Earlier today I watched a Mere Mortals clip that passed on another’s suggestion to use O-rings as router bit insert stop or spacer. The idea was that the fillet between a router bit shank and cutting head affects collet clamping, leading to walk-out or excessive vibrations, and that an O-ring prevents this from being an issue.
The reasoning behind this is right, but the O-ring trick is only effective when working with shorter router bits whose heads would otherwise connect with a collet upon full insertion. Relying on this tip by itself can lead to complacency and frustration. What happens if you have a long bit that bottoms-out and with its fillet well out of the collet, and you still encounter bit walking, vibration, or uneven cuts?
Inserting a Router Bit
- Ensure that there is no grime or debris on the router bit shank and check the collet as well. Make sure that the collet is open and the nut loosened.
- Insert the router bit all the way into the collet as far as it will go.
- Retract the router bit by 1/8″ or so (some manufacturers recommend 1/16″), and then hand-tighten the collet nut to hold the bit’s slightly elevated position.
- Fully tighten the collet nut with the appropriate wrench.
Note: for solid carbide router bits, step 2 is modified – try to minimize the length of the cutting edges that are covered by the collet. I try to insert such bits into the collet by at least 1″, or more than half-way if possible. Always double check with your router’s manual or the bit manufacturer’s guidelines.
If you were to stop at step 2 and then tighten the collet, there is the strong possibility that you will achieve less than optimal collet clamping. Bottoming out a router bit – or any collet-fixed bit, impedes the tightening mechanism of the collet. Then when you try to tighten the collet nut, you may get the false feeling that the nut and collet are fully tightened.
If you have a shorter router bit and insert it until the bottom of the cutting head is close to the collet, and leave it as-is, the fillet at the interface between the bit’s shank and cutting head may affect the grip of the collet. Even if there is no fillet, there will likely be a small segment that is coated. If the collet grips onto this coated area, its grip on the rest of the bare-metal shank may be looser and potentially less than ideal.
With a loose collet, even if it is almost imperceptibly loose, a router bit can “walk” and rise up out of the collet. At the very least, this can cause excessive vibration or inconsistent cutting performance. At worst, this can pose a safety hazard with the potential to cause grievous bodily harm.
This bit-insertion technique can also be applied to other tools that hold bits and burs with collets, such as rotary tools and mills.
Even with these tips in mind, it is always recommended to refer to the tool’s user manual before use and then again periodically as a refresher.
If any of this is unclear, please let me know and I’ll try my best to show this with close-up photographs.