Chuck to extend the router machine shaft.

How to select a CMT 8mm collet extension with chuck

If you have a router table ( <- check out this link on how to make one) or a DIY one made with a plunge router, or if you are thinking of making your own, you may have noticed (or you should know, depending on the case) that when you put the plunge router inverted under a board of about 18mm the router cutter may not protrude far enough above the top surface of the board. One possibility is to use a thinner board, about 10mm (which we may need to reinforce around the base of the plunge router so that it does not warp). Even so, the router bit may not reach the desired height. We can always reduce the length of the cutter shaft that we insert in the collet into the chuck to a little less than the 3/4 of its length. We sometimes can see a line engraved in the router bit shaft as a recommendation for how much length of the router bit shaft must be clamped into the chuck. Although sometimes it may not be enough and it is not advisable to be too short with the length of the cutter shaft that we clamp into the collet. The best solution we have to make the most of our homemade router table can be to use a router collet extension such as those manufactured by CMT. Yes, it is another one “small” expense. Or rather a few more small expenses, because if we take look at the CMT catalog we will realize that we will need to buy several piece.

I have not yet needed to use these collet extensions, as my router cutters always protruded far enough above the router table top. I recognize that this is partly because the router I use, a Power Plus X093, allows the use of 12mm shaft router bits (and it came with a set of such cutters) and these are somewhat longer than the cheap 8mm shaft router bit sets that I have.

Two different router collets and chuck nuts

The wood router that most of us have (here in Spain), and also the first type of router machine that I had, will in many cases be for 8mm router bits at most. And you probably have the 6mm and 8mm collets. With these routers you cannot use 12mm or 12.7mm (1/2″) shaft router bits and there is no collet extension for 12mm or 12.7mm router bits that you can fit in the collet of an 8mm wood router machine.

Cheap router bit sets for wood routers.

These wood router machines often come with a set of router bits, and if they do not, I recommend buying at least one set of cheap router cutters. I have a couple of those cheap sets, one is the typical one that you see everywhere and is usually the one that comes with many plunge routers. The other cheap set of router bits, which I don’t remember where my father got it, is a more complete set that curiously comes with slightly longer shaft router cutters.

For the case where your router accepts at most a collet for 8mm router bits:

To get the router bits to reach the desired height above the top surface of the DIY router table, one solution, as mentioned above, is to use a collet extension such as the ones from CMT. These consist of a shaft reduction plus none chuck for the router bits. You can see in one of the pictures the all parts needed: reduction shaft, chuck, collet and chuck nut. In this type of chuck the largest collet that can be used is 9.5mm (3/8″), and it is not possible to fit larger diameter collets. Neither the 12mm nor the 12.7mm (1/2″) fit inside that chuck.

Combinations for 8 mm collet extensions.

I leave you below the necessary parts, references and prices (outdated) that I found as of today (May 2014):
You need a shaft reduction, a drill chuck and the collets:

Prices are outdated

Reduction shaft diameter S=8mm and metric M10 -> 797.580.00 -> 11,18€

Chuck metric M10 -> 796.100.00 -> 18,97€

You will be able to use the collets for the following cutter shaft diameters:

CMT collet extension collets.

6mm -> 796.560.00 -> 6,15€.

6.35mm (1/4″) -> 796.564.00 -> 6,15€.

8mm -> 796.580.00 -> 6,15€.

9.5mm (3/8″) -> 796.595.00 -> 6,15€.

When I bought the collets they didn’t have the 6.35mm collets, but they gave me the option to buy a reduction that transforms the 8mm collet into a 6.35mm collet -> 8,30€.


And in case you’re also wondering:

The collets that come with your wood router machine are NOT suitable to use in these collet extension chucks (neither the collet extension chuck collets are suitable to be used in the router chuck), so you must also buy the necessary collets according to the router bit shaft diameters that you have or that you think you will have in the future.

> See CMT collet extensions for 12mm wood router machines.

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