How to calculate CNC milling machine speeds?

¿Cómo calcular las velocidades de la fresadora CNC?

To calculate the speeds of the CNC milling machine, a set of parameters must be followed, generally established by the machine tool manufacturers; taking into account the feed speed, cutting speed, number of revolutions per minute, the characteristics of the part to be machined, etc.

Machining parameters of machine tools

Before calculating the speeds of the milling machine for machining parts made of metal, wood and other materials, it is important to know the different parameters to consider in order to make the calculations appropriately. The appropriate router speed depends on the parameters described below.

Feed speed

The feed rate (VF) is a term used by machine tools that refers to the relative instantaneous speed at which the cutting tool (in this case the milling cutter) approaches the surface of the material to remove it; This is the speed of the cutting movement (not to be confused with cutting speed). This parameter is calculated from the path that the cutting tool follows in the feeding direction in one minute.

The forward speed is expressed in meters per minute.

cutting speed

Cutting speed (VC) is the speed at which the cutter edge rotates on the workpiece to be machined. This parameter, which is expressed in meters per second, determines the path of the tooth point of the cutting tool. Cutting speed is usually provided by the tool manufacturer; This will be the one who determines the recommended speed and the limit of the machine that should not be exceeded.

This speed varies depending on the material of the piece, as well as the machining that will be carried out. To calculate the cutting speed, it is taken into account that the larger the diameter of the cutter, the higher the cutting speed. The same goes for revolutions per minute; If these are higher, the cutting speed will also be higher.

Revolutions per minute

Value used to express the speed of the head, that is, how fast the head rotates and how many revolutions or complete turns it makes in one minute. The CNC milling machine usually has between 6,000 and 24,000 revolutions per minute (RPM).

chip load

Also called feed per tooth, it is the thickness of the material that each tooth of the cutting tool removes from the surface of the piece in one complete turn. The adequate chip load allows the temperature of the cutter to be regulated and prevents it from overheating. The feed per tooth depends on the rotation speed and the feed rate.

Axial depth

The axial or pass depth has an influence on the speeds of the milling machine. At a lower depth, milling will be safer, but it requires multiple passes, increasing machining time. When you want to increase the depth, which increases the size of the chips, the machine's feed speed must be reduced.

Radial depth

The radial depth or width of cut varies depending on the robustness of the CNC machine tool. For proper side milling it is important to reduce the feed rate by up to 25%.

Importance of milling machine speeds

The speeds of the milling machine are essential for good machining results. If there is an imbalance between the feed speed and the revolutions per minute of the spindle, poor finishes are obtained and, sometimes, the machine tool can become destabilized and cause the cutter to break. That is why it is important that all the cutting parameters described above are adjusted according to the rigidity of the milling machine, but also depending on its power and characteristics.

It should also be taken into account that the cutters used in CNC numerical control machines can be made of HSS high-speed steel, tungsten carbide, PCD diamond, etc. This implies that, the harder the cutting tool, the cutting speed also increases; but, consequently, the forward speed may be reduced.

The speeds of the milling machine will not only depend on the machine and the different parameters, but also the work carried out on the material and the characteristics of the piece (material, design, shape, etc.) influences it. Thus, the speeds of the milling machine vary if the machining is milling, grooving, etc. How do milling machine speeds affect jobs? For example, a higher feed speed, greater control of chip production, shorter cutting time and tool wear. However, this also increases the risk of the bur breaking and the chances of surface roughness increasing. On the other hand, if the feed speed is reduced, the chips are longer and the machining quality improves; The drawbacks are greater wear on the cutter, greater machining time and cost.