Restoring old lathes, backlash in leadscrews

[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...)

[BillTodd]

Is the compound/topslide LHT?

If you are going to the extent of fitting new screws then adding dials is a must, surely?

And , if you're going to make your own nut, it'd be worth adding some kind of backlash adjustment to it (you can't adjust a tap, the way you can a split die :() .

[irving2008]

Is the compound/topslide LHT?

If you are going to the extent of fitting new screws then adding dials is a must, surely?

And , if you're going to make your own nut, it'd be worth adding some kind of backlash adjustment to it (you can't adjust a tap, the way you can a split die :() .

Yes they are both LH threads. And you are right, it makes sense to add the dials, its not that much more work (!)

As regards backlash adjustment, what do you suggest? the current nut is about 20mm square so I was thinking that a 20mm cylindrical section screwed into the block and locked in place with a small grub screw. It would need to be a fine thread, maybe 20tpi. Not sure I could do that with the changewheels I have today.

Here is a rough pic...

[BillTodd]

As regards backlash adjustment, what do you suggest? the current nut is about 20mm square so I was thinking that a 20mm cylindrical section screwed into the block and locked in place with a small grub screw. It would need to be a fine thread, maybe 20tpi. Not sure I could do that with the changewheels I have today.

Sounds like a good plan to me :)

Can you get at to adjust it in place? (I can just reach the SB adjuster from the back of the crosslide)

An alternative (used on the hardinge) is a brass bar (with the end threaded in place) pressing down on the screw via the nut mounting hole (I'll add a picture later).

[irving2008]

Neat approach there Bill, but sadly the lug on mine goes into a blind hole in the slide and it would involve modification of the slide itself.

I actually measured the screw accurately last night and its actually 7/16" not 3/8", although if I am remanufacturing it makes little difference. What matters is finding some leadscrew of around that size and a matching tap for the nut at a reasonable price.

I have decided that the screwed in section I previously suggested would be hard to manufacture as I have no means to thread it at the moment. So I am going with the split nut and screw idea previously discussed

[ptjw7uk]

Have you considered casting a nut thread using moglice not any idea how much it costs but an idea.
http://www.moglice.com/articles/repl...tethebook.html
(see figure 14)
I suspect you would need a new length of threaded rod to replace the screw.


Peter

[irving2008]

Have you considered casting a nut thread using moglice not any idea how much it costs but an idea.
http://www.moglice.com/articles/repl...tethebook.html
(see figure 14)
I suspect you would need a new length of threaded rod to replace the screw.


Peter

Peter, I've looked at this stuff before, but its hard to get in small quantites. I did wonder about using plastic epoxy metal and packing the nuts, oiling the screw, screwing it in and leave to set... might contain the backlash for a while - worth a try maybe

[irving2008]

Took the other leadscrew (cross-slide) off today. Its identical to the topslide one, although at some time in the past somone has lopped 7/16" off the brass bearing and inserted a (poorly machined) micrometer dial and attempted to solder a pointer (now missing) to the brass sleeve.

I now have everything ordered I need to do this except some 7/16-10 LH leadscrew, wanted to find a UK source as the US source is 10 days shipping and was hoping to make a start this weekend. Kingston Engineering don't go down to 7/16, their smallest is 1/2 but this won't fit through the holes in the slides :sad:

Anyone know of a UK source for 400mm or so 7/16-10 leadscrew? I wanted to go metric but although MD have 10x2D LH at 99p/100mm I cant get a 10x2D LH tap at reasonable cost unless someone has one they can lend me...