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Cross slide acme thread

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Esmith41

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I have a SB 13” lathe with about .080 backlash in the cross slide. I am looking at replacing the threaded portion of the screw along with the nut. Without me taking it apart does anyone know the size of the acme thread 5/8-x? I see replacement nuts on the auction site stating they are 5/8-8. The reason I am questioning that is the travel on a 8 tpi is .125 vs 10 tpi .100 making it fit the dials graduations. Maybe I don’t know what I’m talking about being a noob.... if some one could explain?
 

Technical Ted

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To check your lead, just measure how far your crossfeed moves with one complete revolution of the dial (or just go off the marking on the dial). To physically check the lead and diameter, if you crank the crossfeed all the way towards you does it expose the end of the screw so you can measure it?

Ted
 

RJSakowski

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In addition, you need to check whether the thread is left hand or right hand. All the lathes that I have encountered have left hand threads.
 

MrWhoopee

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You understand correctly, if your dial reads .100/rev, then it has to be a 10 tpi screw. My heavy 10 had .070 lash when I got it. Tightening the screw that retains the cross-feed nut removed most of it. Replacing the missing thrust bearing at the back of the screw removed the rest. Now I have .007 lash. It's not always a worn nut/screw.
 

RJSakowski

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Backlash will be the sum of feed screw wear, feed screw nut wear, thrust bearing free play, and lead screw twist due to frictional drag in the ways. Excessively tight ways will increase backlash. To get a true indication, it would be best that the gib be loosened to the point where any drag is removed.

You can determine if the screw is worn by measuring backlash in the most used region and at the far end of travel. The far ends will have the least wear. Ideally, it will be the same or reasonably close.

A rough assessment of backlash can be achieved by rotating the crank in one direction enough to account for any backlash. Note the reading and slowly rotate in the opposite direction until some resistance is met. Note the difference in the reading.

A more accurate method would be to use a dial indicator or a test indicator. Set the indicator up to measure the cross slide travel. Rotate the crank to feed the cross slide into the indicator and note the indicator position and the cross feed dial reading. Continue moving past, then back until the indicator reads the the same position and note the cross feed dial reading. The difference is backlash.
 

Esmith41

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As soon as I got home I looked at the large dial on my crossslide that read .125 9D85ECAD-25B9-40A3-A6EE-C47AC9CA510C.jpeg I knew it was 8 tpi and measured the threads1C773996-F5E0-4AAA-80F7-7C43AAFD2D3A.jpeg 8tpi
 

Technical Ted

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And in the picture it looks like left hand thread. All the lathe cross feed screws that I've seen are. As a general rule, if the screw is stationary (fixed) and the machine slide (nut) moves away from you when you turn the hand wheel clockwise, it's left hand thread.

Ted
 

Esmith41

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Thanks for the replies everyone. Now to order the threaded rod and nut material. One of my first real projects.
Eric
 
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