In control systems that process one block of CNC code at a time, i.e. one coordinate at a time, it is not possible to end the movement on a given vector at a speed other than zero. This is because the driver does not analyze data on the successive vectors after the current execution. Not knowing what move will be next, he must stop in order to start the next move after downloading the next block.
This results in a situation in which the movement along the tool path is interrupted even though the successive vectors are, for example, tangent to each other. In the case of a tool path in which long vectors dominate, it does not matter much, because when moving along such a vector, the machine has a long enough path to reach the set speed.
The time after which the machine reaches this speed and whether this speed can be reached at all on a vector of a given length, depends on its value and the set acceleration.
The problem appears when the vectors are short enough that the set speed cannot be reached on them. In this case, the average feedrate is much lower than the commanded speed. This results in a significant reduction in machining efficiency and, moreover, due to frequent stops with accelerated tool wear caused by frequent changes in cutting data.
This problem is especially visible when working in the HSM (High Speed Machining) mode, which consists in working with significantly increased cutting speeds.
In this technology, the feed rate is greater than the speed of temperature propagation in the workpiece, which results in almost all of the energy accumulated in the removal of the chip is ejected with it. As a result, the tool and material heat up less during cutting than with conventional cutting.
In order to stay ahead of temperature propagation in the material and at the same time keep the chip thickness at a safe level, the spindle speed must be increased accordingly. So large that in hard materials at low feed rates (below the HSM) it would overheat and damage the tool.
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 results in very fast wear.