Quote:
Originally Posted by craigboez
I considered this, but didn't like the idea of having a separate hub and fins for a few reasons. Strength and additional fastener weight were two, but primarily it was because the "holes" in the hub would actually be slots because of the endmill geometry, and I didn't want to put a rectangular fin into a rounded-end slot.
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If you want to get fancy, you could always round off the edges of the fins. (There's a router tool whose name escapes me that does this.) You'd probably need to use a corner-rounding end mill, and pass each edge through individually—so it's not particularly efficient. This would actually go a lot quicker if you used one of those tabletop chamfering machines, and loaded it with that quarter-round end mill. Then it would be relatively straightforward to make multiple passes quickly. (Either way, you'd probably want to machine the edges of the raw stock, then cut it into pieces.)
Another option might be to cut crosswise slots in the main slot, so that the fin can have a straight-side tab protruding from the bottom, and engaging the sides of the (slightly-deeper) crosswise slot. You don't have much material to work with on such a small hub, but it might work.
Finally, you might want to prototype a version that relies on two or three high-strength bolts to bear the sideways load, and hence resist sliding in the slots (alloy steel socket head cap screws should be perfect).
Quote:
Originally Posted by craigboez
I'm actually considering machining one and using it as a master to create a mold from. Depending on how strong that urethane is, especially if fiber reinforced, it might just be strong enough.
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If you mean casting the hub with spokes from urethane, you'll additionally have to worry about stiffness, and then about friction (against the rollers).
Also, such a complicated mold shape will require some creativity to get the part to release. Much more so if you want to avoid secondary machining to get rid of draft (i.e. angled surfaces which make it easier to remove the part).