Originally Posted by
craigmx5
Or, you could do the calculations (from Machinery's Handbook).
The single-pitch (single form) thread mill is capable of cutting a range of thread pitches, say from 12 to 28 tpi. The form of the cutter is not a sharp vee, but rather carries a radius (or flat) suitable for the root of the thread at the smallest pitch in the range. To mill a thread with a coarser pitch will require it to make a deeper penetration to get the appropriate clearance for the crest of the external thread mating with it.
The specifications in the Machinery's Handbook are calculated from a theoretical sharp vee thread. The flat or rounded root of the internal thread is to be 0.125Xpitch, and occurs at 0.125X the full thread height of a sharp vee thread. The full thread height for a 13tpi thread is 0.0662”, and 0.125 times that is 0.008”. So, with a sharp vee thread mill, it would be necessary to cut the internal thread on a radius 0.008 (0.016 diameter) greater than the major diameter called for, or 0.516”.
But, your thread mill is not a sharp vee. If the flat dimension is not provided by the vendor, figure it to be 0.125 X the finest pitch the thread mill is designed for. Then, do the heavy math (trig) to determine what your additional cut depth needs to be to get the 0.0096 width at the 0.500 major diameter. It will be less than the example above with the sharp vee.