On Tue, 6 Mar 2012 13:11:26 -0500, Wild_Bill wrote:

The desired inch/tpi thread pitches may be a bit more difficult if the
machine is made around metric scales on the dials and a metric leadscrew
pitch, but that doesn't mean that inch pitches can't be done.
I don't have the ability (software and I generally aren't on friendly terms)
but some folks brighter than myself have created threading charts for other
lathe models using spreadsheets or other methods, to show that many more
thread pitches can be attained than are shown in a lathe's threading chart.

Good idea!

I went through the available combinations and found one that will give
36 TPI. There were also several other interesting combinations; 2/3 mm
pitch and 4/3 mm pitch sound like they ought to be good for
something...

No 26 TPI though.

So I started trying "what if" combinations. If I had a 26-tooth
leadscrew gear I would have 26 TPI to the same accuracy as all the
usual inch threads on this machine (.0463%). That would require gear
cutting, and I have several things to learn before I try that.

I can also get close (well under 1%) to 26 TPI with a spindle "gear"
of 17 or 19 or 22 "teeth". This "gear" is actually a timing belt
pulley, which is probably more difficult to make than a gear would be.

I didn't explore a new intermediate gear, since that is a double-layer
thing with a belt on one side and a gear on the other.

Bob