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My router has a 1/2" collet to take 1/2" shank bits. I also have this generic CNC endmill whose shank is 3/8". I want to be able to use this endmill with the router.

Can I apply painter's tape (or masking tape) to thicken the shank of this 3/8" CNC endmill so I can fit it on the 1/2" collet? Would it ruin the collet?

How about in general? Can I use tape to buff up smaller bit to the larger collets/chucks of these power tools?

I know, I should probably be experimenting on this on my own. I never found the time.

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    You really don't want to do this sort of thing. I'm sure there's someone out there who claims to have done this without issues, and some of them might actually be telling the truth. But for all you know some could have tried something similar on a drill — where you can use tape e.g. to try to protect threads held in the Jacobs chuck — and extrapolated up to routers thinking there's no fundamental difference (which they're dead wrong about). And you've got to leave room for the possibility that some might be outright lying. Yeah I know, call the Internet Police, someone is fibbing online!
    – Graphus
    Commented May 14, 2023 at 8:00
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    "I know, I should probably be experimenting on this on my own." No actually, I think this is very much something nobody should experiment with! You could quite easily do some promising tests that lull you into going further. At some point in the future you do this for real on a project piece and when the bit goes through a large knot or across end grain and BAM it spins free. If you're thinking, OK, brown-pants moment but no biggie! that's the least of it: broken chuck, damaged router base and potentially a hole in the wall. And what if instead of the wall the bit impacts your face......
    – Graphus
    Commented May 14, 2023 at 8:05
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    They do make 3/8" router collets, you know. Just spend the 20 bucks and do it right. Commented May 14, 2023 at 12:44
  • Collet inserts are even cheaper than a new collet. Commented May 16, 2023 at 18:13

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That sounds like a good way to get yourself seriously hurt. There is no way Tape can handle the forces generated nor the heat from spinning at 32000 rpm. There is a reason they are steel shanks. The tape might melt or start on fire, and there is no way you can make it tight enough for the bit not to wobble like a top if you tried to cut anything with it, right before it shoots out and smashes something.

Your best bet would be to see if there is a 3/8" collet available for your router. Or to buy a similar bit with the correct shank size.

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