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OK, I had a go on a piece of scrap today. Everything went fine until I
cut the first pass at 40 tpi. Re-reading the chart I corrected the
gears and tried cutting ostensibly 20 tpi. I did, BTW what you
suggested and run the lathe under power both ways just finishing the
last three threads manually.

The big disappointment was that the thread ended up 21 tpi. Once I was
able to get the die on it I was able to correct that with a resulting
pattern of periodic double cuts and a variable major diameter (reminds
me of beat frequency oscillators). I can get the nut on but it is not
pretty!

The other thing that I did not expect was that starting with a rod of
0.493" the final major diameter (before the die application) was
0.507". Even after filing it was still 0.502".

So the big question: Is there a reason for the 21 tpi *other* than
change gears?

Michael Koblic,
Campbell River, BC

BTW what's with the deafening silence on the question as to why these
threads are UNF rather than UNC? Could it be that it does not matter?

You want me to guess this from the other side of the continent?

If the bit didn't cut freely it just might have pushed metal forward,
increasing the diameter and shortening the rod. Did you sharpen the bit edge
to at least the full depth of the thread?

jsw