Mounting a Router Bit

One question that comes up quite often is how to secure a router bit in the router.  After all, it is quite a large chunk of sharp material (HSS or carbide, or combination) to have spinning at speeds up to 20000RPM, and then having it engage with, and cut into a piece of wood.  Certainly for the inexperienced, installing a router bit (and then turning the router on), can be quite daunting.

It is a bit hard to describe just how tight to tighten up the collet – you want it to really grip the shaft of the bit, but not so hard that you can’t get it undone again!  The thread direction does mean that as the router is used, it will have a tendency to tighten further (not that that helps you if the bit is already wanting to slip!).

If you are using a reducer (such as a 1/4″ reducer so you can use 1/4″ bits in a 1/2″ collet) then you need to tighten the collet more than if you are using the correct combination of bit and collet.  This is because you have to squeeze the metal of the collet onto the reducer sufficiently that it squeezes tightly enough on the bit.  Where possible, it is much better to use the right sized bit for the collet you have.

Don’t play games with not setting the bit fully inside the collet – if the bit isn’t long enough for the job that you’d have to only insert it partially in the collet, buy a different bit.  It is way too dangerous not to have the bit properly held.  Alternately, get one of the commercial router bit extenders.

One thing that is worth mentioning is bottoming out of the router bit.  It is quite a common mistake to allow the router bit to drop to the bottom of the collet then tighten the collet up.  There are a number of different theories why this isn’t a good idea, but at least all theories agree on one thing – let the bit bottom out, then raise it up a small amount (1mm or so) then tighten the collet.

Some of the reasons I have heard in the past are: it allows heat to transfer from the router bit to the shaft of the router, it transfers vibration from the routing operation to the shaft of the router, it can cause the router bit to vibrate loose of the collet….and others.

Some, or all of these may be good reasons for not doing so, but I don’t accept them to be the major, or main reason.  Mine is this: as the collet tightens, (given it operates on a thread), it will carry the router bit in the direction that it is tightening, ie towards the bottom of the hole.  If it hits the bottom before it is fully tight (or starts in that location), then there is a possibility that it will feel that the router bit is held tightly because you feel the resistance through your spanner, but it isn’t actually that the collet is fully tight – it is the bit pressing into the base of the hole.  Then during operation, the bit can slip, or even start working its way out of the collet.

Instead, if there is a little bit of clearance between the router bit and the bottom of the hole, then the collet can grip fully around the shaft of the bit, holding it securely and as it is designed.

I have also heard some people drop a small o-ring into the bottom of the collet so the router bit starts off resting against it before the collet is tightened.  I don’t see any problem with this solution – the o-ring can easily compress as the collet is tightened, and if it helps you ensure that you consistently insert the router bit properly, then go for it.

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