Cutting · Feeds and speedsbeginner~3 min read

Feed rate and plunge rate for CNC routers

Short answer

Feed rate is horizontal travel speed while cutting, in units per minute. Plunge rate is straight-down travel speed, and it is usually a third to a half of the feed rate because the centre of most bits cuts badly or not at all. Ramping in at an angle avoids the problem entirely.

Feed rate

Feed rate is how fast the machine drives the spinning tool through the material, measured in inches per minute or millimetres per minute. It is the number you have the most direct control over and the one that sets your chipload for a given spindle speed.

Two limits sit on either side of it:

The low limit is rubbing. Below a certain feed, chips are too thin to cut and you generate heat instead of chips.
The high limit is force. Faster feed means thicker chips, which means more force, which eventually means deflection, chatter, lost steps or a broken tool.

Most hobby users live too close to the low limit, because a slow cut feels safe. It is not safe. It is just slow and hot.

Plunge rate, and why it is separate

When a bit goes straight down, the cutting happens at the very centre of the tool. Most router end mills are centre cutting, so the tool will go down, but the edge near the axis is barely moving relative to the material, so it pushes and scrapes more than it slices. Plunging is closer to drilling with a tool that is not a drill.

Typical plunge

About one third to one half of the feed rate

Why slower

The tool centre cuts poorly, chips cannot escape a blind hole

Better answer

Ramp or helix in, so the tool enters at an angle

Rule of thumb The one third to one half figure is a starting point, not a law. Some bits are centre cutting and plunge happily. Others, particularly some straight bits and any bit with a large flat centre, should barely plunge at all.

Ramping and helical entry

Instead of dropping straight down, the tool moves forward as it descends, so it enters like a plane landing rather than a helicopter. Almost every CAM package offers this as a ramp, a lead-in, or a helical entry for pockets.

It solves several problems at once:

The cutting happens on the side of the tool, where it is designed to happen.
Chips have somewhere to go, because the tool is not in a blind hole.
The load builds gradually instead of arriving all at once, which is easier on both bit and machine.
It removes the burn mark that plunging often leaves at the entry point.

One setting, fewer broken bits

If you break small bits, turn on ramping before you change anything else. Plunge entry is where most small-diameter cutters die, and it is a checkbox in nearly every CAM package.

Feed rate override, and what it hides

Most controllers let you scale the feed rate live, usually from about 10 percent to 200 percent. It is useful for the first pass of an unfamiliar job. It is also a trap, because it changes the chipload underneath you.

If you find yourself running a job at 60 percent override so it sounds acceptable, do not save that as your workflow. Go back to CAM, work out what the real settings should be, and fix them there. The override is a live safety valve, not a substitute for correct settings.

Where feed rate is not what you set

The machine does not achieve the programmed feed everywhere. It accelerates and decelerates for corners, small segments and direction changes. On a detailed toolpath with thousands of short moves, a machine may average well under half its programmed rate. Consequences worth knowing:

Job time estimates based on path length are optimistic. See machining time.
The chipload drops everywhere the machine slows down, which is why burning often shows up in tight detail and corners first.
A toolpath with unnecessary tiny segments, usually from a badly traced image, cuts both slowly and roughly. See vector troubleshooting.