A Little Shop Math, Bench Grinder Speed vs Hand Held Grinder Speed
I have noticed a fairly frequent error occurs when we compare "Dremel"type buffers to bench mounted buffers. The error is understandable because the speed of the hand held devices is considerably higher than that of the bench grinders. Here is a simple guide to accurately comparing the speed of various devices.
Hand Held Grinder vs 8” Bench Grinder
•Hand held (“Dremel” type) grinders have a typical range of 5000 Revolutions Per Minute (RPM) to 20,000 RPM.
•8” bench grinders range from 1700 RPM to 4000 RPM.
While at first glance this may sound like the hand held machine is much faster than the bench grinder, in fact the opposite is true. Our concern is not the speed of the motor but the amount of abrasive material (buffing wheel) passing against the work piece. Therefore, what really matters is the rate of rotation times the circumference of the wheel.
Circumference of the Wheel
Calculate the wheel’s circumference by using the following formula:
C=π·d
C=circumference of the wheel
π=3.14
D=diameter of the wheel
For the sake of this discussion we’ll assume the hand held machine has a one inch wheel and the bench grinder an eight inch wheel.
Hand Held Wheel Circumference:
C=1”x3.14
C=3.14”
Bench Grinder Wheel Circumference:
C=8”x3.14
C=25.12”
So, at the highest speed possible for your hand held grinder, a whopping 20,000 RPM, your work piece is subjected to 62,800 inches of buffing wheel in one minute of buffing. With the bench grinder at its highest speed of only 3,400 RPM, 85,408 inches of buffing wheel passes over your work piece in one minute. If you want to calculate the speed of the wheel in the more familiar Miles Per Hour (MPH), simply multiply your inches per minute by 60 to get inches per hour and then divide that number by 63,360 (the number of inches in a mile).
Hand Held Grinder: 62,800 x 60 = 3,768,000/63,360=59.470MPH
Bench Grinder: 85,408 x 60 = 5,124,480/63,360=80.879MPH