefore,

a ten-roll machine is limited to using two dies.

Number of Dies. Most swagers have either two or four dies, although three-die machines are available. Most swaging

is done in two-die machines, because they are less costly to build and simpler to set up and maintain.

Four-die swaging machines have some advantages. Slightly greater reductions can be made more readily, and cold

working of the dies is reduced, because less ovality or side clearance is required than for two dies. Four-die machines are

especially useful for swaging workpieces from a round to a square cross section. Four dies are generally not used for

workpieces less than 4.8 mm ( in.) across (in either round or square section).

A stationary-spindle usually has twelve rolls, and three, four, or six dies can be used. To change the number of dies in a

swager, the spindle generally must be changed, because the slots in the spindle accommodate only the number of dies

used. Three-die units are typically used to form triangular sections; four-die units, rectangles, squares, and rounds; and

six-die units, hexagonal shapes.

Machine Capacity. The rated capacity of a swaging machine is based on the swaging of solid work metal of designated

tensile strength and is expressed as the diameter--or the average diameter of a taper--to which the machine can swage a

workpiece made from that material. Machine capacity is significantly influenced by the strength of the head. The load on

the head is approximately equal to the projected area of the workpiece under compression multiplied by the tensile

strength of the work metal.

For example, if the strength of the head limits the safe working load of a two-die machine to 51,000 kg (112 500 lb), the

rated capacity (specific diameter) of the machine for a 75 mm (3 in.) long die in swaging solid work metal of 414 MPa

(60 ksi) tensile strength can be calculated using:

Load = specific diameter · die length

· tensile strength

Therefore:

where load is in kilograms (pounds), specific diameter is in millimeters (inches), die length is in millimeters (inches), and

tensile strength is in megapascals (pounds per square inch). Therefore, for the process parameters outlined above, and

using SI units:

Using English units:

For work metal of a higher or lower tensile strength, the capacity or specific diameter would be proportionately lower or

higher, in accordance with the above formula. For a greater die length, the machine capacity would be lower. To swage

parts to a larger final average diameter in this two-die machine, it would be necessary to decrease the working length of

the die proportionately and therefore to decrease the area of work metal under compression.

For the swaging of a tube, the capacity of the machine is limited by the cross section of the die, by the compressive

strength of the tube, and sometimes by the size of hole through the spindle of the machine. The swaging of tubes with a

wall thickness greater than 1 mm (0.040 in.) over a mandrel is considered the same as the swaging of solid bar stock.

Tubes with thinner walls require greater force, depending on tube diameter and length of die, because friction traps the

metal between the die and mandrel, and there is no bulk metal to move.

Machines with dies that produce a squeezing action are rated according to their radial load capacity. The capacity is

usually limited by the stress at the line of contact between the roller and backer. For a reasonable component life, this

stress should not exceed about 1170 MPa (170 ksi). Assuming this stress as maximum when rollers and backers are made

of steel, the radial load capacity is determined by:

where L is radial load capacity in megagrams, N is the number of backers, l is effective roller length in millimeters, D

the diameter of each roller (in millimeters), and D

is the diameter (in millimeters) of the backer crown contacting the

rollers. The coefficient 0.002 converts the Hertz stress formula to megagrams of force based on a value of 1170 MPa for

maximum stress. When English units are used, the coefficient is 1.38 based on a maximum stress value of 170 ksi. Radial

load capacity would be calculated in tons, and all linear measures would be in inches.

For example, a four-die machine having 100 mm (4 in.) diam rolls with an effective roller length of 250 mm (10 in.) and a

915 mm (36 in.) diam backer crown would have a radial load capacity of 180 Mg (199 tons), determined as follows:

Rotary Swaging of Bars and Tubes

Revised by the ASM Committee on Rotary Swaging

Swaging Dies

Resistance to shock and wear are the primary requirements for cold swaging dies. It is sometimes necessary to sacrifice

some wear resistance in order to prevent die breakage due to lack of shock resistance. Numerous materials have been used

for swaging dies. Typical tool steels for cold swaging include A8, D2, S3, S7, and M2 at hardnesses ranging from 55 to

62 HRC. M2 and H13 are frequently used for hot swaging. Shock-resistant grades of carbide are used for high-production

applications. However, the greater density of carbide may lead to increased backer and roll wear.

Types of Dies. Depending on the shape, size, and material of the workpiece, dies range from the simple, single-taper,

straight-reduction type to those of special design. Figure 6 illustrates nine typical die shapes. Specific applications for

each are outlined below.

Fig. 6 Typical die shapes used in rotary swaging. See text for discussion.

Standard single-taper dies are the basic swaging dies designed for straight reduction in diameter. One common use

is to tag bars for drawbench operations.

Double-taper dies are designed for plain reductions, such as those made in the standard single-taper die described

above. A double-taper die can be reversed to obtain twice the life of a single-taper die.

Taper-point dies are used for finish forming a point on the end of the workpiece or for forming a point prior to a

drawbench operation. The built-in cross stop ensures equal length of all swaged points.

Chopper dies are fabricated from heat-resistant alloys. These dies are used exclusively for hot swaging.

Piloted dies ensure concentricity between the unswaged section and the reduced section of the workpiece. The front

part of the die acts as a guide; reduction occurs only in the taper section.

Long-taper dies are designed with a taper over their entire length. However, the length of the taper produced on the

work will be slightly less than that of the die.

Single-extension dies are used for high reduction of solid bars and tubing of low tensile strength. This die produces a

longer tapered section than a standard die.

Double-extension dies are extended at both ends to facilitate the swaging of thick-wall tubing and to provide a longer

taper section.

Contour dies are used to produce special shapes on tubes and bars.

Die Clearance. Virtually all swaging dies require clearance in the form of relief or ovality in the die cavity. Without

clearance, the flow of metal is restricted, and this results in the workpiece sticking to the die.

Ovality in Two-Piece Dies. Dies are oval in both the taper and blade sections. This ovality and side relief provide the

necessary clearance for the die to function. Ovality is useful for applications in order to maximize work hardening. The

disadvantages of using ovality to obtain clearance are:

• Close tolerances are difficult to maintain

• Dies wear rapidly

• Surface finish on the workpiece is inferior to that produced with dies having side clearance

Ovality in two-piece dies is produced by placing shims between the finished die faces and boring or reaming the assembly

to the desired clearance. Smoothly blending the two contours gives an approximately oval shape to the reassembled die.

An alternative procedure for producing ovality is to bore the two die blocks oversize and then to grind the die faces until

the groove in each half is of the proper depth to produce the desired swaged diameter.

The amount of ovality required varies with the characteristics and size of the work metal to be swaged. Table 2 lists

nominal values for determining the amounts of ovality for swaging solid material from 0.8 to 19 mm ( to in.) in

diameter and tubing covering a range of outside diameters. The following sample calculation shows how Table 2 is used

to determine the die ovality required for swaging 12.7 mm (0.5 in.) diam 1020 steel bar to a diameter of 9.5 mm (0.375

in.) using a die with a taper of 8° included angle. From Table 2, the ovality for the die taper for swaging low-carbon steel

is 0.025 mm (0.001 in.) per degree of taper plus 0.5% of the maximum diameter of the bar before swaging. Therefore:

Ovality

taper

= (0.025 · 8) + (0.005 · 12.7)

= 0.2 + 0.064

= 0.264 mm

Using English units:

Ovality

taper

= (0.001 · 8) + (0.005 · 0.5)

= 0.008 + 0.0025

= 0.0105 in.

According to Table 2, ovality of the blade section of the die is 0.075 to 0.1 mm (0.003 to 0.004 in.) less than the ovality of

the taper section. Therefore:

Ovality

blade

= 0.264 - 0.075 = 0.19 mm

Using English units:

Ovality

blade

= 0.0105 - 0.003 = 0.0075 in.

Table 2 Nominal values for computing ovality and corner radius on groove of dies for swaging of bars and

tubing

Percentage of shimming recommended for die diameter of:

Work metal

19-6.4 mm ( - in.)

4.8

(

in.)

3.2

(

in.)

1.6

(

in.)

0.8

(

in.)

Dies for swaging of bars

Low-carbon steels;

hard brass; copper

For die taper: 0.025 mm (0.001 in.) per degree plus 0.5% of max work

diameter. For die blade: above value less 0.075-0.1 mm (0.003-0.004 in.)

(a)

(b)

High-carbon and

alloy steels

125% of value for low-carbon steels 2

(a)

(b)

Lead No shimming required . . . . . . . . .

. . .