Open-loop stepping motor control circuit for a machine tool

Claim 1

A control circuit for a machine tool of the type wherein a workpiece carrier is moved, relative to a tool, in a series of incremental steps, which comprises: 1. means for supplying an alternating potential from an external source; 2. an AC relay connected to said supplying means, said relay having first and second normally open contacts; 3. a starting switch, serially connected with the coil of said AC relay to complete a circuit thereto when closed, said first normally open contacts of said AC relay being wired in parallel with said switch to provide a lockup of said relay when said switch is opened; 4. a transformer connected to said supplying means and having a full-wave bridge rectifier connected to the secondary thereof to supply a source of DC potential; 5. a series circuit connected across said source of DC potential comprising a stepping motor operatively connected to said workpiece carrier, the second, normally open contacts of said AC relay, and the contacts of an electromechanical interruptor; 6. a driving motor connected to said source of DC potential and coupled to said electromechanical interrupter to periodically open and close the contacts thereof; 7. a rheostat, serially connected with said driving motor across said source of DC potential, to adjust the frequency at which said interrupter contacts open and close; and 8. a counting relay, connected in parallel with said stepping motor, to count the number of times said electromechanical interrupter has interrupted the energization of said stepping motor, said counting relay having a normally closed contact in series with said AC relay, whereby when said counting relay has counted some predetermined number of interruptions, said normally closed relay contacts open, thereby releasing said AC relay and thus opening, in turn, said normally opened second contacts in series with said stepping motor and thus preventing further energization thereof.