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.