Top-slide on a HobbyMat lathe

[Sam StonesParticipant @samstones42903]

Attention HobbyMat owners, I need your help.

I’ve managed to borrow one of these lathes to help finish my skeleton clock.

The machine had stood unused for some time, and needed a good clean to bring it up to clock making standard. In the process, I decided to free up various working parts and reset the slide clearances, etc.

All went well until I came to the top slide. On the bench, the top-slide lead-screw fitted into the nut OK, and initially during assembly onto the lathe.

I was surprised to discover that as the tip of the lead-screw reached the pivot pin, the lead-screw began to tighten and will now only move a few turn in and out before it locks up either way. I lowered the pin slightly to clear the lead-screw tip, and at first this appeared to work.

As you will know, the visible part of the nut is eccentric with a grub-screw `keying’ the nut in place. Looking from the other end of the lead-scrw, there is nothing to go on. 

Is there some sort of back-lash adjustment with this arrangement which is causing the problem?

How do I free up the lead-screw?

 

Sam Stones

[Sam StonesParticipant @samstones42903]

Help needed

[Steve GarnettParticipant @stevegarnett62550]

I don’t yet have a good answer for this, but I can tell you that you’re not alone – mine does that too, and I was thinking only the other day about having a go at it. IIRC the whole top slide assembly leaves a lot to be desired, and I don’t recall offhand any means of adjusting the backlash at all, although I’ll take it off again and have a look, just to be sure. The one thing I recall from when I was getting the HobbyMat to run at all was that when I polished the gib strips, things improved quite a bit. But that comes at a price, especially when it comes to locking screws, which seem now to alter the overall ability to move the slide at all after locking, almost as though the locking action bent the strip – which I suppose that it might well have done.

 

Anyway, I’ll have another look at it later, and report back if there’s anything interesting.

[Steve GarnettParticipant @stevegarnett62550]

Well I found out what was jamming mine – just muck. And I didn’t quite remember it correctly either!

 

If you release the nut in the centre of the cross-slide wheel and remove it completely, the wheel will unscrew as well from the leadscrew, and it’s how tightly this is screwed on that determines the amount of backlash; the nut just locks it in the correct position.

 

Chances are that if you remove the wheel, and then release the two screws that locate the wheel bearing assembly to the cross-slide itself, the cross slide will slide off forwards, exposing the lead screw. At this point, you should be able to find out what’s really happening with it and why it’s jamming. Because of the rather flimsy nature of it, I suspect that the first thing to do is to check whether it’s bent or not…

 

Whilst the method of adjustment for both the cross slide and top slide wheel backlash are adequate, I’m sure that the whole thing would run more smoothly altogether if the assembly used proper thrust bearings – I might modify this at some stage. There doesn’t appear to be any way of adjusting backlash in the nuts though – or I’m missing something. More checking required, I suspect.

[Steve GarnettParticipant @stevegarnett62550]

Okay, at the risk of getting boring, a little more information:

 

There is definitely no adjustment in the nuts (actually long threads) that the leadscrews screw into. But it’s the fact that they are quite long, fixed threads that gives a bit of a clue as to why the leadscrew can seize. The trick to getting this not to happen is at the reassembly stage. So far I’ve only done this specifically with the top slide, but the principle is the same for the cross slide too.

 

The reassembly process is to put the slide back in place with its gib hardly tightened at all, so it slides easily. The leadscrew assembly should also not be done up tightly (this is the two screws holding it onto the slide that I’m talking about), but the aforementioned backlash nut should be at least nipped into the correct place. Now the leadscrew can be turned, and the slide moved as far forward as possible, and the nearest gib screw nipped up a bit tighter. Now, the two screws holding the leadscrew assembly can be tightened, because they will be in exactly the correct place – which they wouldn’t have been if the wobbly leadscrew was right out. It would have almost certainly tried to bend this as it was screwed in, but being loose it couldn’t. Now test its ability to move smoothly at the screwed-right-in point, and if it does, then it’s bound to move smoothly over the rest of the travel. Now all you have to do is adjust the other gib screws appropriately and you’re done.

 

Hope this helps…

[Sam StonesParticipant @samstones42903]

Hi Steve,

Many thanks for your quick replies which were over night for me in Australia. I was about to send my reply to your first two messages until I discovered your third one. This is what I was about to say :-

Yes, I’m inclined to agree with you that it’s muck, especially since the only parts now involved are the top-slide lead-screw, the nut (keyed into the rotary bracket with a tiny grub-screw along its edge ), and the rotary bracket (wheel?) used for indexing the top-slide around when taper turning.

The jamming appears to be around the tip of the lead-screw inside the rotary bracket, and I’ll only find that out by removing the nut. Since it’s not my machine, I’m rather concerned that the lead-screw may have partially seized, and that removing the nut may introduce another problem. Like a bad tooth, it has to come out.

When the garage gets a bit warmer this morning, I’ll try to `persuade’ the lead-screw nut to come free. With the grub screw (key) out of the way, I plan to use the tail-stock and a short length of rod to push on the end of the lead-screw. It’s been left with a centring hole so I should use a corresponding rod tip to avoid damaging the lead-screw tip.

Amongst a variety of changes, and if the machine were mine, I would plug the clearance hole for the stop-slide lead-screw since it is ideally placed to catch swarf.

I also agree with your comments about the gib strips which have been cut away in several places to provide flat seats for the adjusting screws. They are weakened as a result, and are prone to bending. And, if the screws are not located correctly, the strips do appear to jam.

My concerns about the back-lash were largely in relation to the possibility of a second nut inside the rotary bracket, whereby the back-lash would be reduced during manufacture. As you indicate, it’s not the case.

As an addendum in response to your third reply:-

I agree that the alignment should be done with the lead-screw wound in as far as possible such that the best alignment is achieved. The fixing screw clearances through the end plates allow for this. I was actually up to this stage when the jamming occurred . Because of the tendency for the gib strips to move/bend, I was more inclined to adjust their final sliding fit before the lead-screw was in position (a method I’ve used when I had my well-loved Myford ML7). Rather than winding the lead-screws in and out, it is much quicker to be able to push the slide to and fro.

Thank you for your patience Steve.

Best regards,

Sam Stones

[Sam StonesParticipant @samstones42903]

Further to the top-slide problem with a borrowed HobbyMat lathe.

With the rotary bracket removed from the cross-slide, and peering through the tiny (3mm) pivot hole in the top of the rotary bracket, it is possible to see that the tip of the thread has been torn by swarf getting into the clearance hole for the leadscrew. This hole faces the chuck, and is ideally positioned as a swarf repository. I’m not sure if this hole was plugged on the original machine, but it should be.

 

What I can’t explain is why the 80mm long lead-screw bottoms out when it is about half way into the nut. This bottoming out is roughly where the tip of the lead-screw passes the pivot hole (the hole which centres the rotary bracket onto the cross-slide). From this point, unscrewing for about 10mm is very smooth, as it should be. Unscrewing for the next 3mm causes the lead-screw to progressively bind, until fear (mine) steps in.

 

The lead-screw is quite soft, and I suspect that although the rotary bracket is cast iron, the nut bush is made from the same steel as the lead-screw. We all know that under these conditions, thread seizure is clearly a potential problem.

 

If anyone knows why the lead-screw bottoms out halfway into the nut, I would welcome comment.

 

Sam

 

[Sam StonesParticipant @samstones42903]

To close on this thread, (pun coming up), the owner of the HobbyMat came to the rescue, and while it still cannot be removed, he managed to run the lead-screw forward.

 

This procedure tore some of the thread peeks. However, there is enough travel for me to continue on the clock project, my primary objective.

 

Sam

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