Boxford Model C leadscrew half-nuts

I'm slowly ticking off a long list of jobs repairing an old Boxford Model C. This lathe has a 3/4 x 8 TPI acme thread leadscrew in reasonable condition. By contrast, the half nuts are worn unevenly, with a very noticeable sawtooth effect and they fail to hold effectively when feeding towards the headstock. The half nuts have a tendency to skip / jump when under the lightest of loads. Everything is done up snug and there is no obvious way to adjust or shim the half nuts any tighter.

My conclusion is the half nuts need to be repaired or replaced. Maybe the best solution would be to bore the thread out of the half nuts, replace with a sleeve nut, fix with epoxy and then split into two halves. I would need to fabricate a suitable jig. I have access to a suitable mill at work but tooling is limited. A ready-made sleeve nut might reduce the work, otherwise I'd have to make one myself including the internal thread. I haven't found any UK based suppliers online, only US based.

The alternative solution might be to build up the thread (how?) and then tap the existing / repaired thread. I don't have a suitable tap and I'm not convinced it would be easy to tap the two split halves, held slightly apart, even if held in a jig.

I suppose it's possible to make an entirely new half nut mechanism, maybe using a plastic insert melted onto the leadscrew thread, but I haven't given this option much thought.

What would you do? Thank you.

make new half nuts or find similar half nuts from other part manufacturers.

The tricky part is the way they are on an arm as a casting and there is not much meat in the bit surrounding the threaded part. Not sure how easy it will be to find a nut in the UK. Then make sure your silver soldering is up to scratch to attach the embryo nut as the residual wall once threaded won't be able to add much support leaving only the back wall of the thead to grip.

Given the number of SBs and Boxfords out there wearing out I wonder if anyone has tried a temporary 3D print nut and arm to enable the lathe to do it's own threading of the replacement.

The original part was made from a single casting that was split after being drilled & tapped for the leadscrew - would it not be easier to make a complete replacement along similar lines than try to add a threaded insert ?

Nigel B

What material are the half nuts ? White metal ? It may be possible to cast new ones round the leadscrew, then unscrew and split. Just a thought ! Noel.

What material are the half nuts ?

Cast iron IIRC

Nigel B.

I would consider soldering them together, possibly with a shim between (if appropriate), and simply re-tapping  the threads. It might only be temporary fix - perhaps only lasting another 30 years or more?

Edited By not done it yet on 03/04/2021 13:03:05

NDIY - that's a really interesting suggestion.

NDIY - that's a really interesting suggestion.

Interesting, but probably not practical. Unlike a Myford, which moves the half nuts vertically to engage & disengage the screw, a Boxford/ Southbend arrangement pivots the two halves :

So the "shim" to allow re-tapping would not be parallel & the engagement mechanism may not have sufficient travel to engage the screw after the modification.

Nigel B.

You could use the same method to repair as is common on Drummond M-type halfnuts. There have been a number of threads on here and at least one article in MEW in recent years on it.

Basically a "bobbin" is turned up out of brass with the internal thread to match the leadscrew. The OD is parallel with the small flange on each end for location.

The old thread is machined out of the halfnut pivoting lever by clamping it to the cross slide of the lathe and boring it with a between centres boring bar.

In the case of the Drummond, the bobbin is cut in half and soft soldered into the machined halfnut lever. With a pair like the Boxford, some care would need to be taken to get the two halves exactly aligned. Maybe as suggested solder it all together first then cut the bobbin in half.

If I could buy similar half nuts at a reasonable price I would consider doing so. But as has been mentioned since you posted your suggestion, the design of this part may not make this practical.

I agree - there is no much material to bore out if I plan to make a threaded insert.

I hadn't even considered 3D printing a nut/arm. I have my doubts it would be viable but I do have access to a printer.

I'm not sure.

If I could make a jig to hold the two arms/nuts at the correct distance and was able to bore out the thread plus enough to install an insert, that might be easier to making a complete replacement. Maybe.

As it stands, the half nuts don't work so there's not much to lose in trying to improve them.

Do you mean soldering the two halves together? They need to be kept at a fixed distance apart and running a tap (if I had one) between the two would fail due to a lack of material. On the other hand, maybe the threads could be built up using solder? I have no recent experience of silver soldering (probably last tried it about 35 years ago). I frequently weld with a stick welder but can't see that helping much in this situation.

Here's a pretty good video of a guy doing a brass sleeve repair on similar sized half nuts for a Craftsman lathe.

Youtube

William.

I watched a video late last night - sorry, I don't have the link to hand - in which a Boxford owner managed to bodge a repair by adding a blob of weld to one of the engagement pins thus forcing the half nuts more tightly together. Seemed to work but was not pretty!

William,

I watched that video too - and was about to comment on it after responding to everyone's replies. Yes, it's a good video and is making me feel more confident about this as a solution. Didn't he use epoxy to fix the sleeve?

Have you checked that the two half nuts are allowed to engage fully? If not, the wear rate would be increased.

If movement is restricted the internal threads will not engage the Leadscrew sufficiently.

So is there any means of adjustment, or facility to increase engagement? Could the heel on each of the two halves be fouling before full engagement?

Howard

This US supplier stocks acme sleeve nuts with left and right hand threads. It would save some work. Shame they aren't in the UK.

https://www.roton.com/product/acme-bronze-sleeve-nut-left-89682/

Both sides appear to be engaging "fully" (in as much as their worn state allows) but the sawtooth effect of the wear allows them to rise and jump the next thread, if that makes any sense.