Automatic tool changers for machine tools having spindle mechanisms which can be driven selectively in opposite directions along the spindle have previously relied on sensing switches to detect spindle mechanism position and to control a spindle mechanism drive. The use of sensing switches has caused problems in attaining smooth control and in reliability.
This invention uses a position signal generator (31) arranged to provide position signals whenever a rotary spindle mechanism drive (8, 9) rotates, and an arrangement for computing (fig. 8) the position of the spindle mechanism (7) on the basis of the position signals and for varying the speed of the spindle mechanism in dependence upon the computed position. No sensing switches need be used.
This invention can be used in an automatic tool changer for a vertical machining center.