Lathe toolpost stud thread repair ideas

Hi all

The toolpost-stud threads on my Churchill Cub lathe are damaged.

It has previously had a hole drilled for a grub screw between the threads, which might make make a helicoil thread-repair questionable.

It's also not possible to fix from underneath as there is no clearance.

My father in law has suggested we machine and freeze-fit then pin a 'plug' which we drill and tap. Also we were thinking of just tapping it for a bolt rather than doing a stud and nut. I was wondering what the reasons were that they always use studs and not bolts. I'm thinking it might be to preserve the threads in the casting!

I've made a short youtube video and was wondering if anyone could offer up any ideas? - https://www.youtube.com/watch?v=FX7ox0a3IQM

thanks very much,

Carl

Edited By Carl Farrington on 10/09/2023 18:18:56

It's to make the thread that wears out on an easily replaceable, consumable part. Wouldn't you rather buy a new stud than make the repair you now have to do?

Yep.. absolutely. I thought it might be that. Thanks for confirming.

Grub screw is probably anti-rotation. I think the side pocket may have been for an indexing system for a tool post that located in the circular pocket.

I think I would see if I could get an M20 x 1.5 or M20 x 1.75 tap. Make a new tool post stud to suit this M20 thread, and tap the slide to M20. If the circular pocket is no longer in use make a collar to secure the stud in place and put a couple of screws through the slide from below into the collar in the pocket.

Martin C

Thanks Martin. I just checked and M20 is the ideal fit for the Multifix holder (if the bushing is removed).

I did just have another, almost obvious idea though. I could make a 'plug' that isn't a press fit, and use 4x M6 or M5 cap-head bolts to fix it in place. It seems like there's ample room. This would be replaceable then if need be

In the picture below I have some M5 bolts (what I had to hand) sat on their heads for ease of photographing, but I would fit them from above, counterboring in the plug.

I like your idea of a collar to come in from the side too.. thanks!

Edited By Carl Farrington on 10/09/2023 19:38:46

I had to stop watching that at about a half minute because i started to feel dizzy at the way the camera was moving.

Yep, sorry. It's pretty hard to film holding a mobile phone in your hand. Much easier with a GoPro.

Can you shim under the tool-post until it lines up exactly with the threads for the grub screw when fitted tightly in the threads? Then fix the grub screw in position to retain the post?

Carl

The Churchill Cub Mk3 had 3 alternative tool posts. Flt top and stud for a simple clamp or block type, T-slotted for American style lantern post and UK style ratcheting 4 way.

See **LINK** , http://www.lathes.co.uk/churchill-cub/

for pictures.

I imagine the deep hole on yours contains the ratcheting and location gubbins.

In your third post you imply that you propose to use a Multifix QC system. This requires a flat mounting surface on top of the slide so sensible option is to fill the aperture with a solid block that will sit flush with the flat portion of the slide top and fit the stud to that.

Which I presume is what you propose in your last post. 4 M5 cap head screws coming up from the bottom will do fine. That said the base appears fairly thin there so maybe counter sunk heads would live more metal to take the loads. I'd glue'n screw with high strength loctite on the base assuming itself reasonably flat. If it's uneven something with a bit more build such as epoxy or JB weld might be better. Its never coming out again is it.

When you fit your stud do receive the first thread to prevent the thread being pulled upwards a touch and making the surface the tool post sits on non-flat. If thats surface isn't decently flat the post will try to rotate. best practice is collar on the stud, like your original, sitting into a recess. A thin aluminium alloy sheet between the toolpost and support surface significantly improves the grip against twist so only m moderate hold down nut torque will suffice.

Gorriloid larruping up a tool post nut in a doomed attempt to stop it twisting as left many a topslide top seriously distorted.

Clive

Hello Carl,

Clive's post above seems to be the most sensible option and I certainly endorse his recommendation for holding the filler block with countersunk screws from below, as well as relieving any threading to the upper flat surface to prevent swelling to interfere with an otherwise flat surface

Incidentally, you have the Cub Mk3A which came as standard with the full Norton gearbox. I don't know the age of your lathe but if the Serial number finishes in 49 or higher it probably has the uprated spindle clutch.

Mine is a salvage job from a Steptoe like scrap yard; it is the Cub Mk3 with the simpler three selector semi-Norton box, making the lathe effectively a change wheel machine with 3 variants on each pitch selection. It was made in 1947 and comes with the Tee slotted insert across the top-slide to carry the 3/4 inch diameter tool post and indexing 4 way tool holder. The insert, tool post and 4 way holder are all hardened.

I would like to find another full gearbox as I am convinced I could do a straight swap with mine, but like hen's teeth they are rare beasts.

I hope you can put yours right, it is certainly not so as it stands.

Regards Brian

Thanks guys.

I understand, however I was planning to fit the screws (cap head hex bolts) from above. Counterboring the filler block.

There is not much (probably not enough) meat in the base of the casting to counterbore to any depth really.

Also, keep in mind that the void that you can see there is a fair bit smaller in diameter than the multifix holder's base. The stud sits in a void, yet the multifix clamps down just fine on what material is left of the casting outside this void.

Does that make any sense?

Edited By Carl Farrington on 12/09/2023 13:06:12

Carl,

All we can do is offer advice on what we see. You are at the sharp end and ultimately the way ahead will be what you feel comfortable with.

Either way, good luck

Brian