Use rubber O-rings to set the correct depth for router bit shafts in the collet. Use 5/32-in. O-rings for 1/4-in. shafts and 3/8-in. for 1/2-in. shafts. They prevent bottoming out (which causes vibration) and avoid tightening on the tapered flange, which can loosen the bit. Measure the collet depth and place the O-ring 1/8 in. shorter than that.