I don't think I have the right words to explain.
Too long, don't read warning....
The thread dial indicates the phase of the cutter with respect to
a thread being cut.
With the half nuts unlocked moving the carriage or rotating the
spindle changes the phase.
With the inch lead screw cutting inch threads the spindle the
ratio between the spindle and the lead screw is simple. More
importantly, you can lock the half nuts anywhere the appropriate
mark lines up and be in sync with the thread you are cutting.
About every 1/8th inch or so. I think. Need to check. In any event
-- places you can lock the half nuts are separated by a fraction
of a lead screw turn. HTRL has some notes on which marks you can
use for which threads per inch (multiples of 2, 4 or 8tpi make a
difference). The marks where you can engage the half nuts are the
ones where the spindle has rotated back around to where you can
pick up the thread.
Add the metric transposing gears, or one of the approximations,
and the ratios get really messy. So messy that there is no simple,
usable, relationship between the phase of the spindle and the
thread dial indication. Spindle rotations don't divide evenly into
lead screw rotations.
There are three solutions to cutting metric gears that I have
1) Don't disengage the half nuts while cutting threads. When done
with a pass run the lathe backwards to return the carriage to make
another pass. This is the most commonly suggested method. Somebody
is sure to pipe up that they prefer doing this with inch threads
even if they have a thread dial. This definitely works.
2) I have heard (but not tried) that starting with the carriage
at the same position (perhaps using a carriage stop to the right
of the carriage, or the tail stock as a carriage stop) AND always
using the same mark on the dial (perhaps the 0 mark) will work.
This makes sense to me mathematically - I won't bore you with the
3) I have also heard (but not tried) of a method where you turn
off the lathe at the end of a pass, note the thread dial position,
disengage the half nuts, move the carriage right to a position
where the thread dial is the same as where it was when the half
nuts were released and engage there. Then turn the lathe back on.
I think there may be more to it than that -- it seems at least
semi plausible to me.
The safe thing is to go with #1.
I'm amazed if anybody has managed to follow all of this. The math
of this fascinates me, but if I start blathering about
Stern-Brocot trees, prime factors, smooth numbers, least common
multiples and largest common factors I'll deserve to be banned
from this list.
On 1/3/21 10:56 AM, john kling via
Hm I don't feel particularly
sharp today and have not in the past thought about why the
thread dial works - But would the imperial still coordinate
the position of the carriage and the lead screw in metric
operations on the imperial lathe. -if this is completely
I have never heard of a metric thread dial. When
cutting metric threads with an imperial lead screw
lathe, one normally engages the half nuts and leave the
half nuts engaged until you are done cutting the thread.
Do not disengage the half nuts. Reverse the lathe to
return the carriage to the starting point after backing
out the cross slide.
On Jan 3, 2021, at 9:57 AM,
Davis Johnson <davis@...> wrote:
The metric thread dial is intended for use with a
metric lead screw, not transposing gears.
Conventional wisdom is that the normal threading dial
is not useful with the transposing gears, no doubt why
you are interested in the metric thread dial.
I have read a contrary opinion, that it will work if
you reposition the carriage the same place for every
pass (perhaps a carriage stop to the right of the
carriage? might have to go on the rear v-way.) AND
start on the same dial mark each time. It sounds
reasonable to me.
I have been wanting to try this for some time, but
haven't done it yet. Next best thing for me would be
for you to try it.
1/3/21 8:58 AM, Al Costich wrote:
For some silly reason, I am in the hunt for a metric
thread dial for a 9A.
Some time ago I acquired a 127/100 transposing gear set
and some additional
Thinking forward, i would like to single point some
I'm not confident enough to make a thread dial myself.
So, should anyone have one that is superfluous to their