Apparatus and method for forming a variable diameter hole in a conductive workpiece

A method for forming a cooling air metering hole in a conductive gas turbine engine component, comprising the steps of:

• positioning an externally insulated hollow electrode having a predetermined inner and outer diameter proximate to a surface of the component into which the metering hole is to be formed;
• pumping an electrolyte through the electrode at a chosen pressure and onto the component surface;
• connecting a first selected DC voltage between the electrode and the component, the first voltage being connected with a polarity to cause the electrode to act as a cathode and the component to act as an anode;
• moving the electrode toward the component at a first selected feed rate to cause a first portion of a radial hole to be formed at a first predetermined diameter;
• sensing a depth of the electrode as the electrode moves through the component to form the radial hole;
• and
• switching at least one of the first selected DC voltage to a higher selected voltage with the same polarity and the first selected feed rate to a lower selected feed rate when a predetermined depth is sensed to cause at least a second portion of the radial hole to be formed with a second predetermined diameter larger than the first diameter and to form the metering hole between the first and second portions.

An apparatus for forming a passage with a variable diameter along its depth in a conductive workpiece, comprising:

• an externally insulated hollow electrode having a predetermined inner and outer diameter;
• a variable DC voltage source connectable between said electrode and the workpiece for supplying a selected voltage, and said voltage source being connectable to cause said electrode to act as a cathode and the workpiece to act as an anode;
• means for pumping an acidic electrolyte through the electrode at a chosen pressure and onto a surface of the workpiece through which the passage is to be formed;
• means for moving said electrode toward the workpiece at a selected feed rate to cause a portion of the passage to be drilled at a first predetermined diameter;
• means for sensing a depth of said electrode as said electrode moves through the workpiece to form the passage;
• and
• means for switching at least one of said variable DC voltage source from the selected DC voltage to a different selected DC voltage with the same polarity and said moving means from said selected feed rate to a different selected feed rate when a predetermined depth is sensed to cause another portion of the passage to be formed at a second predetermined diameter.