Boxford Model C leadscrew half-nuts

[William ChithamParticipant @williamchitham75949]

My greater concern is there is not much "meat" (as mentioned by another poster) to remove for a sleeve.

I'll bet there's enough! Looking at your first photo it is odd how the wear looks heavily skewed towards the bed side as if the nuts are not on the centreline of the screw. I'd say that whether you repair these or replace them you might need to do some fettling to line them up nicely.

W.

[AJAXParticipant @ajax]

Posted by Howard Lewis on 03/04/2021 16:20:15:

It would be too much to hope that the pins are eccentric to enable the arms to be adjusted?

If the two arms are going to be machined, (to remove the worn half nuts and then to bore / screwcut / tap the replacements ), they will probably need to be separated by a spacer, to simulate the fully engaged position.

If you want to Tap the thread, Tracy Tools may be able to supply a Tap, but it will be a high cost per thread, unless you want to make a business, or a habit of the operation.

Howardl

Yes, they will need to be spaced. When placed flush with each other they form something close to an ellipse.

I'm assuming this job will be a one off. If it works once it will see me out, so I'd rather not pay for the tap.

[AJAXParticipant @ajax]

Whilst I'm not convinced I want or need a tap, I decided to have a look online to see what's available.

I found this:

ACME3/4-8 Right Hand tap Trapezoidal thread tap ACME 3/4"-8

but now I'm confused. Aren't acme (29 degrees) and trapezoidal (30 degrees) threads different? Admittedly this was found on a less than reliable auction site.

[not done it yetParticipant @notdoneityet]

Both ACME (imperial) and trapezoidal (metric term) are trapezoidal in thread form. Just different by one degree as stated and, of course, in dimensions – they are either of imperial or metric description (diameter and tpi or pitch).

Edited By not done it yet on 03/04/2021 16:54:42

[old martParticipant @oldmart]

Posted by AJAX on 03/04/2021 16:05:14:

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.

Acme Sleeve Nut, 3/4 – 8, LH, Bronze

They do have a phone number for outside the States callers, so there is a chance they might send things to the UK.

There is a supplier of ACME threaded rod and nuts on ebay, the shipping costs are high, but they don't seem to do 3/4-8 ACME. It might be worth getting in touch with them, though.

[David CauntParticipant @davidcaunt67674]

"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."

Surely if they can rise and jump the next thread then they are not being properly held in the connected position.

The fact that the engagement lever is up when they are engaged must mean unless there is a positive lock somehow the weight of the engagement lever will help them open.

Before doing anything more surely the operating cam etc must be checked thoroughly.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by AJAX on 03/04/2021 16:43:23:

Whilst I'm not convinced I want or need a tap, I decided to have a look online to see what's available.

I found this:

ACME3/4-8 Right Hand tap Trapezoidal thread tap ACME 3/4"-8

but now I'm confused. Aren't acme (29 degrees) and trapezoidal (30 degrees) threads different? Admittedly this was found on a less than reliable auction site.

.

Strangely [but pedantically correct] 29 degrees is a perfectly valid angle for a trapezoid

It's just that those continental chappies have adopted the term Trapezoidal for a particular shape of Isosceles Trapezoid with 30 degree slopes.

… What you would end-up being sold, however, is anyone's guess.

MichaelG.

.

Ref. https://www.splashlearn.com/math-vocabulary/geometry/trapezoid

Edited By Michael Gilligan on 03/04/2021 18:00:47

[Speedy Builder5Participant @speedybuilder5]

just a general comment about the video. When cutting external threads, the school of thought is to set the top slide at 1/2 the thread angle, and here we have an ACME thread plunged in normal to the thread axis. Is that special to ACME threads ?

Bob

[old martParticipant @oldmart]

Its very bad practice to mix ACME with trapezoidal, I would also be suspicious when a vendor mixes them up.

[Michael GilliganParticipant @michaelgilligan61133]

Posted by old mart on 03/04/2021 19:50:06:

Its very bad practice to mix ACME with trapezoidal, I would also be suspicious when a vendor mixes them up.

.

I thought I had explained … ACME is trapezoidal in form … it’s just not a Metric ‘Trapezoidal’ thread.

If you don’t believe me, try this: **LINK**

https://en.wikipedia.org/wiki/Trapezoidal_thread_form

MichaelG.

[BazyleParticipant @bazyle]

Posted by Speedy Builder5 on 03/04/2021 19:44:29:

just a general comment about the video. When cutting external threads, the school of thought is to set the top slide at 1/2 the thread angle, and here we have an ACME thread plunged in normal to the thread axis. Is that special to ACME threads ?

Bob

try drawing it on paper and you will see that teh angled approach doesn't provide the same advantage on this sort of thread.

[old martParticipant @oldmart]

AJAX, your thought on the solution in your first post are sound. Removing the thread, plus enough wall thickness from the existing nuts and then fixing a thin wall inlay with the new thread is similar to my nut on the museums Smart & Brown model A. That design uses a single nut and a plain backing pad. The old nut was a large piece of metal, and I elected to set it up on a faceplate and bore out the threads. First I made a centring plug. I turned the plug until it fitted the core of the worn thread. The other end of the plug was turned to fit in the tailstock. With the plug up against the faceplate, the old nut could be clamped in place. The threads were bored out and the diameter carefully increased until the new nut od fitted it. The parts were tinned with soft solder and then fitted together. S & B use a 1" x 5 ACME thread which I singlepointed before the lathe was dismantled.

[old martParticipant @oldmart]

As already mentioned, with a pair of opposed half nuts, they have to be aligned axially with the leadscrew to work. I would solder the first in place, and then mark the second before soldering it, maybe even cutting the end to match the end of the housing. A lot of fitting and thinking is involved. This would be easier if the second was glued in place to find its own alignment.

Edited By old mart on 04/04/2021 21:12:40

[AJAXParticipant @ajax]

Posted by old mart on 04/04/2021 20:15:25:

AJAX, your thought on the solution in your first post are sound. Removing the thread, plus enough wall thickness from the existing nuts and then fixing a thin wall inlay with the new thread is similar to my nut on the museums Smart & Brown model A. That design uses a single nut and a plain backing pad. The old nut was a large piece of metal, and I elected to set it up on a faceplate and bore out the threads. First I made a centring plug. I turned the plug until it fitted the core of the worn thread. The other end of the plug was turned to fit in the tailstock. With the plug up against the faceplate, the old nut could be clamped in place. The threads were bored out and the diameter carefully increased until the new nut od fitted it. The parts were tinned with soft solder and then fitted together. S & B use a 1" x 5 ACME thread which I singlepointed before the lathe was dismantled.

That looks like very nice work – well done.

[AJAXParticipant @ajax]

Well I've surprised myself and found a very simple fix for this problem. As has been noted previously, there is no way to adjust the half nuts for wear and position apart from opening/closing with the engagement lever. So it dawned on me that all I had to do was replace the pivot bolts (a type of two-step shoulder screw) with an eccentric version. I tried and sure enough it now works perfectly. Admittedly the half nuts are still worn but at present they are not slipping and this is an incredibly easy fix that anyone should be able to reproduce. I posted a video in case none of this makes sense.

Edited By AJAX on 05/04/2021 12:48:20

[AJAXParticipant @ajax]

The other thing I should mention is that whilst looking for a fix I realised that some models of Boxford, Southbend and Hercus lathes apparently share the same half nuts. New spare half nuts for Hercus lathes are available to purchase from Australia for 138 AUD (about £75) and the supplier I found will ship to the UK.

I was offered a set of secondhand half nuts online – no price was mentioned – but the seller has gone quiet since I mentioned new ones can be found for £75. Whilst it may not apply to this seller, I have noticed some astronomical prices being asked for parts from lathes being broken for spares.

[Michael GilliganParticipant @michaelgilligan61133]

An excellent ‘fix’ and a useful video … Well done, Sir

The price of spare half-nuts should have plummeted !!

MichaelG.

[old martParticipant @oldmart]

A good fix which the lathe can still be used with the old nuts. Remember that there may be some backlash between the leadscrew and the nuts and allow for it. That inlay I made was using the old nut, but much easier for me as the leadscrew is 1" diameter and makes internal threading easier. I have done 3/4 x 5 Acme by roughing out most of the thread and then finishing with a tap from Tracy Tools.

[Howard LewisParticipant @howardlewis46836]

Yet another way of skinning the proverbial cat!

Splendid! You now have plenty of time to search out a more permanent solution. Probably several years!

Howard

[AJAXParticipant @ajax]

Posted by Michael Gilligan on 05/04/2021 14:14:11:

An excellent ‘fix’ and a useful video … Well done, Sir

The price of spare half-nuts should have plummeted !!

MichaelG.

Wishful thinking about the price of spares, but here's hoping!

[AJAXParticipant @ajax]

Posted by old mart on 05/04/2021 16:05:59:

A good fix which the lathe can still be used with the old nuts. Remember that there may be some backlash between the leadscrew and the nuts and allow for it. That inlay I made was using the old nut, but much easier for me as the leadscrew is 1" diameter and makes internal threading easier. I have done 3/4 x 5 Acme by roughing out most of the thread and then finishing with a tap from Tracy Tools.

I had a look for Acme 3/4 x 8 TPI taps and the only one I could find was on eBay from a Chinese supplier. Nevertheless, I like your idea of roughing out first and will bear it in mind if I ever need to attempt this job.

[AJAXParticipant @ajax]

Posted by Howard Lewis on 05/04/2021 16:23:41:

Yet another way of skinning the proverbial cat!

Splendid! You now have plenty of time to search out a more permanent solution. Probably several years!

Howard

I'm guessing this "temporary fix" may see me out, but I may do a "proper job" (with a threaded insert) at some stage just for completeness. And then again, if I can buy new half nuts for £75 that doesn't sound like an unreasonable deal either.

[BazyleParticipant @bazyle]

Certainly an interesting fix.
I notice the rotation of the bolt also applies some longitudinal movement so there is potential (with good nuts) for using this to take up backlash for anyone using the lathe and vertical slide for milling.

Re the screwdriver slot. Perhaps one at the front, ie threaded end, or a hex dilled in would be avantageous.

[Speedy Builder5Participant @speedybuilder5]

Not suggesting you buy these, but it shows re-threaded Southbend nuts

SB Half Nuts

[David CauntParticipant @davidcaunt67674]

AJAX

At 3:11 into your video where it is just possible to catch a glimpse of the operating cam it is quite clear that the peg has worn the top right hand end of that slot. I would imagine the same would be true of the other slot which would have allowed the bottom to not be closed up fully. The top is probably the same.

Whilst your remedy obviously has got over the problem it looks as though my thoughts were correct.

Did you examine them? Or is it all just my imagination?

[Author]

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