Jump to content
Non-Gold users will see a video pop-up ad on most screens. This is a test. It should not appear on mobiles etc. ×

Unimat SL keeps blowing house fuse/breakers - is there an economical repair?


Recommended Posts

  • RMweb Gold

Hello everyone,  I have acquired an old Unimat model SL lathe that I would like to use for some scratchbuilding.  The problem is that, when I switch it on, it trips the house power breakers.  I would like to find out if this is a, sort of, common issue that can be repaired economically; or does it mean an expensive replacement motor?

 

I would be grateful for any helpful advice on this please.  In the meantime, I will carry on cleaning years of grime and dust off the  machine, to bring it back to a nice condition.

 

Mike

Link to post
Share on other sites

Doesn't sound too good, at the moment you have a short in the windings of the coils it would be replacement. These motors are very week - if I a remember right they had 95W and on top of it S3 intermittent duty cycle (not sure about the percentage but may be it was 40 - which means intermittent use 40% on 60% off) - should you replace it try to find something more powerful and for constant use (S1))

 

My father had one and I made my first stationary life steam engine with this little lathe...

Link to post
Share on other sites

Sorry I can't help you with the problem but I do hope that it came with a tool post.

 

I recently bought one (Unimat SL that is) and I have had the devil's own job finding one. In the end I had to buy a used one from the US and the postage was 3 times what the item actually cost ($20). I am just hoping that it arrives before we start enforcing tit for tat with the proposed US import duties.....

Link to post
Share on other sites

  • RMweb Gold

Doesn't sound too good, at the moment you have a short in the windings of the coils it would be replacement. These motors are very week - if I a remember right they had 95W and on top of it S3 intermittent duty cycle (not sure about the percentage but may be it was 40 - which means intermittent use 40% on 60% off) - should you replace it try to find something more powerful and for constant use (S1))

 

My father had one and I made my first stationary life steam engine with this little lathe...

Thanks Veccio, I am a complete amateur on this and I'll take note of your recommendations, if I have to buy a replacement motor.

 

Sorry I can't help you with the problem but I do hope that it came with a tool post.

 

I recently bought one (Unimat SL that is) and I have had the devil's own job finding one. In the end I had to buy a used one from the US and the postage was 3 times what the item actually cost ($20). I am just hoping that it arrives before we start enforcing tit for tat with the proposed US import duties.....

Is that the "verical post" assembly, according to my manual?  If so, then yes; it has all sorts of gubbins like that with the lathe.

 

cheers

Mike

Link to post
Share on other sites

Thanks Veccio, I am a complete amateur on this and I'll take note of your recommendations, if I have to buy a replacement motor.

 

Is that the "verical post" assembly, according to my manual?  If so, then yes; it has all sorts of gubbins like that with the lathe.

 

cheers

Mike

 

Hi Mike,

 

No that's for using it as a milling machine - I too got one of those. The tool post is the part that fits on the top slide and holds the cutting tools when you are turning. The distance from the top of the tops slide to the centre line of the lathe is just less than 12mm so you need a very shallow tool post to fit and most that are available are just too deep.

Link to post
Share on other sites

The problem is that, when I switch it on, it trips the house power breakers.

 

Mike, that sounds like a solid short in the machine. Is the motor "trying" to start before the power breakers are tripped?

If so, it is probably the motor, if not, it might be as well a problem in the wiring. Do you know a person that can look into that?

 

Michael

Link to post
Share on other sites

Worn brushes? I brought my Unimat 3 back to life with some new brushes, made by modifying some vacuum cleaner brushes.

 

Or you don't have to spend big bucks on a new Unimat motor, have a look at sewing machine type motors.

Link to post
Share on other sites

The problem is that, when I switch it on, it trips the house power breakers.

Mike

So that we are absolutely clear here do you mean it trips the MCB (indicating a hard short) or do you mean the RCD (which indicates an earth leakage).

Link to post
Share on other sites

  • RMweb Gold

So that we are absolutely clear here do you mean it trips the MCB (indicating a hard short) or do you mean the RCD (which indicates an earth leakage).

I don't really know the answer to that meil.  The sequence is:

I switched on the machine, it did a partial turn then all the electrics fail up to the breaker switch in the house.  The lathe is in the shed, with electrics wired to a breaker box there.  The shed is then wired to the garage, again with its own breaker unit, and all is connected to the mains breaker unit in the house.  By switching on the lathe, the whole circuit tripped right back to the main unit.

 

Hope that helps explain things.  ;)

 

Mike

Edited by Royal42
Link to post
Share on other sites

What size fuse is in the plug? Under normal circumstance this should blow first.

No no no the fuse in a plug is slow acting compaired to a modern trip, a trip will go in milliseconds of a overload where as a fuse takes several seconds to melt and blow, this is why modern electrics use trips as they trip befour your dead, rather than a fuse which blows after you have been killed, not that any of which helps find the problem

Do you have a multimeter? As you could track down a short with that if you know what your doing!

Unhelpfuly with out haveing it in front of us we are on a guessing game that could go on forever

Edited by Graham456
Link to post
Share on other sites

So that we are absolutely clear here do you mean it trips the MCB (indicating a hard short) or do you mean the RCD (which indicates an earth leakage).

Mike putting simply

What he means is is it the?

MCB = the little individual trips in the box that protect a circuit each

Or

RCD =the big master switch that switch it all off

Link to post
Share on other sites

Royal42 Be a bit careful you could end up dead.   I had a similar issue with my Mitre saw, turn on squeeze the trigger and ping goes the RCD. Quick, well not that quick down from loft to basement to reset it and try again. Ping.  If at first you don't succeed try something else so I plugged it in to a different socket on a different ring main. Success!   It ran, and ran, and ran.  We think there was some minor earth leakage on the saw due to damp and plenty more elsewhere on the first ring main so cumulatively it tripped the first circuit.  It is quite possible that you have a similar scenario and if you get the lathe out of a potentially damp/ with condensation, shed and let the lathe dry out thoroughly and plug it in indoors it will be fine.  Personally I would be incredibly careful with mains electricity on a large metal lump like a lathe. My Emco is lethal enough when work flies out of the chuck without getting electrocuted as well/first.

Link to post
Share on other sites

Back to basics. Unplug the lathe (and keep the plug where you can see it so some helpful person doesn't plug it in for you at an awkward moment). First up, check that the plug is wired correctly. Many are not. Make sure it's got the right fuse in. I suspect 3A would be appropriate for the little Unimat motor. Check the lead carefully for damage to the insulation. I'm not familiar with the Unimat motor specifically, but the first thing to do is clean it thoroughly. The sort of crud that builds up around lathes has a high metal content and so is quite conductive. Get it all off the outside first, then have a look in through any cooling slots. If there's rubbish inside you'll have to open it up and clean it internally.

 

While you're in there, check all the electrical connections, looking particularly for stray strands of copper, more conductive crud and making particularly sure the earth core of the mains cable is firmly and conductively attached to the metalwork as it should be. Make sure the live core and anything it feeds only have continuity to anything else through the motor brushes and windings.

 

With the motor apart, make sure the brushes are insulated from each other and from the motor frame. Make sure the commutator is clean. Carefully scrape any deposits out of the insulation gaps. Crud in this area is basically carbon and copper particles so it has the potential to short stuff.

 

All the above can be done without a multimeter just by visual inspection, although a meter (or even just a battery and bulb continuity tester) will help. However, if nothing comes to light up to this point, it's time to check the windings with a meter. None should connect to earth, so that's the first and easiest check. Similarly, none should connect to each other, so you need to identify each end of each individual winding. There should be infinite resistance between separate windings and a smallish but measurable resistance between the ends of any one winding. That resistance should be the same for each. Any that read zero or which are otherwise appreciably different from the others represent a fault.

 

Bottom line is that you appear to have electricity getting to where it shouldn't. The key is to find the route it is taking to do so. In the absence of complex electronics or sealed black boxes this is well within the capabilities of any individual able to work methodically and read a multimeter.

 

Dont plug the thing back in until you've found and rectified all evident faults and then applied your meter to the plug pins to check for three things. First that there is zero resistance between the earth pin and any metal part of the motor that you can touch. Second, an infinite resistance between the active pin and any touchable metal. Third a non zero resistance between the active and neutral pin.

 

Then you can (cautiously) plug it in and try it.

Link to post
Share on other sites

  • RMweb Gold

Thanks everybody, some very useful and sage advice there.  It appears that the previous owner used the lathe for wood mainly and it seem to have clogged in most places.  There also appears to be traces of filings gelled in amongst the woody crud.

Pat, I shall give outer part of the whole area a good clean and then set about going through the stages you mention.  I shall take it slow and methodical as it's been about 50 years since I last stripped any motors down for a home repair.  The things we used to do as teenagers!

 

Thanks again everyone and very much appreciated.

 

Mike

Link to post
Share on other sites

Had the same problem with my U3. Luckily I found an electric motor specialist not far from home who took the job on. They stripped and cleaned the thing out for £20. It was the smallest motor that they'd seen, they usually deal with larger jobs for woodworking equipment.

So, try googling for motor repairers in your area, you might get lucky.

Link to post
Share on other sites

From all that you have said, I would say that dust from the carbon brushes is causing an earth leakage current.  I had exactly this problem on my Unimat 3 when we moved a few years ago from a house that did not have a RCD to one that did.  Cleaning up the insides to remove all the dust would be well worth a try, as I suspect that's what is causing the leak.  I very much doubt that the problem is one of drawing excess current.

 

I had no trouble taking the motor apart, cleaning it and putting it back together again, with new brushes whilst I was at it (universal brushes from eBay that I filed down to suit).

 

I hope this works for you.

 

David

Link to post
Share on other sites

Had my SL since I bought it second hand in  abouit1978, motor finally gave up the ghost recently and I replaced it with a scooter motor as per the thread posted earlier. Also replaced the rubber drive bands with silicone ones - Massive improvement in power, the lathe is around 500% more useful now, wish I'd done it ages ago!

  • Like 2
Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...