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George Hodge
04-18-2006, 11:16 PM
My neighbor hollered at me tonight,'come over and tell me what happened to my 4-1/2" grinder. His brother was grinding something on a metal table,with the trigger locked on.He moved a little to far and the plug pulled out of the wall receptical. My neighbor later noticed the cord unplugged and plugged it back in.The trigger was still locked on and the grinder flew off the table and spun around on the floor until the plug pulled out again. The only damage was to the grinding disc,which he replaced.When he plugged it back in and hit the switch,it now runs WAY FASTER!! than the 10,000RPM that it used to run.Probably more like 15,000RPM. It doesn't make sense. It always ran great before and he has another of the same brand and it runs about 10,000 as the label states.

BillH
04-18-2006, 11:28 PM
How do you know if it is running that much faster? By sound? Not a good guage.

JRouche
04-19-2006, 12:09 AM
I'm thinking the grinder is outta balance and that makes it appear faster due to the greater "excitement" going on. Use your handy dandy non-contact tach to check the speed. JRouche

J Tiers
04-19-2006, 12:19 AM
Possibly the various impacts damaged a bearing, or possibly made the commutator uneven.

Either of those might cause an added high pitched noise, which easily might make a person think it was going much faster. We tend to get used to the pitch of sounds, and associate that with speed in machinery. Usually that is correct, I know I speed more if the radio is on loudly in the car, as I can't hear the engine pitch.

In tis case, it may just be a bearing problem...

IOWOLF
04-19-2006, 04:52 AM
This is a good point,And is why I don't loan tools. Was the guard on it? It should have been, it wouldn't have done the dead chicken then.
I doubt if it runs faster, It just sounds that way,like others have said.

speedy
04-19-2006, 08:47 AM
His brother left the grinder switch closed?:rolleyes:
He plugged it back into a live socket? :rolleyes:

That confirms it, they are definitely related:D
and proves the old adage that "two wrongs don`t make it right"

Ken

motorcyclemac
04-19-2006, 12:17 PM
My Bosch grinder died recently. I opened it up to determine what the cause would be. I couldn't find out what caused the lack of go. I took said grinder to a tool repair shop to have them dope it out. When I returned to pick it up the technician told me that the electronic speed control had died. He could order a new controller for 80 dollars or simply cut the blue wire off the controller and that would allow the grinder to run wide open all the time. He stated that the controller was there to keep the wheel at a given speed and would allow full amp draw under load then when the wheel was allowed to spin free it would throw voltage to a 5th brush and interupt winding to control free speed rpm. After I cut the blue wire from the controller it no longer had the speed control and will wind up like a router to a point where it is almost freaky. The wheel turns so stinking fast that the gyroscopic forces make it hard to move. In this fellows case the grinder thrashing about on the ground may have broken a circuit board trace on the controller or the wire to the interupt winding. I know from personal experience that a hand grinder can run "wide open".

Cheers
Bill

TECHSHOP
04-19-2006, 12:29 PM
I agree with motorcyclemac:

There are quite a few (newer) tools that have sometype of electronic speed control, and when it breaks the tool (usually?, often?) goes "WOT", may question if the thing is spinning up, are your disks and stuff still safely speed rated?

john hobdeclipe
04-19-2006, 12:44 PM
Interesting concept: knock it off the bench and it runs faster. Wish I'd thought of that back when I drove a Volkswagen Camper!

Seriously, if it is actually running faster than it's design speed it may well be exceeding the speed rating of the wheels. Grinding wheels can and will fly apart, and trust me, it ain't always pretty when they do.

motorcyclemac
04-19-2006, 12:46 PM
You have a valid concern about wheel speed. I too question the wheel's ability to handle such wide open throttle conditions. The tech that looked at my grinder told me that if I cut the blue controller wire to always turn on the grinder ...and put it under load at once. He said to never lay the grinder down running as it will continue to speed climb. Which it does. I have tried to let it run and it DOES wind up higher and higher to a point where any guy will hear it and panic to shut off the power. He advised that if left to "run away" it will spin higher to a point where the armeture will violently explode and grind to a halt on the winding shoes!!! He said it will likely pull a helluva lot of amps and kick a breaker. So...as advised...I always put it under load straight away. I will be looking to get a replacement at some point. I never let anyone use this grinder 'cept for me because of the "explosive" potential of the wheel. I think they are rated for 20,000 rpm or there about...and I am quite sure that it is capable of getting far beyond that if the mitre gear will handle it. I have put my amp clamp on the supply wire for the grinder and noted that as the speed climbs the amp draw increases wildly. It will climb to a point where it is pulling WAY more amps than the build sticker says it should. All in all...running in this condition isn't really all that great and most likely should be avoided. I don't much trust my grinder any more. Each time I use it...I brace myself for some sort of catastrophy.

Cheers
-Bill

Scishopguy
04-19-2006, 04:11 PM
I learned, early in my career, to never trust a grinder that someone else has used. I came back from a break and didn't realize that someone had knocked my die grinder off the bench, bending the shank of the stone in it, and had picked it up and placed it back on the bench. When I turned it on it just about ate me up. It took two hands to keep it away from vital parts of me! I learned to check the tool over before plugging it in and also to have a firm grip on it with one hand when plugging it in. This has served me well. A disc grinder could be even more dangerous than the little die grinder that attacked me.

Lets all be careful out there!

Jim (kb4ivh)

Evan
04-19-2006, 05:00 PM
Those motors are series wound. In theory a series wound motor doesn't have an upper rpm limit but in practice it is limited by both mechanical and electrical constraints. If the windings in the field have partially shorted together then that decreases the turns per volt compared to the armature. It acts like a stepup autotransformer and the rpm will increase.

George Hodge
04-19-2006, 11:29 PM
Yup,sounds just like Motorcyclemac describes. And no,there wasn't any guard on it. I told him to use this one for parts. I have several and don't trust using them at all without the guard. With several people in the shop,cutting,grinding and welding,I can see where it could happen if the plug pulled out and someone else just grabbed it and stuck it back in without checking the trigger.

aboard_epsilon
04-20-2006, 09:22 AM
A guess
Some grinders are made in 110 volt and 240 volt versions of the same model.
the difference between the 2 is a resistor.
the 110 volt grinder does not have one, the 240 volts one does.

bet yours is a 240 one and somehow this resistor has failed or is shorted.

all the best.....mark

George Hodge
04-20-2006, 10:57 PM
Aboard,his grinder is a 120v.grinder,(Makita) He's not using it anymore.But,over the weekend,he bought a big Bosch 9in. grinder and it's 220v. Now we're gonna have to tear into it and see if there's a resistor in it,so he can use it on 120v.
Thanks everyone for all the input!! This forum is great!