In control systems which process a single block of CNC code i.e. one coordinate at a time, it is not possible to finish the movement after a given vector at a speed different than zero. This is because the driver does not analyse the data on the consecutive vectors following the ones being currently executed. Not knowing what the next move will be the driver has to stop to start the next move after downloading the next block. This leads to a situation in which the movement along the tool path is interrupted, despite the fact that consecutive vectors are, e.g., tangential to each other. In the case where the tool path is dominated by long vectors it is of little significance, because during the movement along such a vector the machine has a sufficiently long way to achieve the preset speed of the movement. The time after which the machine reaches this speed, and whether this speed is to be achieved at all at a given length of the vector is dependent on its value and the specified speed.
The problem starts to appear in the performance of vectors which are so short that it is impossible to reach the preset speed on them. In this case, the average feed rate is much lower than the preset speed. This results in a significant reduction of processing efficiency and moreover, due to frequent stopping, the accelerated wear caused by frequent changes of cutting parameters.
This problem is particularly visible at HSM (High Speed Machining) operation which consists in working with significantly increased cutting speeds.
In this technology, the feed rate is greater than the speed of propagation of the temperature in the workpiece, which means that almost all the energy accumulated during tearing the chip off is discarded together with it. Consequently, the the tool and the material heats up less during cutting than in conventional treatment.
To precede the propagation of the temperature in the material and to keep the thickness of the chip at a safe level at the same time, the rotational speed of the spindle must be higher respectively. It must be so high that it would lead to overheating and damage to the tool in hard materials at low feed rates (below HSM).
The use of HSM technology on machines with such a control system is not possible because frequent stops of the tool in the material result in its frequent overheating, which lead to very rapid wear.