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Chris Fazio
02-24-2003, 01:12 PM
Saturday I was cutting a 4.5 in. long piece of 2.5 in. dia. 6061 aluminum from an 18 in. long bar which was held in my bench vise. Was using a portable band saw to do the job. When I was close to the bottom of the cut I slowed the saw down and watched the piece fall to the concrete floor. When the piece hit the floor there was a flash of light and a bang like a small firecracker, which made me jump. The piece landed on the 2.5 in. end. When I picked up the piece there were black marks on one side of it looking like those created by a spark. The piece fell on the Al. chips that were lying on the concrete from my cutting of the bar. I've never seen anything like this, and thought maybe static had built up in the piece while I was cutting and when it was a small distance from the floor a spark jumped and ignited a small amount of the Al cuttings on the floor. This is just a theory on what happened, what do you all think happened here? I dropped the piece several more times on the cuttings but nothing else wierd or unusual happened.

Chris

lynnl
02-24-2003, 01:29 PM
That is strange. I don't recall ever seeing any such event.
Where do you live? ...i.e. in a very dry climate where static is common? Was there any oil or other combustible material on the floor?

Tibertus
02-24-2003, 02:09 PM
There was a thread on this a few weeks ago search for "aluminum grenade" and see what you find out.

Peace

Chris Fazio
02-24-2003, 03:25 PM
lynnl
I'm in Pa. and it was raining at the time that this happened. Don't know what the R.H. was at the time. First thing I thought of was the thread on thermite a while back. Floor was fairly clean but who knows what could have been there. Very strange, very strange!

Chris

Forrest Addy
02-24-2003, 05:23 PM
probably handed on a match head. They explode on a sharp impact.

lynnl
02-24-2003, 05:58 PM
We used to make little crude guns with bicyle spokes and matchheads and about a #6 shotgun shot pellet.

gizmo2
02-24-2003, 11:19 PM
Easy. Your al-u-minium landed on an Exploding Spider. Closest cousin is the barking spider. Neither particulary rare. Both have notoriously bad breath.

Oso
02-24-2003, 11:52 PM
Dunno if this is in any way related, but there are points in common.

Today I was doing a long job on a belt/disc sander, sanding 8 edges and 4 corners on each of 34 drawer dividers I had just sheared up. (I am organizing, scary stuff).

Anyway, after every few I would pick up my pile of finished ones and set them aside. After a while I noticed that I got a pretty good shock each time, since the table I set them on was grounded.

The belt was developing a good charge on me and the dividers as it sanded off material. My shoes were not leaky, so it held up until I grounded to another object. The sander has a 3 wire cord and is grounded, but the belt is a non-conductor.

In general, a small object will not have sufficient capacitance to hold much real charge, but it at least provides some support for a static charge theory.

Against that is the fact that your saw probably has the blade effectively grounded thru the pulleys and guides. That should have eliminated any charge.

Well, just like the TV news, "the answer is, there is no easy answer".

darryl
02-25-2003, 04:23 AM
It's possible that the piece dropped in such a way that the full inertial energy in the part had to dissipate into a very small portion on the metal( a corner), rather than into vibration, and that energy would convert into heat. Assuming a sufficiently small impact area, with little room for the metal to 'squish' to , the temperature could have been high enough to actually burn a portion. Also, the impact area on the floor was probably right on a hard spot, a bit of gravel, and not cement. Maybe that spot vaporized minutely, causing the aluminum to ignite on the 'hot spot'. I'd like to see that, as well. Reminds me of the guy that looked down into a pile of magnesium chips by the lathe, and noticed a flashing going on. Turns out there was an extension cord there, and some chips were making it to the gap across the pins of the plug. Could have been a real big flash had it taken off. By the way, I don't buy the 'charged' idea. The surface area of that piece wasn't large enough to store charge sufficient to vaporize enough material to have been seen.

[This message has been edited by darryl (edited 02-25-2003).]

SJorgensen
02-26-2003, 12:04 AM
My bandsaw wheels are rubber coated but the guides are not. If the piece being cut were clamped in an insulator that might lend more support for the static theory. Still considering all of the avenues of charge dissipation it is hard to accept that explaination. Even if a faily large voltage were stored on this material I doubt it could have enough capacitance to do anything beyond a very small mild spark.
Spence

SJorgensen
02-26-2003, 12:07 AM
Do you have any nitrates stored nearby? Is there any leaching of anything building up on the cement? Fertilizer stored nearby?
Spence

Thrud
02-26-2003, 12:12 AM
Chris
I have the same problem when I take too many pain killers - then the cat talks to me...

(Only kidding about the cat!)

Chris Fazio
02-26-2003, 10:20 AM
Spence

The vise is mounted on a wood work bench so I would say it could be considered insulated. Maybe a small spark was all that was necessary to ignite the AL cuttings laying on the floor, definetly speculation on my part. There are no nitrates in the area. The one thing I know for sure is what I observed, but explaining what happened can be a whole other story. I couldn't come up with any theories other than the static theory, so I thought I'd post this to get some other thoughts on what might have happened. May never know what actually happened here. The first step to learning is to say to oneself, I don't know, then to try and find out what it is you don't know. If a person "doesn't know that he doesn't know" then he will learn nothing. Thanks everyone for the input, we may never find out what happened here, but we sure gave it our best shot.

Thrud
I hope you're not machining after taking to many pain killers, remember safety is job one.

Chris

Thrud
02-27-2003, 01:22 AM
Chris
Don't worry bud, I may be ugly but I am not that stupid. Still have all 21 digits. http://bbs.homeshopmachinist.net//biggrin.gif