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  • RPM- Rotations per minute, or speed. This is how many times the cutter revolves in one minute.
  • Feed- How fast your are moving the cutter.
  • SFM- Surface feet per minute. This is how far the cutting edge of the tool advances in one minute. This takes into account the feed rate, the cutter diameter, and the spindle speed.
  • Chip Load- How much material is removed per tooth as you move through the material. This is dependent on feed rate, rpm, and the number of cutting surfaces. It is often reported in IPT (Inches Per Tooth).

SFM is the entry point for calculations. Recommended SFM is based on cutter material and stock material. Given a specific cutter material and stock, you can find recommended SFM values from the cutter manufacturer or from an online table.

Calculating Speed

RPM (Speed) = SFM x 12 / (π * Tool Diameter)

From this calculation, you can get your “speed.” This calculation determines the maximum speed for the tool and material. In other words, this is the “speed limit” rotating faster than this will cause your tool to age prematurely. You will often operate slower than this depending on your machine's spindle capabilities.

Approximation: RPM = 4 x SFM / Tool Diameter

When done with this calculation, pick a comfortable speed and use that to calculate the rest of your parameters.

Calculating Feed:

Chip load is normally determined experimentally by the tool manufacturer. It is reported as IPT. You can use this number along with the RPM you calculated earlier to determine a feed rate.

If you can't find chip load anywhere, a conservative estimate is to divide the diameter of the cutter by 150.

A 1/4“ cutter with 2 flutes would yield the following result:

Chip Load = .25 / 150 = 0.00166”/tooth

Feed Rate = RPM x Chip Load x Number of Flutes

Using a speed of 5000rpm:

Feed Rate = 5000 x .00166 x 2 = 16.66 in/min

Calculating Depth of Cut

The rule of thumb is to cut no deeper than 1/2 of the diameter of the cutter if you're cutting a pocket using 100% of the tool.

speeds_and_feeds_guide.txt · Last modified: 2017/04/08 16:32 by admin