ASM Metals HandBook Vol. 14 - Forming and Forging

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Fig. 8 Effect of impact in forming stretch and shrink flanges by the drop hammer (or trapped-

rubber) and

Guerin processes. Dimensions given in inches.

ASEA Quintus Rubber-Pad Press

ASEA presses, generally designed with force capacities of 50 to 500 MN (5600 to 56,000 tonf), are constructed of wire-

wound frames and have separate guiding columns (Fig. 9). By winding the press frames with prestressed wire, only

compressional stresses are present in the large castings or forgings of the yokes and columns, even when subjected to

maximum forming pressure. Therefore, when the press is loaded, the frame remains in slight compression, and the major

structural components never operate in the tensile mode.

Fig. 9 Schematic of ASEA Quintus rubber-pad hydraulic press with wire-wound frame.

The press is equipped with a forged-steel rubber-pad retainer that has a replaceable insert to allow for forming at even

higher pressures. Although the maximum tool height is sacrificed by using these high-pressure inserts, cutting the work

area in half doubles the maximum forming pressure of the press when needed.

Standard table sizes range from 0.7 × 1.0 m (28 × 39 in.) to 2 × 3 in (79 × 118 in.). These presses provide forming

pressures to 100 MPa (15 ksi). Hard and brittle materials such as titanium, along with the die, can be heated outside the

press by an infrared heater; blanks can also be heated by conduction from the table through the heat transferred through

the die (Fig. 10).

Fig. 10 Schematic of ASEA Quintus rubber-

pad press with provision for heating hard and brittle materials using

infrared heater or heating elements contained in feed table.

Rubber-Pad Forming

Fluid-Cell Forming

Initially developed as the Verson-Wheelon process, the fluid-cell process uses a fluid cell (a flexible bladder) backed up

by hydraulic fluid to exert a uniform pressure directly on the form block positioned on the press table. This process can be

classified in terms of the presses used (Verson-Wheelon and ASEA Quintus) as well as a specialized method (Demarest

process) for producing cylindrical and conical parts. The Verson-Wheelon press has cylindrical press housings of

laminated, prestressed steel that serve as pressure chambers. The ASEA Quintus press has a forged steel cylinder that is

wound with high strength steel wire to create a prestressed press frame with extremely good fatigue properties.

Fluid-cell forming can be used for recessed parts that are beyond the capabilities of rubber-pad forming, for all flange

configurations (including C-shaped flanges), and for complex parts with reentrant features and intricate joggles.

Maximum form block height is 425 mm (16.7 in.), and typical cycle time is 1 to 2 min.

Verson-Wheelon Process

The Verson-Wheelon process was developed from the Guerin process. It uses higher pressure and is primarily designed

for forming shallow parts, using a rubber pad as either the die or punch. A flexible hydraulic fluid cell forces an auxiliary

rubber pad to follow the contour of the form block and to exert a nearly uniform pressure at all points.

The distribution of pressure on the sides of the form block permits the forming of wider flanges than with the Guerin

process. In addition, shrink flanges, joggles, and beads and ribs in flanges and web surfaces can be formed in one

operation to rather sharp detail in aluminum, low-carbon steel, stainless steel, heat-resistant alloys, and titanium.

Presses. The Verson-Wheelon press has a horizontal cylindrical steel housing, the roof of which contains a hydraulic

fluid cell (Fig. 11). Fluid-cell bladders can be of neoprene or polyurethane composition. Hydraulic fluid is pumped into

the cell, causing it to inflate or expand. The expansion creates the force needed to flow the rubber of the work pad

downward, over and around the form block and the metal to be formed.

Below the chamber containing the rubber pad and

the hydraulic fluid cell is a passage, extending the

length of the press, that is wide and high enough to

accommodate a sliding table containing form

blocks. At each end of the passage is a sliding table

that is moved into position for forming.

The rubber pad used in the Verson-Wheelon process

has a hardness of about Durometer A 35. It is

usually protected from sharp corners on the form

block and blank by a throw sheet or work pad that is

harder and tougher than the pad itself. The throw

sheet is much less costly to replace than the rubber

pad below the fluid cell.

Verson-Wheelon presses are available with forming

pressures ranging from 35 to 140 MPa (5 to 20 ksi)

and force capacities of 22 to 730 MN (2500 to

82,000 tonf). Sliding tables range in size from 508 ×

1270 mm (20 × 50 in.) to 1270 × 4170 mm (50 ×

164 in.). The larger machine can form parts having

flange widths to 238 mm (9 in.).

Heating elements can be used with the sliding tables

for producing parts made of magnesium. The

maximum temperature is 315 °C (600 °F), and

special heat-resistant throw pads are used to protect

the hydraulic fluid cell.

Tools. The form blocks for the Verson-Wheelon

process are made in much the same way as those for

the Guerin process. Compression dams or deflector

bars can be used to direct pressure into local areas for forming shrink flanges or return bends, as described in Example 4.

Aluminum alloy or zinc alloy form blocks are recommended. Because of the high pressures, masonite or wood form

blocks may break down from repeated use. More than one form block can be used at a time; the quantity depends on the

size and shape of the form block.

Damage to the rubber pad can be reduced by removing all burrs and sharp edges from the blank. Form blocks should be

smooth; all sharp corners and projecting edges should be well rounded; high tooling pins should be eliminated; deep

narrow crevices or gaps between parts of form blocks should be eliminated; holes in blocks should be plugged during

forming.

Procedure. The sliding table containing the form blocks is loaded and slid into the press. Hydraulic fluid is pumped into

the cell, expanding it and driving the rubber pad down against the workpiece and around the form blocks. The pressure is

released, and the table of formed pieces is slid out, unloaded, and reloaded for another cycle.

Repositioning the form blocks after a few cycles will distribute the wear on the rubber pad and lengthen its life. The use

of a hard-rubber (or occasionally leather) pad slightly larger than the blank assists in the uniform forming of flanges and

prevents wrinkles.

Fig. 11 Principal components of the Verson-Wheelon process.

The cycle time for the Verson-Wheelon process is longer than that of conventional presses, such as those used with the

Guerin process. To reduce cell filling and draining time, it is good practice to load the table to capacity or to have dummy

blocks on the sliding table when only one part is being formed.

In the following example, the time to form a part in a Verson-Wheelon press was less than when the part was made by the

Guerin process. The higher forming pressure completely formed the part in one operation (whereas the Guerin process

had required two operations) and reduced hand work after machine forming.

Example 4: Verson-Wheelon Versus Guerin Process for the Forming of a Complex Part.

The complex part shown in Fig. 12 was originally formed by the Guerin process in a 40 MN (4500 tonf) hydraulic press

from 1.0 mm (0.040 in.) thick alclad aluminum alloy 7075-W. The improved method used a Verson-Wheelon press that

could exert a pressure of 69 MPa (10 ksi) and had a capacity of 360 MN (41,000 tonf). A pressure of 48 MPa (7 ksi) was

needed to form the part. The same tool was used for both processes.

In the Guerin process, forming was done in two

operations. Joggles and stringer tabs were set by hand

after forming. In the first press operation, the outer

flange was formed, and the inner and return flanges

were partly formed. In the second press operation, the

forming was completed with rubber strips confined by

the dams.

In the Verson-Wheelon process, the outer, inner, and

return flanges and the joggles were formed in one

operation. This translated into a 30% labor-cost

advantage for the Verson-Wheelon process over the

Guerin process for this workpiece.

The heat-treated aluminum alloy 6061 form block was

mounted to a baseplate, with the inner and outer rims

acting as dams. Because of the return flange on the

inside radius of the part, the aluminum alloy form

block was split longitudinally, and the outer half was

fastened to the baseplate (see tooling setup in Fig. 12).

The inner half, bushed and located on pins projecting

from the baseplate, was removed from the base with

the finished part. Locating holes in the blank and the

outer form block matched locating pins in the cover

plate.

The 127 × 2240 mm (5 × 88 in.) blank was routed, and lightening holes were individually pierced and flanged, in a punch

press in both processes before rubber-pad forming. After forming, the part was aged to the T6 temper. The production lot

was 20 pieces. Several thousand pieces were produced on the form blocks.

Secondary Operations. Even though higher forming pressures are used, many pieces made by the Verson-Wheelon

process (as well as by the Guerin process) need hand work to remove wrinkles and to add definition to details. A further

refinement in the use of throw pads is a shaped rubber pad. Pad laminations are built up around a cavity that approximates

the shape of the part, so that flow of the rubber is less severe and forming pressure is more evenly distributed than with

the conventional flat rubber pad. The shape of the cavity is only approximate and can be used for similar parts.

Demarest Process

Cylindrical and conical parts can also be formed by a modified rubber bulging punch. The punch, equipped with a

hydraulic cell, is placed inside the workpiece, which is in turn placed inside the die. Hydraulic pressure expands the

punch. Forming with an expanding punch using the Demarest process is described in the following example.

Example 5: Use of Expanding Punches to Form Aircraft Fuel-Tank Sections.

Fig. 12 Complex part that was formed in a Verson-

Wheelon press. Dimensions given in inches.

Aluminum alloy workpieces, rolled and welded into cones (Fig. 13a), were formed into aircraft fuel-tank sections with

expanding rubber punches (Fig. 13b). The cones were lowered into cast iron dies, which weighed 1600 kg (3500 lb) each

and were designed to withstand 10 MPa (1.5 ksi) of forming pressure.

The rubber punch was lowered into the workpiece, and a steel

cover was clamped over the whole assembly (Fig. 13c). The

punch was expanded under 2800 kPa (400 psi) of hydraulic

pressure, which formed the work metal into the curved shape

of the die (Fig. 13d).

The time taken for the entire process, including dismantling of

the die and unloading of the workpiece, was 3 min. In contrast,

spinning requires 15 to 20 min.

ASEA Quintus Fluid Cell Process

The ASEA Quintus fluid cell process is a further development

of the Guerin process, allowing deeper and more complex

parts to be formed. It uses a flexible rubber diaphragm backed

up by oil as either the male or female tool half. The

pressurized diaphragm forces the blanks to assume the shape

of the solid-tool halves. The high uniform hydrostatic pressure

permits the forming of shallow- to medium-depth parts with

complex shapes to final shape, practically eliminating the

subsequent hand forming usually required to apply the Guerin

process.

Presses. The ASEA Quintus fluid cell press (Fig. 14)

consists of a horizontally placed cylindrical press frame, two

rectangular, independently operated press tables on each side

of the press, and all electric and hydraulic equipment necessary

to operate the press placed on top of and alongside the press.

The press frame is a forged steel cylinder that has been wound

under pre-stress with high strength steel wire, so that only compressional stresses can occur in the steel forgings, even at

maximum pressure. Therefore, the fatigue properties of the Quintus wire-wound frame are excellent and the risk of

fracture virtually eliminated.

Fig. 13 Forming of a fuel-

tank section from a blank

using the Demarest process.

Fig. 14

Illustration of the principal components of a new generation ASEA Quintus fluid cell press. (a) Tray

containing blanks inserted into press prior to pressurizing. (b) Pressurized fluid cell diaphragm forming a blank.

A diaphragm (bladder) of natural rubber is located inside the press and covers the entire surface of the press table.

Hydraulic fluid is pumped into the cell created by the frame and the diaphragm, and the diaphragm is forced by the

increased pressure to expand into the table, which forces the blank to assume the shape of the form block, thus forming

the part. Once forming is done, the table is shuttled out to one side of the cylindrical press frame, and the table on the

other side can be shuttled inside the press to allow forming of parts placed on that table.

In the new generation of ASEA Quintus fluid cell presses, the flexible diaphragm can be changed in less than 30 min. No

rubber pad is required, and the diaphragm, which is made of natural rubber, can be repaired on-site should a sharp corner

of a tool or blank penetrate the roll-up pad that rolls out to cover the press table when it is shuttled into the press.

ASEA Quintus fluid cell presses are available with maximum forming pressures ranging from 100 to 200 MPa (14 to 29

ksi) and force capacities up to 1400 MN (157,000 tonf). The forming tables range in size from 700 × 2000 mm (27.5 ×

78.7 in.) to 2000 × 5000 mm (78.7 × 196.8 in.). The large presses can accommodate tools as high as 425 mm (16.7 in.)

and consequently form parts as deep or flanges as wide as 425 mm (16.7 in.).

Tools. Because of the uniform hydrostatic pressure exerted by the press on the tools, they can be made of low-strength

and low-cost tool materials such as hardwood, bakelite, epoxy, zinc-alloy, and so on, as well as of stronger materials such

as aluminum, cast iron, and steel. Depending on the shape of the part, and tolerance and surface finish requirements, parts

can be formed over a male die (form block), in a female die, or in an expansion die. The side of a part requiring a high

surface finish should face the diaphragm, while the side of a part requiring close tolerances should face the tool. Parts

with complex shapes and tight radii or made of high strength materials, demanding high pressures, and parts required in

large quantities may require tools of zinc or aluminum alloys.

Procedure. The ASEA Quintus fluid cell press has two independently operated press tables. This allows parts to be

unloaded and new blanks loaded on tools on one press table while parts are being formed on the other table inside the

press. A press table can be loaded with one large single tool half occupying the full size of the table or several smaller

tool halves; the number is restricted only by the size of the table.

Once blanks are loaded on the tools, the table is shuttled into the cylindrical press frame to a position where the flexible

diaphragm, located in the upper half of the press, covers the entire table area. Oil is then pumped into the cell, causing the

diaphragm to expand, which forces the blanks to assume the shapes of the tools. When the parts are formed, the pressure

is relieved and the oil is evacuated from the cell allowing the table to be shuttled out from the press so that parts can be

unloaded from the tools. The cycle time for a Quintus fluid cell is usually 1 to 3 min, depending on the press size and

forming pressure selected.

Rubber-Pad Forming

Fluid Forming

In contrast to conventional two-die forming, which produces local stress concentrations in a workpiece, fluid forming

(previously classified as rubber-diaphragm forming) employs a flexible-die technique that inhibits thinning and crack

initiation due to the uniformly distributed pressure. In the fluid forming process, a rubber diaphragm serves as both the

blankholder and a flexible-die member. Fluid forming differs from the rubber-pad and fluid-cell processes in that the

forming pressure can be controlled as a function of the draw depth of the part.

Fluid forming was initially known as the Hydroform process. The process, as originally conceived, is incorporated into

the Verson Hydroform press. After a hydraulic pump delivers fluid under pressure into the pressure-dome cavity, the

punch containing the die is driven upward into the cavity against the resistance provided by the fluid, and the workpiece is

formed.

In the SAAB rubber-diaphragm method, hydraulic fluid being compressed by the press piston alone (no moving die is

involved) forces the workpiece to assume the contour of the die. Air vents incorporated into the die facilitate the removal

of trapped air and eliminate blisters on the surface of the workpiece. The ASEA Quintus fluid forming press was

developed through slight modification of the SAAB process and optimization of press specifications with easily

changeable pressure domes.

Another adaption of the fluid forming method is the ASEA Quintus technique used in the ASEA Quintus deep-drawing

fluid forming press. Dome pressure and punch draw are both controlled from below the blank by two concentric rams,

each of which governs one of these two variables. The units have interchangeable pressure domes.

Fluid forming is intended for punch, cavity, hydroblock, or expansion forming of deep-recessed parts (Fig. 15). Cycle

time is 15 to 20 s for most parts.

Fig. 15

Four forming techniques that can be used in a fluid forming press. (a) Punch draw. (b) Cavity draw. (c)

Hydroblock draw (male-die forming). (d) Expansion draw.

Verson Hydroform Process

This process differs from those previously described in that the die cavity is not completely filled with rubber, but with

hydraulic fluid retained by a 64 mm (2½ in.) thick cup-shaped rubber diaphragm. This cavity is termed the pressure dome

(Fig. 16). A replaceable wear sheet is cemented to the lower surface of the diaphragm, as shown in Fig. 16. More severe

draws can be made by this method than in conventional draw dies because the oil pressure against the diaphragm causes

the metal to be held tightly against the sides as well as against the top of the punch.

Reductions in blank diameter of 60 to 70% are common for

a first draw. When redrawing is necessary, reductions can

reach 40%. Low-carbon steel, stainless steel, and aluminum

in thicknesses from 0.25 to 1.65 mm (0.010 to 0.065 in.) are

commonly formed. Parts made of heat-resistant alloys and

copper alloys are also formed by this process.

Presses. A special press, called a Hydroform press, is used

for this process. A lower hydraulic ram drives the punch

upward; the upper ram is basically a positioning device. A

hydraulic pump delivers fluid under pressure to the pressure

dome. The blankholder is supported by a solid bolster and

does not move during the operation.

Dome pressures range from 41 to 103 MPa (6 to 15 ksi), and

punch force capacities vary from 3.2 to 19 MN (368 to 2089

tonf). Special guide pins and a platen adaptor convert a standard Hydroform press into a single-action conventional

hydraulic press with a force capacity of 6.2 to 40 MN (700 to 4470 tonf). This variation of the process has the punch

stationary and the blankholder actuated by the die cushion of a single-action hydraulic press, as shown in Fig. 20.

Maximum blank diameters are 305 to 1020 mm (12 to 40 in.), maximum punch diameters are 254 to 864 mm (10 to 34

in.), and maximum draw depths are 178 to 305 mm (7 to 12 in.). The maximum rating is 1500 cycles per hour. The

practical production rate in cycles per hour is usually about two-thirds the machine rating. However, the operation often

takes the place of two or three conventional press operations.

Tools. Punches can be made of tool steel, cold-rolled steel, cast iron, zinc alloy, plastic, brass, aluminum, or hardwood.

Choice of material depends largely on the work metal to be formed, number of parts to be made, shape of the part, and

severity of the draw.

Blankholders are usually made of cast iron or steel and are hardened if necessary. Clearance between punch and

blankholder is not critical; it may be 50% or more of the thickness of the metal being drawn.

For short runs, an auxiliary blankholding plate can be placed on a blankholder that is already in place. The auxiliary

blankholder plate should not overhang in the punch clearance more than its thickness, and it should not be larger than the

blankholder.

Rubber strips are placed on the blank to break the vacuum caused by dome action during drawing. Blankholders can be

contoured to match the shape of a preformed blank or to preform the blank as an aid in forming.

Procedure. The blank to be formed is placed on the blankholder. The pressure dome, filled with the hydraulic fluid and

covered by the rubber diaphragm, is lowered over the blank, and preliminary pressure is applied through a pump in the

hydraulic supply line. The preliminary pressure can range from 14 to 69 MPa (2 to 10 ksi), depending on the part to be

formed.

The punch is raised and pushed into the blank from underneath. As the form in the blank rises into the hydraulic chamber,

the pressure in the chamber increases sharply, reaching as high as 103 MPa (15 ksi). A pressure control valve keeps the

pressure within programmed limits. When parts are formed of thin metal, a vacuum release valve can be built into the

punch to aid stripping after forming.

Three cams are programmed to control the operation of the machine. The first controls the height of rise of the punch, the

second controls edging or sharpening of the corner radii, and the third returns the punch at the end of the stroke while the

blankholder strips the finished part from the punch. The forming of a complex part by the rubber-diaphragm process is

described in the following example.

Example 6: Rubber-Diaphragm Forming of a Complex Jet-Engine Part.

Fig. 16 Fluid-cell forming in a Hydroform press.

Fuel-nozzle swirl cups for high-performance turbojet engines were originally produced by welding six press-formed

sections of type 310 stainless steel, AMS 5521 (Fig. 17a). Forming the six sections was difficult, and the finished parts

were expensive. The rejection rate was also high.

Fig. 17 Original and improved methods of forming a fuel-

nozzle swirl cup for a turbojet engine. (a) Part formed

by original method; six press-formed sections welded together. (b) Partly drawn blank ready for rubber-

diaphragm forming. (c) Punch of six similar wedge-

shaped segments doweled into bottom plate, used for

rubber-diaphragm forming. (d) Swirl cup as formed by the rubber-diaphragm method an

d subsequently pierced

and trimmed. Dimensions given in inches.

Rubber-diaphragm forming in a Hydroform press was tried. This press formed the part from one blank 1.1 mm (0.043 in.)

thick by 324 mm (12 in.) in diameter. Less press force was used, and costs were reduced 50%.

Before forming, the blank was rough drawn (Fig. 17b) in a 1330 kN (150 tonf) hydraulic press to a depth of 35.6 mm

(1.40 in.), and its thickness was reduced to 0.99 mm (0.039 in.). After degreasing and annealing, the partly formed blank

was drawn in a 305 mm (12 in.) Hydroform press, using the punch shown in Fig. 17(c). The blank rested on a blankholder

mounted on a subbolster. Diametral clearance between punch and blankholder was a minimum of 50% of the work metal

thickness. The production rate was 30 pieces per hour.

After forming, six equally spaced 36.50 mm (1.437 in.) diam holes and a 63.50 mm (2.500 in.) diam center hole were

pierced in a 490 kN (55 tonf) mechanical press. The outside diameter was trimmed in a lathe after the part had been

pierced, annealed, and restruck. The completed workpiece is shown in Fig. 17(d).

Lubricants. The following example shows the importance of the lubricant and its application when the depth of draw is

near the limit for the rubber-diaphragm process.

Example 7: Use of Lubricant to Eliminate Tearing and Wrinkling in Severe Rubber-

Diaphragm Drawing.

The stepped cover shown in Fig. 18 represented the limit of forming severity for the rubber-diaphragm equipment that

was available. The material was 1.0 mm (0.040 in.) thick cold-rolled drawing-quality 1008 steel. The shell was 102 mm

(4 in.) deep and had a step in its outer contour. Attempts to draw the stepped shell in one operation in a Hydroform press

were not successful. Subsequently, two Hydroforming operations were developed in which the larger width of the cover

was drawn first, and then the narrower portion above the step was produced in a redrawing operation to complete the part.