# Lead Screw Deflection??



## 38Bill (Aug 3, 2016)

Newer style Craftsman 12" X 36" lathe. Today while cleaning and oiling I noticed that the lead screw deflects out away from the lathe bed when the half nut is engaged.  The lead screw looks to be straight and parallel to the bed and runs true until the half nut is engaged. Is that normal or is something in the apron out of alignment?  Thanks.


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## David S (Aug 3, 2016)

Hi Bill.  I notice that you haven't had any replies.  I have a 6" Atlas not the larger one you have.

Now this my seem like a stupid question but are you sure you are engaging the half nuts to engage with the roots of the lead screw?  If I don't get mine quite in sync the lead screw will deflect.

David


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## 38Bill (Aug 3, 2016)

Yes even with the half nuts fully engaged and the apron moving I can see that the lead shaft is pulled forward away from the lathe bed bowing the shaft.


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## JimDawson (Aug 3, 2016)

You may need to shim the half-nut assembly towards the bed.  I did this once on something many years ago, but can't remember the details.

Also, make sure the current engagement is the same at both ends of the bed, you may need to shim the bearing support at the end of the bed.


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## Rob (Aug 3, 2016)

When I first got my lathe I had a similar issue. I found my problem to be the bearing on the right side of the lead screw. It was one my grandfather had made and he didn't fit it correctly. Are your bearings the original ones?


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## wa5cab (Aug 4, 2016)

Before you start shimming on the carriage, determine whether or not the lead screw is parallel to the side of the bed.  As the machined side of the front way is well above the screw, this may be difficult to measure directly.  So position the carriage near the headstock and near the right end of the bed.  In each position (motor not running), slowly close the halfnuts until the inner nut just touches the leadscrew (rock the carriage back and forth as needed in order to be able to engage).  Make a pencil mark on the apron to indicate the angle of the lever at touch up at each end of the bed.  If the tailstock end or the screw is closer to the bed, shim the support bearing to make the arm positions equal.  If the reverse is true, I don't think that you can move either the QCGB or the left support bearing in the horizontal plain.  So unless a PO had previously shimmed the right bearing carrier giving you something to remove, the only solution would be to machine the mounting face of whichever type bearing carrier you have.

Once the screw is parallelto the bed, proceed with shimming the scroll and nut carrier away from the apron.  If this results in the lever hub binding against the face of the apron, you will need to also shim in the square hole in the hub.


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## 38Bill (Aug 26, 2016)

Follow up: Well I pulled the apron and found the problem. One half nut had a hard, grease like stuff stuck in the threads while the other was clean. This caused the lead screw to deflect when the half nuts were engaged. Once cleaned up they look like new, no wear at all so I'm guessing that they were very rarely used and that's why they were gummed up. 

FYI: This model lathe has a brass lead screw bearing/bushing that adjusts in and out  for adjusting the lead screw end so you don't need to shim to get it parallel to the bed.


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## T Bredehoft (Aug 26, 2016)

Good on ya for doing a thorough investigation. Pulling the apron is not an everyday job.


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