There is a nice bit I would love to get/use but it uses a 6mm shank. My router uses 1/4 inch shank. It is a 0.014 inch difference...is it a bad idea to get this? Is there an adapter or something if not?
From the comments on an answer:
My router is a Performax trim router, there is 1/4 collect and 3/8 collect. Not sure of the model.
In short: No, no, no.
You can technically do this, unluckily, but it is highly inadvisable. It is a pity that it just looks like one could do it (1/2'' is not far off compared to 12mm, and 1/4'' is even closer to 6mm) and that with some luck, it indeed seems to "work just fine".
That is true for the other way around, too. You can hardly tell a difference with the naked eye, and the bits will fit in "just fine" either way.
I've seen people do it, too, and they so far got away without accident or even physical injury. (Which makes the whole matter worse because that "proves" that it works.)
However, using an imperial bit in the corresponding (well, almost corresponding) metric collet is dangerous both during the operation and when using the collet with your 6mm bits as intended afterwards.
What happens is that you irreversibly widen your collet by a tiny amount (too small to see, but certainly enough to be on the dangerous side). If you are unlucky, the whole thing blows to smithereens at 25k RPM with shrapnel flying all over the place and the bit come off flying in a random direction.
If you are lucky it "works fine", but now the collet no longer grips the original bits firmly and safely, so you have the risk of a bit flying away in a random direction every time you use the tool "properly" with the correct bits. Plus, you never know if the collet isn't going to break a month or two later, anyway.
The same is true for the opposite case, using a metric bit in an imperial collet. Only in this case, you force the clamps together much more than originally intended. There is a bit of tolerance, so you can certainly fasten the screw harshly enough to do those extra 0.3 millimeters. But it's not what the tool manufacturer intended you to do, or planned in.
The bit isn't going to have a sufficiently firm, safe grip, and you cannot be sure whether you have structurally damaged the collet by bending the little clamps too harshly.
You NEED a different collet.
Your router could be spinning the bit at 20,000 rpm. Ignoring the possibility of damaging your work piece, the personal risk is extremely high.
edit: On @AstPace's good suggestion, I've added a couple of relevant points from the comments trail.
As this router also has a 3/8" collet, you could use an adapter to get down to a 6mm shank. Now, adapters aren't great, as they're one more thing that can go wrong, but if you were desperate, you can find a 3/8" > 6mm adapter here.
(It's much easier to find adapters for 1/2" collets, but I digress.)
The most obvious thing would be to find the cutter you want in a shank size you can easily use.
It doesn't look like there is a commercial adapter available for this. The smallest I could find are 1/4" to 4 mm. This makes sense due to the wall thickness of a 1/4" to 6 mm adapter would be just 0.175 mm (about 11/64") thick.
The best option would be to find a 6 mm collet for your router (if available).
My Makita 3709 has only one collet and the instructions booklet say that it's valid both for 6mm AND 1/4" bits. Clearly, if the collet fits 1/4" it can also fit 6mm, probably by just a tiny extra tightening.
I'm going to go against the grain here and say it's probably within spec for an ER collet.
But one should always refer to the manufacturer for the last word. The ER collets have a "clampling range" which the manufacturer usually specifies. For example, see Techniks specifications. I've noticed that those ranges appear in other websites, but I'm not sure if they come from Techniks' catalog or from another source.
To give an example, if a 1/4" shank measures something between 0.236" and 0.276", then it should still be within the manufacturers recommendation for a 6.0-7.0mm collet (or within 0.216"-0.255" for a 5.5-6.5mm collet). And vice-versa, if a 6mm shank measures anything between 0.211"-0.25", then it is within Technik's 1/4" collet recommendation.
The OP does not have a "trim" router but a "compact" router by Performax which does provide 1/4" and 3/8" collet capacity. The trim router has ONLY the 1/4" size. Now, the answer to his question IMHO is this: If you are going to be doing this regularly, then it is not advisable. If you just want to do this job and this job ONLY, it should be fine. Just take some precautions, and try it momentarily with a slower speed first. If you hear or feel anything out of the ordinary then it's the shank vibrating. If that's the case then stop immediately and get the proper size or adapter.
My router has a slotted collet with threads on the outside...wherein a reciprocal threaded nut can be screwed down to tighten the collet grip on a shank that is inserted. This allows for a shank of close..but not exact same size to be used. Each manufacturer of tools use different tooling dies. This is why you can buy say a 1/2 inch socket from two different companies...and one fits perfectly on a 1/2 inch nut...and the other may be a little loose...or not go on at all...the size of the nut can even vary from one manufacturer to the next. If a shank fits into a collet easily...and the tightening nut easily locks that shank tightly into the collet...you should be fine. Simply put router collets are designed on the same principle as a drill chuck that accepts different size bits...although not as wide ranging.
