South Bend Toolroom Lathe Tool Post Keeps Rotating while cutting

Does any person have a good fix for keeping the tool post on the lathe from rotating while cutting? Especially when I am using a cutoff tool, the tool post rotates no matter how tight the tool post nut is. I have an Aloris quick change tool post. Typically, I dial in the face of the tool post, tightened nut on the T-nut. Because my cutoff tool is hanging out a ways from the center of the tool post, it causes the whole tool post to rotate. Any ideas on modifications to the tool post to prevent the rotating of the tool post?

First of all check the top of the compound where the tool post sits for flatness, ...over time the tee slots can be damaged so the tool post is only riding on a few high spots.

My first thought is the tee nut below the compound slot ? and as above is it flat and no high spots.

That really should not be an issue, especially on that size/type lathe, if things are set up right.
Some things I would check first include the T nut and the mating surfaces of the tool post and top of the compound. If the T nut is too thick, or improperly made, it will bottom out on the bottom of the tool post before the tool post is pulled securely down. There should be a gap between the top of the T nut and the tool post WHEN EVERYTHING IS TIGHT. Also, if the bottom of the tool post is convex, the top of the compound is worn, or the area around the stud is pulled up, then the tool post could be bearing only on a small area in the center. This would also limit it's holding power against rotation. Check that the tool post sits flat on the compound and has even bearing all over.

On edit: others beat me to it, I guess I type slow!

Rather than rewrite previously made suggestions:
https://www.practicalmachinist.com/...ee-nut-fit-lathe-compound-291143/#post2376382

Relieving the center bottom surface of the toolpost does improve its grip even if the top of the compound is perfect---see above thread.

Denis

Thanks for all the input!!! I will check for flatness and repair if needed. Being that this a newer South Bend made overseas, I assume the compound probably is not flat and I'll check T-nut and everything else. Thanks again.

Another thing to check is to make sure you have plenty of good threads on the bolt that you are tightening that goes through the holder and into the T nut. If you are running out of threads or the threads are bad the nut may tighten up without tightening the holder down. An easy way to verify is just to put a couple of washers under the nut and see if that makes a difference.

Just a thought,
Ted

Was having the same problem on my Nardini lathe, my friend suggested putting a thin piece of aluminum between the tool post and compound for some grip.

Does the toolpost have a 3/4" or larger stud going through? If not, I doubt there is much you can do to make a puny bolt not stretch because it is being tightened to its yield point.

I assume all toolposts have a serrated finish on the bottom side? They need to. A dowel pin could be used but that limits quick adjustments to the toolpost position.

Many times, either the Tee-nut was not made correctly or there are not enough threads on the stud. I can't tell you how many tee-nuts I've made or remade over the years.
JR

HuFlungDung said:

I doubt there is much you can do to make a puny bolt not stretch because it is being tightened to its yield point.

Click to expand...

Good one.. On a Wells Index, 4 5/8" studs hold the head from tilting side to side.. Tighten 'em,
and a few days later the head was off and they were loose again.. Rinse/repeat.. Finally pulled the
studs out, nice hour glass shape, they had reached that yield point.. Tighten 'em, and they would
just loosen/stretch.. Replaced them with some grade 5 studs, never had a problem again...

Is the nut loose after the tool post moves?? If it is, I'd replace the stud. And also everything
else everybody has already said.

There is a root cause here and I highly doubt that it is because a smaller stud is not tightened enough. First a caution, before just tightening down on that stud, look at all the photographs of broken Tee slots on lathes and mills and other machines. Over torquing is a SURE path to a broken Tee slot. DON"T tighten it to it's yield point: that will break it after two or three such tightenings. NO bolt or stud should ever be tightened to it's yield point unless that is specifically specified by the design engineer and then it is considered a single use bolt and is thrown away if loosened.

Try to find that root cause. I highly suspect that it is a high spot on either the compound's Tee slot area or on the tool post itself. Such a high spot would act as a pivot point and would prevent the outer area of the tool post from contacting the compound and stop the rotation. I would check for that first.

Remove the stud and Tee nut. If the stud will thread all the way into the hand held Tee nut or at least 3 or 4 turns, then it is probably OK and I would not worry about that.

If you must take a make-shift measure, I do like the aluminum gasket idea. I would use aluminum foil as it will not compress much over a period of days or weeks. And I would cut out the center and use only the outer 1/4" or 3/8" periphery for the grip area. This should overcome a moderate sized high spot in the center.

For the long run, I would make sure that the top of the compound is flat and the bottom of the tool post is slightly concave. That will ensure that it rests on it's edges, not on a central high point. This alone should prevent any rotation. You should not need any aluminum gasket or need to over torque the stud.

true temper said:

Was having the same problem on my Nardini lathe, my friend suggested putting a thin piece of aluminum between the tool post and compound for some grip.

Click to expand...

EPAIII said:

There is a root cause here and I highly doubt that it is because a smaller stud is not tightened enough. First a caution, before just tightening down on that stud, look at all the photographs of broken Tee slots on lathes and mills and other machines. Over torquing is a SURE path to a broken Tee slot. DON"T tighten it to it's yield point: that will break it after two or three such tightenings. NO bolt or stud should ever be tightened to it's yield point unless that is specifically specified by the design engineer and then it is considered a single use bolt and is thrown away if loosened.

Try to find that root cause. I highly suspect that it is a high spot on either the compound's Tee slot area or on the tool post itself. Such a high spot would act as a pivot point and would prevent the outer area of the tool post from contacting the compound and stop the rotation. I would check for that first.

Remove the stud and Tee nut. If the stud will thread all the way into the hand held Tee nut or at least 3 or 4 turns, then it is probably OK and I would not worry about that.

If you must take a make-shift measure, I do like the aluminum gasket idea. I would use aluminum foil as it will not compress much over a period of days or weeks. And I would cut out the center and use only the outer 1/4" or 3/8" periphery for the grip area. This should overcome a moderate sized high spot in the center.

For the long run, I would make sure that the top of the compound is flat and the bottom of the tool post is slightly concave. That will ensure that it rests on it's edges, not on a central high point. This alone should prevent any rotation. You should not need any aluminum gasket or need to over torque the stud.

Click to expand...

You don't have to over-torque the stud if it's 3/4" If you make a good fitting T nut that is as long as the slot in the compound, and the toolpost is sitting directly on top of the compound, there is no way in hell that tightening the toolpost down is ever going to break the top out of the T slot. That happens with abuse where the toolpost gets wacked hard by something.

I don't think I saw anyone mention the length of the clamping bolt. If the bolt is too long, it can bottom out in the compound slot, with the T-nut hard up against the T-slot, but the bolt will not be clamping the toolpost down with enough force at the top. It could be a very small amount too long, but this would be another possible cause. If it is bottoming on the T-slot, there may be obvious marks.

specfab said:

I don't think I saw anyone mention the length of the clamping bolt. If the bolt is too long, it can bottom out in the compound slot, with the T-nut hard up against the T-slot, but the bolt will not be clamping the toolpost down with enough force at the top. It could be a very small amount too long, but this would be another possible cause. If it is bottoming on the T-slot, there may be obvious marks.

Click to expand...

Yup, seen it.

Dimpling the hole (with a prick punch) in the bottom of the tee nut doesn't seem to hold up.

Placing the stud in the proper amount, and then a small tack weld does work.