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View Full Version : 7 inch grinding wheel on an 8 inch bench grinder



GrahamC
01-23-2009, 10:20 PM
Did a search and came up empty.

So, any problems using a 7 inch grinding wheel on an 8 inch bench grinder?

I know that it is not recommended to use an 8 inch wheel on a 7 inch grinder but how about the reverse keeping in mind the rpm limit of the wheel and that the already larger wheel guards.

cheers, Graham

oldtiffie
01-23-2009, 10:22 PM
8" start their way down to 7" (and lower!!) as soon as you start using them.

Should be OK

GrahamC
01-23-2009, 10:42 PM
8" start their way down to 7" (and lower!!) as soon as you start using them.

Should be OK

Ah, of course; very good point!

Sometimes, no matter how hard you think about something you end up over thinking and you can't see the obvious!

Thanks for the kick in the butt.

cheers, Graham

Teenage_Machinist
01-23-2009, 10:47 PM
My ceramics teacher has an 8 or 10 inch grinder with 4 inch wheels from all the glaze grinding. As long as the rests adjust...

chipmaker4130
01-23-2009, 11:05 PM
Biggest problem I see is if you have a large diameter motor. The better grinders have a slim but strong motor enabling one to use the wheels up and still hang a workpiece square in front of the housing. The budget grinders, however, often have a weak motor nearly as big as the rated wheel, making it tough to grind a longer piece anywhere but the end.

Fasttrack
01-23-2009, 11:10 PM
The speed of the grinder, in RPM, will be less than the speed of an actual 7" grinder. No problems with safety, it just means your cutting efficiency will be slightly reduced. On the other hand, it means less heat generated. Furthermore, as OT says, an 8" wheel only stays 8" for so long! :)

Personally, once I get about 2" smaller than the size grinder (6" for an 8" or 4" for a 6" etc), I get a new wheel. They aren't that expensive (at least when they last as long as mine do - I don't do alot of off-hand grinding, just tool sharpening) and I'm too impatient to dick with a tiny little wheel that's hardly spinning. :D

38_Cal
01-24-2009, 12:15 AM
Fasttrack, thwap yourself in the back of the head! RPMs are RPMs, no matter what size wheel is on there, within the limits of enough horsepressure to spin it. Surface speed will lower with a smaller wheel.

David
Montezuma, IA

Fasttrack
01-24-2009, 12:35 AM
Edit: Oh wait - I wasn't completely wrong, I just might have been misleading.

I'm not sure what type of grinder uses a 7" wheel - a surface grinder I expect. Maybe this isn't true for surface grinders, but I was thinking about the difference between a 7" angle grinder and say a 4.5" angle grinder. The motor rpm of the 4.5" grinder is higher than the motor rpm of the 7" grinder. This means that the surface speed of the 4.5" grinding wheel in a 7" angle grinder is much less than the surface speed of a 4.5" wheel in a 4.5" grinder.

Thats what I was trying to say. The 7" wheel (meant for X rpm) is now on a motor that is turning an rpm that is less than X since its meant for a larger wheel.

So when I said "the speed of the grinder, in rpm is less than the speed of an actual 7" grinder", I meant that the rpm of the 8" grinder is less than the rpm of a smaller grinder (usually), which corresponds in a lower surface speed than the wheel was intended to be used at.

oldtiffie
01-24-2009, 01:07 AM
Don't sweat it Fasttrack.

Motor speeds on most HSM grinders are a function (%-age) of theoretical RPM at the supply frequency and the number of "poles".

The US frequency is 60Hz (cycles/second) but it is 50 in OZ so our grinders run at about 50/60 ~ 0.83 (~83%) of the speed in the US.

Perhaps we should use the new wheels and send them to the US when we wear them down from 8" to 7".

You are correct to as regards 7" wheels on surface and Tool & Cutter grinders as it is a standard size. But again, we are on 50~ whereas most motors etc. seem to be sized/speed for 60~ (US, UK, Europe etc.).

We are just the "odd lot" here in OZ!!

Ghop Shop
01-24-2009, 01:17 AM
My Dad's twelve inch grinder.

Gayle Hopsonhttp://i190.photobucket.com/albums/z256/GHopShop/May62007039.jpg

Fasttrack
01-24-2009, 01:46 AM
:) I wasn't "sweating it", just pointing something out.

Holy smokes Ghop Shop! How can you get anything done on those tiny little wheels. I get fed up when mine are at ~ 4"

Teenage_Machinist
01-24-2009, 02:46 PM
LOL!

I am somewhat intruiged by you not being able to find a 8 inch wheel. Maybe you are looking at surface or toolroom wheels. Do they all have a 1.25 inch bore?

PackardV8
01-24-2009, 02:53 PM
Your father's grinder could be a poster for his generation's life lesson:

"Use it up, make do, do without."

GrahamC
01-26-2009, 01:05 PM
Thanks for all the comments.

I was given a couple of new 7" wheels and they will get put to good use - the rests are adjustable (shop made in fact) and the wheels are going to fit just fine.

cheers, Graham