When machining threads on a lathe, a “Fishtail” is used to “square” the tool to the part. For external threads, the Fishtail’s long edges are held against the O.D. of the part, while the tool is aligned to fit in one of the Fishtail’s “V” notches. When cutting internal threads on a short workpiece, it is often easier to use the fishtail, with its end against the face of the part, as shown in picture #1. However, first, check that the end of your fishtail is square with its longer edge (see Pic #2).
Tom Nichols
Automotive Machine & Supply, Inc
Cleburne, TX
