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Why didn't you just use pegboard instead of the custom jig? That is, get a piece of pegboard a little bigger than you plywood, clamp them together, and start drilling? Seems like that would be faster and cheaper. Just curious.
A tip I learned from my Dad and seldom see written up: To avoid chip out and burs when drilling holes, drill fromboth sides. First drill a small pilot hole all the way through, then drill afull size hole part way. Finish by drilling full size hole from the other side.Works particularly well with hole saws in plywood. In metal, the problem isburs, but the same trick works. Takes more time drilling but saves lots of timesanding/deburring. Also, small pilot holes are easier to drill accurately.
I know of two advantages to the wedge method. First, as UnCivilEngineer notes, you can tighten the joint when it loosens, which it can do from the stress of use and material shrinkage. Second, it allows you to replace the handle when it breaks. More of an issue with (historically expensive) metal heads & (historically cheap) wooden handles.I suspect tradition also plays a role. Modern glues are much better than tool makers had when hammers were first designed.