Router bit won’t fit after one use.

Just a reminder as posted on this forum many times... Do not tighten a collet without a bit in it. Doing that could deform the inner part of some collets. If this has happened, you have to be very careful when trying to pry it open. Over doing it could break or weaken the part. It would be safer to replace the collet. I wouldn't run a router without a bit because the collet would be loose, right?.

There's another common problem and that is bits that aren't the correct shaft size. Be aware that there are metric, imperial and poorly manufactured ones available. If you tighten a collet on one that's too small, it's similar to tightening a empty collet. Or if the bit is oversize it's obviously not going to go in. As @mgmine mentioned: Try different bits and see if they all have the problem. If you have a calliper, try measuring the shafts.

A different issue is people bottoming out the shaft of the bit. This can cause a collet to not tighten properly. I pull the bit out about 1/8" and then tighten it. Some people use O-rings either on the shaft or in the collet to prevent this (member Harry made bit storage with holes the right depth to set the position of the O-ring). Many have had bits wander in depth or come flying out. At around 20000 RPM this can be quite dangerous.

In your case I suspect either:

• The collet has been over compressed
• The bit isn't the correct size

Just for clarity's sake, here is a diagram of a collet, nut and spincle. The collet is a very precise thing. The difference between tight and loose is a matter of a few thousandths of an inch. The nut pushes the collet down into a tapered spindle so deforming it is pretty easy to do, and I suspect a quarter inch collet is even more easily damaged. Once damaged, a collet can't be trusted for safety reasons Paul explained.

One other issue could be that you are using an Imperial bit in a metric collet, or vice versa. The difference is tiny, but then so are the tolerances.

I don't ever use a collet that is giving me any signs of problems. New collets for good brands are cheap compared to a hospital bill. For a few off brands that are more than a few years old, even finding a collet that fits is difficult to impossible. Another reason to buy a spare, several if it's a cheapo router or Crapsman.