Blown Fuse

The fuse blowing surely indicates the cause was electrical. Inclusions in Aluminium alloy are unlikely, believe it or not commercial metals are rarely made by blokes melting stuff in a pot any more! But even if Keith was machining a highly active metal and it ignited, it wouldn't blow a fuse. There's no way of translating chemical energy in a fire into electrical energy in the fuse.

On the other hand electrical faults do start fires and blow fuses!

I agree with others suggesting failure of a suppressor capacitor on the mains input; they are highly stressed, and not any old capacitor will do the job. If the dielectric fails a heavy current flows to earth, very likely enough to blow a fuse, but then leaving the capacitor open-circuit so everything works again. (Apart from the exploded suppressor function!) An electrical fault could create Keith's symptoms, which suggest the headstock isn't properly bonded to the bed electrically. Bonding ensures all metal parts are at the same potential if a fault occurs, and, earthing the whole, makes the machine safe. If bad bonding is behind the event, Keith would have been electrocuted had he been touching the chuck when the capacitor let go rather than cutting metal.

I'd check the earth wires are all properly connected.

Dave

Hello,

I had in my workshop some drill machines and the lamps with surpressor main capacitors. They worked very fine till an age of 20 years then they fail with a shortage of the mains. The capacitor had a self healing feature. That means after the first shortage the blown the plastic film in the capacitor gets very hot and build a new film. The film is thinner then before an after some time it happens again. And in the time between you find with normal measurement possibillities no fault. You have to have a meter which measure the quiscent current at a high test voltage and this is expensive. I cut them out and make bridges or put some in with a higher voltage.

The next is the harmonizing of the main voltage of 220 V to 230 V for the first look no big difference, but they change the tolerance, as I know before 220 V +- 10 % and then 230 V + 10% . In our house with solar on the roof we have till 256 Volt main voltage. The suppresor capacitors have a print on of 220 V, it is only a question of time when they fail.

Werner

One possibility occurs to me which I haven't seen mentioned. Are there any mains conductors with chafed insulation, possibly at entries or corners? If the insulation finally wore through - vibration could cause this - and the conductor made contact with earthed metal, it would give a big spark, and could very easily burn itself clear, for a while at least. I cannot explain the spark at the tool point, however.

George

Was the spark strong enough to pit the metal?

The surpressing capacitors have three lines two for the mains and one for earth. this make them effectiv for surpressing. The shortage is in the capacitor direct from main to earth.

As it happen to me there was such a thunder that I could not recognize anything other special things.

Werner

It would have to be very fine swarf to burn in air.

What type of lathe is it? I can't think of any machine that doesn't have a metal to metal path between tool and spindle.

Was the machining being done dry, without coolant or lube?

One of the forum members here has previously commented about the build up of aluminium dust on a bench sander which ignited when he later held a ferrous metal to the sander without cleaning off the aluminium dust.

I wonder what size aluminium has to be reduced to and at what temperature it becomes ignitable?

Edited By Ed Duffner on 14/08/2019 18:13:00

They are probably not going to turn up in the home workshop but machines have been built that run on PTFE pads, most likely there is still an earth path through the ballscrew or other equipment mounted on the table like the inductosyns etc. One thing welders were careful about was where the weld current return path was going, we did have a few occasions when the return clamp made a poor connection and the weld current fried the earths in the equipment wiring, they were also careful not to have weld current flowing through ball races as they could arc and it was goodnight to the bearings.

Mike

I have just tried to answer most of the queries, at some length, but when I went back a page to read some of the answers my new post vanished, This has happened previously and I would like to know what I am doing wrong.

The lathe is a Clarke 300M mini lathe. I have checked the earthing of every part 4 times now and am happy that all is correct. I cannot see any signs of chaffing of any of the wiring. I had been using White Spirit as a lubricant, but as this was the final polishing fine cut ,it was dry. The only swarf that was near the tool was from this pass and was very fine and lacy. The "event" happened half way across a 1 inch diameter, so the tool was already in contact with the item. No marks of the flash were left on the item.

I will, tomorrow, stop everything and take the board out and have a look underneath. I am looking for physical signs of failure, but other than a multi meter I have no means of checking whether a capacitor is half failed, as suggested.

Keith

More evidence that physics doesn't work quite the same in your part of the universe, I'm afraid.

Or you may be navigating away from the page without waiting for the new posting to load?

Neil