To calculate the speeds of the CNC milling machine, a set of parameters must be followed, generally established by the manufacturers of the machine tool; taking into account the feed rate, cutting speed, number of revolutions per minute, the characteristics of the piece to be machined, etc.
Machining parameters of machine tools
Before calculating the speeds of the milling machine for machining metal, wood, and other materials, it is important to know the different parameters to consider to be able to make the calculations properly. The appropriate speed of the milling machine depends on the parameters described below.
Feed rate
The feed rate (VF), is a term used by machine tools that refers to the instantaneous relative speed at which the cutting tool (in this case the milling cutter) approaches the surface of the material to remove it; it is the speed of the cutting movement (not to be confused with the cutting speed). This parameter is calculated from the path followed by the cutting tool in the feed direction in one minute.
The feed rate is expressed in meters per minute.
Cutting speed
The cutting speed (VC) is the speed at which the edge of the milling cutter rotates over the piece to be machined. This parameter, which is expressed in meters per second, determines the path of the tooth point of the cutting tool. The cutting speed is usually provided by the tool manufacturer; they will determine what the recommended speed is and the machine's limit that should not be exceeded.
This speed varies according to the material of the piece, as well as the machining to be performed. To calculate the cutting speed, it is considered that, the larger the diameter of the milling cutter, the greater the cutting speed. The same applies to the 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 a minute. The CNC milling machine typically 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 workpiece in a complete rotation. The appropriate chip load helps regulate the temperature of the cutter 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 influences the speeds of the milling machine. With less depth, milling will be safer, but it requires multiple passes, increasing machining time. When you want to increase the depth, which enlarges the size of the chips, the feed rate of the machine 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 rate and the spindle revolutions per minute, poor finishes are obtained and, at times, the machine tool can become destabilized and cause the cutter to break. Therefore, it is important that all the cutting parameters described above are adjusted according to the rigidity of the milling machine, but also based on its power and characteristics.
It should also be noted that the cutters used in CNC numerical control machines can be made of high-speed steel HSS, tungsten carbide, PCD diamond, etc. This implies that the harder the cutting tool, the higher the cutting speed; however, consequently, the feed rate may decrease.
The speeds of the milling machine will not only depend on the machine and the different parameters, but also the work performed on the material and the characteristics of the piece (material, design, shape, etc.). Thus, the speeds of the milling machine vary if the machining is milling, grooving, etc. How do the speeds of the milling machine affect the work? For example, at a higher feed rate, there is greater control of chip production, less cutting time and tool wear. However, this also increases the risk of the cutter breaking and the chances of surface roughness increase. On the other hand, if the feed rate is reduced, the chips are longer and the quality of the machining improves; the drawbacks are greater cutter wear, longer machining time, and increased cost.
