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Old 04-18-2019, 11:30 AM   #18
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Join Date: Nov 2008
Posts: 1,078

The tool to make it is a quarter rounding bit, so I too just call it rounded, but I like the description "intrados", which is an architectural term applied to arches.

But I think architecturally the "extrados" refers to the top of the arch rather than the other side of the arch. In other words, when both sides of the center hole are rounded, they would both be "intrados" and the outside of edge of the hub would be "extrados".

I was mistaken in my belief that the purpose of the rounding was to make it easier to attach to the shaft of the hub. As PM has pointed out, it's actually to accommodate the metal fill where the center shaft is joined to the hub plate (welded, I presume).

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