Milling Table Feed Rate

The table feed is the number a milling machine is programmed in, and the chip load is the number the cutter cares about. This equation is the whole of the relationship between them: each tooth takes a bite of $f_z$, there are $z$ of them, and the spindle brings them round $N$ times a minute.

Because the table feed is derived and the chip load is fundamental, raising the spindle speed without raising the feed in proportion is a mistake — and it is the mistake most often made, because rpm feels like the safe thing to increase. Thinning the chip does not make the cut gentler. Below a chip thickness comparable to the edge radius of the tool, the edge stops shearing and starts ploughing: the metal is pushed down and burnished rather than cut, the tool heats, and it wears out by rubbing. Too small a chip load kills more milling cutters than too large a one, and it does so while the cut sounds beautiful.

Count the teeth honestly. $z$ is the number of teeth actually engaged and cutting, which is not always the number on the cutter. A fly cutter has one. A face mill with inserts set at two heights presents half of them at a shallow depth. And on an indexable cutter whose inserts run out relative to one another, the highest tooth takes a chip larger than this equation says while the others take less — which is why runout shortens insert life so disproportionately, and why measuring runout is worth more than most of the other things done to a tired cutter.

Two effects make the real chip thinner than $f_z$, and both mean the feed can be raised rather than that the equation is wrong. Radial chip thinning: when the stepover is less than half the cutter diameter, each tooth enters and leaves at a shallow angle and the maximum chip it takes is smaller than the programmed feed per tooth, roughly by the sine of the engagement angle. This is the arithmetic behind trochoidal and dynamic milling, where a small stepover is fed at what looks like an impossible rate. Axial thinning does the same for a round-insert or ball cutter taking a shallow depth of cut.

Finally, climb or conventional changes what the chip does at each end of its life. In climb milling the tooth enters at full chip thickness and leaves at zero; in conventional milling it enters at zero, which means it rubs before it bites. Climb milling gives better finish and tool life on any machine with the backlash taken out of the feed screw, and was the reason the ball screw arrived.