[Henry Anthony]

Reply to Bob,

"so far I have not discovered a way in Inventor to constrain the length of the arc"

I believe that's true but you could work backward from pi * d = c

"But working with it bit, it seems that the arcs may need to be sprial arcs not circular arcs"

That's a brain twister for those smarter than I about these matters such as a die designer. On a practical note, the amount of error will probably not matter that much. However, for the sake of discussion and doing thing right, this is a good question. Brings to mind a spring. Does it change diameter as it compresses and lengthens? I suppose yes, if you deform the length enough.

Am I making any sense at all?