Restoring old lathes, backlash in leadscrews

[BillTodd]

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.


[edit]

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

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)

[John S]

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

[irving2008]

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?

[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/

[irving2008]

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

http://www.homeworkshop.org.uk

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...

[John S]

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

[irving2008]

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

Good point... but then you need a lathe and thread-cutting ability to make one. Do you know of a source of 3/8 x 10tpi ACME threaded rod other than buying a Myford leadscrew?

[BillTodd]

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

Quite correct. You'd be better off just bodging something with a bit of threaded rod!

Irving;
I've just been looking at Tony's lathe site: If the lathe has any historical value I suggest you do not cut the nut as I suggested, it would devalue the lathe considerable.

I'd offer to make you a LH square threaded nut (sounds like a challenge :)) , but my Hardinge will not cut 8 tpi (a major PIA).

I think the best thing to do would be to replace the whole handle, screw and nut. Perhaps using the TR10x2 trapezoid with a metric dial, although I agree with John, adapting a part from another lathe could save a lot of work.