When we did the 3 way 1020 bench comparo, I made a fixed radius plate out of high density particle board with the radius cut in with a router. The funny thing was that the fixed radius plate flowed worse than any of the clay adapters.
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I have witnessed that many times. In fact I machine a plastic radius plate when it is a serious effort. Everytime it flows less with the plate as opposed to a hand formed clay radius. J.Rob