MIL-HDBK-660B
4.4.1.2.5 Universal mandrel. The universal mandrel provides twisting while draw bending. Individual bends need not be separated by short straight sections. The freedom of movement of this mandrel permits an H bend in one plane to closely follow an E-bend in the other plane. The mandrel can withstand the heavy pressures yet can hold the internal dimensions to very close tolerances.
4.4.1.2.6 Small line mandrel. Nylon and polytetrafluoroethylene mandrels may be used on a waveguide with very small cross sections and complex shapes.
4.4.1.3 Mandrel selection. The plug or form mandrel is sometimes more useful than the complicated mandrels since a perfect fit is not necessary for a specific size waveguide. An undersize mandrel extends into the waveguide beyond the tangent point. This type of mandrel provides greater support beyond the tangent of bend compared to other mandrels. Sharpness of bend and clearance between mandrel and inside of guide determine the tip placement.
4.4.1.4 Improper tooling and setting. Improper tooling and setting can result in flattening, wrinkling, or rupturing problems (see figures 10 and 11) as follows:
a. Tapering of mandrel by improper machining or through wear. b. The clamp die may be causing slippage.
c. Insufficient pressure on the pressure die. d. Mandrel improperly set.
e. Adjustment of wiper die differs in the same setup for various materials, f. Wiper die is not as close as possible to the tangent point.
g. Wearing of bending form and the machine spindles. Where guide tolerances are difficult to maintain, or flattening and thinning must be rigidly controlled (see figure 12) because of "off-size" waveguide, different sizes of mandrels could solve the problem.
FIGURE 10. Wrinkling caused by tapered mandrel.
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