By the time you get spindle & LS at exactly the right place you'll
have spent far more time than if you simply reversed with the LS
engaged. Cutting integer imperial threads with an imperial LS there
are lots of positions where things line up. When you start getting
into n 1/2 threads (like some pipe threads) it gets more
restrictive. When you're cutting bizarre decimals, like metric
approximations, it's functionally impossible to get things lined up
for subsequent passes.
In practice, it's quicker to cut threads on these machines by using a
handcrank on the spindle & keeping the half-nuts engaged. If it's a
relatively long thread, you can use the motor to return to the
starting point for each pass, taking care to not get hit by the
handcrank.
Roy
--- In 7x12minilathe@..., "steam4ian" <fosterscons@...>
wrote:
G'day John et al.
Thank you for replying. I have had a few moments to digest what you
are saying. Spindle angle is also important in the equation.
Thus the Spindle must be at a set angle, the saddle at a set
position
and the LS at a set position for a repeatable outcome. The set
position of the LS is implicit in the half nuts being able to close
(assuming a single start thread on the LS). Missing from my
assessment was the spindle angle.
So you set up by putting the saddle against the stop, rotate the
lathe by hand to engage the half nut, mark the spindle against a
reference, mark the LS against a reference then take the first
pass.
Subsequent passes would require the saddle to be against the stop,
the spindle to line up with its reference and the LS marker to line
up with its reference, engage the half nuts as the marks line up.
Please comment.
One good turn deserves another.
Regards,
Ian
--- In 7x12minilathe@..., John Dammeyer <johnd@> wrote:
Hi Ian,
You are making an assumption that when the lead screw is at a
specific
point when the spindle is at an equally repeatable location.
That's
essentially how the thread chasing dial works. If you were
cutting
a
thread that required engaging the half nut at any line it means
that the
lead screw and spindle are linked in multiples of your lead screw
pitch.
But if you are cutting a thread where the #3 position and only
the
#3
position is the right place it's possible that the spindle is 90
degree
offset for the #2 position, 180 for the #1 position and 270
offset
for
the #4 position.
Now follow that same line of thinking for metric and you may find
that
the nut would have to be engaged at the #1 position the first
time. The
1.27 position the next time and so on.
John Dammeyer
Automation Artisans Inc.
Ph. 1 250 544 4950
