By: Josh Rucinski
One of the hardest things to find online is the speeds of which you should cut materials. The main reason is because the information is protected. The secondary reason is because RPM spindle speed is not actually the same as cutting speed.
Cutting speed, when you can find it online usually reads like this 600 fpm which means 600 feet per minute. Machinists use this because the diameter of the cutting head of the tool changes the speed of the cut dramatically. Consider the following. I have a 10” diameter tire and I rotate it once on the ground it will have travelled 10”x pi = 31.4”, if you have 40” diameter tire, the distance is four times the length on the ground. If you rotated both tires during the same length of time, the 40” diameter tire is moving 4 times the speed!
Therefore cutting speed is expressed in rpm by using an equation. Pi is simplified as 3, and the inch/foot converter is factored in. There the expression is this (fpm*4)/diameter of tool= rpm. CNC push speed is a small fraction of the rpm, The Feed Rate in "INCHES Per MINUTE" is determined by multiplying the number of cutting teeth by the RPM, multiplying that product by the Feed per Tooth, and dividing by 3. The calculation is as follows:
Feed Rate = (Number of Cutting Teeth x RPM x Feed Per Tooth)
Example #3 Use the information and RPM calculated in Example #2 for a Milling Machine, ball-parking the FPT(Feed per Tooth) of .005 (see table), and a cutting depth of .050"
Feed Rate = (2 x 2000 x .005) = 20 inches/minute
This is really just rules of thumb, adjustments are always required to each product.