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View Full Version : Grinder RPM v. disc rating



RussZHC
05-31-2012, 08:59 PM
Looking for a second angle grinder, leaning towards Makita hoping against all odds I will get something near the value I have gotten from the smaller version I've had for years.

Anyway, looking at their 7" versions (finding many manufacturer's difference between 7" and 9" is the size of the guard...) and am a bit concerned that most of them have a no load speed rate of 8500rpm. The bulk of 7" discs and wheels I have come across in catalogs are rated at 8600rpm.

To me, compared to other sizes of grinders where the margin is often 1200 or 1500 rpm difference between the tool and the wheel/disc (in favor of the abrasive), the room for "error" is really small.

Thoughts?
Am I being too cautious?

oldtiffie
05-31-2012, 09:17 PM
That Makita should be ideal.

Speed difference is minor and will effectively become closer and then under as the disc diameter reduces with wear.

If you you are using "known" brands instead of "no-name" or el-cheapo wheels you will be OK.

Same applies to "flap" wheels, "wire" wheels and "cut-off" wheels.

All wheels should be "run up" and held for a minute when you first fit them - but be sure the guard/s is on and the opening away from you.

lakeside53
05-31-2012, 09:45 PM
Don't worry about it - as Tiffie says - buy quality abrasives...

wierdscience
05-31-2012, 10:24 PM
So long as your grinder's rating is under the wheel's rating your good.

Black_Moons
06-01-2012, 01:31 AM
Its less about exact rpm and more about 'class' of grinder.

ie, 8600rpm means 7" or 9" grinder only, Don't use on a 11,000rpm 4 1/2 or 5" grinder. While its safe to use a worn 5" wheel on a 4 1/2" grinder, its not technicaly safe to use a worn 7 or 9" disk on a 4 1/2 or 5" grinder.

Theres also 20,000rpm high speed sanders. My brother learned first hand why you do NOT put a 14,000rpm rated grinding disk in a 20,000rpm sander, Several stitchs.

chipmaker4130
06-01-2012, 01:45 AM
. . .While its safe to use a worn 5" wheel on a 4 1/2" grinder, its not technicaly safe to use a worn 7 or 9" disk on a 4 1/2 or 5" grinder. . . .

I'm quite sure that by the time a 7 or 9 in. wheel has worn enough to fit a 4 1/2 in. grinder, it will be just fine at the smaller unit's higher speed. The max speed ratings are developed at max diameter, and for a given thickness wheel construction is pretty much the same.