CNC Feed Rate Calculator

Feed rate, feed per revolution, and plunge rate from rpm, flutes, and chip load.

What it does

Feed rate is the speed at which the cutter moves through the material, usually written in inches per minute or millimeters per minute. It is not the same as spindle speed. The spindle turns the tool; the feed moves the tool or the work into the cut. Both have to be set together, because the chip that forms at each tooth depends on how far the tool advances during one revolution.

Chip load, also called feed per tooth, is the thickness of the slice taken by one cutting edge. It is the number that connects the other variables. A two-flute cutter at 18,000 rpm with a 0.005 in chip load advances 0.010 in per revolution, and at 18,000 revolutions per minute that becomes 180 in per minute. The same cut in metric uses a 0.127 mm chip load and produces about 4,572 mm per minute.

The practical range matters. A chip load that is too small rubs the edge against the material, generates heat, and produces dust instead of chips. A chip load that is too large overloads the flute, causes deflection or chatter, and can snap a small bit. The right value depends on the material, the tool diameter, the depth of cut, the rigidity of the machine, and the quality of the workholding. Tool manufacturer charts are the best starting point.

This page returns the feed rate, the feed per revolution, the chip load used, and a starting plunge rate equal to half the feed. The plunge figure is a rule of thumb for entering the material, not a recommendation for every operation. Ramping, helical entry, and drilling cycles use different feed logic.

The result is a starting point. A test cut in scrap material, a listen for chatter, and a look at the chips will tell you whether to adjust. Small machines, long tools, and hard materials often need a lighter chip load than the chart suggests. A rigid production machine with a short tool can sometimes run higher.

How to use

1. Choose imperial or metric before entering the chip load. The unit must match the spindle speed unit you are using for feed.
2. Enter the actual spindle speed in revolutions per minute.
3. Count the flutes or cutting edges on the tool. Do not count the shank or the flutes that are not cutting.
4. Enter the chip load per tooth from the tool maker, the material chart, or a proven job.
5. Read the feed rate and feed per revolution. Use the feed rate for the program and the per-revolution value to check the chip thickness.
6. Start with a conservative depth of cut, make a test cut, and adjust the feed or speed based on chips, sound, and finish.

Formula

spindle speed

Revolutions per minute of the cutter.

flutes

Number of cutting edges that remove material.

chip load

Feed per tooth, in inches or millimeters per tooth.

feed rate

Linear speed of the cutter through the material, in inches or millimeters per minute.

feed per revolution

Distance the cutter advances during one full turn of the spindle.

plunge rate

A starting vertical feed for entering the material, set at half the cutting feed.

The units must be consistent. If the chip load is in inches per tooth, the feed rate is in inches per minute. If it is in millimeters per tooth, the feed rate is in millimeters per minute. Feed per revolution is not the same as chip load unless the tool has one flute; with two flutes, each revolution removes two chips. The half-feed plunge is only a starting point. A hard material may need a much slower entry, and a ramp or helical path may allow a faster one. The formula does not account for radial chip thinning, deflection, spindle power, or the maximum feed the machine can accelerate through.

Worked example

Reference table

Limits

• A starting calculation, not a substitute for the tool manufacturer's speeds and feeds chart.
• Does not model depth of cut, radial engagement, material hardness, coolant, or machine rigidity.
• The half-feed plunge is a rule of thumb and may be too fast for hard materials or small tools.
• Does not check spindle power, axis acceleration, tool deflection, or the maximum feed rate of the machine.

Common mistakes

• Using feed per revolution as the chip load on a multi-flute tool.
• Counting flutes that are not cutting, such as the unused edges of a compression bit.
• Mixing inches per tooth with millimeters per minute without converting.
• Starting at the maximum depth of cut and the chart feed at the same time.
• Ignoring tool runout, which makes one flute take a heavier chip than the others.
• Running a very small chip load to be safe, which rubs the edge and dulls the tool.

Try it yourself

Quick checks. Answers sit tucked under each question.

Questions

Sources

• Walla Walla University: Machining lecture, feeds and speeds
• Fab Academy: Selecting the Right Bit, Feeds and Speeds Charts

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