As a sort of follow up to the Machine Tool Technology thread I thought I would put up the calculation for drill speeds. Not one speed fits all materials and if you want your tools to last and your holes to be round you should know what speeds are best. The following is a very condensed selection from the Machine Tool Technology book.
Do not confuse cutting speed with RPM. Cutting speed is the circumferential speed of the drill and it is expressed in surface feet per minute (sfpm). Too fast for a given material will over heat and dull the drill, too slow can cause it to break.
You should understand that a 1/4" drill turning 1222 rpm has a cutting speed of 80 sfpm. A 1/2" drill at 80 sfpm will be turning only 611 rpm.
Here are some typical cutting speeds for the materials you are most likely to encounter using high speed drills;
Low carbon steel-100
Medium carbon steel-80
High carbon tool steel-50
Stainless steel-40
Brass/bronze-250
Hard bronze-100
Aluminum-most alloys-250
Magnesium-325
Bakelite and similar plastics-125 (with proper drill sharpening)
Wood-350
The formula for RPM uses cutting speed (CS) with D as the diameter of the drill;
__CSx12
Dx3.1416 =RPM
Example for 1/4"drill at 100 sfpm;
__100x12
0.250x3.1416 =RPM
1200_
.7584 =RPM
RPM= 1528
After you figure a couple of these for the sizes you use most you will develop a feel for what you need to use on your own drill press. Since few have an infinitely variable drive you pick the speed of your machine that is reasonably close. You should also use the appropriate cutting fluid as needed.
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