The is the rear end plate of reel S/N 52, screwed to a tool plate while I round off the corners of the pillar lugs.
This is a fussy operation, with fine positioning of the Y and rotary axes to make all the arcs meet on tangents. A step between arcs of .001 inch would be highly visible.
Prior to this operation, I cut the 4 large arcs between the pillars, plus the notch between the two lugs at the reel foot. The milling cutter diameter sets the radius of the concave arcs on either side of the lug. At this point I could, in theory, cut a single arc spanning about 180 degrees around each lug. But to do this, I have to remove the end plate from its centered positon on the tool plate and reattach it so that one lug is at the center of the tool plate. This amount to “re-chucking” the part, and always involves a positioning error. So a single convex arc around the top of the lug does not match up with both concave arcs.
My solution is this: I cut a small flat across the top of the lug, then two corner rounding cuts. Each of these corner rounding cuts has to match up with only one concave arc, a much easier positioning problem. The corner rounding arc and the flat across the top of the lug do not meet on a tangent, but rather form a “sharp” corner that is still acceptable in finished appearance. Here is the end result:
It is problems like this that make me think about acquiring a CNC mill. The G code that runs such machines has convenient commands for making arcs around specified centers, without having to re-chuck the part.