Problem Parting Off On A 101.28980 Lathe

[AMDtrucking]

Hello gentleman, I'm new here and need some help.
I also have Craftsman Commercial 101.28980 The bed is 42" long and the cross slide, travels about 6" Is it a 12" lathe or 6"?
There is visible wear on the X-Slide, mostly on the front rail and my cross slide Y-Slide, gets tighter as it goes dipper in away from me. Obviously it is worn on the front. But I've learned to deal with it and not complaining.
However, the biggest problem I have is when I'm parting off a piece. I use KDK tool holders and ISCAR parting blade with inserts and 5C collet chuck, as well as 5" six-jaw Buck chuck. Needless to say, that I have my tool aligned exactly in the center, perpendicular to the work, 50% speed, and plenty of cutting oil. When I'm parting, my work, sometimes jumps on top of my blade and breaks it. When I free the blade and spin the chuck - it seems to wobble, although it wasn't wobbling before. I would unscrew the chuck and screw it back again, then it quits wobbling, until the next parting off accident. Correct me if I am wrong: There could only be two reasons for that. It's either my tool holder is not sturdy enough and it dives under the work, or my spindle bearings are gone. I tightened all the gibbs but it didn't help. The last time I looked at the bearings, was when I replaced the belts and they looked OK to me. Please advise, what am I missing? It only happens under a lot of pressure, when parting. It never happens when I'm turning, even under a lot of pressure.
UPDATE: I forgot to ask: It seems like every time, I move my tail stock, it gets off center. The question is: Do I loosen the nut, that's holding the tail stock, before I start adjusting my screws (one on the front, one on the back), or I could adjust my tail stock, while it's holding my work tight?

 

[wa5cab]

To answer your last question first, loosen the clamp nut, then adjust the tailstock gib screws.

Back to the first question, if you live in the US or US territories, it's a 12". US practice is to class the machine by the maximum diameter of work piece that can be swung over the ways. If you live in Britain or a (former?) Commonwealth country, it is a 6". British practice was to class the machine size by the nominal distance from the inner corners of the ways to the spindle axis. Elsewhere, your guess is as good as mine. In US practice, it would be called a 12x24.

When the parting off problem happens, does the part seem to wobble or the chuck body?

 

[AMDtrucking]

To answer your last question first, loosen the clamp nut, then adjust the tailstock gib screws.

Back to the first question, if you live in the US or US territories, it's a 12". US practice is to class the machine by the maximum diameter of work piece that can be swung over the ways. If you live in Britain or a (former?) Commonwealth country, it is a 6". British practice was to class the machine size by the nominal distance from the inner corners of the ways to the spindle axis. Elsewhere, your guess is as good as mine. In US practice, it would be called a 12x24.

When the parting off problem happens, does the part seem to wobble or the chuck body?

Thanks for my first two questions answered very clear.

When I'm parting off, neither the part or the chuck wobble at all. Only after it breaks the tool, then it starts wobbling. If I remove the chuck and put it back, it doesn't wobble any more.

 

[Mwmx54]

I could be wrong, but it sounds like the threads in the chuck, or the threads on the spindle are loose enough that it's allowing the chuck to come loose, skip a thread is the best way I can describe what I would call it. How tightly do the threads fit on the chuck and spindle.

 

[AMDtrucking]

I could be wrong, but it sounds like the threads in the chuck, or the threads on the spindle are loose enough that it's allowing the chuck to come loose, skip a thread is the best way I can describe what I would call it. How tightly do the threads fit on the chuck and spindle.

The threads in the chuck are brand-new and the threads on the spindle look OK. I use a a truck oil filter remover tool to grab around the chuck and tighten it, followed by few knocks of hammer on the handle. It's tight to the point, that I can switch in reverse and the chuck doesn't come loose. I don't think it is that. Maybe the problem is that I just can't part off on a lathe? I don't know what to think. I'm so scared to part off, when it is not brass, aluminum or delrin that I grab a hack saw and cut it off with it, especially when it is stainless steel, although I stick to 303 and 304 instead of 316.

 

[Mwmx54]

You're not parting off with an upside down tool and the lathe in reverse right? Don't make any cuts were the lathe has to be in reverse, without doing research into how it can kill you. Sorry I'm not much help. Just reminded me of what stripping threads out feels like. Tightens up nice and tight, then you think just a little more.. click too far and the threads are dickered.

 

[wa5cab]

MW's idea is one of three or four that occurred to me. But you (AM) did not answer my question. So I will repeat and add some more that I did not get into my previous because the doorbell rang about that time indicating that our dinner was here.

1) Has this ever happened when you were using the 5" chuck, or only with the 5C collet chuck?
2) After it happens, does it look as though the chuck (and work piece) is wobbling or only the work piece?
3) You said that loosening and then re-tightening the chuck removes the wobble. Do you loosen the collet as well or does it remain tight?
4) How much of the work piece extends up into the spindle (if any)?
5) Approximately how far away from the collet nose is the parting tool?
6) At approximately what diameter does the jump occur?

I will also add that it is not necessary and is in fact undesirable to tighten the chuck to the spindle with anything other than inertia. When you screw a chuck onto a threaded spindle, tighten it by hand until it is about 1/8 of a turn (45 degrees) shy of being tight. Then grasp the chuck jaws or the collet chuck nose with your right hand and with your wrist and elbow smartly or rapidly twist it home. You should hear a snap or a ringing noise as the rear face of the chuck hits the shoulder part of the register (solid flange on the nose of the spindle).

 

[AMDtrucking]

MW's idea is one of three or four that occurred to me. But you (AM) did not answer my question. So I will repeat and add some more that I did not get into my previous because the doorbell rang about that time indicating that our dinner was here.

1) Has this ever happened when you were using the 5" chuck, or only with the 5C collet chuck?
2) After it happens, does it look as though the chuck (and work piece) is wobbling or only the work piece?
3) You said that loosening and then re-tightening the chuck removes the wobble. Do you loosen the collet as well or does it remain tight?
4) How much of the work piece extends up into the spindle (if any)?
5) Approximately how far away from the collet nose is the parting tool?
6) At approximately what diameter does the jump occur?

I will also add that it is not necessary and is in fact undesirable to tighten the chuck to the spindle with anything other than inertia. When you screw a chuck onto a threaded spindle, tighten it by hand until it is about 1/8 of a turn (45 degrees) shy of being tight. Then grasp the chuck jaws or the collet chuck nose with your right hand and with your wrist and elbow smartly or rapidly twist it home. You should hear a snap or a ringing noise as the rear face of the chuck hits the shoulder part of the register (solid flange on the nose of the spindle).

I'm sorry, I guess I didn't understand your question the first time around. I apologize for that. I'll try to answer them in order:

1) It happens with every chuck I own: 5" 6-jaws Buck chuck, 4'" 4-jaw no name chuck and my brand-spanking-new chinese collet chuck.

2) The work piece was still being held tightly, by the chuck and wobbled together with it.

3) If I remember correctly, I unscrewed the chuck to see if my spindle threads were damaged. The threads were fine and I screwed the chuck back on the spindle. Then I noticed, that it doesn't wobble anymore. My work piece was still in the chuck.

4) Since I was parting off 3 inches of a foot long rod, 9 inches were inside the head stock bore.

5) So close, that it left a mark on my collet.
6) the rod was 1/2". It "drove" onto my tool and broke it, when I was half way trough. So, I'm going to say: 0.25"

I will definitely take a note of your suggestion on how to tighten the threaded chuck and will use it in the future. Thank you. I honestly did not know that.

At first, it was parting just fine. I liked those nice looking little rolls of chips coming out. I even thought of engaging my cross slide auto feed.

And all of a sudden - kaboom!

My $10.00 Iscar insert is gone and the $78.00 blade is bent. The chuck jammed and stopped turning.

 

[AMDtrucking]

You're not parting off with an upside down tool and the lathe in reverse right?

Right.

 

[mikey]

Looks to me like you're doing everything right, AM. If the work is climbing on top of the blade then there has to be clearance somewhere to allow that, which makes the spindle bearings suspect or excessive slop in the saddle/compound gibs. Another possibility is that the tool holder is moving in your tool post. Try eliminating these things first.