Fig. 2.16. Brake Forming
Fig. 2.16. Brake Forming to be made in two or more directions, it is best to make all bends at an angle to the direction of rolling. Relatively simple and long parts can usually be press brake formed to a tolerance of 0.030 in., while for larger more complex parts, the tolerance may be as much as 0.063 in.
Punch presses are used for most deep drawing operations. In a typical deep drawing operation, shown in Fig. 2.17, a punch or male die pushes the sheet into the die cavity while it is supported around the periphery by a blankholder. Single action presses can be operated at 90-140 ft/min., while double action presses operate at 40-100 ft/min. for mild draws and at less than 50 ft/min. for deep draws. For the higher strength aluminum alloys, speeds of 20-40 ft/min. are more typical. Clearances between the punch and die are usually equal to the sheet thickness plus an additional 10% per side for the intermediate strength alloys, while an additional 5-10% clearance may be needed for the high strength
Blankholder X / Punch
Blankholder X / Punch
alloys. Excessive clearance can result in wrinkling of the sidewalls of the drawn shell, while insufficient clearance increases the force required for drawing and tends to burnish the part surfaces. The draw radius on tools is normally equal to four to eight times the stock thickness. If the punch radius is too large, wrinkling can result, and if the radius is too small, sheet fracture is a possibility. Draw punches and dies should have a surface finish of 16 ^in. or less for most applications. Tools are often chrome plated to minimize friction and dirt that can damage the part finish. Lubricants for deep drawing must allow the blank to slip readily and uniformly between the blankholder and die. Stretching and galling during drawing must be avoided. During blank preparation, excessive stock at the corners must be avoided because it obstructs the uniform flow of metal under the blankholder leading to wrinkles or cracks.
The rate of strain hardening during drawing is greater for the high strength aluminum alloys than for the low to intermediate strength alloys. For high strength aluminum alloys, the approximate reductions in diameter during drawing are about 40% for the first draw, 15% for the second draw, and 10% for the third draw. Local or complete annealing for 2014 and 2024 is normally required after the third draw. Severe forming operations of relatively thick or large blanks of high strength aluminum alloys generally must be conducted at temperatures around 600° F, where the lower strength and partial recrystallization aids in forming. This is possible with alloys such as 2024, 2219, 7075, and 7178, but the time at temperature should be minimized to limit grain growth.
In stretch forming (Fig. 2.18), the material is stretched over a tool beyond its yield strength to produce the desired shape. Large compound shapes can be formed by stretching the sheet both longitudinally and transversely. In addition, extrusions are frequently stretch formed to moldline curvature. Variants of
stretch forming include stretch draw forming, stretch wrapping, and radial draw forming. Forming lubricants are recommended except when self-lubricating smooth faced plastic dies are used; however, the use of too much lubricant can result in workpiece bucking. The 2XXX and 7XXX alloys can be stretch formed in either the O or W condition. Material properties that help in stretch forming are a high elongation, a large spread between the yield and ultimate strengths (called the forming range), toughness, and a fine grain structure. Alloys with a narrow spread between the yield and ultimate strengths are more susceptible to local necking and failure. For example, 7075-W has a yield strength of 20ksi, an ultimate strength of 48 ksi, a forming range of 28 ksi (48 ksi - 20 ksi) and a stretchability rating of 100, while 7075-T6 has a yield strength of 67 ksi, an ultimate strength of 76 ksi, a forming range of only 9 ksi, and a stretchability rating of only 10.
In rubber pad forming, a rubber pad is used to exert nearly equal pressure over the part as it is formed down over a form block. Rubber pad forming and a closely related process, fluid cell forming, are shown in Fig. 2.19. The rubber pad acts somewhat like a hydraulic fluid, spreading the force over the surface of the part. The pad can either consist of a solid piece or may be several pieces laminated together. The pad is usually in the range of 6-12 in. thick and must be held in a sturdy retainer as the pressures generated can be as high as 20 ksi. The 2XXX and 7XXX alloys are formed in either the O or W temper. Rubber pad forming can often be used to form tighter radii and more severe contours than other forming methods because of the multidirectional nature of the force exerted on the workpiece. The rubber acts somewhat like a blankholder helping to eliminate the tendency for wrinkling. This process is very good for making sheet parts with integral stiffening beads. Most rubber pad forming is conducted on sheet 0.063 in. or less in thickness; however, material as thick as 0.625 in. thick has been successfully formed. Although steel tools are normally used for long production runs, aluminum or zinc tools will suffice for short or intermediate runs. Fluid cell forming, which uses a fluid cell to apply pressure through an elastomeric membrane, can form even more severe contours than rubber pad forming. Due to the high pressures employed in this process, as high as 15-20 ksi, many parts can be formed in one shot with minimal or no springback. However, fluid cell forming presses are usually expensive.
Superplasticity is a property that allows sheet to elongate to quite large strains without localized necking and rupture. In uniaxial tensile testing, elongations to failure in excess of 200% are usually indicative of superplasticity. Micrograin
Rubber Pad Forming i
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