snip
Gear is threaded on the shaft so it can easily be removed. Normally,
one would just face off the slotted portion to flat then center bore
using center drill, drill then ream. In this case I want the slot (or
what would be left of it) to remain. Shaft is about 1/2" and bore will
be 1/4" so 1/8" of slot will be left on either side.
What would be the best starting procedure in this case seeing that the
tip of the center drill won't be reaching the flat? I'll be pressing a
1/4" shaft in the bored hole that will be coupled to a motor so the
bore has to be fairly accurate.
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This is one of the very few cases where you can successfully use
an end mill in a jacobs style chuck. I suggest you get a 4 flute
[two flute end mill or drill will tend to grab in the slot] 1/4
inch end mill, and slowly and carefully feed the endmill in the
required depth.

GmcD