To optimize chip removal, the rotating speed to the cutting tool and the linear axis speed at which the tool moves must be balanced to maximize metal removal, machine performance, and cutting tool performance. The following provides background information 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. A frequency inverter controls the speed of the spindle. 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, which 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. To achieve optimum results, the best feed rate is determined through experience and trial-and-error. 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 recommend 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 — the radial depth of cut of the cutting tool in one revolution — is 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 ascertain cutting speed (rpm) and feed-rate values for the setup. For wood, a chip load that is too low generates excessive 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. Some say it 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

On new spindle applications, we strongly encourage you to consult PDS for assistance 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.