The threads on cutters intended for Clarkson style holders are Whitworth form and 20 tpi, irrespective of whether the cutter is imperial or metric. I was under the impression that the shanks of such cutters were of a limited range of sizes. My smallest holders are 1/4" and 6mm and have 1/4" and 6mm by 20 tpi threads respectively.

I'm not aware of special taps for any of these threads (other than 1/4" BSW) and 5.65mm seems non-standard. Not much help I'm afraid.

Andrew

Just drill and if possible ream the extension for a close fit on the shank and then use Loctite to fix it. If you need to get it apart just boil hard in water for a few minutes, of gently heat with a blowtorch.

As Jason implies, good luck trying to drill a counterbore with an unsupported milling cutter.

Some of the older textbooks describe grinding a drill bit with a flat end as a "bottoming drill".

Could be worth trying with a long series drill, unless you need carbide for the metal you are working with.

If you are counterboring, it doesn't have to be flat bottomed, you could just use an ordinary drill?

Although the thread pitch on all the threaded cutters is 20tpi, the size of the thread is the cutter shank size. That 6mm cutter will have a 6mm X 20tpi thread. I very much doubt if the taps would be available easily. All of the threaded shank cutters up to 16mm have the 20tpi thread, I cannot say whether the larger ones have the same pitch as I do not have any to check.

I would Loctite the 6mm into an extention to do the milling. Make the extention as large diameter as possible for stiffness and take light cuts.

Yes, they are. I've just double checked 25mm/1" to 50mm cutters and they're all 20 tpi Whitworth.

Andrew

Just measured a 6mm shank Dormer and that's a similar 5.65mm over the crests of the threads.

1/4" shank Dormer is also well under at 0.236" over the threads

I see that the likes of Tracy do M6 x 1.25mm taps which would be close to the1.27mm cutter pitch and if as slack as they measure the difference in angles may not be a problem over a short length of thread

It is worth reading the datasheet for the proposed 'glue' before you begin.

To work best, it needs some clearance and surface roughness to give a film thickness and a good bond. Hence a reamed hole might not be the best and a rub of the cutter shank with a coarse diamond stone will not harm anything.

For tight tolerances, it is useful to know that a slot drill, or endmill will not produce a perfectly square counterbore. This is because the end cutting edges are not square to the tools axis. The counterbore will be slightly deeper at the outer diameter than the inner. This probably does not matter if a screw is to be fitted in the counterbore.