------ Original Message ------
From: "DJ Delorie" <dj@...>
To: [email protected]
Sent: Thursday, 16 Feb, 23 At 18:48
Subject: Re: [SouthBendLathe] Metric threads/Imperial threads

"Andrei" <calciu1@...> writes:

All this discussion of metric threads has raised a question in my mind. Sorry for being simple, but the only metric threads I ever cut were small enough that they were done with a tap or a die, never on the lathe.

Did you know that spark plug threads are always metric?



That's a spark plug plug for servicing chain saws. I don't recall the
exact thread, but finding a die for it would likely be impossible.

Part 1 - Metric leadscrew. How do you cut imperial threads? Part 2 - Metric leadscrew. How do you cut metric threads? Is it as straight forward as imperial on a imperial lathe? Part 3.- Imperial leadscrew. Allan had a great explanation for how to cut metric threads. Thank you.

The key thing to understand is what the thread dial does. It indicates
the *difference* between the cut thread and the tool. In general, it
does around once per four inches of leadscrew (and thus tool) travel.
The gearing determines how many times the spindle goes around in those
same four inches; an 8 TPI thread would go around 32 times in 4 inches.

A metric lathe would have a thread dial that goes around, for example,
once per 12cm.

When doing "matched" threading (imperial-on-imperial or
metric-on-metric) you can disengage the thread dial, move the carriage
by some whole number of threads, and re-engage the thread dial; without
messing up your threading operation. The key is "whole number of
threads". If you have a whole number of threads in 4 inches, starting
on the "same number" always drops you into a thread, etc. Same for a
whole number of threads per 12cm for metric lathes.

The problem with mis-matched threading is that there is no metric thread
that is a whole number of threads per 4 inches (or the reverse) so
there's no way to move "a whole number of threads" away. Hence the
extra effort taken to keep the thread dial in the "same leadscrew
thread" so that it stays on the one cut thread you started with.

If you had a leadscrew with 9 threads per attoparsec (tpap), you could
turn any thread that was a multiple of 9 tpap (like 27 tpap) but you'd
need to use the "mis-matched" method for either inch or metric threads,
assuming the change gear fit in your local spacetime.

I suggest using an electronic leadscrew for those ;-)