Guide to calculating working speeds of milling cutters for CNC routers

Before setting up any cutter for cutting, there are certain parameters you must take into account if you want to achieve the required finish and precision. One of the most important of these factors is the chip load per tooth, or chip thickness (Cpt). Chip load can be defined as the size or thickness of the chip removed by each flute per revolution.

When machining the material, the cutter must spin at a certain speed and feed at a certain feed rate to achieve proper chip loading. There are also several factors to consider when choosing the appropriate speed and feed rate.

The feed rate used depends on a variety of factors, including the power and rigidity of the CNC router, the way the workpiece is clamped, the power of the milling spindle, the depth and width of cut, the sharpness of the cutting tool, the design and material of the cutter, as well as the material being machined.

To achieve the optimal chip load, you must consider these variables, together with the CNC router milling machine and the materials you intend to cut. This will help you find the best feed rate and the best RPM for any given tool and material.

One thing to keep in mind is to produce sawdust, not dust. Sawdust chips help carry away the heat generated during cutting, thereby extending tool life and improving edge quality. Feeds and speeds are usually set in the programming software used to create the machine program. Many resources are available to help determine the right settings for specific material/router bit combinations.

These often provide a good starting point, but can usually be further improved through a small amount of trial and error. Most milling machine controllers allow you to adjust the feed rate while a program is running, and by listening to the sound the cutter makes, this can be a good way to optimize the parameters.

The feed rate is calculated using the following equation:

Feed rate = N x T x Z

N = number of cutting edges (grooves or flutes)

T = Chip load is the amount of material that must be removed by each flute of the cutter as it rotates and advances into the work (mm per tooth)

Z = Working speed (RPM), the speed at which the cutter rotates.

Now we will break down the relationship between feed rates, number of cutting edges, chip load and RPM. For most materials there is a recommended chip load.

If you run at 18000 rpm using a 25mm two-flute cutter and a recommended chip load of 0,1mm/tooth:

Feed = 2 x 0,1 x 18000 = 3600 mm per min

If we increase the RPM to 24000, the new calculations will look like this:

Feed rate: 2 x 0.1 x 24000 = 4800 mm per min

Based on this equation, as RPM increases, feed rate will also increase if all other settings remain the same. If the number of cutting edges changes, feed rate will either increase or decrease depending on whether the number increases or decreases. The same is true for chip load; if the recommended chip load is 0,1 mm/tooth, RPM, feed rate, or number of cutting edges can increase or decrease to maintain the required chip load. Therefore, if the chip load remains the same and feed rate increases, either RPM or number of cutting edges must increase to maintain the recommended chip load.

When calculating the feed rate for any material being machined on a CNC router, chip load is therefore one of the most important factors to consider because chip load determines the amount of material each tooth will remove, plus the load each tooth will have to support. Another factor that affects chip load is the diameter of the cutter. A larger cutter will be able to handle a higher chip load.

Therefore, depending on the tool diameter, if the RPM and number of cutting edges remain the same, the amount of chip will increase with a larger diameter cutter, and thus the feed rate will increase. When machining softer materials or using a stiff cutter, the amount of chip can be increased. If a very long router bit is used, the amount of chip should be reduced.

For most materials you will be cutting on a CNC router, you will typically set the RPM between 12000 and 24000 and adjust the feed rate to achieve the desired results. The speeds and feeds chosen can be affected by the power of the spindle being used. Higher power spindles will produce more torque, allowing the machine to operate at a variety of RPMs (torque decreases as the RPM is reduced).

Although there are formulas for calculating feed rates, you will find that the optimum feed rate will be determined by experience. Typically, you will start with the calculated feed rate. Ideally, it is usually suggested that the actual feed rate be set at about half the calculated amount and gradually increased to the capability of the machine and the desired finish.

Processing department

Once you have determined what feed and speed to start with, there are other factors to consider. The next thing to consider is the cutting direction, which is the direction in which the cutter is inserted into the material. Conventional milling or forward cutting is the most commonly used method. With this method, the work is fed against the direction of rotation of the cutter. The other method is up-cut milling or reverse cutting. For this machining method, the workpiece and the CNC router must be rigid. When machining non-ferrous materials, up-cut milling should be used to achieve a good finish.

Another factor is the depth of cut. The depth of cut will affect the edge finish as well as the life of the tool. You will need to adjust your depth to achieve the desired results depending on the type of material and the size of the cutter. Typically, a depth of cut equal to the radius of the cutter is a good starting point when cutting non-ferrous metals.

To do:

– Make sure you have the right cutter for the material to be processed

– Look for recommended settings when programming a new cutter type for your CNC router

– Check with your woodworking machinery supplier if you are not getting the expected quality or productivity

– Make sure your tweezers are not worn and that the tool is mounted correctly

– Start with lower feed and plunge speeds to avoid possible tool breakage or machine damage

To avoid:

– Making test cuts on a piece of scrap material is not a good way to check your settings before running the main program

– DO NOT use worn or damaged cutters, which can cause overheating and poor cutting quality

– DO NOT continue cutting if you hear an unusual cutting noise.

– DO NOT stop the machine and check the bit and the router settings

– DO NOT cut too deep in a single pass. Sometimes it can be more efficient to use a higher feed rate and two or more passes, rather than a single cut at a low feed rate.