Restoring old lathes, backlash in leadscrews

[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.

[irving2008]

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.

I think you are right, tho what historical value the lathe has is moot.

I can get a 18" of 3/8 x 10 LH ACME (enough for both top and cross-slides) for $20 inc shipping and a LH tap from RDG for £10. A brass block off eBay (enough for 5 nuts) is £6, and some steel stock to make the shaft I have already.

Plan would be to turn a new shaft (basically 3/8" dia rod with a 7/16" dia x 1/8" wide section which locates in the base of the brass support. The end of the shaft turned down to 4mm. Centre drill the new screw 4mm dia and loctite/pin the screw to the shaft.

Mill a block of brass to size with a lug on top, turn the lug round on the lathe. Drill out the block and tap (remembering to turn it backwards!)

How hard can that be? :whistling::nope:

Later I may remake the brass support to incorporate a dial (but then I'd have to make a 4th axis indexer to engrave it on the mill... hmmm small chuck, a bearing and a 50tooth gear would give me something I could engrave 50marks, equal to 2 thou steps...)