Static or electrical problem with lathe

I have been using my mini lathe for five years with no problem. A week ago I was turning a piece of BDMS 30 mm dia with a TCT tool which was producing long stringy swarf. Suddenly there was a flash and a bang. The garage and the house RCDs fell out - but not the 16 amp circuit breakers.

I looked into the control panel box of the lathe but there seemed no visual problem. The lathe 10 amp fuse had not blown. I tested the earth connection from the lathe to the mains and all is ok. There are no chafed or bare wires. After re-setting the RCD’s I tentatively tried again and the lathe worked ok.

Yesterday doing a similar operation there was a repeat bright flash, bang and RCD’s falling out. But once reset all worked again. Visually although very fast the flash seems to come from the tool and position where the BDMS is being turned. These incidents were turning BDMS bought from two different steel suppliers so it is just standard stock material.

I contacted the machines reputable suppliers but they say they have not heard of this before.

Has anybody else had this problem or any idea as to what the cause may be.

The flash is very worrying though - suggesting full mains voltage rather than a small earth leakage. Maybe the long swarf is getting into the motor or switch somewhere - touching mains voltage somehow?

I would strongly suggest not using the lathe and disconnecting it from the mains until you can get it fully PAT checked. It might be that the body of the lathe is getting up to mains potential somehow  

I would also double check and electrically verify the earthing arrangements in your workshop

I would not risk using the lathe until then - if the chassis is live or sometimes live, it could kill you  

Edited By John Doe 2 on 28/06/2023 10:04:05

I had a similar problem with my mini-lathe. A ribbon of swarf penetrated into the control box through the lead-screw hole and shorted out to earth. Presumably the rotating screw pulled a dangling ribbon inside.

Didn't do any damage - I was lucky. I took the front panel off to reveal the control board, and removed some obvious bits of swarf ribbon. Then I hoovered the innards and went round the components with a Q-tip to make sure no tiny bits were left behind.

I stopped it happening again by taping around the hole to reduce the gap and by sticking a short plastic shield over the lead-screw at the headstock end. Some mini-lathes come with a plastic bush to keep swarf out. I also cleaned the lead-screw more often in case the ribbon had been drawn in by catching on dirt.

Good chance this is your problem. I can't think of an electrical fault that would cause sparking between chuck and tool-post. In a mini-lathe both are firmly bonded together and can't be at a different potential. But after being drawn into the control-box. my ribbon of swarf burned out after earthing near the chuck. Hard to tell exactly where because a a few centimetres disappeared in a flash.

Dave

If you think about it a 5A cartridge fuse will blow by melting at a lot less than 16A and if it was not inside a ceramic tube there would probably be a flash and a bang. This means a short from mains to earth caused by a fine ribbon of swarf can trip the RCD which only requires a small current to trip it but the 16A fuse may not blow. There will probably be some evidence somewhere of the short and, as pointed out above, it may be hidden in odd corners or places you have not thought about checking.

I imagine the short was to the earthed metalwork of the lathe so I would think in this case the earth connection has worked the way it is supposed to.

Martin C

Yes - that is not an effect of the lathe or turning operation; but different metals produce swarf that can behave in its own ways, depending on the particular operation.

Brass and gunmetal produce fine needles that can penetrate anything.

Others, including aluminium, phosphor-bronze and some steels, give long strands that can worm their way into motors or switch-boxes.

How do I know? Errr....

Phosphor-bronze. Most of the swarf helices fell towards me but they were the decoys to allow one to creep round the back of the headstock, on an Myford ML7, and into the motor. All stop, house as well. The breakdown did not hurt the 3ph motor but wrecked the inverter.

So....

Have, as other advise, the electrics tested for any faults.

Ensure the motor and switch are protected from swarf - but do not obstruct cooling air-flow.

Thank you all for responding so quickly to my query. Although I had thought that I had all nooks and crannies for swarf taped up there are always the 'parts that other swarf cannot reach' !!.

There may well be an electrical problem I cannot detect so taking your advice I have contacted Warco again who say they are happy to check the lathe over. It has otherwise given me five years trouble free service. Wouldn't you know they are in the middle of a major move so I will have to wait two weeks. Such is life.

Thank you all agian your help is very much appreciated.

Some times filter components can fail with a flash bang and smoke, the equipment can keep working without the filter being operational but will now be putting noise onto the mains supply. A failed capacitor can be hard to spot as the only damage may be a crack that is not easy to see.

Mike

How bright was the flash, and how loud was the bang?

If the RCDs have tripped, the fault must have been to earth: a L-N fault, however big, will not trip an RCD.

I suspect that a bit of swarf might have caused an earth fault, but it was sufficiently thin that, in effect, it was its own fuse. The flash vaporized the swarf, which is why you found nothing.

Problem hopefully solved.

It was the second occasion when I opened the control panel and now recal that the fuse holder cap, which is on the side of the box, was slightly loose so I had tightened it. I thought little of it at the time as I was more intent to look inside. I presume I may have caught the cap with my hand when changing the chuck but not noticed.

After all of the replies I looked further to see were swarf may get into places it shouldn't although I have been carefull to block anythhing up. However, when I checked the fuse holder cap I found that it can be unscrewed to give a gap up to 2 mm of exposed brass screw which of course is a terminal of the fuse holder, while still being functional. Although certain that it was not this loose it is certainly possible that a randon thin swarf string waving about had got into the space. I presume the electricty ran from the fuse holder to earth via the tool position on the work, which is where I saw the spark.This would explain why I had been using the lathe for some time but it only took one errant string to cause the problem and afterwards the lathe continued working. As Chris Pearson says it became its own fuse.

So that it doesn't happen again I have turned a circular plastic shield to prevent the cap being accidentaly opened and held this on the side of the box by double sided foam tape.

Many thanks again to everybody for your help.

Glad you found the probable cause Geoff. Might still be a wise move to let Warco check over the lathe to give you piece of mind.

regards

2 mm of exposed brass screw which of course is a terminal of the fuse holder

Is that a satisfactory arrangement? An accessible live conductor at mains voltage?

Not good is it! Though the fuse holder in question may be the 110VDC motor, not mains, accessible conductors are an old problem.

Exhibit A is a well-made British fuse-holder of the last century:

And here it is successfully rendered dangerous by a whisker of wire despite the cap being fully screwed home.

By far the most dangerous electrical devices I grew up with were valve TV sets. Users were protected by a flimsy compressed cardboard back, punched full of long ¼" wide slots for ventilation, and held on with a few self-tappers. Inside, the power supply was super-dangerously rectified direct off the mains, no transformer, and included an 15kV plus Extremely High Tension output to drive the Cathode Ray Tube. Many early TV sets emitted dangerous levels of X-rays too!

I've never understood why the safety standard for 13A plugs and sockets was so much stricter than those applied to TVs, Wireless, and Gramophones.

Dave

Edited By SillyOldDuffer on 29/06/2023 19:08:07

A fuseholder carrying above 50V that can be undone without using a tool or removing a separate cover first does not meet current electrical standards.

Robert.

A screwdriver slot constitutes "needing a tool". Being loose isn't its intended use.