Milling Cutting Force Calculator | Feed and Power Guide

Cutting force is the amount of pressure that the material puts against the cutting tool as the tool shears through the material. Cutting force is an important factor in milling operations because cutting force can dictate whether the tool will break or the part will be ruined. If the feed rate is pushed to hard, the cutting force increases which can lead to the spindle of the machine beginning to whine or the chips begins to weld to the cutter.

An understanding of cutting force is an essential part of controlling the machine, and an understanding of cutting force allow for an increase in the speed of the machine without going beyond the capabilities of the machine itself. The variable Kc represents the specific cutting pressure. The value of Kc vary with the material that is being cut.

Cutting Force in Milling and How to Control It

For instance, soft woods like pine have a resistance of approximately 400 N/mm² whereas stainless steel have a resistance of 2200 N/mm². You can calculate the total cutting force by multiplying the cutting pressure by the depth of cut, the chip thickness, and the number of teeth of the cutter that is biting into the material. If you increase the axial depth of cut, the cutting force will linearly increase.

The radial engagement of the cutter, also known as the stepover width, also play a role in the cutting force. If shallow side cut are made, the chips that are cut will be thicker which increases the cutting force. To compensate for this increased cutting force, a lighter feed per tooth should be programmed into the machine.

Aluminum slot are one of the most common areas where cutting force can create a problem. For instance, if an aluminum slot is milled at half of the tool’s diameter, a 0.125-inch feed rate with a quarter-inch endmill will cause the thin slice of aluminum to turn into a fat chip that increases the cutting force of the endmill by 20% to 30%. To avoid this increase of cutting force, the radial engagement must remain under 20% of the tools diameter.

This is more effective than changing the RPM or the number of flutes of the tool. Woods tend to allow for faster feed rates than metals but can easily become tear-out if overloaded with cutting force. Tools cutting steel must be rigid because the metal is more likely to deflect than other metals.

A 10% safety margin is used with steel to avoid such deflection. The number of flutes of the tool impacts the milling operation. Two flute cutter are used for softwoods when setting the table for the cutter to allow for the softer woods to be bitten at depths between 0.008 and 0.015 inches per tooth.

Four flute tools are used for cutting steel with a chip thickness of between 0.005 and 0.015 inches. Using more teeth increases the feed rate at which the tool move through the material at the same chip thickness. A higher feed rate increases the material removal rate from the work piece.

However, if the wrong number of flutes are used, rubbing heat into the work piece can occur or the cut can become starved of proper coolant. Slotting operations require more conservative feed rates than other milling operations but finishing operations can use deeper radial steps to smooth out the area to provide a shine to the part. The power and torque in the machine are directly related to the cutting force created by the tool.

By multiplying the cutting force and the cutting speed for the material being milled, you can calculate the power demand of the spindle. This value is often the bottleneck for smaller routers. For instance, milling aluminum at 600 N/mm² with a decent speed will only require 1 to 2 horsepower.

However, milling stainless steel will quickly require more than 5 horsepower. The torque is strongest at the lowest RPMs in the machine. Using oversized endmills on high speed spindles can create stress on the bearings.

The cycle time for the machine is related to the cutting force. The longer the machine travels a certain path or the number of passes the tool makes through the work piece, the longer the cycle time for the machine to complete the task. Cycle time can be reduced by using presets to move the tool to certain positions within the work piece.

For instance, presets may allow the machine to move to 19k RPM and 20 IPM speed for cutting hardwoods. There are several common pitfalls with milling operations. Chatter can occur with endmills with long stickouts which will dull the cutting tool’s edges quickly.

Using shorter tool holders will restore the cutting stability to the machine. Using dull cutters will double the cutting force of the tool and turn clean chips into stringy chips. The sharpness of the tool is absolutely necessary for cutting operations due to the laws of physics.

Heat can be a problem when cutting plastics like acrylic. In this case, you must use cool air and the engagement of the tool with the work piece should be kept light to avoid melting the acrylic work piece. Cutting chips that have already been cut in a work piece will lead to the process becoming gummy so using climb milling techniques along with providing good evacuation will ensure that the process continues smoothly.

Machines have limits to the rate at which they can remove the material from the work piece. The material removal rate is a calculation of the amount of material that is removed per minute. These engagement states will allow for an understanding of the depth of the cut that should be made.

For instance, if the radial engagement is under 20% then the depth of the cut can be increased. If the engagement rate is over 70% then the feed rate must be decreased. The RPM that is used with the cutter should be checked against the diameter of the cutter.

Safety margins of 10% to 20% can be used as a buffer for vibration or other inconsistencies in the machine. By cutting force as the primary concern and increasing the speed of the machine after ensuring that the forces acting upon the tool are within safe parameters, the tools will last longer for the operator and the parts will be produced correctly.