Single point threading problem

[Hopefuldave]

stevec Wrote:I always believed the reason for 29° was so that the trailing edge of the 60° threading tool would be 1° shy of parallel with the compound. That way all the cutting will be done with the leading edge of the cutting tool.

Not quite *all* the cutting on the leading edge - the odd degree means the trailing edge takes a *very* fine shaving too, which keeps the thread profile clean and gives a nice shiny finish :) It also means that the tool forces act only in one direction to close up any backlash in the leadscrew and half-nuts, and the chip is (almost) all from one edge and curls better (picture a straight-in cut, the chip will be "v-shaped" and won't clear as well).

A lot of people take the final finish cuts "straight in", which has the same effect of ensuring the right profile, and some only ever feed straight in...

Re the topslide angle, it needs to be swung 29 degrees anti-clockwise from "straight in", 61 degrees clockwise from parallel with the ways (assuming an external, right-hand thread).

Re the clearance angle on the side of the tool, I make 1/6th inch (lead) in roughly-a-foot (circumference) to be less than a degree of helix angle, so your 10-degree clearance should be plenty! Bear in mind (if you haven't already) that you need front clearance too, as otherwise the bottom of the tool will be dragging against the trailing edge of the thread (just cut by the leading edge of the tool).

Just my ha'pennorth,
Dave H. (the other one)