MIL-HDBK-660B
FIGURE 7. Draw bend.
4.4.1.1 Draw bending. In draw bending, the waveguide is clamped to a rotating die. When the form is rotated, the guide is drawn against a pressure die. The mandrel follows the radius while supporting the walls preventing collapse of the guide. The guide is boxed in on all sides by the outer dies. The wiper die used prevents wrinkling or buckling of the guide at the tangent point of the bend. The inside dimensions of the waveguide can be held to tolerances of ±
0.1 percent with the articulated mandrel. Tolerances approaching those of the articulated mandrel can be realized using a polytetrafluoroethylene mandrel. Small radius bends may be formed by applying the basic techniques very carefully. Complex shapes can be formed to meet design requirements of limited space. Instead of bends separated by straight sections, space can be saved when bends are placed tangent to each other.
4.4.1.2 Metal mandrel. The use of a mandrel is necessary to bend the waveguide. The various types of mandrels are plug, formed, ball, cable, and universal (see figure 8). The inside surface finish on bending depends on the type of mandrel finish.
4.4.1.2.1 Plug mandrel. The plain or plug type is the simplest form of mandrel made from solid bars rounded on the end. This mandrel may be used on moderate wall waveguide for bends having a large radius. The tip of the plug mandrel is inserted up to or just beyond the bend tangency (see figure 9). During bending, the guide is drawn around the form and pulled off the mandrel, thereby removing the internal support. However the actual bending point, the area of greatest tendency for wrinkling, is fully supported.
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