In the case of collision between the grinding wheel and the workpiece or other machine components, the motor current and acoustic emission increase to levels that are usually more than three times the amplitude of rough grinding processes. What must be evaluated is the acoustic emission or the effective current of the grinding wheel drive. Ideally, collision monitoring should be constantly active as an elementary safety function. Yet this implies that both setup operation and fast traverse motions of the grinding spindle support must be monitored. Several ball screws in rapid traverse however create an acoustic disturbance signal which can amount to ten times the roughing signal. In such cases, collision recognition is not possible with acoustic emission. Here, we must fall back upon monitoring motor currents.
Potential collision should be checked with a static threshold, which is set at 2.5 to 3.5 times the roughing signal. In order to avoid false alarms, delay times of t = 5 to 50 ms should be included for this threshold.
The damage to the machine resulting from the collision is the smaller the faster the correspondingly moved axis is stopped. As previously used systems show, the sensor-bound reaction time of the monitoring system is negligible compared with the delay time caused by the PLC of CNC control. The latter only checks the pertinent alarm input in cyclical intervals. According to the range and performance of the PLC, the cyclical input check causes delays of t = 100 to 800 ms as opposed to an average reaction time of monitoring systems of t < 20 ms till the point the alarm output is set. It is thus advisable for a rapid reaction that the pertinent alarm output of the monitoring system is directly connected to the control release of the position control of the relevant axis. In this way, the CNC-axis can be stopped with practically no delay.
A rapid pullback of the axis should be avoided if possible, since situations are certainly imaginable (internal cylindrical grinding) in which the rapid pullback would lead to further damage.