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Craftsman 618 Half Nuts Disengage While Auto Feeding

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DaveBarbier

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#1
Hi everyone, it appears as though my lead screw is worn right at the spot I want to use to do a simple boring job with the auto feed. Mic'ing my lead screw on the ends (still on the threads) I get .498", but right at the spot where it "pops out" it's .491". Is that normal?

If I engage the half nut at the far right end of the bed and give a little back pressure to the carriage it powers through with no issue. But when I do the same where the worn area is, I can stop it easily and watch as the half nuts lift over the threads.

I removed the half nut and cleaned it as well as the entire lead screw.

I see lead screws on eBay but before I buy I want to be sure mine is overly worn and a non worn lead screw should fix the issue.

Thanks in advance!
 

markba633csi

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#2
I'm guessing the threads are worn enough to be rounded- have you looked closely at the thread profile?
Atlas 618 (not sure about other models) leadscrews seem to be rather soft- I had to replace mine too with a better used one off Ebay (about 25$)
Mark
 

westsailpat

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#3
I don't know so I'm watching with interest . .007 doesn't seem like all that much , when I got my Craftsman 6" I also got a box of parts there was a half nut in there and it was really worn out . So my thought is maybe it's a combo of nut and screw . Or maybe just a new half nut would make the fix .?
 

Glenn Brooks

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#4
Can you make consistent cuts on round stock? My new to me SB Fourteen wouldn't hold a consistent OD when turning a test shaft. Sometimes varied by .010", and it was absolutely all over the place. Nothing consistent at all. The leadscrew looked OK,but had some moderate wear in the middle, where all the machining takes places. I had a new leadscrew and bronze nut made up, and now the lathe is dead on - every time. Cost about $150 for both parts. Personal opinion - buying used parts off eBay is just asking for instant wear and tear, better to have sometihing like a leadscrew screws and nut made up new, to proper tolerance.

Glenn
 

wa5cab

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#5
The only two circumstances that I can think of for there to be any difference in the OD between the worn and unworn area is a significant number of hours of the half nuts popping open and sitting grinding on the screw. And hundreds or thousands of times closing the half nuts when they weren't aligned with the threads. Under normal circumstances, the thread flanks wear but not the diameter. Starting to do it now certainly won't help your problem any but the latter case is why I haven't disengaged the threading dial but once or twice in more than 35 years. I watch the dial and engage the nuts on the mark. And never grind on the threads and nuts.

Anyway, I'm inclined to agree with Glen. I would be leery of a used lead screw, too. Unless I knew how it was used (or not used). I have a spare lead screw for my 3996, but I only kept it because I knew that the lathe that it came off of belonged to a transformer manufacturer. And aside from the rare times when the machine was actually used to turn or thread something, knew that it wasn't being used.
 

DaveBarbier

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#6
I'm guessing the threads are worn enough to be rounded- have you looked closely at the thread profile?
Atlas 618 (not sure about other models) leadscrews seem to be rather soft- I had to replace mine too with a better used one off Ebay (about 25$)
Mark
I don't think I really studied the threads, but I did look them over. No obvious rounding off. But as said below, it's probably more likely that the width of the threads decreases from wear and not the total diameter. I'll have a closer look when I have time.
 

DaveBarbier

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#7
I don't know so I'm watching with interest . .007 doesn't seem like all that much , when I got my Craftsman 6" I also got a box of parts there was a half nut in there and it was really worn out . So my thought is maybe it's a combo of nut and screw . Or maybe just a new half nut would make the fix .?
Definitely possible the half nut is also worn, but not obviously so. When cleaning it it seemed normal.
 

DaveBarbier

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#8
Can you make consistent cuts on round stock? My new to me SB Fourteen wouldn't hold a consistent OD when turning a test shaft. Sometimes varied by .010", and it was absolutely all over the place. Nothing consistent at all. The leadscrew looked OK,but had some moderate wear in the middle, where all the machining takes places. I had a new leadscrew and bronze nut made up, and now the lathe is dead on - every time. Cost about $150 for both parts. Personal opinion - buying used parts off eBay is just asking for instant wear and tear, better to have sometihing like a leadscrew screws and nut made up new, to proper tolerance.

Glenn
$150 for a new half nut and lead screw? That's cheap. I doubt I can get a price like that around southern CT. I'll try and look around. Or maybe a member can make them for me. I'll do. A bit of research.


Yes, I get pretty consistent cuts on round stock, though I'm not turning anything very long.
 

DaveBarbier

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#9
The only two circumstances that I can think of for there to be any difference in the OD between the worn and unworn area is a significant number of hours of the half nuts popping open and sitting grinding on the screw. And hundreds or thousands of times closing the half nuts when they weren't aligned with the threads. Under normal circumstances, the thread flanks wear but not the diameter. Starting to do it now certainly won't help your problem any but the latter case is why I haven't disengaged the threading dial but once or twice in more than 35 years. I watch the dial and engage the nuts on the mark. And never grind on the threads and nuts.

Anyway, I'm inclined to agree with Glen. I would be leery of a used lead screw, too. Unless I knew how it was used (or not used). I have a spare lead screw for my 3996, but I only kept it because I knew that the lathe that it came off of belonged to a transformer manufacturer. And aside from the rare times when the machine was actually used to turn or thread something, knew that it wasn't being used.
I'm definitely guilty of engaging the half nut randomly. But this can't be all from me as I've only been using this lathe for about a year and a half with occasional use. But from now on, I'll be sure to do it properly. As well as the diameter being off by .007", I'm sure there is a little rounding of the threads allowing for a ramping up of the half nut.

And I agree with you and Glen, of course I'm taking a chance with any used eBay part, but I could ask to have it mic'ed up. But I'd have to trust them...

Lead screws seem to be fairly inexpensive, so I may go the used eBay route with one I feel comfortable with before having one made up. A lead screw seems easy enough to make, but a half nut is a much more involved part.
 

RJSakowski

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#10
I'm definitely guilty of engaging the half nut randomly. But this can't be all from me as I've only been using this lathe for about a year and a half with occasional use. But from now on, I'll be sure to do it properly. As well as the diameter being off by .007", I'm sure there is a little rounding of the threads allowing for a ramping up of the half nut.

And I agree with you and Glen, of course I'm taking a chance with any used eBay part, but I could ask to have it mic'ed up. But I'd have to trust them...

Lead screws seem to be fairly inexpensive, so I may go the used eBay route with one I feel comfortable with before having one made up. A lead screw seems easy enough to make, but a half nut is a much more involved part.
I have an Atlas/Craftsman 6 x 18.

I would pull the half nuts and examine them. They are made of Zamak and more likely to be the problem. The 6 x 18 has a fairly fine thread as lead screws go and it is easy to fill the threads with a combination of fine chips and oil. The threads are hidden from view and the problem goes on unnoticed.

Long ago, I had a similar problem and ordered new half nuts. A few months ago, I found my old ones and realized that the the threads were filled with gunk. This prevented them from properly engaging the lead screw and furthermore, prevented the cam from dropping over center which allowed a slight to moderate machining pressure to disengage the half nuts. After cleaning with a solvent and a fine bristled wire brush, they look like new and I wouldn't hesitate to use them again.

On the lead screw, I measured mine and my greatest decrease in diameter was .002" The unworn portion miked at .5021" and the area of most wear was .4998". There was a visually noticeable decrease in thread thickness at the worn area though.
 

markba633csi

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#12
I wouldn't have thought a worn leadscrew would cause such large diameter changes when turning (cutting)- I wonder what the
explanation for that phenomena is?
Alas, Ebay is risky but my old screw was really, really bad and the replacement is nearly like new
Mark
 

wa5cab

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#14
I have to toss in here that Zamak half nuts, other things being equal, will outlive brass ones. I bought a new set from Clausing a couple of years ago but upon examining the originals (and they are originals, my 3996 is a one owner lathe), I had to conclude that they didn't need replacement yet.
 

DaveBarbier

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#15
I found a decent lead screw on eBay (apparently). The ad says it's like new and I messaged the seller and had him measure it. He says it's .4995" and for the entire length it doesn't vary more than .001". Here's hoping it works! I'll keep you all posted.

An unrelated note, I'm now selling a worn lead screw. :)
 

DaveBarbier

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#16
Just to close this thread out, I got the lead screw and the seller was correct and honest that the lead screw is in good shape. I also measured his same measurement. Installed the leadscrew just now and it works without issue. No popping out.

I did a few boring passes on my aluminum part taking up to .020" and it went nice and smooth. Speaking of the depth, is that too much for my lathe? Other than the motor power, what determines max depth? If my lathe does the pass and finish is acceptable is that ok? Do these little lathes wear out more and it's advised to use light cuts?

Thanks for all the help, much appreciated.
 

RJSakowski

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On a small lathe like the 6x18, excessive cuts will severly stress the tool post, compound, and cross slide. Early on, I used the lantern tool post and had broken the tool holder. Later on, Clausing sold a tool holder which was much stronger but that just transfered the stress to other parts. Deep cuts are hard on bearings and the lead screw, if you're using power feed. Deeper cuts are also more prone to chatter. As you use your lathe, you will get a feel as to what is an acceptable depth of cut. It will vary with material and with the cutting tool being used.
 

DaveBarbier

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On a small lathe like the 6x18, excessive cuts will severly stress the tool post, compound, and cross slide. Early on, I used the lantern tool post and had broken the tool holder. Later on, Clausing sold a tool holder which was much stronger but that just transfered the stress to other parts. Deep cuts are hard on bearings and the lead screw, if you're using power feed. Deeper cuts are also more prone to chatter. As you use your lathe, you will get a feel as to what is an acceptable depth of cut. It will vary with material and with the cutting tool being used.
Ok, I figured my question wasn't able to be imperially answered. All that makes sense and I guess I'll just have to gain more experience and comfort.

Thanks for the reply, RJ, I really appreciate it.
 
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