Lead Screw Problem - Craftsman 6”

[Voda2000]

Hi, I’ve finally got back to working on my Craftsman 101.20140 lathe I got last year. So far I have had to replace the cross feed nut and the pinion gear on the rack. I also added a QCTP to replace the broken lantern and I’m quite happy with how it’s working.

I had noticed it was cutting a taper so I attempted to level the lathe last night. I don’t have a machinist level so I did a few test cuts on a 12” bar and was working back and forth to see if I could get the cuts the same. After realizing the tail stock was way out of alignment I was able to get it to about 0.002” which I was happy with given my method.

When I went to take a pass along the whole bar with the power feed the half nuts started “skipping” in the middle. I’m not sure if that’s the right term but it’s like the are crawling up and over the threads on the lead screw.

When I got the lathe, the half nuts wouldn’t hold the screw at all. I shimmed them a little between the back of the carriage and the nuts and this seemed to help but I was assuming it was a temporary fix. The lead screw threads don’t look pointy but they are not square either. I have cleaned the nuts and the lead screw and picked out all the debris. Given the number of odd things when I got the lathe I wouldn’t be surprised if the previous owner had used the half wheel with the half nuts engaged.

My question is about what my next steps would be. I had got a quote a year ago from Clausing and the lead screw is about $150 and the half nuts are $50. I’m in Canada so the shipping will be steep and I prefer to try and minimize the number of orders. I am planning on keeping the lathe long term as it is the appropriate for the small work I do so I don’t mind putting some money into it.

Is it likely just the half nuts are worn?

Should I just bite the bullet and replace both?

With a worn lead screw ware out new half nuts quickly?

 

[markba633csi]

When I got mine, the leadscrew was badly worn. I found a very nice used one on Ebay. I also replaced (I think) one of the half nuts, and cleaned everything completely, using a string to clean the screw while running slowly.
The nuts really need to be in good shape for this to work well. Also the leadscrew should be decent and not bent. Check the carriage also for binding- it should move freely. On a lathe with a worn bed, you may have to make a compromise with the shims; you may not have full easy travel all the way down to the tailstock, depending on the degree of wear.
-Mark

 

[Voda2000]

Thanks Mark. I checked Ebay but all the used ones are coming from the US and the shipping plus the item is as much as ordering it new. Ebay kills Canadians with shipping sometimes.

Carriage is moving freely but I’ll double check. Maybe after leveling the lathe it is binding a little.

I’m going pull the half nuts tonight and take another look at them but I think they are pretty worn.

Would the easiest way to check the lead screw for a bend to be to just run it with a straight edge? How straight does it need to be?

I’m really torn between just trying new half nuts and try to save $150 or just replace it all...

How much wear on the lead screw is too much?

 

[phubbman]

The half nuts are intentionally softer than the lead screw so that they wear out before the lead screw does. They're meant to wear out. Not that the lead screw couldn't be worn, but it's probably the half nuts.

If the threads are worn, they will look rounded off instead of having crisp edges at the peaks and valleys of the threads. If you need to buy new ones, consider getting two pair. That may take some of the sting out of the shipping costs, and you'd have another set on hand for when the new ones wear out.

 

[pontiac428]

Lead screws are easy to make for the Atlas. Acme threaded rod is pretty cheap. I replaced all of my lead screws with stock from McMaster. Just copy what you have on your machine while it still runs, then do the replacement. The Acme rod will cost less than the nuts do. It will make a positive difference if you replace all of the screws, nuts, and half nuts while you're doing it, and will last for your lifetime under hobby use.

 

[Voda2000]

Thank guys.

I just took another look at it and it seems most of the wear on the lead screw is in the middle. It sure the history of this lathe so I don’t know why there particularly. Not sure how bad this would be considered.




The half nuts look done to me. Here’s a few photos. Any thoughts?


I had a look at McMasters site but I could find Acme precision ground 1/2-16tpi. Anyone have a link? Seems like it would be simple enough to machine it myself from a threaded rod.

 

[pontiac428]

Are you sure it's 16 tpi? 10 is a more reasonable pitch for a lead screw, especially since your handwheels are marked .100 per turn...

You could machine it. Lots have, even on this site. It's just a matter of making an Acme thread cutting profile tool. I prefer to buy commercial for lead screws, since it is a "standard" to which the movement of your lathe is calibrated. It may be a throwback to my lab days, but a commercial operation can grind a more precise and uniform lead screw than my lathe can by the time I factor in gear play and all the wear points along the way on my old lathe. Four feet of 1/2-10 is $10 from McMaster. At that price, it isn't even worth my time to try!

 

[Voda2000]

I read it was 1/2”-16tpi in another site but perhaps that is wrong? Maybe some can confirm that one way or the other. Turning down the ends for the bearings would be easy enough if I had a pre threaded rod.

I could try to make my own from a scratch but I’m fairly new to machining and have yet to do any single point threading. Also I would need a functioning lathe to cut the threads on.

 

[francist]

 

[wa5cab]

Assuming that the model number of your lathe is 101.21400 and not 101.20140 as you wrote, then the only differences between it and the Atlas 618 are the printing on the badges and nameplate. The lead screw diameter and pitch are 1/2"-16 acme as you said. The pitch of the lead screw has nothing to do with the markings on the cross feed. That screw is (where there is a relation) is 3/8"-10 Acme.

Both your lead screw and half nuts are worn out, Even were the lathe bed long enough to make a new lead screw (which it isn't as the lead screw is longer than the maximum center to center rating of the lathe), you would not be happy with the accuracy of the results. It is long enough, however, to machine the ends of a 2 or three foot long piece of precision Acme threaded rod given that the lead screw will just fit through the hollow spindle. To do it, you will need to have a steady rest or rig up an outboard bearing to support the left end if the rod. However, I would not recommend trying to do it despite what a few others have done. You might try to find someone close enough to the factory to pick it up and who has a Stamps.com account. The reason for the Stamps.com account is that they recently added for shipping to Canada a way to prepay the customs and tax so that you don't have to go through all of that. However, I don't know what the cost would be as the only time I have used it was on a radio part that cost less than $10.00 US. The fee was $6.95. But I don't know whether that was a flat rate or based on the declared value of the part.