Thread turning question

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.
Reply to
john j
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Greetings John, 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. Cheers, Eric
Reply to
Eric R Snow
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 particular aplication?. 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 happening. 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 place.
Reply to
Ken Davey
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.
Reply to
Jon Elson
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 volia!...
--.- Dave
Reply to
Dave August
I agree.
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.
Reply to
Roger Shoaf
You dont have a die? A die is far far easier to do then trying to do this on a lathe
Rule #35 "That which does not kill you, has made a huge tactical error"
Reply to
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 ;)
Reply to
Rex B
I STRONGLY suggest a bottle of dychem and a thread file or a good 3 cornered swiss file.
Rule #35 "That which does not kill you, has made a huge tactical error"
Reply to
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!
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Reply to
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 time . If thread is 1/8" deep , advancing blade will always take out 1/16" 1) left 2) Mid 3) right 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 .
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