Is a 6mm router bit shank the same size as 1/4"?

January 6, 2015 1:24 PM

I'm looking to buy an unusual router bit to put in a CNC router which takes 1/4" shank bits, but the bit I've found is listed as having a 6mm shank. Can I expect that the vendor (or manufacturer) is just pretending at being metric and ignoring the 0.35mm difference in their listing, or are there incompatible metric router bits out there in the world?
posted by kiwano to Technology (8 answers total)

Apparently this guy bought a router with a metric collet by accident. So yes, there are imperial and metric collet/shank sizes, and they are not compatible.
posted by aubilenon at 1:39 PM on January 6, 2015

I don't have specific experience with CNC routers, but it probably depends on how the bit is held. If it's the sort of 3-jaw clamp that's common on drills, it's almost certainly fine; if the bit is held in a collet or similar, 0.35 mm can be a pretty big difference. The shop manager who taught me to use a milling machine wanted people to check the shanks and come get him if they were more than 5 thou out of spec; 0.35 mm is nearly 14 thou, which seems like plenty to keep a collet from gripping properly.
posted by dorque at 1:39 PM on January 6, 2015

Can you get a replacement metric collet and nut for your router? I bought a half inch collet and nut for my Freud router that came with quarter inch collet and nut.
posted by X4ster at 1:48 PM on January 6, 2015

They are different sizes, and you'll need a metric collet to use that bit.
posted by mr vino at 2:08 PM on January 6, 2015

As other people have said it should be pretty easy to get a different collet for that router to allow metric bits and vice versa. But you do need the right collet - metric for metric and all that.

Neither metric or Imperial collets are 'uncommon' though, they're just different. Metric is more pervasive outside the US, Imperial within the US, but common in their respective places, if that makes sense. So you should be able to buy a bit to fit the collet you have, or get a collet to fit the bits that are easy for you to get hold of.
posted by Brockles at 3:09 PM on January 6, 2015

Lee Valley has a ½" to 6mm reducing collet.

This LumberJocks thread has people reporting both that some collets have enough throw to handle both, and success with wrapping some shim stock around the shaft.

I'd lean more towards the Lee Valley solution, especially since "unusual router bit" usually says to me "I've got my wallet way open anyway..."
posted by straw at 3:53 PM on January 6, 2015

And if, like me, your small handheld router only has ¼" and 8mm collets, Amana makes an 8mm to 6mm reduction bushing.
posted by straw at 4:27 PM on January 6, 2015

I wouldn't run any router bit, regardless of how small, without the right size collet. Shimming is just asking for an injury.
posted by werkzeuger at 7:09 AM on January 10, 2015