If I have a threadded shaft that needs the threads cleaned up by
re-tracing the threads on a lathe, what is the best method of
realigning the part to the threading tool? Threads are standard 60
degree Vee type, 28 threads per inch.
Don't know if it matters, but I have a Smithy CZ-239 lathe.
I will assume you know how to make sure the part runs true and that
the threading tool is on center and square to the part. Set up the
gearbox to cut 28 TPI. With the tool away from the work engage the
half nuts and turn the spindle slowly, by hand or at low speed. When
the tool is past the end of the part stop the spindle and crank the
threading tool in towards the part. The tool will be off a little so
use the compound to advance the tool until it fits perfectly in the
existing threads. This will require cranking in the cross slide a
little, then cranking in the compound a little, etc. Use a magnifier
to make sure. Once the tool is bottomed out set the cross slide dial
to zero. Now back the tool out of the threads. If you only have one
part to do run the lathe in reverse with the half nuts engaged until
the tool is past the end of the part .100. Crank the cross slide in
but not all the way to zero, say .020 over. Now turn the spindle by
hand and watch carefully (I'd use that magnifier) to make sure the
tool is centered in the threads. You may have to adjust it a little
with the compound even though you already did that. Back the tool out,
run the lathe in reverse, or turn the spindle in reverse by hand,
until the tool once again clears the end of the part by at least .100.
Crank the tool in about .010 over and turn the spindle making sure the
tool is centered with the threads. So what you want to do is sneak up
on the part to avoid scrapping it. After you have a little experience
and get to know your machine it will be lots easier and you won't need
to be so cautious. I'm assuming, since you asked this question, that
you don't have lots of experience threading. That is why this reply is
so long and detailed.
OK - I'll have a go at this.
Assuming you do not have an appropriate die or thread-restoring tool (which
your first approach should be) this is how I would do it.
The shaft in question has center holes at each end Yes/no.
If yes; Set up to turn between centers.
If no abandon all hope - or at least do not expect uptown accuracy - and
that is needed information - just how critical is accuracy in this
With buggered threads there is no way you can pick up an accurate center to
do this on a lathe without pre-existing center holes in the shaft.
Lacking said center holes skip to the last sentence in this post.
Having the above mentioned holes and having set up to turn between centers;
Your tool is mounted accurately as to angle and center height.
Using the cross and compound feeds eyeball the tool into the thread (power
off and all lash taken up).
A piece of white paper placed under the work will help to see what is
Once you have eyeballed the tool into what seems to be the right place (the
lathe is, of course, set to cut a 28 TPI thread) run the lathe (VERY slow or
by hand) along the shaft/thread. The worst part of the buggered thread may
get shaved a little. Observe and adjust until it seems like the tool will
follow the original (undamaged portion) of the thread. Advance the cross
slide a half thou or so and run it again. This will indicate whether or not
you are getting it right. An even shaving (actually just polishing of the
undamaged thread is what you are looking for. Carefully approached it is
possible to restore the thread without re-sizing the whole thing.
Having said all that it might be the better part of doing things to simply
machine a whole new shaft. That is why you have the lathe in the first
Set up the correct thread pitch on the lathe, and engage the
leadscrew. Turn the spindle over by hand a turn or two to get
the gear train to take up slack. Advance the tool until the
point is near the work. Adjust the compound and crossfeed
together to bring the point into the center of the existing thread.
If you have an aligning gage (this is a little stamped metal
piece with some 60 degree Vees cut in the sides) you can use that
to align the angle of the cutter to point straight into the work.
If not, you can use the existing threads. A small magnifier or
Jeweler's loupe is real handy for both parts of this alignment.
what you really need is a thread chaser.. usualy sold as a square bar with
'teeth' on it... again usually with two different pitchs of thread on either
end on all four sides.. i.e. 8 different pitchs.. you manually hold the
appropriate size 'chaser' against the damaged threads.. (severl times) and
It is easy to screw up trying to learn to chase threads with the single
point, but running a die to clean them up, using the thread tool Dave
suggested or even using a small triangular file is much less of a risk.
Of course plan "B" would be to cut off the boogered end of the shaft and
drill and tap to install a stud.
Well, yes, unless the boogered part is close to the start of the threads.
I have the same issue with an Atlas 618 I'm restoring. Despite my care
at disassembly (brass drift etc) somehow the spindle threads became
unhappy on both ends - near the start. The left side threaded collar
would only thread on about 1/2-3/4 turn.
The left end is 3/4-16, so I got a good die for that. Starting it
straight was very difficult. I used a lathe and tailstock to align them,
though I did not have a die holder for the tailstock. I used the flat
end of the tailstock spindle against the die, with the spindle in the
chuck. When it felt right I turned away. I ended up with usable but
imperfect threads. It essentially cut "phase-shifted" threads, so the
nut fits loosely.
Now I need to fix the chuck threads, last thing needs doing to this
lathe. The check threads on about 1 turn, then stops. I cannot find a
visible burr on the spindle threads. The chuck threads onto another
spindle just fine.
I can order a thread file, or I can set up the change gears to 10 TPI
and let it clean up it's own spindle. I've also chucked this spindle
into another lathe and tried to clean it up using the thread-chasing
technique described above, but never could get it to track correctly
before I ran out of patience (not one of my admittedly few virtues).
I keep hoping a spindle will turn up on ebay so I can avoid this ;)
With your job set up in the lathe, correct pitch setting, and 60
degree thread cutter, set the toll post cross slide to be parrallel
with the job.
move the cutter close to the job, and engage the feed, now adjust the
other cross slide so that the cutter lines up with the threads.
start the lathe up keeping feed engaged all the time, double check
that the cutter is running in the correct place and your pitch is the
same as your old thread.
Commence cutting as per normal!
No you wont be taking out more metal on the advancing
face of bit .
Suppose you could cut a thread with 4 passes .
Take out 3 passes w/o moving cross slide , then cut final
with cross slide moved to final position .
This makes the advancing blade take same bite each
If thread is 1/8" deep , advancing blade will always
take out 1/16"
Now advance cross slide
4) cut final pass
You use Cmpnd to select a small area to take out .
Dont take out to much at one time .