How to Calculate Feed Rate for CNC Milling

When it comes to feed rate, people may have already know what is feed rate. However, for how to calculate feed rate, most people will have no idea about it. This short article aims to show you the related information of feed rate, including what is feed rate, why is it important, how to calculated feed rate and the limitations when calculate it. Hope it could help you a lot.

List: 

1. What is feed rate?

2. Why is feed rate important?

3. How to calculate feed rate?

4. Limitations

What is feed rate?

Feed rate is one of the most important factors to consider when implementing any CNC strategy. Simply put, the feed rate is the speed at which the tool engages the part and is usually measured in units per minute. Depending on the type of material you are cutting (aluminum, steel, wood, acrylic, etc.), the material of the cutter (hard alloy, high speed steel, ceramics, etc.) and many other materials, the recommended cutting feed rate will vary. The cutting factor includes the required surface and the characteristics of the CNC machine itself.

Why is feed rate important?

The feed rate is very important because it is directly related to almost all aspects of CNC machining, from safety and productivity to tool life, surface finish and part quality. It can also cause mechanical parts of the CNC machine to wear out.

Think about it. The cutter engages the material and accelerates along a linear path at a speed of 100 inches per minute. When it comes to changes in the direction of the corner, you must decelerate exactly to the dead point at the corner, change direction, and then accelerate to 100 inches / minute to continue cutting. This rapid deceleration causes an increase in the torque load on the CNC machine. Just as we like to drive a car at a certain speed for optimum fuel efficiency and minimize tire wear, we cut at a certain cutting speed in CNC milling to minimize tool wear and optimize cycle times.

How to calculate feed rate?

Feed is generally measured in Inches Per Minute (IPM) and speed is measured in Surface Feet per Minute (SFM). A feed of 10 IPM would drive a tool 10 inches in one minute. If you took an end mill and rolled it along the floor like a tire at 100 SFM it would travel 100 feet in one minute. Some other key terms include spindle speed, measured in Rotations Per Minute (RPM) and chip load, which is the amount the tool advanced or moves per cutting tooth per revolution..

These all relate to each other through some relatively basic equations. They can look a little intimidating at first, but knowing how these parameters relate to each other is a good foundation to understanding feeds and speeds.

· In the CNC program we however write the RPM. This equation calculates the RPM N from the cutting speed V in meters per minute (m/min)and the tool diameter D (the 1000 in the numerator meters to mm).

N=1000V/πD

· The Feed rate (F in the picture) is the linear speed of the tool as it travels along the part contour. A cutting tool is designed to cut a certain amount of material in each revolution, and the insert’s chip breaker geometry is designed to break chips within a particular range of chip thickness. The chip thickness directly depends on the feed rate in mm/rev. The feed rate is therefore specified in cutting tool catalogs in mm/tooth/rev (mm per tooth per revolution). It is converted to mm/min for the CNC program (note that in turning it is programmed as mm/rev).

F=fz x z x N

fz=free rate, mm/tooth/rev

z=number of teeth

N=RPM

According to the above formula, you can calculate the theoretically ideal feed rate and spindle RPM speed based on surface speed, number of tool slots and required chip load. Now that we have completed all of these equations, the good news is that you can't do any math manually. Modern CAM tools like Fusion 360 can do all the math for you. You only need to enter the two required parameters and the computer does the rest.

Limitations

To be honest, finding the right feed rate did not make the cutter answer. The ideal feed rate may depend on many variables that are difficult to control, including but not limited to:

Tool holder rigidity and grip

Workpiece rigidity and grip

material

Tool materials, geometries and coatings

Machine rigidity

Machine tool spindle horsepower and torque at different speeds

Obviously, this is a complex issue involving many variables. Feed rate that may work well in one job may not work for another. The good news is that there are some general guidelines and starting points that can help you achieve the best feed and speed without too much growth pain.

This short article relates to the basic knowledge about feed rate, which is the way to build your own knowledge. Hope you could have rough impression about how to calculate the feed rate. For more information, please contact us.