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.

7 Answers 7


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.

  • I wanted to comment on the accepted answer because this doesn't seem to be always the case. Manufactures specify the "clamping range" for the collets, and many times imperial and metric do overlap. I've given an example and references in my answer.
    – Esteban
    Nov 5, 2021 at 1:36
  • I can assure you that you will not "widen your collet" in any irreversible way. Collets by design widen and contract in order to allow tool changes, and you wont cause any additional wear. 1/4" collets have a tolerance range of 0.236—0.276" and 6mm is at the bottom end of that tolerance. Furthermore I invite you to throw some calipers on a range of 1/4" shanks to see how sloppy the tolerances on them are and most likely you have already put a "6mm" shank in your collet.
    – jesse_b
    Feb 24, 2023 at 13:11

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.

  • Is there a adapter I could use?
    – Ljk2000
    Apr 24, 2016 at 17:01
  • Adapter -- not unless you also have a 1/2" collet. Different collet? Maybe. Tell us what make/model your tool is. Apr 24, 2016 at 21:33
  • 1
    Never never ever mismatch bits and collets or metric/ imperial.... very bad!
    – Matt
    Apr 24, 2016 at 21:43
  • My router is a performax trim router, there is 1/4 collect and 3/8 collect. Forgot to mansion that. Not sure of the model.
    – Ljk2000
    Apr 24, 2016 at 21:53
  • 1
    Well, adapters aren't great, (extra things to go wrong at the aforementioned 20,000 rpm) but there's one 3/8" > 6mm here: elairecorp.com/colletadapter.html Apr 24, 2016 at 22:19

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.

  • 3
    If your instruction manual claims that it supports both 1/4" & 6mm, then it's on the manufacturer if something goes wrong. Unfortunately, you're still the one that may be hurt, but they would be held liable. If your instruction manual doesn't claim that it supports both sizes, then the conventional wisdom says DON'T DO IT.
    – FreeMan
    Nov 19, 2018 at 20:38

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.

  • Even after looking at the link, I'm not sure what an "ER collet" is. Is that a brand, a style, a marketing term? Could you expand on that?
    – FreeMan
    Nov 4, 2021 at 18:52
  • 2
    It is a style of collet. I'm not an expert on the different tool holding mechanisms (my equipment either has a chuck or an ER-32 collet), but the ER type collet is the type of geometry, with the number following it is the overall size. Some history on the wiki page: en.wikipedia.org/wiki/Collet#ER_collets And a blog post that goes more in-depth: blog.tormach.com/…
    – Esteban
    Nov 4, 2021 at 21:15
  • 1
    This is very interesting, thanks for the addition.
    – Graphus
    Nov 5, 2021 at 9:59

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.

  • 2
    Note that even if you can't hear ,or see it, any wobble does cause wear on both collet and bit shaft and tends to become worse and start affecting the "right" bits too. As a one-time kludge, ok, maybe, but even then you might want to get another collet and keep track of which one you are abusing.
    – keshlam
    Dec 23, 2016 at 16:07
  • I wonder what the "precautions" are for a chunk of sharp tool steel suddenly converting rotational energy into radial energy.
    – user5572
    Nov 4, 2021 at 23:22

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.

  • 2
    Router collets are not designed on the same principle as a drill chuck. A drill chuck has movable jaws to engage bits of various sizes. A router collet has a fixed size that tightens to clamp onto shanks of a given diameter. Damon's answer from 2016 correctly points out the easily overlooked dangers of using a router in this way. May 7, 2019 at 18:10

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