MIL-HDBK-660B
FIGURE 13. Depth of support of plug; form and ball mandrels.
4.4.1.5 Lubrication. Effective operation entails frequent lubrication of tools. Proper adjustment of tooling is impossible if slide wear is permitted. Lubrication of the guide and mandrel reduces galling of mandrel and frictional drag. Likewise, clamp slippage, guide flattening, and rupture can be prevented. Lubricating the wiper die decreases friction and wear on the die. Slippage can be caused by allowing the lubricant to penetrate into clamping areas. Different types of material require various lubricants.
4.4.1.6 Twisting and bending. Space saving that imposes design restrictions may result in contorted shapes of waveguides. Individual bends need not be separated by straight lengths, that is, bends could be positioned tangent to each other. If it is desired to have an H bend in one plane follow an E bend in the other plane, then a mandrel with two flexing modes or a universal mandrel is required. Production of various twists during bending results from the ability to adjust the form die to different forms. The electrical characteristics of the formed shapes are superior to those of assembled shapes.
4.4.1.7 Mandrel material and finish. Tool steel is used for bending copper, brass, and aluminum. The inside surface finish of the waveguide on bending depends on the type of mandrel finish. Since scoring or marking the inside surface of the waveguide is intolerable, a tool steel mandrel may be ground finish or chrome plated. Plating extends wear life and can be replaced without major rework of the original tool. Chromed mandrels should be replated when the chrome is worn away at any point to provide many more bends from the mandrel.
4.4.1.8 Polytetrafluoroethylene mandrel. Mandrels of polytetrafluoroethylene or fluorinated ethylene propylene (FEP) are used to support the walls of the waveguide during bending, thus eliminating the costly method of filling with rosin. It may be used for bending waveguide sizes of 3 by 1.5 inches and smaller.
a. The mandrels are made from a solid block of polytetrafluoroethylene or fluorinated ethylene propylene machined to the internal dimensions of the waveguide, minus .005 to minus .008. Tests have proved that the amount of clearance is critical in producing distortion-free bends. The leading edges of the mandrel are rounded slightly to aid in inserting it into the waveguide.
b. The length of each mandrel is governed by the longest bend normally made on the applicable size waveguide.
At least an extra 6 inches (152.4 mm) is added to this dimension for ease of handling and to attach a device to withdraw the mandrel from the bent section.
c. Two methods of attaching the pulling gear are used. In the first, used on small size waveguide, a hole is drilled in the center of the wide plane of the mandrel, approximately 1 inch (25.4 mm) from the end, large enough to
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