Just make the collet part too long so that it extends above the surface and clamp it in place endwise with a G clamp or toolmakers clamp while you drill the 8 mm hole. I found this is good enough to hold it in place while you carefully drill the hole. Cut the collet up to suit afterwards.

Alan

Edited By Alan Jackson on 05/02/2021 18:24:59

Hi Peter, Guys,

The way I would make that split cotter is like this.

Drill a hole say 10 mm deeper than the collet length with a tapping size drill say 5 mm for an M6 thread.

Open out the hole to the collet diameter for the length of the collet.

Thread the 5 mm diameter in the bottom of hole.

Drill the collet right through 5 mm tapping size for M6

Drill one half of the collet M6 clearance, and thread the other half M6.

Thread an M6 screw into the collet and screw the whole lot into the bore.

Tighten up so that the collet is locked in the bore.

Then mark out and drill the cross hole making sure that you leave a mm of clearance between the outside of the M6 collet through screw.

Disassemble and clean.

Cut the collet in half by either sawing or parting.

Make sure that you properly deburr the edges of the scallop after parting in half.

If you do it properly it will take a quarter of a turn or less from loose to locked solid on the shaft.

Here is one I made for my Norman Patent tool holder.  12 mm diameter, M6 through screw.  Note the edges at the bottom of the scallop.  Any burrs will cause those edges to dig into the bar that its clamped onto and make it hard to loosen.

Edited By John Baron on 05/02/2021 20:39:24