OK, With the billet annealed, I need to locate and drill two starter holes for the wire EDM. I use an edge finder to locate the center of my wrist pin hole, which is X0, Y0 (remember basic geometry from school)for all operations from this point on. This is just a starter hole, and will be opened up with the EDM. Drill the wrist pin hole, then move to the tang cavity starter hole and drill it as well.

The billet is then loaded in the wire EDM, and useing the same method as the edge finder on the mill, I find X0, Y0...the center of the wrist pin.

Once I have that location, I thread the wire through the cavity starter hole and make the cut. Here is a picture of the program path on the EDm screen.

I would shoot a picture in the EDM work area, but you would only see water spray everywhere. EDM stands for electrical discharge machining. It works like and arc welder in reverse. The machine uses a .010" brass wire, with fresh wire being constantly feed through the work, with an electric charge, which causes an arc between the wire and the work piece, to erode the metal in front of the wire. The wire never touches the work, but maintains about .001" gap. It is VERY accurate, but VERY slow.

Once I cut the tang cavity, I clean up the wrist pin hole in a spiral path. Produces a better hole than a drill and reamer, and since the rest of the profile of the action will be cut next, the hole will be perfectly parallel with the action face.