ASM Metals HandBook Vol. 14 - Forming and Forging

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Dies for swaging of tubing

When OD equals a minimum of 25 times wall thickness, use no shimming.

When OD equals 10 to 24 times wall thickness, use 60% of values for bars (see above).

When OD equals 9 times wall thickness or less, use same values as for bars (see above).

Corner radius on die grooves

For solid work metal, of blade diameter to nearest 0.13 mm (0.005 in.)

For tubing, corner radius should be equal to wall thickness.

(a)

Percent of average diameter of work.

(b)

Stone edges of die groove

These calculated values determine the thickness of shims that must be used between the die faces during machining of the

cavity to produce a die of proper ovality for swaging 1020 steel bars. These values also apply when the alternative method

of producing ovality is used.

In addition to ovality, die halves should be provided with corner radius at the exit end of the blade section as well as at the

die entrance. Table 2 indicates that the corner radius on the groove should be of the blade diameter to the nearest 0.13

mm (0.005 in.) for swaging solid sections, or equal to wall thickness for the swaging of a tube. Therefore, the die for

swaging the 12.7 mm (0.5 in.) diam 1020 steel bar referred to in the sample calculation above would require a corner

radius of about 0.64 mm (0.025 in.).

The included angle for the taper section of oval dies should be no more than 30°. An included angle of 8° or less is

preferred.

Two-Piece Dies With Side Clearance. Workpieces swaged in 240° contact dies have better surface finish and closer

tolerance. The service lives of these dies are longer, and the work metal is cold worked less rapidly than in oval dies. Dies

with 240° contact can be used for straight reductions of solid bars or thick-wall tubing.

Figure 7 shows the design of 240° contact dies with die clearance. The dies are first bored or ground without shims to

produce the area of work contact. Shims are then inserted to produce side clearance only. Side clearance is then bored or

ground until dimension E (measured diagonally across the mouth of the die) = , where d is the initial

diameter of the taper at the entrance to the die, and s is the thickness of the shim stock placed between the die faces. The

maximum thickness of the shim should be one-tenth the swaged diameter of the workpiece. This will produce a total

contact of 240° along the taper and blade sections. The intersection between the taper and blade must be well blended for

best results in feeding and finishing.

Fig. 7 Design of die with side clearance. See text for discussion.

When swaging tubing, the shim thickness varies with the ratio of outside diameter to wall thickness (D/t ratio) so that the

side clearance is nearly zero for thin-wall (D/t = 30 or more) tubing.

The same procedure is followed in determining the side clearance for the blade. The diameter of the swaged workpiece is

used instead of the large diameter of the taper. The same shim is used for both taper and blade.

Ovality in Four-Piece Dies. Each piece of a four-piece die makes approximately 90° contact with the surface of the

workpiece when the die is not provided with ovality or side clearance. Dies without ovality are used for sizing thin-wall

tubes (D/t = 30 or more). For swaging solid sections or thick-wall tubing or for mandrel swaging, oval dies are required;

ovality influences circumferential flow of the work metal and reduces the load on the machine.

Oval dies are produced by various methods. A common method involves grinding the dies, which are held by a fixture

mounted on the rotating face plate of an internal grinder. The taper is produced by pivoting the grinding wheel slide to the

appropriate angle and traversing the surface.

Rotary Swaging of Bars and Tubes

Revised by the ASM Committee on Rotary Swaging

Auxiliary Tools

Swaging machines may require auxiliary tools for guiding and feeding the workpiece into the die, holding it during

swaging, and ejecting it. These tools range from simple hand tools to elaborate power-driven mechanisms. Some of the

common types of auxiliary tools are illustrated in Fig. 8, 9, and 10; their uses are described below.

Fig. 8 Three types of mechanisms for feeding the workpiece in rotary swaging. See text for discussion.

Fig. 9 Principal components of a spring ejector mechanism with an adjustable rear stop.

Fig. 10 Principal components of an adjustable stop-rod mechanism.

Rack-and-pinion mechanisms (Fig. 8a) are designed for manual operation and provide more force for feeding the

workpiece than can be obtained by hand feeding. Operator fatigue is reduced with these mechanisms, and the workpiece

is guided straight along the centerline of the machine.

Feed attachments for long workpieces (Fig. 8b) consist of a carriage with antifriction rollers mounted on a fixed

bar that extends from a bracket on the entrance side of the machine for the length of the longest workpiece to be swaged.

The outer end of the bar is aligned by leveling screws in the base of a triangular support. The carriage provides a means of

attaching plain or antifriction workpiece holders and adapters, as well as a handle for manual feeding toward the swager.

An adjustable stop is provided on the support bar to control the length of the swaged section and to reproduce accurate

tapers.

Roll-feed mechanisms (Fig. 8c) have rolls at the entrance and at the exit end of the swager. The rolls at the entrance

feed the workpiece, and the rolls at the rear pull the workpiece from the machine. Roll-feed mechanisms are used for

continuous swaging. Rolls can be made from either metal or a nonmetallic material (such as rubber). Some roll-feed

mechanisms have four soft rubber rolls at the entrance to the swager and no rolls at the rear. This arrangement is ideal for

swaging small-diameter bars whose surface finish is critical, because it prevents marking of the swaged surfaces when the

bars are pulled from the rear of the machine.

V-shape work guides (Fig. 8b) are used to support and center the ends of long tubes or bars as they enter the dies.

This type of guide is mounted on the front of the machine and can be adjusted vertically to accommodate a range of

workpiece diameters up to the capacity of the machine.

Spring ejectors are required for the removal of short workpieces when size prevents manual withdrawal from the dies

or when workpieces are swaged over their entire length and cannot be passed through the spindle to the exit end. Figure 9

shows the principal components of a spring ejector mounted on the rear of a swaging machine spindle. As the workpiece

enters the die against the workpiece stop, the ejector rod is forced backward until it contacts the preset stop screw. As

soon as the swaging cycle is completed, the spring-loaded ejector forces the workpiece from the front of the machine.

Spring ejectors reduce operator fatigue and shorten the swaging cycle in many applications. A similar mechanism can be

used on large machines with power feed. The ejector maintains contact with the workpiece stop on the return stroke, thus

supporting the workpiece until it is free from the dies.

Stop rods (Fig. 10) are often used to improve uniformity of swaged pieces in production runs. These rods can be

adjusted and locked so that subsequent workpieces will have swaged sections of equal length. The swaged length can be

held within 0.025 mm (0.001 in.), depending on the speed and on the feed pressure.

Rotary Swaging of Bars and Tubes

Revised by the ASM Committee on Rotary Swaging

Automated Swaging Machines

Work-holding fixtures were originally designed for manual operation. These fixtures include a variety of grippers that

facilitate workpiece alignment parallel to the feed direction, provide constrained rotation to prevent finning or flashing

between the dies, and have capabilities for feed-stroke control. Work holders have been designed for use on a variety of

workpiece sizes.

The feed, stop, and ejector mechanisms shown in Fig. 8, 9, and 10, as well as a variety of manual work-holding fixtures,

have formed the basis for contemporary automated systems. The range of equipment available includes single-station

machines (Fig. 11), which form parts automatically in one or more setups, and multistation transfer machines (Fig. 12)

that use different types of swaging heads to perform multiple operations in a single setup.

Fig. 11 Automated die-

closing swaging machine with a gravity parts feeder, hydraulically operated feeding

unit, and part transfer system.

Fig. 12 Multistation automatic swaging transfer machine combining forming and machining operations.

The automatic machines are assembled using a modular concept, and the number of stations can be varied to suit a

particular application. Programmable control allows different stations to be actuated, bypassed, or exchanged to process a

family of parts on one system.

Secondary operations, such as drilling, turning, reaming, splining, thread rolling, or marking, can also be incorporated

into the manufacturing process. Such a sequence of operations is illustrated in Fig. 13 for a torch nozzle.

Fig. 13

Torch nozzle produced using a sequence of operations on a multistation transfer machine similar to that

shown in Fig. 12.

The material for automatic operation can be supplied either from different types of magazines (such as vibratory feeders,

conveyors, and gravity chutes) or directly from coiled stock. This allows the machines to operate unattended for long

periods of time, resulting in machine efficiencies of 90% or more. The stroke and speed of each feeding unit can be set

according to tolerance and surface finish requirements.

This closely controlled process of automatic swaging provides highly repeatable results and consistent part quality.

Automatic operation allows one operator to operate several machines.

Rotary Swaging of Bars and Tubes

Revised by the ASM Committee on Rotary Swaging

Tube Swaging Without a Mandrel

Tubes are usually swaged without a mandrel to attain one or more of the following:

• A reduction in inside and outside diameters or an increase in wall thickness

• The production of a taper

• The conditioning of weld beads for subsequent tube drawing

• Increased strength

• Close tolerances

• A laminated tube produced from two or more tubes

The usual limit on the diameter of tubes that can be swaged without a mandrel is 30 times the wall thickness. Tubes with

an outside diameter as large as 70 times their wall thickness can be swaged, but under these conditions, the included angle

of reduction must be less than 6°, and the feed rate must be less than 380 mm/min (15 in./min). Under any conditions, the

tube must have sufficient column strength to permit feeding. Squareness of the cut ends, roundness, and freedom from

surface defects also become more critical as the ratio of outside diameter to wall thickness increases.

Types of Tubes for Swaging. Seamless and welded tubing can be swaged without a mandrel. Seamless tubing is

available in greater wall thicknesses in proportion to diameter than welded tubing. However, seamless tubing is the more

expensive and may have an irregular and eccentric inside diameter, which will result in excessive variation in wall

thickness of the swaged product. When purchasing seamless tubing, it is possible to specify two of the three dimensions:

outside diameter, inside diameter, and wall thickness. Therefore, the disadvantage of varying dimensions can be partly

overcome by specifying the two dimensions that must be controlled for an acceptable product.

Welded tubing usually has a more uniform wall thickness than seamless tubing and therefore has an inside diameter that

is more nearly concentric with the outside diameter. The swaging of certain types of welded tubing (for example, as-

welded and flash rolled) can result in bending, because the metal in the weld area flows less readily than the remainder of

the tube material. If the weld is defective or if the metal in the weld area is harder than the remainder of the tube, splitting

will occur during swaging. Welded tubing must be held in the centerline of the feed direction during swaging to produce a

straight product.

Die Taper Angle. In best practice when swaging low-carbon steel, the included angle of die taper should not exceed 8°

when using manual feed. For thin-wall tubing of low-carbon steel or for more ductile tubing, such as annealed copper, the

included angle may be as great as 15°, provided both pressure and feed are decreased proportionately. When the angle of

taper exceeds 15°, mechanical or hydraulic feed should be used.

Reduction per Pass. Multiple passes are necessary to swage tubing in dies with a taper exceeding 30°. Steep taper

angles generate excessive heat and feedback and radial pressures. This condition may result in metal pickup by the dies

and is more pronounced when swaging aluminum tubing.

Effect of Reduction on Tube Length. In swaging tubes without a mandrel, wall thickening is usually more

significant than increase of length. Lengthening of about 5 to 15% can be expected for typical swaging operations on low-

carbon steel, copper, aluminum, or other readily swageable metal tubes with outside diameters of 15 to 25 times wall

thickness. Lengthening increases as the amount of reduction per pass increases. Because of the uncertainty about the

relative amounts of radial and axial movement of metal, percentage reduction is frequently designated in terms of

diameter reduction, rather than area reduction. When the tube is reduced to the extent that it approaches a solid, the

endwise flow of metal increases. When total reduction in area is greater than 65 to 75% (depending on the ratio of outside

diameter to wall thickness), the tube should be considered a solid, and swaging dies should be designed accordingly.

Effect of Reduction on Wall Thickness. Swaging of tubing without a mandrel results in an increase in wall

thickness. The increase in wall thickness is greater for larger reductions in outside diameter. Increased ductility of the

tube material promotes wall thickening.

The wall thickness that will be produced by swaging a tube without using a mandrel can be calculated to about ±10%

from the empirical relation:

where D

is outside diameter before swaging, D

is outside diameter after swaging, t

is wall thickness before swaging,

and t

is wall thickness after swaging.

Swaging of Long Tapers. The method used for swaging long tapers depends on work metal hardness, outside

diameter, wall thickness, and overall length, because these variables determine required machine size, die design, and

type of feed mechanism.

Welded tubing sometimes causes difficulty in swaging long tapers because of variations in hardness between the welded

seam and the remainder of the tube. Postweld heat treatment is recommended when swaging long tapers from welded

tubing.

Almost any reasonable length of taper can be swaged on any length of tube that has a diameter within the capacity of the

machine. Long tapers usually require multiple operations.

Table 3 compares the lengths of taper that can be formed in a single operation and in multiple operations on tubes with an

outside diameter of 57 mm (2 in.) or less, using standard-length and extended-length dies. Standard-length dies refer to

manufacturers' catalog sizes; extended lengths are greater than those shown as standard. The longest taper formed in a

single operation is fairly close to the length of the die. However, when dies of the same length are used in multiple

operations, a smaller portion of the usable length is used for forming the taper, because of the allowance required for

blending subsequent passes.

Table 3 Tapers swageable on 57 mm (2 in.) maximum OD tubes in single and multiple operations

Length of taper swaged, mm (in.)

Multiple operations

Die length, mm (in.)

Single operation

First operation

Intermediate operations

Final operation

114 (4 ) 105 (4 ) 79 (3 ) 63.5 (2 )

89 (3 )

162 (6 )

152 (6) 127 (5)

111 (4 )

136.5 (5 )

213 (8 )

203 (8) 178 (7)

162 (6 )

187 (7 )

380 (15)

375 (14 )

. . . . . .

. . .

455 (18)

451 (17 )

. . . . . .

. . .

610 (24) 584 (23) . . . . . .

. . .

Extended die lengths (standard plus 38 mm, or 1 in.)

152 (6)

143 (5 ) 117 (4 )

102 (4)

127 (5)

200 (7 ) 190 (7 ) 165 (6 ) 149 (5 )

175 (6 )

250 (9 ) 241 (9 ) 216 (8 ) 200 (7 ) 225 (8 )

The number of operations needed to produce a specified taper, in addition to the length of taper and length of dies used, is

influenced by the following:

• Minimum length of die entrance is 9.5 mm ( in.)

• Each succeeding taper must overlap the preceding taper by 25 mm (1 in.) to permit blending

• All operations

except the last must allow a straight section (blade), with a minimum length of 25 mm (1

in.) on the tube in addition to the taper being swaged

Example 1: Forming a 760-mm (30-in.) long Taper in Four Operations.

Figure 14 shows the sequence of operations for swaging a 32 mm (1 in.) OD low-carbon steel tube to 12.7 mm ( in.)

in diameter over a taper length of 760 mm (30 in.). Extended dies 250 mm (9 in.) long were used for the first three

operations, and a standard die 210 mm (8 in.) long was used for the final operation. An allowance of 9.5 mm ( in.)

was made for die entrance, a 25 mm (1 in.) overlap was used for each succeeding taper, and each operation except the last

allowed a blade section to remain. The same machine was used for all four operations.

Fig. 14

Sequence of operations for swaging a taper on a long tube. Extended dies are used in the first three

operations; the final operation uses standard-length dies. Dimensions given in inches.

In each operation, the tube was fed through the die to a stop, reducing the tube in each operation to the diameters shown

in Fig. 14. Each feed length was controlled by a stop so that the newly formed taper blended with the preceding one.

Figure 15 shows how a taper 760 mm (30 in.) long can be formed in two operations by dies 455 mm (18 in.) long. The

rate of feed for swaging long tapers is usually 25 mm/s (1 in./s), withdrawal time is 100 mm/s (4 in./s), and handling time

requires about 4 s per operation.

Fig. 15 Swaging a 760 mm (30 in.) long taper in two operations using dies 455 mm (18 in.) long.

Dimensions

given in inches.

An accurate feeding attachment is necessary to swage long tapers. The attachment must feed the tube to the proper

length for each operation to produce a uniform taper. This is accomplished by registering the infeed position of the tube

from the butt end by means of stops on the attachment (Fig. 14).

Manually operated feed attachments are generally used for producing tapers longer than 405 mm (16 in.). Either hydraulic

or air-actuated feed attachments are more convenient for tapers up to 405 mm (16 in.) in length.

Cost is the deciding factor between using standard or extended dies for swaging a given taper. Cost also usually

determines the number of operations to be used. However, when tapers exceed 510 mm (20 in.) in length, there is no

alternative but to use multiple operations, because few swaging machines can hold dies longer than 405 mm (20 in.).

Any swaging machine can handle extended dies that are longer than standard for the machine size (see Example 1). A

given machine can also accommodate shorter dies when die box fillers are used. Therefore, each machine has

considerable flexibility in terms of the length of dies it can handle.

Extended dies cost more than standard dies (usually about one-third more). Therefore, it must be decided if it would be

more economical to pay the higher cost for extended dies and use fewer operations, thus increasing productivity, or to use

less expensive dies and accept lower productivity. Similar consideration must be given to the use of a larger machine that

will accommodate a longer standard die.

Rotary Swaging of Bars and Tubes

Revised by the ASM Committee on Rotary Swaging

Tube Swaging With a Mandrel

For some applications, it is necessary to reduce the wall thickness of tubing by swaging over a mandrel. A mandrel is

used to maintain the inside diameter of a tube during the swaging of its outside diameter, to support thin-wall tubes during

reduction in diameter, and to form internal shapes. When extended through the front of the dies, a mandrel can also serve

as a pilot to support one of the tubes that are to be joined by swaging.

Mandrels are made from shock-resistant tool steel and high-speed steels. They are hardened, ground and polished, and

sometimes plated with about 5 μm (0.2 mils) of chromium to improve wear resistance and surface finish on the inside

diameter of the tube. A combination of hardness and toughness is needed for the larger mandrels. Tungsten carbide

mandrels are used for superior wear resistance when production volume justifies their increased cost. Mandrels are

commonly produced from S group tool steels hardened to 59 to 61 HRC or from A2 or W1 tool steel hardened to 60 to 62

HRC and ground to a finish of 0.06 to 0.075 μm (2.5 to 3 μin.).

Types of Mandrels. The types of mandrels most often used are illustrated in Fig. 16 and are described in the following

sections.