MIL-HDBK-660B
receive a nut of the size suitable for the pulling rod. A second hole is drilled from the center of the end of the mandrel into the first hole to receive the pulling rod. The threaded pulling rod is inserted from the end of the mandrel and screwed into the nut placed in the first hole. The second method, used on larger size waveguides, is to undercut the mandrel about 3 inches (76.2 mm) along its length and rivet to it a fabricated metal clip to which a rod is attached.
d. The self-lubricating qualities of polytetrafluoroethylene or fluorinated ethylene propylene offer a low coefficient of friction which prevents seizing, galling, and binding. It is also non=hardening and readily compressed. These characteristics give the mandrels a long life expectancy. After 200 bends, mandrels in use have shown no appreciable change in dimensions or shape. Life expectancy is estimated to be approximately 500 bends before wear will affect use of the mandrel. When that point is reached, the mandrel can be remachined to the nearest smaller size practical. The new mandrel is used in bending operations as follows:
i. Cut waveguide tubing to size and file the edges smooth at the ends. ii. Anneal tubing in the area to be bent.
iii. Acid clean the tubing.
iv. Inspect tubing to assure that it is clean and free of foreign matter or chips.
v. Insert the mandrel in waveguide tubing by pushing until the center of the mandrel is in the center of the bend area.
vi. Make the bend in the conventional (cold) manner. vii. Clamp the bent tubing in a machinist's vise.
viii. Extract the mandrel from the bent tubing, using a small winch or puller hooked to the rod in the mandrel.
e. Bends of a smaller radius can be formed with less distortion by using a polytetrafluoroethylene mandrel. Time is reduced by 61 percent because this method eliminates waiting time for the rosin to solidify, steps required to melt and burn out rosin, and acid cleaning after rosin is burned out. It also eliminates damage to bent sections during melt out, as well as the potential fire hazard and noxious fumes caused by rosin.
f. Mandrels of polytetrafluoroethylene or fluorinated ethylene propylene offer a higher degree of accuracy and ease of handling than other types.
4.4.2 Wallace bending machine. The Wallace bending machine involves an extensive set-up time and can be used to advantage only where several sections of waveguide with the same type of bend are being manufactured. The advantage of using the Wallace bending machine is that the waveguide can be bent without using a filler inside the guide. The waveguide, however, must still be annealed on the portion that is to be bent. Some of the special forms that are used with the Wallace bending machine are shown in figures 14 through 18.
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