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Spindle RPM and Feed Rate Calculations | Chip Load Formula

To optimize chip removal the rotating speed to the cutting tool and the linear axis speed that the tool moves must be balanced to maximize metal removal, machine performance, and cutting tool performance. The following information provides background information of their relationship to help maximize machining performance.

Spindle Speed 

Incorrect spindle speed is a common error in CNC machining. Each material type and type of cut has an ideal tool profile and cutting speed. Larger diameter tools require slower cutting speeds. The spindle speed and feed rate for a given cut must be balanced for best work quality, tool life and spindle life. The speed of the spindle is controlled by a frequency inverter. All spindles are 3-phase asynchronous motors with infinitely variable speed from 0 rpm to the maximum rated rpm. The speed can be achieved by correctly programming your frequency inverter drive to match the spindle.

Feed Rate 

The feed rate of the cutting tool must be balanced proportionally with the spindle rpm. Changing one influences the other. Feed rates set too slow decrease tool life by causing overheating and this condition may leave burn marks on the work. A tool moving too slowly causes the tool to heat up because not enough material is removed to cool the cut interface. Determining the best feed rate is determined with experience and trial-and-error to achieve best results. The feed rate chart shown here provides a range for starting point values. For first time setups, begin at the mid-range. Your cutting tool supplier can advise the cutting data for your specific application.

Chip Load 

Chip Load is a term used to describe the thickness of a chip removed by one cutting edge of the tool. Chip load is sometimes referred to as ‘feed per tooth’.  The chip load is the radial depth of cut of the cutting tool in one revolution and calculated as follows:

Chip Load = feed rate ( ipm ) ÷ ( cutting rpm x number of cutting edges )


Chip load is one factor used to determine a starting point to determine cutting speed (rpm) and feed rate values for the setup. For wood, a chip load that is too low generates too much heat, the chips produced are ‘dust-like’ and may leave burn marks on the part. A chip load that is too high is pushing the cutter through the material. This generates high radial loads on the spindle bearings, and over time, can cause spindle failure, excessive tool wear, and tool failure. There are two styles of milling: climb milling and conventional milling. For woodworking, ‘climb milling’ produces a better finish with less tear out, and some say, provides longer tool life. 

Chip load is one of many factors used to size the spindle and determine machine requirements, such as:

  • The speed range and power requirements for the spindle
  • Tool load to determine type and grade of cutting tools
  • The amount of torque generated to make the cut

We strongly encourage you to consult PDS on new spindle applications to assist in establishing your baseline cutting values. Our staff is here to discuss your technical questions by phone or email. The chart below provides chip load values for common router tools. These are ‘not to exceed’ starting point values for first time setups. Actual values will vary as a result of many machine factors, such as: rigidity, horsepower, collet condition, spindle integrity, part clamping, hold down, and others.