Kent,
This is all about the chipload you are cutting. Every CNC bit has a certain designed chipload value at which it runs best. Staying within the design range is hugely important to the job setup. If you go beneath the design range, you’ll start making dust instead of chips, heat the hell out of the tool (and dull it quickly, or break it). If you push the tool too fast, you’ll get chatter, some dust, and even more tool killing heat. Most broken bits are caused by heat, not the pressure of the bit against the stock (which is what most people assume, and why they chronically want to overdo it when slowing down the tool).
Basically, all chipload means is “the length of a chip cut in one revolution of the bit, by one flute, in inches”.
Another way to say it is, “This bit should be cutting chips “chipload” inches long, if I set up my feed and speed correctly”.
Lets say you are using a 1/4" upcut 2 flute bit in hardwood. Your chips should be somewhere between .005-.007 inches long. Knowing that, here’s the simple math to calculate the feeds and speeds:
Chip load = Feed rate / (RPM * # of Cutting Edges)
Feed rate = RPM * Cutting Edges * Chip load
RPM = Feed rate / (# of cutting edges * Chip load)
Most often, I see vendors recommending an RPM on the packaging. For example, the 1/8" compression bit I bought from Amana, said to run it at 20,000 RPM on the packaging. The chipload for that bit is .002-.004 inches, so my feedrate would be:
20,000 * 2 * .002 <= feedrate <= 20,000 *2 * .004, or a speed of 80ipm to 160ipm. So the first time I used the bit, I would setup my toolpath to run at 80ipm, then use the fob to adjust upward until I get nice looking chips at runtime. Next time, I’d just program in whatever worked the last time I used the bit.
Onsrud publishes a great reference on the chiploads for their bits, and you can usually apply them to other manufacturers bits for a given design without too much trouble:
https://www.onsrud.com/xdoc/feedspeeds