Know Your Feed Rates for Machines

By Kip Hanson

This one’s a little easier to understand. While the milling cutter is spinning at its recommended cutting speed/rpm (or the workpiece rotating against a fixed lathe tool), each is also moving around, through, or into the workpiece at a commanded feed rate.

Here again, machining centers win the simplicity award: Just tell them to move at however many inches or millimeters per minute you desire, and the machine will do its darnedest to achieve that feed rate, even when zipping through tight corners.

Of course, that value is determined by another calculation: the recommended chip load per tooth x the number of teeth x spindle rpm. On a four-flute end mill with a 0.005-inch “chip load” running at 2,000 rpm, that comes out to a sedate 40 IPM (inches per minute) feed rate. Kick up the rpm to 4,000 and you’ll need to double the feed rate if you’re to maintain the same chip load.

Similarly, lathes can be commanded in G98 mode (feed rate per time) or G99 (feed rate per revolution). The latter is the default, and about the only time you would ever use G98 is when feeding bar stock (because the spindle is either turned off or moving very slowly). Want to feed a rough turning tool at 0.012 IPR (inches per revolution)? Easy, just put the machine in feed mode (G01) along with an F.012 and G99 command (it’s modal, so you only have to tell it once). And because turning tools have only a single cutting edge, there are none of those cumbersome chip load per tooth calculations.