3-81. DRAW FORMING

T.O. 1-1A-9 3-81. DRAW FORMING. Draw forming is defined as a method where a male die (punch) and a female die is used to form a sheet blank into a hollow shell. Draw forming is accomplished by forcing the male die and the metal blank into the female die. Generally mechanical press either single or double action and hydraulic presses are used to perform the drawing operation. Results will depend on die design, radii of die forming surfaces, finish of die, surface clearance between punch and female die, blank hold down pressure, shape of blank, material allowance on blank, elongation factor of material, temper, shape of part being formed, drawing speed, and lubricant. Optimum results usually requires experimentation and adjustment of one or more of these factors. Drawing of very deep shells require more experimentation and the utilization of a succession of limit draws. Because of the work hardening resulting from each draw, reduction in successive draws must be less. In severe conditions an intermediate anneal is sometimes used. Condition "O" material of the heat treatable alloys can be heat treated after drawing to obtain higher strength and to relieve the effect of work hardening. However, the non-heat treatable alloys can only be annealed to relieve the effect of work hardening. This material should not be annealed if high strength is the major requirement. 3-82. The recommended material to manufacture drawing dies is hardened tool steel for large scale production; kirksite and plastic for medium or short run production; and phenolic and hardwood for piece production. 3-83. STRETCH FORMING. This process involves stretching a sheet or strip to just beyond the elastic limit where permanent set will take place with a minimum amount of springback. Stretch forming is usually accomplished by gripping two opposite edges fixed vises and stretching by moving a ram carrying the form block against the sheet. The ram pressure being sufficient to cause the material to stretch and wrap to the contour of the form block. 3-84. Stretch forming is normally restricted to relatively large parts with large radii of curvature and shallow depth, such as contoured skin. The advantage is uniform contoured parts at faster speed than can be obtained by hand forming with a yoder hammer or other means. Also, the condition of the material is more uniform than that obtained by hand forming. The disadvantage is high cost of initial equipment, which is limited to AMA level repair facilities. 3-85. Material used for stretch forming should be limited to alloys with fairly high elongation and good spread between yield and tensile strength. Most of the common alloys are formed in the annealed condition. It is possible to stretch form the heat treatable alloys in tempers T4 or T6, where the shape is not too deep or where narrow width material is used. For the deeper curved shapes, the material is formed in the annealed "O" temper, heat treated and reformed, to eliminate distortion resulting from heat treatment. As previously stated the material should be reformed as fast as possible after heat treatment. In some instances the material is formed immediately after heat treating and quenching. Selection of a system or condition of material to be utilized will require experimentation and the subsequent utilization of the system that gives the best results. 3-86. HYDRAULIC PRESS FORMING . The rubber pad hydropress can be utilized to form many varieties of parts from aluminum and its alloys with relative ease. Phenolic, masonite, kirksite and some types of hard setting molding plastic have been used successfully as form blocks to press sheet metal parts such as ribs, spars, fans, etc. The press forming operations are usually accomplished by setting the form block (normally male) on the lower press platen and placing a prepared sheet metal blank on the block. The blank is located on the block with locating pins, to prevent shifting of blank when the pressure is applied (the sheet metal blank should be cut to size and edges deburred prior to pressing). The rubber pad filled press head is then lowered or closed over the form block and the rubber envelope, the form block forcing the blank to conform to the form blocks contour. This type forming is usually limited to relatively flat parts having flanges, beads and lightening holes. However, some types of large radii contoured parts can be formed with a combination of hand forming and pressing operations. It is recommended that additional rubber be supplemented in the form of sheets when performing the above to prevent damage to the rubber press pad. The rubber sheet used should have a shore hardness of 50-80 durometers. The design of foam block for hydropress forming require compensation for springback even through the material normally used is Condition "O" or annealed. Normal practice is to under cut the form block 2-7° depending on the alloy and radii of the form block. 3-87. DROP HAMMER FORMING . The drophammer can be used to form deep pan shaped and beaded type parts. Kirksite with a plastic surface insert is satisfactory for male and female dies. The surface of kirksite dies used without plastic insert should be smooth to prevent galling and scratching of the aluminum surface. When forming deep pans and complicated shaped parts it is often necessary to use drawings rings, pads or 2-3 stage dies. An intermediate anneal is sometimes used to relieve the hardened condition (cold work) resulting from the forming operation. 3-88. JOGGLING. A joggle is an offset formed to provide for an -overlap of a sheet or angle which is projecting in the same plain. The inside joggle radii should be approximately the same as used for straight bending. 3-27