Inches Per Minute
Inches per minute (IPM) is a linear feed-rate unit meaning the tool or axis travels one inch in one minute. In CNC milling and routing, IPM is the standard for programming cutter advance through the workpiece. It is commonly calculated as RPM multiplied by chip load per tooth (IPT) and number of flutes, forming the basis for mill and drill feed calculations.
On the CNC shop floor, IPM is the production link that ties spindle speed, cutter geometry, chip evacuation, surface finish, tool life, and cycle time together. Programmers enter IPM values for each motion block, using different feeds for plunge, ramp, contour, and finish passes. The goal is to keep chip thickness within the toolmaker's recommended range while maintaining stable cutting. Higher IPM reduces cycle time, but only up to the point where tool deflection, chatter, spindle load, or chip packing degrade cut quality. In millwork and routing involving wood, MDF, plywood, and composites, IPM must clear chips fast enough to avoid burning, edge tear-out, and premature cutter dulling. The feed rate is the router bit's advance through the material in inches per minute, directly affecting throughput and part quality.
Is IPM a spindle speed?
No. IPM is linear axis feed, while spindle speed is RPM. IPM describes how fast the cutter advances through the work, not how fast the tool rotates.
Why does chip load matter when setting IPM?
In milling, the cutter removes material tooth-by-tooth. The correct IPM is derived from desired chip thickness per tooth, spindle RPM, and flute count, which keeps cutting forces and heat in the correct range.
When is IPM used instead of IPR?
IPM is standard for milling and routing where the workpiece is stationary relative to cutter rotation, while IPR (inches per revolution) is more common in turning and boring where feed is referenced to each spindle revolution.