How to use 1/2" shank diameter tool?

Hey all,

Proud owner of a new Shapeoko 3 and brand spankin’ new at CNC milling. There are some tools I want to occasionally use that are only available in a 1/2" diameter size. Is there such a thing as a 1/4"-to-1/2" adapter to use them with a 1/4" collet?

This is for a face mill for aluminum. Clearly a 1/2" diameter is meant for a bigger machine but cutting time isn’t an issue, so I figure with very shallow cuts and exercising something resembling patience it wouldn’t be a problem, all I need is an adapter. Any advice would be appreciated as my Google-fu is letting me down at the moment. If this is a bad idea even with extremely shallow cuts, I’d very much like to hear that as well, of course.

My understanding is this is a bad idea.

There is a 3/8" collet available for the Makita:
but it’s not a precision unit — I haven’t gotten around to ordering one yet (and I did buy one of every other size from Elaire Corp. (3mm–8mm, incl. the 3/16") and having a hard time justifying it since I’m concerned about runout and so forth. (Apparently Elaire won’t make one since it leaves too little material for reliable mounting?)

But to get 1/2" tooling one pretty much needs a larger (heavier) router and an upgraded Z-axis — the HDZ has an 80mm mount option, so that gets one to ER-20 collets — but folks have experimented with heavier units and so forth, but I don’t know that anyone has reported replicable success.


Sounds like it is indeed a bad idea, then. Thanks for your thoughts on the matter and saving me from myself.


Never EVER use an adapter in a Router… An object spinning at 30,000 rpms is under a LOT of stress, I know of a story where a router bit broke off and hit the operator in the chest…yeah, he died.


Here’s a 1" insert face mill with 3/8s shaft.

Its very important to understand SFM and to only run something like this around 10,000 rpm max.


Upgrade to an HDZ and a 2.2kw spindle that uses ER20 collets. ER20 clamps up to 13mm (.512").

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If you get a 3/8" collet be sure you mark it plainly. The rest of the world has 8MM collets and a 3/8 is 9.5MM. From sight only you could put an 8MM bit in the 3/8 collet and that could be a major problem. So if you have a 3/8 inch collet be careful not to put metric bits in it. I have seen people ask if you can put a 1/4 bit in a 6MM collet. 6MM is .236 inches, so you might be able to cram a .25 inch bit in but you are headed for trouble.

The bottom line for metric collets is only use metric bits and for imperial collets only use imperial bits.


One consideration though is 8mm and 5/16" are essentially interchangeable:

5/16 == 0.3125 in == 7.9375 mm

since the difference is w/in typical manufacturing tolerances:

0.0625mm == 0.00246063 inches

A further consideration is the 3/8" collet is actually just an angled ring, lacking the central shaft — and since it’s usually black oxide, readily distinguished.

I will note that when I put the full set of sizes together, I did feel obliged to label the holder:

(1/8" is from Carbide 3D, as is 1/4" (not shown), the balance is from Elaire Corp. (3/16", 4, 6, 8mm — still surprised there isn’t a 3mm) — the 5/16" has a lip thickness of just 0.78mm or so)

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There is a great difference between an 8mm and a 3/8 in collet for the Makita, they don’t look the same at all.

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