How to calculate CNC milling machine speeds?
To calculate CNC milling machine speeds , you need to follow a set of parameters, generally established by the machine tool manufacturers, taking into account the feed rate, cutting speed, revolutions per minute, the characteristics of the part to be machined, etc.
Machine tool machining parameters
Before calculating milling machine speeds 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 correctly. The right milling machine speed depends on the parameters described below.
Feed rate
The feed rate (VF), is a machine tool term 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 cutting speed). This parameter is calculated from the path followed by the cutting tool in the feed direction in one minute.
Feed rate is expressed in meters per minute.
Cutting speed
The cutting speed (VC) is the speed at which the cutting edge of the milling cutter turns over the workpiece. This parameter, expressed in meters per second, determines the path traveled by the tip of the cutting tool's tooth. Cutting speed is usually provided by the tool manufacturer, who determines the recommended speed and the machine limit that must not be exceeded.
This speed varies according to the material of the part and the machining to be performed. When calculating cutting speed, bear in mind that the larger the diameter of the milling cutter, the higher the cutting speed. The same applies to revolutions per minute: the higher they are, the higher the cutting speed.
Revolutions per minute
A value used to express the spindle speed, that is, how fast the spindle turns and how many revolutions or complete turns it makes in one minute. The CNC milling machine usually runs at between 6,000 and 24,000 revolutions per minute (RPM).
Chip load
Also called feed per tooth, this is the thickness of material that each tooth of the cutting tool removes from the surface of the workpiece in one complete revolution. The right chip load helps regulate the temperature of the milling cutter and prevents it from overheating. Feed per tooth depends on the rotational speed and the feed rate.
Axial depth
The axial depth, or depth of cut, has an influence on milling machine speeds. The shallower the depth, the safer the milling, but it requires multiple passes, increasing machining time. When you want to increase the depth, which increases chip size, the machine's feed rate must be reduced.
Radial depth
The radial depth, or width of cut, varies according to the rigidity of the CNC machine tool. For proper side milling, it is important to reduce the feed rate by up to 25 %.
Why milling machine speeds matter
The milling machine speeds are essential for good machining results. If there is an imbalance between the feed rate and the spindle speed in revolutions per minute, the result is poor finishes and, sometimes, the machine tool can become unstable and cause the milling cutter to break. That is why it is important for all the cutting parameters described above to be adjusted to the rigidity of the milling machine, as well as to its power and characteristics.
You should also bear in mind that the milling cutters used on CNC machines can be made of HSS high-speed steel, tungsten carbide, PCD diamond, etc. This means that the harder the cutting tool, the higher the cutting speed; but, as a result, the feed rate may be reduced.
The milling machine speeds will depend not only on the machine and the various parameters, but also on the work carried out on the material and the characteristics of the part (material, design, shape, etc.). Thus, milling machine speeds vary depending on whether the operation is milling, grooving, etc. How do milling machine speeds affect the work? For example, the higher the feed rate, the greater the control over chip formation, the shorter the cutting time and tool wear. However, this also increases the risk of the milling cutter breaking and the likelihood of surface roughness. On the other hand, if the feed rate is reduced, chips are longer and machining quality improves; the drawbacks are greater cutter wear and longer machining time and higher cost.