8" vs 10" contact wheel

6

69camaro

Joined
Dec 7, 2006
Messages
35
Getting ready to purchase a Coote grinder. Is there any good reason to purchase the 8" wheel vs the 10" wheel. I intend to learn to hollow grind and have a 1750 rpm 1.5hp motor. Expecting to purchase 3-step pulleys and ceramic platen from Norm.

Also, do any of you have input on the Knife rest? How well does it work?
 
I have a 10" wheel on my Bader III. I like it. I've used an 8" too. I'll probably get an 8" wheel, because I like the grind line height better on the small blades with it. I think the 10" wheel is a little better to keep you from getting the edge too thin where the blade starts to curve up, but practice helps a lot with that issue. I really don't think it makes a lot of difference.
 
Where the contact wheel starts to make the biggest difference is with larger blades. To do a hollow grind on a larger blade you need a larger diameter contact wheel. I purchased the 10" contact wheel just for this purpose, because I've always loved big bowie knives and the like, and to hollow grind a wider blade, you want a bigger contact surface (more gradual curve to the hollow grind).
 
All of the above but unless you have variable speed the 10" wheel is spinning so fast (surface feet per minute) its hard to control and keep things cool. You can sure hog off steel in a hurry though. I dont have vs and cant use my 10" on the Burrking.
 
What I did was determine the speeds (fpm) that were reasonable for grinding (asked some people what speeds they typically use) and then found the pulleys to get my 10" wheel to drive the right RPM for the right belt speed.

with a 10" direct driven contact wheel, it's easy to do the calculations. Take the motor RPM multiply it by the diameter in inches of the pulley on the motor, divide by the pulley diameter you're driving on the coote shaft, then multiply that by 2.6 (10" * 3.14159 * 1foot/12") to get the FPM.

(example, what i've been using, 1725 rpm motor, 2" step of a 4-3-2 pulley on the motor, 6" step of a 6-5-4 pulley on the 10" coote)

1725x2/6 = 575 RPM
575 RPM * 2.6 Feet per revolution ~= 1500 fpm belt speed.

the other 2 speeds which I have available to me are

3inch -> 5inch ~= 2700 fpm

4inch -> 4inch (direct drive equivalent) ~= 4500 fpm

I was told by several people with KMGs that they run theirs with step pulleys at 800 fpm 1750 fpm and 3500 fpm for the three speeds.
 
You will find grinding to the left side of your contact wheen on you coote a lot easier with a 10 in wheel. I have used a 10" on a Burr King for at least 20 years with no problem. The 8" seems way too slow for me. It is just in what you get use to. But I really am enjoying the now Vari- Burr-King I just insalled. Mike
 
Thanks for the input. I ordered it this morning with a 10" wheel and 3-step pulleys. Now to decide if it will be freestanding or bolted to a wall bench.
 
I guess I wasn't thinking about the speed. I have a variable speed 1 1/2 hp motor. I never got over 50% when I grind on it. I usually stay around 35-45% escpecially with the high grit belts.
 
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