Surprise, once I return the carriage to the beginning of the screw, I can't
register the cutting tool on the next pass with my first cut.
This is on my new old lathe (Reed Prentice, 100 yrs. old), the Monarch at my
last job had a nifty indicator just to find the proper moment to engage the
screw, but what does one do without that.
And there must be a decent way cause they had screws 100 yrs.
Worst case you can leave the half nuts engaged and put the lathe in reverse
to wind the apron back for the next pass. If you don't have a threading dial
it is possible to make one. Google tells all. - GWE
They had thread dial indicators like that 100 years ago!
Seriously, without a thread dial indicator, you need to leave
the half nuts engaged and reverse the spindle to bring the
carriage back to the start point.
There is a very neat article in HSM magazine describing how
to fabricate a thread dial indicator for nearly any machine,
it ran about two years ago or so. Maybe another poster has
There is a way to do it without a thread dial indicator *and* be able to
disengage the half nuts.
Once all is set up to make the first pass position the carriage where you
want to start (LATHE OFF).
Pull the lead screw to the LEFT by hand to cancel out any end play.
Engage the half nuts (hand rotate the chuck until they drop easy).
Position a carriage stop against the right of the carriage. Make a register
mark on the chuck/workpiece/or whatever (top, side or whatever is
Make your first pass (just a scratch pass to test the setup).
Disengage and return the carriage to the stop.
Lathe off now.
Again pull the lead screw to the left..
Hand turn the chuck (normal direction) until the register mark lines up in
the original postion.
At this point the half nuts should engage. If not rock the chuck back and
forth slightly until the half nuts drop easily into engagement.
All should be well now to resume threading.
Do another (or several more) scratch cuts to verify.
For left hand threads reverse left/right hand instructions.
Hope this helps.
All the other posters are correct: without a threading dial, you will have
to leave your half-nuts engaged throughout the entire threading operation;
reversing the spindle between cuts to back the carriage up to the starting
point for the next cut. (Btw, you must also use this procedure if cutting
metric threads on a lathe with an inch-calibrated threading dial.)
However, they failed to point out one critical procedure. (I'm sure they
assumed you knew this, and you probably do.) Make sure you completely
disengage your cutting tool from the workpiece thread you're cutting (by
backing off the cross slide or compound) before reversing the spindle. It
might seem that this would be unnecessary since reversing the spindle (with
half-nuts engaged) should cause the tool to follow the path of the
previously cut thread backwards to the starting point. The problem is that
tolerances in the lathe drive train will result in the reverse path not
coinciding exactly with the forward (cutting) path. If you fail to withdraw
the cutting tool from the work, then at best you will bugger the thread you
are trying to cut; at worst, you will bugger the thread AND you will
damage/break your cutting tool.
Good point, except for one minor detail: Do not back off the cross slide.
Do all the backing and feeding with the compound. Feeding with the cross
slide will usually result in the point being broken off the tool...
No, one uses the cross slide for that, and feeds successive passes with the
compound. By setting the cross slide dial at 0 you have a ready reference
to which you return each time. It's not only faster, but easier. That's
the way threads are properly chased. By the way, it's a very good idea to
set your cross slide such that the handle registers at roughly 10:00 and
then set your 0 reference. That way if you're chasing a thread that
requires a rapid withdrawal of your tool (such as chasing to a shoulder)
it's very easy to withdraw the tool. After setting your cross slide and
dial, pick up the item to be threaded by feeding the compound until the tool
Yep. It's a sort of bicycle pedal thing. You can find a spot to
put the handle such that an applied force can get it going in
either direction, with about as much effort.
Not that I've ever *done* that of course. But it sure does make
a mess when it happens!!
IIRC, there is one case where one does not need to use a threading
dial - that is when one is cutting the same tpi as the lead screw.
I think you can close the half nuts at any time then, and the
tool will pick up properly.
The only way I know is to back the tool out, and run it in reverse
to get it back to your starting point - without disengaging the carriage
from the leadscrew, of course. If there's another way, I'm looking forward
to learning it.
This won't always work. Imagine cutting an 11 TPI thread on a
machine having an 8 TPI leadscrew. Let's say you cut exactly 5
threads, having progressed axially 5/11 inch. The leadscrew will
have turned 5/11 * 8 turns or 3-7/11 turns. The register mark on the
workpiece is in the right place but the leadscrew won't line up with
the halfnut when you return the carriage to the start position.
A method that would work is to use a 1" dial indicator on the carriage
and have it always travel exactly 1 inch (or an integral number of
inches) even if it isn't cutting the whole way. For threads
divisible by 2 it could be an integral number of half inches, for
threads divisible by 4 it could be an integral number of 1/4 inches,
and so on.
I figure that when copious spare time becomes available (yeah, well...)
making a threading dial for add-on to the old FE Reed is a project well
worth trying. I suppose one might be able to bastardize one from another
lathe with the same pitch leadscrew, but that would take a pretty good
BTW, the fractional inch trick only works if the thread pitch is
divisible by a factor of the leadscrew pitch. For an 8 TPI leadscrew
the factors are 2, 4 and 8. If you had something wierd like a 9-TPI
leadscrew, then you could cut thread pitches divisible by 3 by moving
in increments of 1/3 inch.
When cutting metric threads with a TPI leadscrew or inch threads with
a metric leadscrew, the only course is to leave the halfnuts
engaged from start to finish.
There is one other way, using a single point engagement dog clutch
like hardinge uses on the HLVHs.
I'm pretty sure that inch/metric HLVHs have a single leadscrew, and
it's english. And the threading is carried out in the same way
in either inch, or metric mode.
Basically the leadscrew phase is always maintained when its feed
clutch is released. When it closes back up, the spindle and
leadscrew phase is exactly the same each time.
On a HLV, the half nuts are never opened during threading.
You power forwards and backwards. Rather ingenious, very quick, bit
hard to get used to if you have used the normal style.
"That which does not kill you,
has made a huge tactical error"
The half-nuts are always closed, right. But the top lever
actually unclutches the leadscrew from the spindle! Think
about it, the spindle is turning, and the leadscrew is stationary
when the lever is straight up.
The only way I could do that with my southbend (halfnuts closed
of course) is to put the reverse tumbler in the center postion.
Once I do that, I've lost the thread lead, if I re-engage it
again the thread dial won't be right. The gears can engage
at any random point besides the point where they were the first
But because the hardinge has a *single*point* engagement clutch
on the shaft that feeds the gearbox it can only drop in in one
position - which preserves the relative phase between leadscrew
and spindle. Ingenious. The only other lathe that I've ever
seen do that, was a round bed Drummond, I think.
I always wondered how hard it would be to retrofit a system like
that to my south bend.
It was clear that hardinge bros figured out that threading was a
'big deal' and designed their machines to do it well. Even their
very old toolroom lathes (like that one that sold on ebay recently)
had the quick-retract for the compound slide, so that threads
could be done up near a shoulder easily.
I'm intrigued by this concept (especially since I cut mostly metric
threads)! Two questions:
1. Does it matter where in the leadscrew power train that the single-point
engagement clutch is located? For example, does it need to be at the input
(where the spindle drives the gear train), or could it be at the output
(where the QC gearbox drives the leadscrew), or perhaps even somewhere in
2. After you disengage the clutch (spindle still turning, but leadscrew
stationary) how do you reset the carriage position since the half nuts are
kept engaged? I'm guessing there is some additional reverse drive mechanism
for the leadscrew. Do you just throw the tumbler into reverse and re-engage
the single-point clutch? The disadvantage there is that the reverse motion
of the carriage would be no faster than the forward. Or is there a
higher-speed reverse driven by the spindle or, perhaps, by an electric