If the angular groove in the part requires the processing of a triangular file, but only one side with a notch (nearby to not participate in it), tape it temporarily any adhesive tape (e.g. Scotch tape or duct tape).
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By analogy with existing devices for welding and brazing working "with the light", this device is also called a burner. However, it acts on the mating parts do not flame, and hot air...
