[TheForge] rpm/horsepower

[Ries Niemi]

On Wednesday, October 6, 2004, at 10:33 AM, Schade wrote:

>
> I need a new bench grinder. The one I have is 1/2 HP 1750 RPM.
> It seems like everything I have seen is higher horsepower and 3600
> RPM. Is this marketing? Is a 1/2 HP 1750 RPM the same (torque)
> as a 1 HP 3600 RPM?
>
> Bob
>
>

Where the heck did you get a 1750 rpm bench grinder?
Grinding wheels are designed and rated in surface feet per minute, with 
the average stones wanting to run around 5000sfpm.  A 6" grinding wheel 
on a 1750rpm grinder is running at about half that- too slow, in other 
words.
Commercially, almost all bench grinders are 3450 rpm until you get up 
to the big dogs- say 12" or 14", at which point they drop down to 1750.
Now if you are gonna use this for buffing, or wire brushing, that might 
be different. But for grinding, in a 6" or 8" size, you want 3450, and 
thats what they make.
As far as torque goes, yes, its true that a slower motor has more 
torque for the same horsepower. But you need the speed, so you do need 
a bigger motor. I have a 1 1/2hp Jet that I am quite happy with- cant 
seem to slow it down.

If you are wet grinding woodworking tools, of course, throw all this 
out the window, and buy a special machine for that. But for general 
purpose sharpening and grinding, 3450 is right.

ries