Built a template to guide plunge routing of mortises, a first for me.
I had a set of Bosch guide bushings one of which had an OD of 5/16 and
an ID of 17/64. As I was planning 1/4 inch wide mortises and had a
5/16 straight router bit, that's the bushing I built the template
around.

Lots of fussing, measuring, calculating and squaring ensued, but I got
it done. Clamped a board into the jig, inserted 1/4 inch bit in my
Bosch 1615 router in the plunge base, rigged up the DC and off we go.

Jig base was 1/8 hardboard to minimize loss of plunge depth - bad
idea, too flexible, allowed router to wobble a bit and felt like the
guide was riding on the work a bit 'cause it extended maybe a scosch
more than 1/8 below the base. Ran first cut anyway (1 1/2 inch long,
about 1/8 deep), brought the router back, plunged another 1/4, started
the cut, and the bit contacted the inside of the bushing! In a flash
the neat little peened over end of the guide bushing was gone and the
guide was loose in the bushing plate. OOPS! I could't have drilled
that guide out as cleanly or nearly as fast!

Question is, how much clearance does one allow between the inside of
the bushing and the bit? Apparently 1/128 isn't enough, especially if
things are going to wobble. I did not expect a 1/4 bit to deflect
anything like 1/128, especially as I was just playing in Poplar end
grain, not Bubinga or something tough.

And yes, the bit was properly tightened down and the guide bushing was
locked in with that fancy Bosch spring loaded lock. If it matters, the
bit was a relative no-name cheapy, 1/4 X 1 on a 1/4 inch shaft, but
what it cut, it cut cleanly.

Any thoughts appreciated, I'm trying to learn.

Merry Christmas to all and all the best for the New Year.

Tom