Carriage slipping

I recently bought a used mini lathe on ebay, but when trying to turn a piece of aluminium under powerfeed, I found that the carriage was slipping. At first I thought the leadscrew was bent, but I don't think so. Today I engaged the half nuts and held my hand on the lever to feel for reaction, but I discovered that the pinion gear was actually slipping on the rack (hope I'm using the correct terms). I cannot figure out what can cause this. Below is a photo of the piece of aluminium I tried to turn.

I don't have a mini-lathe any more Charlene so can't check, but I believe the rack is screwed to the body of the lathe and by slackening the screws the rack can be adjusted for height to engage the pinion properly. Perhaps your rack is set too high or is sloping up to the chuck end.

Hope a mini-lathe owner can confirm.

Dave

Welcome to the forum Charlene.

It could be that the 'forward/reverse' lever on the back of the headstock isn't fully engaged up or down.

The problem could also be that the gear train in the gearbox on the left of lathe headstock is slipping. Most likely this is caused by the 'banjo' - the plate that carries several gears - slipping so the gears don't engage fully, you just need to tighten its fixing screw.

Look round the right side of the apron, the half nuts should close almost completely on the leadscrew. If they don't, there is a long adjusting screw in one half nut that keeps them apart - this should be adjusted until they don't quite close fully on the leadscrew.

Finally, your lathe tool looks odd, can you take a photo that shows clearly how it is arranged, I suspect it might be one reason for poor finish? If it's struggling to cut for any reason it can force the halfnuts or gear banjo out of engagement.

Hope that helps,

Neil

It looks like a right hand insert tool mounted upside down. It looks really odd like that.

It does, indeed, look as if the tool is upside down.

Apologies and respect, in advance, Charlene, if that's not the case. But, worth

pointing out.

ATB,

Daniel

What do you mean by "pinion slipping on the rack" please? If the half nuts are engaged the carriage should be driven via the lead screw, and you would see the handwheel that drives the pinion engaging with the rack will rotate as the carriage moves. But in power feed the rack and its pinion play no role in moving the carriage.

Could the nut be clogged up with swarf which is stopping it closing completely on teh screw.

Which, of course, means what I said earlier about adjusting the rack is completely wrong. Oh dear, I'm having a bad day...

Dave

Check that if fitted the half nut lever detent is working correctly, mine fell out recently.

What lathe are you using? Knowing this will help.

Were you using the Leadscrew to provide the power traverse, or the Rack and Pinion?

If the machine is what is normally meant by "mini lathe" then the power traverse will be provided by the Leadscrew, there being no other form of power to the Saddle.

If the Forward / Neutral /Rreverse selector for the feed fully engaged? The spring loaded handle should engage with one of three indents, to lock it in the Forward / Neutral / Reverse position.:

Being a mini lathe, it is likely that the gears are plastic, rather than metal. Are all the teeth intact? One or damaged or missing teeth on one or more of the gears may the problem.

If this is the case, replacement gears should be available from one of the importers.

If this is the case, you MUST specify the machine, since some machines use 3mm keys and keyways, whilst others use 4mm, and the bore may be different.

This assumes that the gears are all correctly meshed. Firstly, the banjo should be set so that the gears are correctly meshed. Normally it should just be possible for the gears to rotate with a piece of paper passing through where each pair mesh. Excessive backlash may allow the gears to "slip". This is likely to damage both the gears.

If the pinion is "slipping" on the rack, presumably, you cannot apply the feed by moving the Saddle with the Handwheel. If this is the case then the Pinion is probably not engaging the Rack properly, or the gears within the saddle are damaged (probably missing teeth) but less likely since these are usually metal.

If the saddle moves using the Handwheel, but "slips" under power traverse via the Leadscrew, then either the Half Nuts are closing completely, for some reason, or there is a problem in the gear train driving the Leadscrew. The half nuts may be damaged, in which case they will, need to be replaced. Again, it is presumed that the le3adscrew is clean and not clogged with swarf, which would prevent proper engagement of the half nuts.

When the "slip" occurs, does the Leadscrew stop rotating? If so, the problem lies in the drive to the Leadscrew.

There should be a key in the Gear providing the drive to the Leadscrew.

All the gears should have full face engagement. If not, then it is possible that a spacer is missing, allowing a gear to move endways and disengage.

There should be a key uniting the gears in any compound gear gear in the gear train.

The tumbler reverse gears under the mandrel should be engaging fully, whichever direction is selected, and be intact and undamaged .

The Pinion on the Mandrel should have a key, so that the drive is taken positively, without slipping.

Hope that this is helpful in finding the source of the problem.

Howard

Thank you for all the replies. I cannot really adjust the rack and I've checked the forward/reverse lever, but that was not it. I can assure you guys the tool is not upside down. Probably the angle the photo was taken at. I'm adding another photo at a better angle. Even without the tool, it is doing exactly the same. It's also not the gears slipping. As far as I'm concerned it must be something between the half nuts and the leadscrew, because the carriage is pushed from the leadscrew via the halfnut, as mentioned. I spent more than half an hour cleaning the leadscrew and the half nuts yesterday as well. I think that the half nuts becomes "stuck" to the leadscrew at those positions and that causes the pinion to slip on the rack. Half nut lever detent is also good.

Hi Charlene, I still cant get my head around how you are trying to drive the saddle. In your first post you mention using powerfeed but the pinion slips on the rack..? and you have mentioned it again in your last post...?

If you are using the leadscrew and half nuts to drive the saddle then the pinion and rack play no part in the drive apart from the pinion handwheel free wheeling along the rack.

This may sound a daft question but you are not trying to drive the saddle via the handwheel/pinion whilst you have the leadscrew half nuts engaged are you..?

Edited By Ron Laden on 09/09/2018 15:51:45

Hi Ron, my apologies. It's just the way I'm trying to explain it. I realise the drive comes from the leadscrew.

I have seen some improvement now by loosening the gears a bit, cleaning the leadscrew even more and making sure everything is rigid.

Hopefully the finish I'm getting is clear enough in the photo. My original thinking was that the leadscrew is also bent and I still think so. It also looks like it a bit when it is turning.

O by the way, I am trying to find out what lathe it is because there was nothing on the front cover when I got it.

Hello Charlene,

Post a picture of the whole lathe on here. Someone will surely identify it.

Daniel

Hi Charlene, well done some progress. If you can post a picture of the lathe I,m sure we can work out the make and hopefully the model.

regards

Ron

Charlene, have you tried:

- run the carriage to the extreme right

- loosen the the leadscrew support at that end

- engage half-nuts

- tighten leadscrew support

Seems to me, when I had a minilathe, that fixed just about anything.

I got it wrong suggesting the rack might be the problem, but the leadscrew is adjustable. Possibly it's not level and is flexing when pulling the carriage along on the half-nuts.

With the lathe turned off, try engaging the half-nuts with the carriage at extreme right, extreme left and in the middle of the bed. You may have to turn the chuck by hand until the half-nuts drop into place on the leadscrew's thread. The thing to look for is any sign of the leadscrew bending as the half-nuts engage. If it does, it may be necessary to adjust the height of the bearing block on the right hand side to level the leadscrew, i.e. adjust for minimal bending right, left and centre when the half nuts engage.

On my my mini-lathe it was possible to see the half-nuts by angling a hand-mirror under the saddle. Does that give any clues.

Another possibility: lathes are protected from damage due to accidentally powering the saddle into the chuck with a pin designed to break during an overload. Sometimes the pins don't sheer cleanly and you end up with a leadscrew that looks as if it's working but slips under load. With the half-pins engaged and the chuck spinning slowly under power make sure your leadscrew is turning properly. As it moves towards the chuck, try pushing the carriage backwards by hand to see if the leadscrew stops turning: if it does, check the pin and the gears to see where the drive stops. Although the mini-lathe isn't very powerful take care not to get caught in the works!

The leadscrew isn't very stiff. If it is bent, not too difficult to remove and straighten in a vice provided it's not wildly out. Depending on the model it may be possible to replace it.

Dave

Edit: Bandersnatch got in while I was making coffee...

Edited By SillyOldDuffer on 09/09/2018 17:11:31

Ah, I see your tool is not the more usual type we see on mini lathes.

The handwheel and its pinion should just freewheel round when the leadscrew half nuts are engaged. It's not unusal for the backlash in the handwheel gear train to cause the handwheel to 'stop and go' as the weight of the handle make sit turn forwards, then wait for the gears to catch up. You can ignore any irregularity in its movement when using power feed, as long as the saddle itself doesn't 'stutter'.

If you feed using the handwheel, then the backlash is taken up and you won't notice any problems.

The finish in that last shot looks OK reasonable. What depth of cut and speed are you using?

It's worth loosening the two cap screws that hold the apron to the front of the saddle. Close the half nuts and the apron will 'align' with the leadscew. Close up the screws gradually, checking that the half-nuts engage easily without pushing the leadscrew in and out. This will give you the best possible alignment.

Dare I suggest you may find my 'Mini Lathe' book useful?

Take a photo of the whole machine for us to try and we will ID it!

Neil

Hi Charlene, if the change gears are plastic, is possible for the one on the leadscrew to slip over the key. It may look OK and you may not be able to make it slip over by hand. I had this problem on my mini mill, took me a little while to discover that the gear on the motor would slip on occasions and while the gear looked OK, when measuring with my digital callipers, it became apparent that it was very slightly egg shaped at the keyway. fitting a new gear solved the problem.

Regards Nick.

I really appreciate all the suggestions, thank you very much.

The lathe below. Seems like about 12" between centres, if I measured correctly.

Maginification of the finish I'm getting: