another then as is shown in fig4 in a next step 312 the maximum admissible diameter d max of the tool from the restraint capacity or restraint energy e r u0169ck of the protection booth is calculated as a function of the rate of rotation it being assumed that the protection booth can absorb a certain amount of energy without fracturing this energy can be determined by means of shot trials using a predetermined projectile thus for example inspection windows of 12 mm thick polycarbonate sheet can absorb a dose of energy of some 1100 nm if a loss of strength resulting from the aging of the material is taken into account a maximum restraint energy of 500 nm can be assumed for polycarbonate sheet this maximum restraint energy yields the following formula for the calculation of the relationship between the tool diameter and the revolution rate n of the work spindle d lt 60 u00b7 2 e r u0169ck 0 5 u00b7 1 u03c0 m n eq 3 assuming that the mass m of a flying part is 0 1 kg the maximum admissible tool diameter d max for the programmed desired speed of rotation n soll can be calculated fig6 shows the relationship between the maximum admissible tool diameter and the maximum speed of rotation of the spindle for different restraint energies under the action of a standard shot of 0 1 kg the maximum diameter d max calculated in step 312 is compared in the next step 313 with the measured or stored tool diameter or both when the measured or stored tool diameter exceeds the admissible maximum the work spindle is stopped in.