Stripped the drive belt on my Chinese Mini lathe

[Jim Dalton 1Participant @jimdalton1]

Hello all

Great forum I've been viewing for a while, this is my first post, looking for help I guess.

So I am somewhat of a beginner, but I've been using my Chinese mini lathe (Sieg SC2 from ArcEurotrade) for a number of months now, with reasonable success.

Today however the motor was stalling. I stripped back the motor end to find a stripped drive belt, on the motor to headstock drive.

I have no doubt this is down to my lack of skill on the lathe, but my questions are

1/how might I prevent this from happening again?

2/ is the immediate fix just a case of slipping on a new belt?

Many thanks

JD

[Jim Dalton 1Participant @jimdalton1]

[not done it yetParticipant @notdoneityet]

If the motor is actually stalling, I would expect a problem, downstream of the belt, that needs finding and fixing.

Either a mechanical issue or overload (by the operator) issue, I would think.

Replacing the belt is an obvious requirement, but I doubt that alone will solve the problem of stalling the motor.

If it is not actually stalling the motor, the problem might have been only an improperly tensioned drive belt.

[SillyOldDufferModerator @sillyoldduffer]

As NDIY said, and the immediate fix is to fit a new belt. As that's done by slackening the motor fixing bolts you can take the opportunity to tension it correctly; not loose, just slightly tight.

How hard to drive a lathe is a learned skill. Two beginner mistakes are being either too gentle or too brutal. I run my lathe by ear; l like the motor to sound as if it's having a bit of trouble doing the work, but not labouring or being significantly slowed down. Not a good idea to flog the machine and it shouldn't be necessary.

Another possibility is the suitability of the metal you're turning. I started practising on odd bits of scrap which turned out to be a bad mistake because a lot of scrap metal doesn't machine well. Apart from Brass, DIY store metal is horrible too. Better to get a machinable Aluminium Alloy, Brass or Leaded Mild-steel of known specification. These cut nicely with sharp HSS or carbide inserts.

I wouldn't recommend strengthening the lathe. Trouble with doing that is something more expensive is likely to break next time. A stripped belt is much easier to fix than a broken Hi/Lo gear, damaged motor, or fried control board. Don't ask how I know…

Dave

[Michael GilliganParticipant @michaelgilligan61133]

Jim,

When you remove the stripped belt, could you please share some detail photos of the damage. … These might help identify the cause of failure.

Thanks

MichaelG.

.

Edit: Some useful notes here …

 https://www.acorn-ind.co.uk/insight/top-6-causes-of-timing-belt-failure/insight/top-6-causes-of-timing-belt-failure/

and this is the page I was looking-for …

https://fear.al/wp-content/uploads/2017/01/Probleme_Diagnostikimi_Zgjidhja_Rripa_Faze.pdf

Edited By Michael Gilligan on 02/09/2019 10:19:30

[LainchyParticipant @lainchy]

Posted by SillyOldDuffer on 02/09/2019 09:45:44:

Another possibility is the suitability of the metal you're turning. I started practising on odd bits of scrap which turned out to be a bad mistake because a lot of scrap metal doesn't machine well. Apart from Brass, DIY store metal is horrible too. Better to get a machinable Aluminium Alloy, Brass or Leaded Mild-steel of known

VERY true. I bought some offcuts from my local steel stockholders… bad move. If you want some bits to play with, I suggest eBay… selection of EN1a / Brass / Aluminium of varying sizes… perhaps 10, 15, 20, and 25mm round. If you get it all from one supplier, you may find the postage is OK. You don't have to get it all at once either, and the bits you don't use can go in a box of bar stock. That's how I've done it anyway. I think I paid £7.50 for the last piece of 25mm En1 – 300mm long. Maybe to avoid too much expense, start with 15mm brass, and a piece of 15mm En1a. That should give you a good start maybe.

Enjoy

[Martin of WickParticipant @martinofwick]

Ex factory, the supplied belts are not always of the highest quality. I have a C1 on which the first belt simply disintegrated as if attacked by some sort of contamination. Replaced with UK sourced T5 belt, not had a problem since.

Was the lack of drive due to motor stalling or the belt slipping?

I suspect that you need to look at alignment and tension in the first instance. Prolonged high speed running on tight belts can cause accelerated wear. Have some mechanical sympathy, avoid loading up the lathe by checking/reducing the RPM drop under cutting load. Oh, and avoid hard mechanical 'stops' (jus sayin!)

Easy to replace, unscrew the plastic plate on the end of the motor pulley, slip the old belt off and the new belt on, then check alignment of the motor with the lathe bed , that the motor pulley is in line with the countershaft pulley and a reasonable tension on the belt – say 1/2 an inch play

ex factory the motor on my CJ18 the back of the motor was an inch higher than the front ( how do they do it?). Detected by the slight whine as the pulley wheel tried to pull the over tensioned belt off the countershaft pulley. Had it been left in this condition, the belt would have failed quite quickly.

Ah the mini lathe – dontcha luve em!

Edited By Martin of Wick on 02/09/2019 11:24:20

[Brian GParticipant @briang]

I suspect a lot of us have been there, they seem to be designed to make the belt difficult to tension properly. I removed the headstock on my son's Warco mini-lathe before realising the sound was a slipping belt and not the back-gear. The belt was so loose I didn't need to loosen the motor bolts to remove it, but as it was undamaged I didn't think it could be the problem.

I couldn't find any information on how tight the belt was supposed to be, so used the same method as for Volkswagen timing belts, setting it so that I could nearly twist the belt 90 degrees in the middle of the run, since which the same belt has been trouble-free

Brian

[old martParticipant @oldmart]

Assuming the belt is a toothed type and some of the teeth broke off, they may have been responsible for the stalling. These belts have a number on the outside, and are very easy to get. They don't have to be very tight as the teeth do the driving, but as already mentioned, check whether the pulleys are in line and parallel.

[Jim Dalton 1Participant @jimdalton1]

Thanks all. I don't think the motor actually stalled. I'm having trouble trying to upload a photo of the belt, bit suffice to say that almost all of the teeth are stripped clean off!

[HOWARDTParticipant @howardt]

I shredded my belt after three years of use. As I couldn't get a replacement I bought a 5M set of belt and pulleys. Fortunately I had a mill so was able to bore out both pulleys. Still working six months later. I prefer the 5M setup as the teeth are a more positive drive form, a little more noise but not obtrusive. Also the belt is readily available.

[Ian PParticipant @ianp]

Posted by Jim Dalton 1 on 02/09/2019 14:12:35:

Thanks all. I don't think the motor actually stalled. I'm having trouble trying to upload a photo of the belt, bit suffice to say that almost all of the teeth are stripped clean off!

Hard to imagine why so many of the teeth have been removed. I am not conversant with that lathe but its only the driving (motor) sprocket that can do the actual stripping, the driven sprocket has no power (other than inertia) to impart any shearing force on the belt teeth. My expectation would be that once a continuous group of teeth have been removed the motor sprocket would not be able to impart any significant force in section as it would just slip.

I can imagine that if the motor was running and the chuck was stationary (because of some stripped teeth) and the chuck was turned by hand bringing the first good tooth into play, it would most likely get sheared (snatched) off.

Timing belt drive is very robust and reliable technology. My money would be on either a poor quality belt or possibly the wrong pitch belt was fitted in error, some metric and imperial pitches look almost identical, also there are belts and sprockets that have different tooth forms (for low backlash, power transmission etc).

Ian P

[Michael GilliganParticipant @michaelgilligan61133]

Posted by Jim Dalton 1 on 02/09/2019 14:12:35:

Thanks all. I don't think the motor actually stalled. I'm having trouble trying to upload a photo of the belt, bit suffice to say that almost all of the teeth are stripped clean off!

.

Start here, Jim: **LINK**

https://www.model-engineer.co.uk/forums/postings.asp?th=103028&p=1

… it's easier than it looks.

MichaelG.

[Jim Dalton 1Participant @jimdalton1]

With thanks to Michael, here's the belt;

[Ron LadenParticipant @ronladen17547]

Jim, was yesterday the first time you experienced the the problem i.e. had the lathe been fine until then. Also what sort of work has the lathe been doing to date, any heavy work..?

[Martin of WickParticipant @martinofwick]

Hmm…

always slightly worried when I read that replacement belts are hard to get… on both my Chinese specimens I have used a 5T replacement belt, readily obtainable from the usual belt and bearing factors. Maybe I have used the wrong belts…If the originals were not 5T then they must be very close as never had a problem with the 5T replacements.

HTD 5M is better for power transmission, (and belts are about half the price of synchroflex) but as pointed out, you have to change the pulleys.

The dodgy plastic motor pinion is a potential source of problems. From new, the supplied belt and replacement belts did not fit that well on my mini lathe (well the teeth fitted into the gaps, but not what I would call a precision fit). I have ignored the issue in the absence of any problems to date. May replace it one day with a metal pulley as I think could help reduce heat build up in the belt.

[Jim Dalton 1Participant @jimdalton1]

Ron, it wasn't running very well of late. However, I was attempting to part off and had the workpiece dislodge from the chuck, so perhaps that's the root cause.

While I am ordering the replacement belt, is there anything else I should order to improve the setup and machine performance in general, or are the best mods the diy ones?

[Martin of WickParticipant @martinofwick]

……….I was attempting to part off and had the workpiece dislodge from the chuck

Aha! after remorse comes the confession… Don't worry we've all been there!

Thing is, given the nature of the belt, at the kind of RPM and torque load needed for parting, the 5T belt is right on the limit of its design parameter and close to tooth stripping territory and a lock up is the last straw.

Drilling larger holes is another activity that puts a large load on the belt and motor.

As above, switching to 5M system may help a bit, but the difference to T5 at low RPM is not huge.

Although it is a PITA, (particularly setting motor alignment and position) it is well worth ensuring all alignments and belt tension is appropriate to give the belt the best chance of survival. Loads of other DIY mods available on the net to suit any particular requirement.

Often you can hear the protest from the speed controller or see the RPM drop when the motor is struggling. Minimising strain on the machine comes with practice and experience and breaking a few tools is just part of the learning process.

Just crack on, and soon you will be lathing away like a pro!

[Jim Dalton 1Participant @jimdalton1]

Thanks Martin. Appreciate the words of encouragement. There are so many mods, I don't know where to start. My work is mostly turning down round bar, drilling cross holes, tapping all the way through cylinders of approx 4 inches long. And some knurling. So anything that might improve performance in these operations.

[Jim Dalton 1Participant @jimdalton1]

Thanks Neil. By the way, I've enjoyed your book on lathes.

[MadMikeParticipant @madmike]

Jim have you spoken to Ketan at Arc Euro? He is very helpful.

[JasonBModerator @jasonb]

Posted by Martin of Wick on 02/09/2019 17:18:56:

 

The dodgy plastic motor pinion is a potential source of problems.

I thought the SC2 had an aluminium pulleys, at least that is what ARC show.

And the T5 belts are not the same pitch or profile as the stock belt

Edited By JasonB on 03/09/2019 07:40:26

[Neil WyattModerator @neilwyatt]

Thanks Jim

[Neil WyattModerator @neilwyatt]

These belts actually need a fair tension, There should be a push-pull screw arrangement for the motor (subtly different between lathe models).

Belts should be easy to source from the sellers of these lathes.

If the motor pulley has worn so the teeth are no longer square you may need a cheap replacement to stop it slipping again. A temporary fix is reversing the pulley, although it may need modifictaion.

Neil

[Martin of WickParticipant @martinofwick]

Oops,

Guilty as charged of assuming all CMLs are the same. Should have read the OP more closely so treat all my previous comments as not applicable to the Seig flavour of mini lathe.

Interesting to know what the belt standard is. Certainly the T5 does not look like the OEMs belt that I had (kind of looks halfway between a T5 and an M5). As it fitted into the pulleys OK I used the T5 anyway.

So Jim, best stick to OEMs spares unless changing the drive system. Looking at the work you do, it seems to be biased towards low speed high torque end of lathe operations (tapping, knurling, drilling etc). Because of the lack of back gear arrangement on CMLs, I think it is fair to say the CML struggles in this region and will always put large loads on the belt.

In terms of mods there is not much to assist high torque operations with this type of lathe without totally remodelling it. Some designs of auxiliary counter shaft are available, but while these help match motor and controller for low speed operations, the torque loads on the final drive to the spindle remain the same.

One change I made to the leadscrew drive was to use a Ralph Paterson mod that places an extra change gear reduction step on the banjo. This then makes some very fine feeds available (down to .001 thou). I am not sure if the SC2 has the same banjo arrangement as the older style non brushless CML, but you could take a look.

Some people have fitted an ammeter to judge how much load they are putting on the machine.

If you are doing a lot of tapping, as an accessory I would recommend making a hand crank for the spindle rather than attempting to bully taps through the job using the motor, as that places the drive under a lot of strain.

I don't know what arrangements are available for adjusting the gibs on the carriage on your SC2. On my CML, this was a total pig, In the end I got fed up with having to partially dismantle the lathe to adjust and then having it loosen off a week later etc . I used a design from the internet and replaced the original arrangement with a tapered gib arrangement (forget who – google minilathe tapered gibs). There was nothing fundamentally difficult in the job, but you need access to a mill and another lathe (I had to reduce the shaft diameter of the carriage drive gear shaft). A great deal of faffing and fitting to and fro was required – but what joy to be able simply adjust carriage play without having to take off the apron.

S

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