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noel shelley12/03/2020 19:57:29
116 forum posts

This point should be made from time to time for the benefit of those new to engineering of any scale !!! NEVER GRIND ALUMINIUM AND STEEL ON THE SAME MACHINE. The powdered aluminium gathers in the guard to be subsequently blasted by red hot steel particles as the oxide of iron! At some point a violent exothermic reaction will start. I won't bore you with the chemistry but it will blast your hands ,chest and face with a flame at several 1000 degrees C. IF your lucky you may live to tell the tale after much plastic surgery.

For graphic illustrations of what takes place look up Thermit welding or the Thermit process. And YES accidents of this type do happen !!! Noel.

Tony Pratt 112/03/2020 20:25:28
1234 forum posts
5 photos

Who has rattled your cage?frown

Tony

mgnbuk12/03/2020 20:29:20
800 forum posts
61 photos

NEVER GRIND ALUMINIUM

Is all that needs to be said ?

Taught that grinding aluminium was a no-no very early on, though more for the propensity to clog the wheel & cause a wheel burst than for exploding dust. An exploding wheel can spoil your day just as well, though.

Nigel B.

Harry Wilkes12/03/2020 21:53:37
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981 forum posts
63 photos
Posted by Tony Pratt 1 on 12/03/2020 20:25:28:

Who has rattled your cage?frown

Tony

Maybe he as had 'the free ride' 🤣

H

Hopper12/03/2020 23:24:30
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4804 forum posts
105 photos

Has this ever actually happened to anyone? Big step from thermit welding to knocking the burrs off a bit off ally angle on the grinder.

Mark Rand12/03/2020 23:24:55
923 forum posts
6 photos

But the coolant on the surface grinder washes all the fines into the tank anyway. There isn't anything left to catch fire...

I.M. OUTAHERE13/03/2020 06:03:13
1468 forum posts
3 photos
Posted by mgnbuk on 12/03/2020 20:29:20:

NEVER GRIND ALUMINIUM

Is all that needs to be said ?

Taught that grinding aluminium was a no-no very early on, though more for the propensity to clog the wheel & cause a wheel burst than for exploding dust. An exploding wheel can spoil your day just as well, though.

Nigel B.

Quickest way i know of to bugger a grinding wheel other than bashing it with a hammer !

David Colwill13/03/2020 07:11:06
651 forum posts
34 photos
Posted by Hopper on 12/03/2020 23:24:30:

Has this ever actually happened to anyone? Big step from thermit welding to knocking the burrs off a bit off ally angle on the grinder.

In a word yes.

I have had this happen to me.....Well sort of!

The linisher had been used for pretty much anything including wood aluminium and rusty steel (that was what I was doing )

Anyway all of a sudden there was a bit of a flash. I'd like to be able to say that I was blown clean across the workshop, through the brick wall and into the pub but alas it wasn't so.

The most serious aspect of this was the loss of productivity as I spent at least half an hour trying to repeat it.

I always thought it was a thermite reaction but in reality it was more likely dust being ignited by a spark.

Regards.

David.

David George 113/03/2020 07:21:49
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1340 forum posts
448 photos

I have ground aluminium with special grinding wheels. We used a Norton wheel to machine inserts which were held on a vacuum chuck and surface ground them and used paraffin as a coolant. Also you can get an aluminium cut off wheel. You don't get that chip flying every where when using a saw blade have a look on Norton site. But always right wheel for right job.

David

Robin13/03/2020 09:33:49
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355 forum posts

I didn't know you could grind aluminium. About 54 years ago I was told aluminium would clog the stone and if I was seen grinding brass something heavy would be flung at my head from behind.

This is a really good website, wish I could contribute more but I am seriously out classed.

I've been buying my Thermite on e-Bay...

peak413/03/2020 10:56:32
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1197 forum posts
143 photos

Folks, don't forget, that when some folks talk about "grinding" aluminium or steel, not everyone is thinking of a bench grinder with a wheel.
Knife makers for instance often use a 2"x72" (or 48" belt "grinder"

Other engineers may consider this as a linisher or belt sander.

When I was writing articles some years ago for a specialist motorsport safety newsletter, I did mention a particular accident where someone using a linisher had ended up with burnt fingers due to some sort of exothermic reaction when he's swapped from "grinding" aluminium to steel without cleaning the machine first.

I vaguely remember reading about it originally in the model engineering press, but this is some 20+ years ago.

Bill

John Purdy13/03/2020 17:19:03
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212 forum posts
75 photos

Yes it VERY DEFINITELY can happen, I know from personal experience as it happened to me. Unbeknownst to me, my son had ground of the heads of a number of aluminum pop rivets on my 1" belt sander. Later while fairly aggressively grinding the sawn edges of some angle iron, a large bright white fireball engulfed the sander, my hands and the front of my shirt. My hands suffered fairly sever burns with the skin hanging off the heals of both hands and the ball of my thumbs. The front of my shirt was smoldering. I lost most of my mustache and eyebrows, My face looked like I had a good sunburn ( except around my eyes which were protected by my glasses). and the workshop was full of white smoke. Luckily the burns weren't quite bad enough to require plastic surgery. As has already been said, essentially what you have made in the grinder is thermite with the mixture of aluminum dust and the iron oxide from grinding steel, ignited by the hot sparks. After it happened I wrote it up in ME (09 Mar 2001) as a warning to others. I have pictures of my burned hands somewhere. My son has been educated and I now leave the side cover off the belt sander and brush out any residue on a regular basis. Also never use it on aluminum!

John

Edited By John Purdy on 13/03/2020 17:23:24

Tim Stevens13/03/2020 17:33:09
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1268 forum posts

The answer, of course, is not to have a guard on the machine where dust can collect. Just in case this seems a good idea, remember that the hot sparks will then go straight into your eye.

Tim

Mike Poole13/03/2020 20:00:22
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Moderator
2746 forum posts
64 photos

Well despite the piss taking this would seem to be a useful heads up from Noel, although as with others, grinding aluminium was forbidden on any shop grinder but the usual reason was wheel damage. It would seem a thermite reaction can be initiated by accident as Johns incident demonstrates. It would be a wise precaution to clean a linisher before and after any aluminium activity, I would still avoid grinding aluminium on a grinding wheel that is not specifically for aluminium.

Mike

Edited By Mike Poole on 13/03/2020 20:00:45

old mart13/03/2020 20:24:59
1995 forum posts
151 photos

The general danger from grinding aluminium is from the wheel rupturing as already mentioned. Dust build up is mostly forgotten, which reminds me of an incident with the toolroom vertical bandsaw. Apprentices had been busy cutting tooling lugs off of titanium workpieces when the saw burst into flames. The fire was put out with CO2 and after that incident, the saw was kept vacuumed out regularly.

At the museum, bench grinders are about to be banned, as there are so many people who don't seem to be able to use them safely. The only ones left will have diamond laps and one will have a slow moving wet wheel for wood chisels and plane blades. I won't miss the damn things.

Edited By old mart on 13/03/2020 20:29:37

SillyOldDuffer13/03/2020 21:54:30
Moderator
6330 forum posts
1389 photos

Pedant Alert!

Unlikely to be the Thermite reaction - there's a simpler alternative. Thermite is a mixture of Iron Oxide and Aluminium Powder. When ignited the Aluminium robs Oxygen from the Iron Oxide leaving molten Iron behind, which can be used to weld steel.

Iron Oxide is unnecessary in this case because air is a much better source of Oxygen.

It's normally difficult to ignite Aluminium because an impermeable Oxide skin forms instantly to snuff out the reaction. It stops air getting to the metal. Finely powdered Aluminium is the exception. Once started, the flame blows nearby unburned powder into the air and ignites it. In the right conditions a pile of Aluminium powder goes off like old-fashioned photographic flash powder. Lots of heat, close to an explosion.

Aluminium is so hard to ignite it's safe to machine it without any special precautions. Not so certain other metals. Magnesium and Titanium have to be treated with extreme caution. Grinding either is likely to cause fireworks. Never pour water on an Magnesium fire - it explodes!

Perhaps the most dangerous material to machine is Plutonium. Ebay don't sell it! Fortunately Model Engineers are unlikely to need to machine any reactive or toxic metals.

Dave

Mike Poole13/03/2020 23:07:37
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Moderator
2746 forum posts
64 photos

The aluminium superstructure of the ships hit by Exocet missiles was blamed for the horrific burn injuries to the crews of the ships attacked in the Falklands war. Aluminium is not an inert material if the right conditions are present.

Mike

wrinx17/03/2020 14:44:59
14 forum posts

Wow...didn't know this thanks for the warning! I tend to avoid working with aluminium anyway, but I'll know to thoroughly clean up afterwards in future.

wrinx

Robert Atkinson 217/03/2020 15:06:43
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771 forum posts
17 photos
Posted by SillyOldDuffer on 13/03/2020 21:54:30:

Pedant Alert!

Unlikely to be the Thermite reaction - there's a simpler alternative. Thermite is a mixture of Iron Oxide and Aluminium Powder. When ignited the Aluminium robs Oxygen from the Iron Oxide leaving molten Iron behind, which can be used to weld steel.

Iron Oxide is unnecessary in this case because air is a much better source of Oxygen.

It's normally difficult to ignite Aluminium because an impermeable Oxide skin forms instantly to snuff out the reaction. It stops air getting to the metal. Finely powdered Aluminium is the exception. Once started, the flame blows nearby unburned powder into the air and ignites it. In the right conditions a pile of Aluminium powder goes off like old-fashioned photographic flash powder. Lots of heat, close to an explosion.

Aluminium is so hard to ignite it's safe to machine it without any special precautions. Not so certain other metals. Magnesium and Titanium have to be treated with extreme caution. Grinding either is likely to cause fireworks. Never pour water on an Magnesium fire - it explodes!

Perhaps the most dangerous material to machine is Plutonium. Ebay don't sell it! Fortunately Model Engineers are unlikely to need to machine any reactive or toxic metals.

Dave

 

 

This is not totally correct, but having ignited bulk thermite (not easy, nomal approch is to use magnesium ribbon) I can see were you are coming from. "Thermite" sparks are actually common but not often recognised. They were identified as a cause of ignition in refinery fires when aluminium (or titanium) objects were dropped on rusty steel. Hot steel sparks thrown onto a pile of mixed aluminium and rust can and does cause ignition (not when you want it to).

There are other hazards with light alloys I once set "fire" to a 5" diameter by 8" long piece of unknown composition alloy when tappng some blind holes. I used some spray solvent to clean chips out of the blind hole and noticed smoke coming out. The solvent was trichloreythlene and reacted with the alloy. A handy bucket of water stopped it.

Robert G8RPI.

Edited By Robert Atkinson 2 on 17/03/2020 15:07:39

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