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My router supports 6mm shank bits. However the bits that I have are 6.35mm shank diameter.

Can I safely and reliably change the collet of the router to accept a 6.35mm tool without compromising safety?

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6.35mm is exactly 1/4".

While the two are very close in size (I mean 0.35mm - how bad could it be?) when you have a chunk of metal spinning at ~20000RPM, it can get very bad very quickly. Your question about safety is very valid.

Can you change to a 1/4" collet?

  • Yes, if your router supports changing to a 1/4" collet.
  • Otherwise, NO.

If your router does not support installation of a 1/4" collet, then do not attempt to do so. It will be far cheaper to sell/give away/trash the unusable 1/4" bits than it would be to repair the damage to flesh caused by the bit coming loose and careening around the room.

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  • Don't make me say the thing I always say about power tools!
    – jdv
    Aug 10 at 23:28
  • What, @jdv? That their sole purpose in life is to separate meat from bone in their operators?
    – FreeMan
    Aug 11 at 11:37
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    Oh, they have many purposes. But when they sit in your shop at night, unplugged and in the dark, they dream only of one thing: meat of unwary operators. Think about it: they are essentially dead until you reanimate them with electricity. Power tools are basically zombies.
    – jdv
    Aug 11 at 13:06
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Can I safely and reliably change the collet of the router to accept a 6.35mm tool without compromising safety?

Probably. It would help to know what router you're working with, but we can at least say that routers from all major brands use removable collets so that you swap the collet to use different bit sizes. That may not be true for very small routers, like "palm" routers and laminate trimmers, but if your router has a 1 hp motor or larger, it almost certainly has a removable collet. Check with your router's manufacturer to see if they offer a 6 mm collet for your router.

If the manufacturer doesn't make a 6 mm collet for your router, there are several 3rd parties that manufacture collets in a variety of sizes for major-brand routers. One of those is Elaire Corporation. I've never used their products, so can't vouch for them, but they have many sizes available. There are also collet reducers available to let you adapt, say, a 1/2" collet to a 6 mm bit.

The reason that you want a collet that fits your router bit exactly has to do with the way collets work. Take yours out and look at it: it's basically a tapered cylinder with a hole bored through the center and several slits cut vertically. When the collet nut is tightened, it pushes the collet into a taper, squeezing the sides of the collet around the bit's shaft. If the radius of the shaft matches the radius of the hole in the collet, the inside surface will match the curve of the shaft, and there will be lots of friction between those surfaces to hold the bit. If the radii of the hole and shaft are different, the collet will touch the shaft only in a few places and there will be much less friction holding the bit in place.

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