Use rubber O-rings to help you position router bit shafts the right depth in the collet. Use 5/32-in. dia. O-rings for 1/4-in. shafts, and 3/8-in. dia. for 1/2-in. shafts. O-rings prevent two problems: 1. “Bottoming” the bit’s shaft in the collet where it can vibrate and loosen when the router is running. 2. Tightening the collet around the slightly tapered ﬂange where the shaft enters some bits, creating the possibility that the bit might loosen when the router’s turned on. Measure the depth of your router’s collet and position the O-ring on the shaft 1/8 in. less than this depth.