On Mar 18, 12:39 pm, "David" cosmosatnointerbaunspamdotcom wrote:
Thanks for those suggestions, but as I previously posted
a flat bottom on the mortise isn't my concern.

I'm at a loss as to what you're really trying to accomplish. You're
working on a stair railing. Stairs are inclined, the railing is
inclined, the bottom of the hole will be inclined. If you use a
mortising machine the uphill end of the mortise will be where the
interference occurs. The spindles are metal. Cut the spindles on an
angle. You don't have to worry about having insufficient tenon
length compromising the strength of the railing. The mortise will
hide the cut edges. This will be far faster than trying to work
inside a mortise.

Unless there is some other consideration you haven't mentioned, you
seem to be making this needlessly complex.

R