Cross Slide Lead Screw Backlash

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SolarFreak

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In the process of doing a CNC conversion on an antique Clausing lathe... I have the cross slide + compound slide tore apart, cleaned and in bags... While I'm tore into it, I'm evaluating the backlash in the cross slide. Compensating for it in software (Mach3) is certainly an option, but I'm wondering if maybe I should take care of it in the "hardware" layer. There's currently about .022" of backlash. There's no visual wear, it just seems to have been made that way. What you see in the pictures that may LOOK like wear is not, that's the way it was made - it's very well preserved.

I'm seriously considering sooting the lead screw and casting a zinc (or zinc based alloy) directly on it. It should give me a replacement with practically ZERO backlash.

Thoughts?

Should anything go wrong, removing the casting will be easy. More than anything, I'm concerned with a fit that is too tight. I'm doing a CNC conversion on the lathe, and as long as I have it apart, I think shoring up the slop would be smart.

Here's some pictures of the parts currently:

CrossSlideDrive-1.jpg

CrossSlideDrive-2.jpg
 

Swifty

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The first problem I can see is that the screw may be worn different amounts along its working area. This is fairly normal as we all tend to work within a reasonably close area, when turning a larger diameter you are using an area of the screw that's seldom used, so the wear is less. Forming a zinc nut in one area will most likely bind somewhere along the length of the screw. Besides this most nuts are made out of bronze, which is a lot better wearing than zinc.

Paul.
 

chipenter

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On my watch makers slides there are two bronze nuts fixed on one side and adjusted on the other .
And on my mill a slot with two screws see below , I can't get all the back lash out or gets to stiff .

feed nut.jpg
 

lensman57

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Hi,

May I ask what is the reason you wish to convert an antique lathe into a CNC machine? These things were designed to work as per normal, I think that you will always have issues with repeatability with old hardware such as this. In any case one way to reduce the backlash is to install a 2nd nut either outside of the cross slide ( have a look at the Sherline design for inspiration ) or jst adjacent to the main nut itself and work one nut against the other, easier said than done. The photo posted by the other gentleman who replied shows a nut from a possibly a Sieg Chinese made lathe and the split is duplicating the 2nd nut, as the grub screws are adjusted the "two nuts" work against each other and reduce the backlash, the buzz word is reduce.
I have also seen conversions that the main screw and nut have been removed and a modern ball screw and nut is installed externally. To get the idea look at the CNC conversion kits that are sold in the US for the Chinese lathes.

A.G
 

Davo J

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I would leave the original screw out and use a ball screw. I have bought a few from the Chinese seller and they are reasonably cheap, even a used one would be better than the original one. In the long run you will thank yourself you went to the trouble to fit them.

Dave
 

starnovice

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Some time ago I bought a used BF30 (Jet). Recently I had to replace the x axis screw because if I adjusted the lead nut to take the back lash out in the center of the table travel, it would bind really bad once it got past the center section. You probably will run into the same problem. Also, lead screws are not always exactly the same throughout the whole length. If you custom cast a nut that fits one part of the screw it may not turn at all on another part of the screw.

Pat
 

n4zou

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My Lathe cross slide has two bronze Nuts and a wedge between them. They attach to the cross slide with two allen head cap screws (one screw each). Between the two nuts is a wedge that is adjusted with a set screw. This allows removing almost all backlash. Adjustment is easy. Fully tighten one of the nut mounting screws leaving the other screw slightly loose. Then you start adjusting the set screw as you rotate the cross slide handle one way and then the other until you can feel all the backlash disappear without binding the cross slide. At this point you tighten the slightly loose lead screw nut. If you did it properly there is only about .003 backlash and no drag. It would not be difficult to make a matching nut and then machine an angle on one side of the nuts that match the angles of the wedge that go between them. Below is a picture of them on the lead screw and the wedge between them with the cross slide removed.

DSC00496.JPG
 

canadianhorsepower

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If I can give my 2 cents Ive been very sucessful by simply adding a sring Valve spring from a motorcycle motor
that I cut for the lenght i wanted. T was tightening it to remove the full back lash. Of course it is located on the inner
side of the saddle this way when Im cutting it's the pressure of my nut that I feel and not th spring tension.
hope it help
 

SolarFreak

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Wow! Thanks for all the great feedback. To much to process and keep up with until later this evening...

This is looking to be a great forum! Hope I can return the favor(s) in the future.
 

etard

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If your lathe is currently working, you could go the ballscrew route to get rid of all the backlash relatively cheaply. For one this small, it should cost less than $50 then you would just need to machine the screw to adapt to a motor presumably off the backside of the lathe and something to retain the front where the handle resides currently. I would also like to do this in the future, but I would want to start with a newer chinese lathe so that I wasn't dealing with bedwear etc...
 

MachineTom

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Take your crossfeed screw, all clean, go to the far end, and with numeric drill lay the shaft of a drill into the cut of the thread, find one that fits such that it is level with the top of the threads, work down the lenght of the screw inch or two at a time, likely that you will find a place where the drill is loose in the threads, sitting in the bottom portion, now find a drill that will fit in the loosest thread level with the top, the difference in the diameter of the drill shanks, is the wear of the threads, the nut will be worn at least twice as much.

The problem with using software to compensate for backlash, is the huge amout of variance in the wear on an old machine. As others have said toss the screws, go ball screws.

I don't know how you define antique, but if it doesn't have gearhead headstock with roller bearings on the spindle, I won't invest a dime in making it CNC.
 

SolarFreak

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Take your crossfeed screw, all clean, go to the far end, and with numeric drill lay the shaft of a drill into the cut of the thread, find one that fits such that it is level with the top of the threads, work down the lenght of the screw inch or two at a time, likely that you will find a place where the drill is loose in the threads, sitting in the bottom portion, now find a drill that will fit in the loosest thread level with the top, the difference in the diameter of the drill shanks, is the wear of the threads, the nut will be worn at least twice as much.

The problem with using software to compensate for backlash, is the huge amout of variance in the wear on an old machine. As others have said toss the screws, go ball screws.

I don't know how you define antique, but if it doesn't have gearhead headstock with roller bearings on the spindle, I won't invest a dime in making it CNC.
You bring up some good points that I wanted to elaborate on:

First, my definition of antique. Granted a bit vague. It's an older lathe, I would peg it in the mid to late sixties. Gear head headstock? I don't know the exact definition of that... but, all gears, save for a belt between the motor and the drive. Timken tapered roller bearings on the head stock.

As far as investing money in making it CNC - Too late...

I have four 425 oz-in steppers, DRO panel, control box, computer, etc, etc. My intention is to do the conversion such that the lathe can be reverted back to it's original state (aside from new holes, etc) In that case, the motors and what not are free to put back on the market, or attach to some other piece of equipment.

I've somewhat digested all the input (I'm very grateful - thank you again!) Installing a ball screw is very appealing, and adapting what I have for a ball screw wouldn't be too tough. For the sake of economy, I'm currently thinking of casting and/or machining a spring loaded dual nut setup. Similar to what n4zou mentions, but the wedge will be replaced with a spring. Just so happens that I have some valve springs from a motorcycle engine that should work nicely.

Did some more inspection tonight on the parts, I have a couple of worn thrust bearings I need to address to. I should just be able to shim them and move on.
 
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