- Shear forming
Shear forming, also referred as shear spinning, is different from conventional
metal spinningin that in this latter the area of the final component is approximately equal to that of the flat sheet metal blank and little or no reduction in the wall thickness occurs, whereas in shear forming a reduction of the wall thickness is induced. This final wall thickness is achieved by controlling the gap between the roller and the mandrel.
Before the 1950's, spinning was performed on a simple turning
lathe. When new technologies were introduced to the field of metal spinning and powered dedicated spinning machines were available, shear forming started its development in Sweden.
Figure 2 shows the schematics of a shear forming process.
1. A sheet metal blank is placed between the mandrel and the chuck of the spinning machine. The mandrel should have the interior shape of the desired final component.
2. A roller makes the sheet metal wrap the mandrel so that it takes its shape.
As can be seen, s1 which is the initial wall thickness of the workpiece is reduced to s0.
Workpiece and roller tool profiles
In shear forming, the starting workpiece can have circular or rectangular cross sections. On the other hand, the profile shape of the final component can be concave, convex or a combination of these two.
A shear forming machine will look very much like a conventional spinning machine, except for that it has to be much more robust to withstand the higher forces necessary to perform the shearing operation.
The design of the roller must be considered carefully, because it affects the shape of the component, the wall thickness, and dimensional accuracy. The smaller the tool nose radius, the higher the stresses and poorest thickness uniformity achieved.
Spinnability, sometimes referred as shear spinnability, can be defined as the ability of a metal to undergo shear spinning deformation without reaching its ultimate
tensile strength. Published work on spinnability is available from the authors Kegg and Kalpakcioglu.
Kegg predicted that for materials with a tensile reduction of 80%, the limiting spinning reduction will be equal or greater than 80%. Kalpakciouglu, on his side, concluded that for metals with a true fracture strain of 0.5 or greater, there is a a maximum limit for the shear forming reduction. For materials with a true strain below 0.5, the spinnability depends on the
ductilityof the material.
Highly spinnable materials include ductile materials like aluminum and certain steel alloys.
Importance of shear forming operations in manufacturing
Although lately shear forming and conventional spinning have been losing ground to other manufacturing processes such as deep drawing and ironing, being able to achieve almost net shape thin sectioned parts makes out of spinning a versatile process used widely in the production of lightweight components. Other advantages of shear spinning include the good mechanical properties of the final component as well as very good surface finish.
Typical components produced by mechanically powered spinning machines include rocket nose cones, gas turbine engine and dish aerials.
C.C. Wong, T.A. Dean and J. Lin, A review of spinning, shear forming and flow forming processes, Int. J. Mach. Tools Manuf. 43 (2003), pp. 1419–1435
B. Avitzur, Handbook of Metal-Forming Processes, John Wileyand Sons, Inc., Canada, 1983.
R.L. Kegg, A new test method for determination of spinnabilityof metals, Transactions of the ASME, Journal of Engineering forIndustry 83 (1961) 119–124.
S. Kalpakcioglu, A study of shear-spinnability of metals, Transactionsof the ASME, Journal of Engineering for Industry 83(1961) 478–483.
[http://www.pmfind.com/sheet-metal-spinning.asp Metal spinning process video]
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