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Question/thoughts for you re slot roughing finish- when doing the cut, were your table gibs locked? On my old mill, if I don't lock the gib on the axis that doesn't need to move, I get a slot that looks like I cut it with a Cockshutt 3 bottom plow rather than a mill. Also, if the endmill is held in a collet I get much better results than if I do a light cut with it held in the drill chuck. (yes, sometimes I cheat and do a small slot cut with a endmill in the drill chuck - even though it is a no no and I should know better )Your final slot cut looks just fine anyway.
As for slotting, I use carbide endmills, and for small slots like these chain drilling doesn't make sense.
Using a drill chuck for plunging an endmill seems perfectly OK.
When I made the yokes I seem to remember milling out of one plate each.