I have decided that to make a 13" hoop I need to make a bending form that is 11" across. My 10" form resulted in a hoop that was 12" across, so it looks like I need to subtract 2" from the final size when designing the form.
I won't bore you with the details this time. I did the same stuff as I did on the previous two forms with circles and screws and angle irons. Here is all the stuff.
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The Stuff
Drawing circles on plywood, etc.
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Drawing Circles Again
You know the rest. I ended up with a form that was 11" across.