Q.: When sawing concrete with a 30-inch diamond blade the cutting has been very slow. We've been told that we should reduce the motor speed for large blades to make them cut efficiently. Is there a rule of thumb on how fast to run a saw?

A.: The Concrete Sawing and Drilling Association gives the following suggested spindle speeds for various sizes of blades:

### Blade diameter, inches/Spindle speed, rpm

12in/2547rpm

14in/2183rpm

18in/1698rpm

20in/1528rpm

24in/1273rpm

30in/1019rpm

These speeds are usually printed on the blades but sometimes wear off. They are the appropriate speeds for a blade tensioned for 9600 feet per minute total travel of a point on the circumference. They are somewhere in the range of 9000 to 11,000 feet per minute. For larger blades the speed can be calculated from the formula:

```           12 x feet per minuteSpeed  =  -------------------------           3.14 x diameter in inches
```
When cutting more than 20 inches deep, the Concrete Sawing and Drilling Association suggests that the contractor install an intermediate shaft between the spindle and the engine. In general the speed should be increased for soft, abrasive material but decreased for hard, dense material. Further information can be obtained from the blade manufacturer.