Lathe Breakdown - Source for dovetail bolts or nuts?

My lathe started suddenly making a lot of chatter this evening after adjusting the topside angle.

Tracing it back two of the dovetail nuts which connect the topside to cross slide have snapped in half at their weakest point. Looking at the break they have been failing for years, and clearly decided that being re-tightened tonight was the final straw.

As a complication I was making a new drawbar for the Harrison at the time (it has needed one since I got it and has now deteriorated to the point of being unsafe) so the mill is also out of action.

I cannot for the life of me find any UK suppliers of dovetailed nuts (or bolts), other than for decking purposes, any suggestions?

Is there any chance you could silver solder the two halves together for now to finish your job then the mill is available to make new ones???

If you send me a diagram/drawing of what you need jelly ill knock you some out in the morning for you to collect..

Ian …nether edge

Ian, You're a very kind bloke (who now has a private message with some drawings)!

Most of my drawbars are suitable lengths of threaded rod, with a double nut at one end. I used to think they needed to be really tough, but they don’t actually need to be that tight to work…..

In a world where Loctite exists, is the second nut still necessary?

And I wonder why the OP couldn't have quickly made some temporary shaped nuts with a hacksaw and file. Spend a bit more time on them, and they could have been permanent replacements.

all done

This was my initial thought, until I realised I'd used all my M12 threaded bar.

Again, it's not that I didn't think of it, but I didn't have any suitable stock, most of what was to hand in sane sizes was either too weak for such a highly stressed design (brass, copper) or too hard for sensible hand working (817M40 pre-hardened, or EN24W in old money) without ruining tools prematurely.

If I was going to have to buy steel I wouldn't normally use, then looking for the parts themselves seemed just as sensible.

A big thankyou to Ian who has made this much easier than anticipated.

Edited By Jelly on 14/03/2023 11:56:23

I made four for the museum's Smart & Brown model A, to replace the original two. This was because the cross slide was cracked and doubling up the dovetail bolts allowed them to work without tightening so much.

The problem with these bolts is they would not be strong enough if made from nuts, they have to be one piece. Also, the dovetail is curved, so the inside radius is smaller than the outside. This is how I made them, the picture is of one of the originals.

It is my strong suspicion that TOS used nuts, and straight nuts at that, very deliberately as a sacrificial part to prevent over-tightening from damaging the cross slide.

The three contact points between the nuts and the circular dovetail creates a significant stress riser directly through the thinnest part of the wall, creating a failure point which is immediately obvious to the operator, and the cross slide is designed to make them easily replaceable without any disassembly, just wind it out fully and pop them in from underneath via a little pocket.