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Harrison lathe half nuts dont stay engaged under load

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Nick Chase 113/04/2022 19:06:20
6 forum posts

Hi guys - can anybody help?

i have a harrison vt330 lathe which is basically a M300 and its saddle is very similar to the colchester student i believe.

it was a pretty worn old thing when i got it but ive patched it up some what and reground the bed so its in reasonable shape for the most part.

however when screwcutting the half nut lever has a tendency to drop under load and disengage the nuts and ruin the thread. The lead screw is a tad worn near the head but it looks like the half nuts are pretty new. the lead screw end float is fine.

Firstly does anybody know if there is supposed to be some kind of spring indent to hold the lever in place? there doesnt seem to be much to this bit of the mechanism and i cant really see why it would be a problem

and secondly does anybody know how to adjust the half nuts?

Ive played with the screws on them, and it seems to have improved things a lot but i dont really know what i am doing. The half nuts appear to now be centered and dont move the lead screw more than a couple of thou when they are engaged, but they are impossible to adjust in situ

thread cutting is something of an adventure at he moment as if you dont pay attention the saddle position can shift a bit.

any words of wisdom would be greatly appreciated

DC31k13/04/2022 19:53:42
686 forum posts
2 photos

Do you have a manual for it? The Harrison M300 manual is online in more than one place. Go to the page that shows the apron and halfnut mechanism and verify that everyt component shown on the that page is present in your machine.

Silly question: have you verified that it is imperial leadscrew and imperial halfnut (or metric-metric) and not a mixture?

See also: https://www.model-engineer.co.uk/forums/postings.asp?th=141350

Nick Chase 114/04/2022 10:52:20
6 forum posts

Hi

thanks for the suggestions

i do have a manual and the exploded diagram doesnt seem to have any thing on it that looks like an indent mechanism to me. But i may just be being a bit thick.

i think its all there though. there are only a few bits anyway

the best plan i can come up with is to over tighten the grub screw that holds the bush bearing for the handle and maybe that way make it a bit stiffer and less likely for the handle to drop.

I havent verified the metric-metric status, but i assume it wouldnt even go together if it wasnt? or is that a wrong assumption. the halfnuts must be 8 or more threads long so id expect then to totally misalign if they were imperial

There is a little leadscrew wear near the chuck (its a high milage lathe) but with the half nuts close this seems to translate to 0.4mm of saddle backlash at the unworn end and 0.62mm near the chuck.

i didnt think this seemed unreasonable, and didnt think it would cause the problem. but what do i know?

i did see that post (ive scoured the internet for hours ) but there are a couple of screws on the halfnuts that do provide some adjustment in position (not backlash). I just cant understand how im supposed to adjust them, plus i have to take the saddle box off to adjust them so its like shooting in the dark

cheers

N.

KWIL14/04/2022 11:50:49
3554 forum posts
70 photos

Nick,

I have a Harrison M300 which is fortunately in near mint condition.

In the Harrison Manual on page 98, IIllustration 94/10/2.Half Nut Part No 39, behind that Part No 44, Plate Bridge.

Also illustrated are parts 40 - 43 which appear to provide some form of Detent mechanism.

As the Half Nut Shaft, Part No 30 is inserted solely from the front of the Apron there must be a matching hole/depression for the ball and pad to contact. No obvious way to adjust, other than it fits where it should!

Note there is only 1 half nut listed, not a pair.

Regretfully only examination of the physical parts will provide a full answer.

Hopefully this will help you in some way.

 

Edited By KWIL on 14/04/2022 11:55:19

Nick Chase 114/04/2022 16:04:55
6 forum posts

Hi

I hadnt considered looking at other manuals . good idea.

however all the old m300 manuals seem to be 68 pages. (and look pretty much identical to my VT330 manual)

i eventually found a version for what looks like a newer version of the m300 (on vintagemachinery.org) which has 128 pages and diagrams like you describe,

but this is a somewhat different apron and as you say it only uses 1 half nut, not the 2 halves that i have.

So i think you are perfectly right. I will have to take the apron off yet again and strip it and strain my brain to work out what is going on.

Thanks for the input.

p.s to be fair had i bought a lathe in good condition instead of this crapped out POS

I wouldnt have been forced to slowly understand why it was producing such poor results and to work out how to fix it up, so i guess its been a good learning experience after all, and isnt that half the point?

cheers

N

DC31k14/04/2022 16:49:31
686 forum posts
2 photos

You said in the original post that the leadscrew is a little worn near the headstock. Perhaps try some pseudo-screwcutting up near the tailstock end of the bed to either eliminate or confirm the leadscrew's contribution to the problem. Try and do something that mimics the normal cutting forces on the nut/screw interface (e.g. weld down the tailstock and tie a bungee cord between it and the saddle).

See if you can design something that fits on the apron to keep the lever in its correct position (with a suitable release mechanism for when you do want to disengage in a hurry). If dropping it disengages the nuts, rotate the handle 180 degrees and put a weight (lead ball) on the end to bias it closed.

Nick Chase 114/04/2022 18:47:17
6 forum posts

test on the unworn end . ....genius

ill give it a go

ta muchley

Nick Chase 120/05/2022 12:40:42
6 forum posts

For the record i got this sorted.

Testing on the unworn leadscrew end showed the lead screw was ok, so it had to be the adjustment or the halfnuts. plus it was rock solid moving away from the chuck and useless moving towards it.

There is no indent mechanism to locate the thead engage leaver, so it wasnt that.

The halfnut mechanism has a fixed pin on one nut and an adjustable one on the other. They ride in a slot on the other end of the lever so when you turn the lever it moves one nut up and one nut down.

There is a grubscrew on the top and bottom of the adjustable pin so you can change its position on the halfnut and this means that it engages at a different rotation of the lever which essentially changes the height at which the 2 nuts meet. (the height range is tiny though)

It turns out you can just about reach the top adjusting screw insitu with an alan key if you remove the saddle plate on that side, so you can tune its position with everything assembled.

The adjustment was maxed out though.

So I ended up making an offset pin for the press fit fixed pin as it allowed the handle to rotate a bit more and meant the adjustable side was more in the center of its range.

However the main problem seemed to be that in its previous life, bed wear meant the saddle was running low, and clearly the half nuts wernt fully engaged and as a result the lead screw had worn some slight steps on the flanks of the half nut threads.

Now the nuts arnt all that worn, they must have been recently replaced as there is plenty of meat left on them.

But now ive reground the bed and raised the saddle (the bed wear was horrible) its all in better alignment, and with the new pin the leadscrew is now fully engaged in the halfnut. However this means that the leadscrew just touches on the unworn bit at the top of the halfnut thread and it seems like a slight vibration will make it jump down to the next step in the nut thread. This lets the carridge move sideways a bit and the thread is ruined.

Under heavy loads it can even disengages completely.

I could try to skim the halfnut thread, but id need a second lathe to do that.

So I smeared a dab of jb-weld on the thread face, greased up the lead screw, and clamped it on, and thusly smoothed out the thread flank a bit.

Its still a bit sensitive to the adjustment, but with a bit of fiddling, and several heavy runs up and down the lead screw with some resistance applied to the carridge to bed it in a bit, it now seems to stay engaged nicely in bith directions.

Now this isnt a blindingly good fix, but hopefully now it will wear on the metal high spots of the nut (only a few tens of thou high i think) and over time wear back to a decent full metal thread face. And jb weld is reasonably tough stuff, so with the low mileage ill be putting on it it will probably be fine.

if not ill have to find a second lath and try and skim the nut.

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