Abrading apparatus having zero reference sensing means

An improved apparatus for forming electrical discharge machining electrodes said apparatus being of the type including a first platen, a second platen mounted in superimposed relation and adapted to be driven vertically with respect to said first platen, a die member having an abrasive surface mounted on one of said platens, a workpiece member mounted on the platen opposite the platen on which said die member is mounted in working alignment with said die member, means for applying horizontal oscillatory rotary motion to said first platen, vertical drive means for infeeding said workpiece member and said die member into contact with each other while said oscillatory rotary motion is being applied to thereby abrasively shape said workpiece member to the form of said die member, and wherein said improvement comprises

• an electrically conductive coating disposed over at least a lower portion of said abrading die;
• a source of electric potential coupled to one of said electrode workpiece and said electrically conductive coating;
• and
• switching means coupled to the other of said electrode workpiece and said electrically conductive coating,
• whereby when said coated abrading die is first driven into contact with said electrode workpiece said electric potential is coupled to and operates said switching means to thereby indicate said contact.