This was the set up for boring the sprocket. The alloy block was first drilled and tapped M6 in two locations then faced on the back and mounted in the chuck, then the center was bored out to give me somewhere to hold in a later step. The recess was turned to a snug fit on the sprocket.
The sprocket was then mounted in the fixture and the centre section faced to suit the boss and rough bored. I the reversed the sprocket in the fixture and finish bored to 21mm.
I set up the rotary table on the mill, clocked it true and then offset 15mm (half the PCD of the vernier holes). I then removed the assembly from the lathe and mounted it on the chuck of the rotary table, gripping on the bore of the fixture.
The 16 3mm holes were first centre drilled, indexing 22.5 degrees between each hole, then the process repeated to the final size
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