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View Full Version : A VERY dangerous lil' machine!



torker
12-01-2007, 12:14 AM
Hey guys..thought you all might like to see this.
Was using one of my electric die grinders today to grind the reliefs bigger on a big Birfield type U joint from a big ol' Hough loader.
This was my industrial strength Makita grinder...25,000 rpm with lots of jam.
The bit must have slipped suddenly in the collet and came part way out.
It bent almost immediately before I could even react. The bit hit the side of the Birfield and shattered! Then it was a real handfull. Thing was shaking so bad I could hardly hang on to it with one hand to shut it off with the other.
I saw the bit and looked right across the shop to see if the gurl was ok. A chunk that big could really mess someone up.
I checked myself after that to see if the chunk was stuck in me somewhere.
I heard of a guy getting a piece of one of these right in his heart. Guess it killed him.
I later found the piece inside the Birfield cup.
From now on...full safety gear(maybe even a leather jacket) and a plywood guard around wherever I'm using this thing if anyone else is in the shop.
I should add...this is a 1/4" shank and a 1/2" diameter burr on the end....or it WAS anyway.
Whew!
http://img.photobucket.com/albums/v210/torker/DSC00004-6.jpg
Russ

matador
12-01-2007, 12:30 AM
Another reason I hate hand held grinders.Glad you're ok,Russ.Similar thing happened to me with a Dremel type thing(NOT a real Dremel,I hasten to say!).Broke the casing,after frightening the cr#p out of me.The bit was a 1/8 shaft.Never again.

Bill Pace
12-01-2007, 12:31 AM
WHOA!!, thats impressingly scary, gives me chills thinking about it....

darryl
12-01-2007, 02:07 AM
Some years ago a co-worker brought me a die grinder to fix. Seems he had gotten the collet nut jammed between the vise jaws and broke the coupler that connects the output shaft to the motor shaft. So I fix it, I don't remember how. Anyway, not long later he brings it back again- same problem. I fix it up again. This time I watch him use the thing, and he's got virtually no control of it. It's a wonder it stays in his hand as he does the same thing yet again. Collet nut slips down between the vise jaws and stops the shaft in maybe a couple microseconds. Bang- it's broken again. Just watching the end of that thing take off as it bites into something scares me.

Anyway, I fix it again, only this time I borrow a trick from the model engine field, and make a centrifugal clutch for it. It works based on the speed of the output shaft. If it's spinning fast enough, the clutch is engaged strongly enough to handle significant torque. If you stop the output shaft by some means, it instantly de-couples from the motor and requres you to shut it off, have no load on the output shaft, then start it again. Once up to speed you have torque again.

So that worked. He managed to test it a few times over the next several minutes while he worked on his 'technique'. I fully expected we'd be taking him to the hospital with some large gaping arc shaped gouge in a leg or somewhere, but luckily for all of us it didn't happen. A full scale die grinder has to be one of the most dangerous tools.

Alistair Hosie
12-01-2007, 06:50 AM
even in dentistry that happens with smaller cutters of course but you must pick the correct collet no point in blaming the tool sorry.Alistair

IOWOLF
12-01-2007, 07:44 AM
I have done the same thing with the correct collet, Got to keep them tight.
Glad to see the Gurl and CAT are ok, And you of course. :)

Benesesso
12-01-2007, 08:26 AM
Keeping collets tight is one thing, but there is another reason why burrs and grinding stones can disintegrate suddenly. I forget the technical term, but it will happen nearly every time when grinding inside a hole that is just slightly larger in dia. than the burr. What happens is that the burr/stone ricochets all around the ID. If the tool is running at full speed, the violence is very strong.

We had a problem with a 500 HP vertical water pump. Motor was attached to the pump which had no bearings--only the 2 in the motor. The impeller had wear rings at top and bottom. There was a window with an opening of ~4" in the mating housing between the pump and motor where the motor shaft was visible. Pump was started at full voltage-4160V.

After a few starts the pump failed to start-tripped the breaker. Hmmm, new motor installed (we have 6 of these pumps), no help. Motors made weird sounds as they tried to start, with the shaft making only a few jerky turns before the breaker would pop. But with no voltage applied, you could reach in the window and easily rotate the shaft! Now what? I got involved and found the wear rings were made from K monel. I got the pump manuf. to make some new ones from a "slippery" stainless steel, Nitronic 60. NRC said we had to do a 100 sequence start to prove everything (nuke safety-related pumps here). Pump started fine 98 times in a row, then popped breaker again. Back to square one. Then vibration engr. got involved. Examined contact pattern on lower wear ring and his light bulb lit right up. Correct, low speed ricochet problem, even with my slippery rings.

But why was the impeller even touching the ring during starts? Seems that when the 4160 hit it, the magnetic field would suddenly pull the whole rotor sideways--hard enough to cause shaft deflection and slam the impeller into the ring. The motor torque would then cause the impeller to bounce around inside the ring hard enough that friction would prevent it from gaining speed.

Solution? Couldn't reduce starting voltage-too many design changes involved. Couldn't increase impeller/ring gap either. Then we noticed that we had 6 other pumps that looked very similar, except they had 800 HP motors from the same manuf. OK, we didn't need the 800 HP, but those motors had much thicker shafts. Got the OK to use these motors, end of problem!

Moral: Be careful when grinding anything where the stone/burr can ricochet around, because the frictional forces can get enormous.

torker
12-01-2007, 08:47 AM
Benesesso...That's exactly what happened here. The pockets in the Birfield joint are about a 1" semi circle. I was grinding on them for a total of prolly 2 1/2 hours. Lots of "ricochets"! I'd wore out one burr on this job already...this was a brand new one and I guess it grabbed too hard.
Alistair... I have two of these big industrial die grinders. These aren't little namby pamby grinders. They are big and have a lot of power. You have to use two hands with a death grip at all times. I rarely use my Hitachi die grinder. That thing is just rude for power. I've busted off quite a few bits with it. And I am using the right collets. The 1/4" ones are the only ones I have.
I have a couple of smaller air grinders. I like these far better as they are a lot safer. If you lean on them at all they slow down. Not so with the electric ones.
The air grinders use too much air for a big job like this. The compressor is constantly running so I use the electrics.

torker
12-01-2007, 08:48 AM
Jay...ya my cat just had to get into the picture...lol!

IOWOLF
12-01-2007, 10:51 AM
shame the Gurl didn't.

ProGunOne
12-01-2007, 01:45 PM
shame the Gurl didn't. :D :D




I use an air die grinder and carbide deburring bit almost daily. Safety glasses with side shields & death grip a must, but an in line air regulator like the one shown has worked greatly for me in helping to control the grinder. A good pair of leather gloves is a plus.

http://img253.imageshack.us/img253/11/airregulatorbe6.jpg (http://imageshack.us)

torker
12-01-2007, 03:13 PM
shame the Gurl didn't.
LOL! Ya...that's hard to look at all day!

Dawai
12-01-2007, 10:28 PM
Got a buddy with a joker type scar across one eye..

He walked into a garage and a zip blade exploded and it hit him right across the eye.
No insurance, No disability.
His friends had to help him survive till he could go back to work.

NOW, that cnc mill of mine? make a typo? you need a rip cord to pull as you are running away. That's why they built plexi boxes around the newer ones. Carbide can stick in plywood.

Rich Carlstedt
12-01-2007, 11:21 PM
this can happen for several reasons.
It is imperative to keep collets as tight as posible

First, the spiral on the burr is "Pulling" the burr out of the collet, so the forces are to make it come out.
Second, using a foriegn made burrs requires that you check shank size with a mike before using . I HAVE FOUND AND THROWN OUT undersize shanks !
Third, never use the burr without a full grip "choked" up as much as possible. holding the burr at the end of the shank is a no-no !. If you must to reach a difficult area, reduce speed to 1/3 of normal.
Fourth, small holes require 120 % of attention and hand strength.
Fifth, use a second cut (crosscut) burr in small holes to lessen 'takeoff " of the grinder

Rich

motorcyclemac
12-02-2007, 03:39 AM
I have a momentary on foot switch attached to my drill press. When I use my 1/4 horse die grinder I plug it into that foot switch. That keeps me safe when something like this goes wrong. I put one of those flap wheels in the collet one time and had it bend the shaft. I know what ya mean about SHAKE!! That is when I started with the foot switch.

Cheers
Mac.

wierdscience
12-02-2007, 10:18 AM
Hey ya those things are dangerous,I slipped with a Dremel and an 1/8 burr once and snagged the bottom edge of my tee-shirt pocket.It caught in the fabric and twisted the shirt up tight to my neck before it stalled in less than 1/10 of a second.Even that dinky little Dremel armateur has significant power spinning at 25,000rpm.

Russ,I switched over to smaller diameter burrs in my 1/4 shank grinders,less chance of a snag and more control.I find the double cut style 1/4 od burss cut just as fast as the 1/2" single cut.