Machinery's Handbook, 31st Edition
1432
Stretch Forming
Formed workpiece
F, v
Die
Fixed adjusted grippers
Blank
Form block
F, v
a) Starting Position b) Ending Position
Fig. 23. Stretch Draw Forming.
The advantages of this process are the following: a) The tensile forces applied always act tangentially to the body of the blank b) The lack of springback in the finished part c) Flexible low-cost tooling d) Increase of yield stress up to 10 percent e) Less forming pressure required f) The die can be made of inexpensive material The disadvantages are these: a) More material required for gripping b) Difficulty of adaptation to modern high-speed automated lines c) Reduction of material thickness by 5 to 7 percent Spinning Metal spinning is the process of forming various seamless and axially symmetrical parts from flat circles of sheet metal (blanks) or from a length of tubing over a mandrel with tools or rollers. There are three types of metal spinning processes: conventional spin- ning, shear spinning, and tube spinning. Conventional Spinning.— In conventional metal spinning, a disc of metal is rotated at controlled speeds on a specialized machine similar in design to a machine lathe. Instead of the clamping chuck common on a machine lathe, a wood or metal spinning mandrel is used, the form of which corresponds with the internal contour of the part to be pro- duced. The blank is clamped between the spinning mandrel and a follower on the tailstock spindle, as shown in Fig. 24. The mandrel, blank, and holder are then set in rotation. Spinning tools or spinning rollers are forced against the rotating blank by hand or by a computer controlled hydraulic mech anism. The process requires a series of crossing steps, as indicated in Fig. 24, to complete the shaping of the workpiece. With this forming technique, a material’s thickness gener ally does not change from the blank to the finished component. If the spinning forces applied with the hydraulic mechanism are higher in comparison to hand spinning, man drels made of a harder material have to be used, e.g., boiler plate, chilled cast iron, or hardened tool steel.
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