Blown Fuse

I would appreciate peoples opinions and possible explanation of the following.

This afternoon ,machining the aluminium prop driver for a model aero engine I am making, taking a final light facing cut, 400rpm,slow hand feed, there was a sudden loud bang and a flash of flame at the tool point and everything stopped. I checked the lathe fuse and it had blown. Replaced it with new one and all started working I then switched everything off, took the cover off the electronics,suspecting a stray piece of swarf had shorted something, but all looked clean with no smell of burning, so I put it all back together and continued where I had left off.

I am completely mystified. The tool was a brand new,shiny tip for ali. cut was about 3 thou, slow feed and was about half way to the centre of 3/4"dia H 15 aluminium, so friction was minimal.

Keith

Hi Keith, this sounds a bit dodgy, and tbh almost like a lightning surge, as there should be no potential difference between the work and the tool point! Check the earthing on the lathe, and also check the earthing to your workshop and your house, or wherever the lathe is fed from. I suppose it could be a static discharge, but very unlikely, and impossible if the lathe and the installation is correctly earthed. Get it checked to be on the safe side!!

Phil

Thanks for that Phil, I will certainly get things checked out,as you suggest. The bang was very loud and the lacy swarf at the tool tip burned. It was not a "pop" that you get when a fuse blows. The failed fuse did not show visual signs of burning out..

Keith.

What type and rating of fuse are you using ?

Emgee

As Phil says, it should be almost impossible for there to be a potential difference between tool and work . . . unless there was, say, a plastic sleeve protecting the workpiece from damage in the chuck jaws Even then the tool should conduct any charge away from the workpiece while it is cutting.

The metal-to-metal contact between component parts of the machine, including the bearings, should equalise any static charge build up. I'm not sure any sort of static charge would blow the mains fuse anyway as there should be no connection between that and the machine chassis. The fuse 'blows' when it passes excessive current in the live supply lead.

Very strange . . . can we asume you don't have a modern consumer unit with MCBs etc? If you did, surely this would have tripped out if there was a short-circuit anywhere in the power supply.

Just some random thoughts to throw in the mix . . .

Here is a radical and perhaps fanciful suggestion,

The aluminium being machined was from a faulty batch having alloying elements like magnesium or titanium included that had not integrated properly and Keith's tool tip ran into just such a piece of segregated material.

I am unable to think of any electrical cause for what happened that would manifest itself as a one off in this way.

One thing he hasn't described was what did the discharge, to give it a name, do to the tool tip and workpiece, other than he carried on with a new fuse in place.

Certainly very curious.

Brian

I have this morning, checked the earthing of the lathe by connecting the earth on the 13 amp plug to every bare metal part of the lathe I can see and getting a positive ring on my meter.. I also have one of those plugs for checking correct earth on 13 amp ring main sockets and all sockets in the workshop and house are correct.

Emgee,The lathe fuse is 6.3 amp HRC, 20MM cartridge.

Farmboy, I do not have a consumer unit . Still with original fuse box.

Brian, The discharge did not damage the tool or the workpiece,other than to ignite the swarf on the tool. Would your suggested mix up with the metal cause the fuse to blow?

Keith.

Hard to imagine how the tool and work on a lathe could be at a different potential but that seems to be what happened.

If you have a multimeter check for electrical continuity between all the metal parts of the lathe back to the earth pin on the plug. Everything should be the same low resistance to earth.

Older lathes may rely entirely on being bolted together to provide an electrical earth and perhaps over time corrosion or oil somehow formed an insulating layer. Modern lathes are explicitly earthed. All the main parts of my Chinese lathe are interconnected with earth wires BUT these could conceivably come loose or be broken. So in both old and new lathes it's possible one part of the lathe might get to float electrically above above the rest. Very unlikely I would think, but not impossible.

Another thing to check is where the unwanted power came from. Natural to assume it came out of the wall-socket in the normal way and the blown fuse suggests that's the case, but worth making sure nothing else in the workshop could have earthed itself via the lathe. Not, for example, a good idea to earth a welder via the body of a machine tool!

If the phenomenon was caused by lightning, a power surge, or a neutral or earth fault on the supply side you were lucky the electronics survived!

I quite like the swarf inside theory combined with an earth fault. If a bit of swarf shorted out the mains input to the headstock ( and disappeared in the process), AND the headstock ISN'T properly electrically bonded to the bed, AND the bed IS correctly earthed then a blue flash at the tool would be possible.

Very odd though.

Dave

PS I see Keith did a multimeter test while I was dithering.   To prove my theory, he should have found differences.  Ho hum , wrong again...

Edited By SillyOldDuffer on 13/08/2019 10:45:14

Is it possible the flash when viewed was behind the tool ie within the motor casing which from the viewing angle gave the impression of being at the tool tip?

An arc sufficient to blow the fuse should have left some mark on the tool tip and work item

It is entirely possible for a fuse to fail due to mechanical reasons when no electrical fault is present.

I have had a spark fly past the motor pulley on my rear drive Rockwell 10" bench lathe during a stall, the tip was also chipped. Motor? Nope, that is a sealed externally cooled one. Turned out to be an errant out of view QCTH jamming the vari drive pulley and also producing the spark.

This in the OP The tool was a brand new,shiny tip for ali. sounds like a carbide tip, which 99% of the time will chip or flake if the drive stalls.

Totally unqualified musing - was the material aluminium? ... Might there have been a magnesium/mag alloy inclusion which heated to explosion point (hence the flash and burned swarf) and a concurrent electric surge to blow the fuse?

retires to comfortable ignorance

rgds

Bill

I have had a second look at the electronics board and it is perfectly clean with no sign of any over heating. The flash and explosion,for that is what it was, was definitely at the tool tip, hence the burning swarf. The bang was so violent that I was still feeling the effect over an hour later.

The loading could not have been lighter and still be cutting.Low feed,fine cut and new tool tip rule out any chance of overload, surely. Everything is working perfectly at the moment, but I will admit to being somewhat apprehensive. I have now started to make the crankshaft, with high tensile steel, so hope I do not get a repeat.

We clearly have a mystery here and I thank you all for your input.

Keith

I don't suppose you are machining magnesium alloy, although that has NOTHING to do with the fuse blowing.

Whoops !  Just seen Cornish Jack's post

Edited By Speedy Builder5 on 13/08/2019 14:01:35

My audio amplifier gave a loud bang and cloud of smoke, after a good look round I could see no problem and powered it up and it worked perfectly. After a second coat of looking at I found the capacitor across the mains switch had failed, apart from the ear splitting bang and smoke the only physical damage to the cap was a hard to see split. It might be worth a very close look at things as not everything fails with dramatic burn marks and catastrophic physical damage.

Mike

Just a further thought, pursuing the lightning theory: is the workshop supplied by an overhead power cable? Or is there anything else that might act as a lightning conductor in the vicinity of the lathe? Having asked that I'm thinking you should have felt something if it was a bolt of lightning while you were hand feeding!

Reading the original post again, and some of the answers, I'm more incliined to wonder if there might have been some sort of inclusion in the aluminium which caused a spark. I seem to remember reading that fine aluminium swarf/dust can ignite violently when suspended in air, although I have no experience of it. I still can't think of a connection between either event and the fuse blowing though . . .

Interested to see if this can be solved.

If the swarf burned, it isn't aluminium, it's probably a magnesium alloy.

If you were hand feeding is it possible that you were taking an over heavy cut and the shock of seeing the pop made you jump, advancing the tool and stalling the lathe?

Neil

Finally found a reference here to prove my memory still hasn't completely failed regarding aluminium burning.

It clearly requires a particular set of circumstances, but I would suggest it is a possibility.

I would be really interested to find out exactly what did happen.

Edited By Farmboy on 13/08/2019 23:02:18

I have had aluminium burn on the lathe ( it was very thin like spiders web ) Aluminium again like a web was included with magnesium in photographic flash bulbs

Roy

We had a new cleaner at work and he decided to burn some papers in a skip which had aluminium swarf in and the aluminium caught fire. The fire brigade could not use water to put out the fire with a hose as water reacts with the aluminium fire and produces oxygen they covered the skip with fire blankets and cooled the outside of the skip and building nearby.

David

If it was caused by  something with the H15 aluminium, would that have created a situation that blew the fuse also? The lathe has an overload trip and on the few occasions when I have stalled it for some reason,the overload device has tripped out. I have not had reason to change the fuse for many years. So far as the event is concerned, I remember the flash of flame to be yellow and the burning of the lacy swarf to be almost instantaneous.

Keith

Edited By KEITH BEAUMONT on 14/08/2019 09:32:52