Restoring old lathes, backlash in leadscrews

Hi all,

This is one for the experts, JohnS, Robin, etc. I suspect.

I need to fix the backlash on the cross- and top-slide on the lathe (the 85y old one)

It currently has 8tpi ACME (or something like that, square topped) threaded screws working onto a tapped cast-iron block which is the slide itself. In other words there is little opportunity to make another part. I'll get some pics/drawings done in due course but I'm guessing the wear is in the cast-iron block (nut) and not the screw as I can move the slide back and forth 2mm on the screw.

What are my options if I:

1. want to retain the existing screws
2. fit new metric trapezoidal screws, say TR10x2

A couple of ideas I had:

1. bore out the cast iron block (with my new boring head on the mill), assuming there is space and fit a bronze/delrin/whatever sleeve.
2. mill out a section of the block and drop in a new 'nut'

I think the advantage of the second approach is I can trial different nuts and maybe get some form of anti-backlash approach in there...

The Cross-slide is likely to be a left-hand thread. Are they really 8tpi? surely both are more likely 10tpi (How are the dials scaled?).

If you have room, make two new lead screw nut for each slide thread them on the OD. drill the blocks and thread it to take the OD of two new bronze nuts. Use the OD thread to take up the backlash.

Alternatively, again if room allows, a new nut could be screwed on to the existing screw and fixed to the existing block.

Pictures would help :)

Like you said one for the experts. :smile:

I'll be watching this one tho. Need to fix mine at some point.

So lots of pics please.

OK, a closer inspection and the application of some cleaning cloths showed I was wrong in assuming the SuperRelm was like its smaller brother. Where the RelMac has the leadscrews working directly into the cast iron, the SuerRelm is more sophisticated and I found, after moving the tool carrier and cleaning 85 years of gunk off, a screw head. Further inspection revealed a bronze/brass nut held in by a screw - see pics.. this is the topslide, but the cross-slide also has a screw head only visible once the topslide is removed.

It is 8tpi LH x 3/8", I measured it accurately with the digital calipers. RDG do a 3/8 x 10tpi LH tap for £10 but not an 8tpi :sad:, then again i dont want to be making a new screw if I can help it...

I can move the nut back and forth about 1.0mm on the screw. And the screw's endfloat in the brass bearing/mount is about 0.3mm but thats easily rectified with a washer under the handle

How do I measure wear on the screw - is it likely to wear or can I assume the wear is all in the nut?

Very similar to my Southbend. I sawed across the nut, down toward the mount (right-angle to bore), a thread or two from the back edge. Then drilled and tapped two small holes either side of the lead screw at the split end, so that two screws will push the split apart and reduce the backlash (I'll try to get a picture tomorrow).

Alternatively, you may be able to cut the nut completely in two and screw one part against the other (see attached pictures of my old mill's y nut). You'll have to find some way of fixing the two parts together to stop the adjuster nut turning.


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How do I measure wear on the screw - is it likely to wear or can I assume the wear is all in the nut?

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Both will be worn. You should be able to detect the difference between the middle and the less-used ends of the thread. That said, the adjuster suggested above works quite well on my 72 y.o. SB 415 - it goes between tight, loose and tight over the whole travel, but the backlash is workable (perhaps ~10-20 thou at worst)

Quite a few ways to take up wear in a nut but if the screw is worn there isn't really a lot of use.
Square thread of 8 tpi means it has crests and spaces of 1/16" equally, second pic shows ? the spaces wider to the left of the nut ?
Hard to tell could be camera angle.

One alternative is to replace the screw and nut with a Myford ML7 part which is also 3/8" but 10 tpi acme to give 100 thou per rev

Think its just the camera angle John, tho its hard to tell... The calipers are in the garage now and can't be assed to go get them, but it looks fairly even - cetrtainly not 1.5mm wear's worth!

Can't find a 3/8 x 8tpi LH tap anywhere... and quoted $201 to have one made! So thats not going to happen then...

The threads inside the brass block, if they were ACME square top originally are very much triangular now... so much so I wonder if they were ever ACME - well after 85 years a bodge or two wouldnt surprise me... but as you say John, maybe not worth bothering with, althopugh cutting the block 1/3 way downwould not be hard to do

Replacing them with 3/8 x 10tpi LH as per Myford had occured to me, I can get a tap from RDGTools as mentioned, but where can I get a die?

Why do you need a die ? Just buy a spare screw from Myford or Jim Marshall who breaks Myfords.

http://www.homeworkshop.org.uk/

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Originally Posted by John S

Why do you need a die ? Just buy a spare screw from Myford or Jim Marshall who breaks Myfords.

http://www.homeworkshop.org.uk

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I could, but thats £30 or more, plus if I am going that route I'd rather replace with something metric as its more useful. A 150mm length of TR10x2D and a bronze nut would be cheaper...

You will never cut an accurate thread for a leadscrew with a die be it imperial or metric.