Q: While routing half-blind dovetails, the bit slid down onto the jig after the third drawer. I cleaned the collet and started again. After about two more drawers, the bit had slipped again. I had to reset the bit after about every other set of drawers. I needed eight. The router is an older one, but it has never given me a problem like this. Any help on what the problem might be?
A: The likely cause of your problem is a worn collet. But start with the bit. Measure its shank with a machinist’s caliper; its diameter should be spot on, but two or three thousandths undersize is acceptable. That is, a 1/4″ shank should be 0.250″ in diameter; if less than 0.247″, replace it and see if the problem is solved. If the bit shank is accurately sized, the culprit is the collet, and you need to replace it instead.
An obvious follow-up question is: “If the problem is the collet, why doesn’t it occur with all my bits?”
The dovetail is a harbinger. As a collet wears, it doesn’t grip a bit tight enough to ensure it spins at the same speed as the router motor. Under load, the bit slows down, but we don’t notice that happening. Scoring or galling on the shank are signs of this, but again, we don’t always notice. Eventually, the grip deteriorates to the point that bits begin to work out of the collet. Ah, then we notice!
Because a dovetail bit is designed to pull itself into a cut, trying to burrow deeper, and because the dovetail shape traps the bit in the cut, it’s more prone to be pulled out of the collet by the resulting tension stress.