Calculating the Material Removal Rate

The feedrate and spindle speed together quantify a metric known as the Material Removal Rate (MRR), measured in mm3/min or in 3/min.

For information on the feedrate and spindle speed, see the introduction to CNC milling knowledge base page.

The MRR is the volume of stock material being removed per minute. MRR is a direct measure of the efficiency of a cutting operation. A higher MRR means the job is being completed more quickly.

However, maximizing MRR is a balancing act. While a high MRR is faster, especially in roughing operations where the goal is to remove a large amount of material quickly, it can come at the cost of a poorer surface finish, increased tool wear, and greater stress on the machine.

For finishing passes, the MRR is typically much lower to prioritise accuracy and a smooth surface.

You do NOT need to know how to calculate the material removal rate in order to use the CNC!

But keep reading if you are intrigued! 

The fundamental equation for calculating the Material Removal Rate (MRR) is:

MRR=da×dr×f

Where:

• da​​ is the Axial Depth of Cut (ADOC): This is the depth the tool is engaged in the material along the spindle's axis (how deep you are cutting).
• dr is the Radial Depth of Cut (RDOC): This is the width of the tool engaged in the material, perpendicular to the spindle's axis (how wide of a path you are cutting). For a slotting operation, this would be the full diameter of the endmill.
• f is the Feedrate: This is the velocity of the tool as it moves through the workpiece.

The spindle speed is related to the feedrate by the equation:

f=S×N×CL​

Where:

• S is the Spindle speed (in RPM).
• N is the Number of flutes on the endmill.
• CL​ is the Chip Load, which is the recommended thickness of material that one cutting edge (flute) should remove in a single pass. The ideal chip load is usually listed on the datasheet for the endmill.