Bench grinder motor stalling

Hello all
I got a cheap chinese bench grinder. Since I never had used one before in my life, I was stunned to discover that I can stall the 3/4 HP motor by
merely pushing the workpiece against the wheel with quite moderate force. Is that how bench grinders are supposed to behave? 'Cause I know for sure I cannot stop a horse, or even 3/4 of him with just my hand :-)
Mongke
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Been there, done that. Ancient Chinese secret. How much current does it draw? I've seen garbage on eBay sold with HP "ratings" worse than the ones Sears uses.
The import bench grinders I've owned that didn't have a bigtime name on them were useless crap.
GTO(John)

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On Wed, 31 Dec 2003 01:02:01 +0000, GTO69RA4 wrote:

No current rating anywhere and I'm too lazy to go and measure it. I'll better return it. Are B&D's worth it? Just for fun, I tried to stop the 1/3 HP motor in the drill press (taiwanese). Couldn't. Oh, I guess I could have tried with more force but I was afraid of damaging the thing (or myself)
Mongke
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On Tue, 30 Dec 2003 15:47:09 -0500, the renowned "mongke"

The Ryobi ones are not much more expensive than the no-name ones and work fine.
Best regards, Spehro Pefhany
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my 6" chinese is like that, par i guess, at least it was 20 yrs ago when i bot it. still use it, keep thinking an actual good deal will present itself some day. i wonder how the large 8" with the larger shaft does?
--Loren

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wrote:

No, you should not be able to stall it that easy., if it was a good grinder, and 99% of import bench grinders are junk, and they don't have enough weight in them to turn them into a decent boat anchor. I have a small 6" Wissota brand over 20 years old and its a real bear. IIRC its only 1/3 or 1/2 hp. I had a china made 6" that you would be hard pressed to be able to grind a screwdriver tip on it. I would not pay any attention to the HP ratings on any of these type of tools, and as someone already stated, look for amp draw to give you an indication of its power. I see its a wattage war with lots of those import tools. I know you can convert the watts to amps but just how many folks know how? I guess that a 500 watt grinder looks better than advertertising its only 4 amps.
I rarely use any of my bench grinders since I got my belt grinders. They just work so much better for most things I need to grind. Visit my website: http://www.frugalmachinist.com Opinions expressed are those of my wifes, I had no input whatsoever. Remove "nospam" from email addy.
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On Wed, 31 Dec 2003 01:36:05 +0000, Roy wrote:

Actually I do look for the amps rating if possible. And nicely enough, my new Bosch roto hammer, just arrived today, claims a whopping 6.9 amps in big red on black letters on the box.
Regards,
Mongke
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I'm not going to defend your bench grinder, but I do feel compelled to point out that power is proportional to force x velocity. So a horse, going two MPH can produce tremendous thrust. Your grinder's surface speed is probably about 30 MPH (or maybe 60), so the tangential force at the wheel edge is way less.
Your drill press may have had the belt drive set for a speed reduction, and, of course, the chuck diameter, where you grabbed it is only a couple of inches in diameter. Under those conditions it WOULD be hard to stall a 3/4 HP motor.
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Got one of those one time.... got it home, balanced and running (although not very smooth) and very shortly lost bearings in it. Tore it apart and even though it had a *huge* housing, the armature inside was smaller than a 1/4" electric drill. I think you will find the same thing if you take the motor apart. They install spacers around the inside of the motor and you really don't have much inside. Threw it in the garbage after I saw how it was made. Ken.
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No useful power is only part of the problems with these.. the electrical connections on the one I bought were wire nuts tightened onto the plastic wire insulation. Another tool had about 20 ga wire that was folded back over the insulation so the screws would tighten down. I always check the electrical connections of anything used or of questionable quality. These China electrical tools are the only times when I've seen a 3 conductor plugs on 2 conductor power cords.
As someone else said, the rotors are really small, about 1/8th the size of a real 3/4 hp motor. The motors are essentially the same as might be found in old vinyl record turntables or a small cheap fan. I get more power from a 1/8 hp 3400 rpm blower motor that I've adapted to use as a bench grinder.
There is no thermal protection in most of these tools. If they get hot enough to smell them, they're usually scrap. If it's not used for grinding, the case should be vented with screened holes. When it burns out, make a cutout on the bottom or back side, and use the housing as a belt driven arbor.
With the China grinding wheels mounted, the thing would drive itself around like a hovercraft, even after truing the wheels with a diamond dresser. A suitable use for the "grinder" is to mount a combination of buffing wheels and/or fine wire wheels on them, then they're OK for light duty use. Another use might be to drive a flex cable for small Dremel-type cutters.
WB ..............

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replying to Wild Bill, Confused wrote: Hi I have a bench grinder that I converted for polishing. Recently the thing has lost power when I push against the polishing mop and I can even stop it rotating if I grab hold, can anyone loll me what?s up. I?ve opened it up and had a look, no debris or broken parts, no bearing wobble. Is it the capacitor or is it toast?
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replying to Wild Bill, Confused wrote: Hi I have a bench grinder that I converted for polishing. Recently the thing has lost power when I push against the polishing mop and I can even stop it rotating if I grab hold, can anyone loll me what’s up. I’ve opened it up and had a look, no debris or broken parts, no bearing wobble. Is it the capacitor or is it toast?
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    Almost certainly the capacitor. And they are cheap, so get a replacement and swap it in.
    You probably cooked the capacitor when pushing too hard against the mop and stalling it, which results in the start centrifugal switch closing, and applying power to the capacitor for too long. They can only take a very short application of the AC which drives the motor.
    Get several spare caps, as you will probably be killing more of them in this application.
    Better if you have a three-phase motor and a VFD to drive it. That can take the extra load a lot better than a cap-start motor.
    Good Luck,         DoN.
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