I appreciate that this thread is a bit old now, but as still somewhat a novice, I wanted to say thanks to Darren and Boggy for their help in explaining how to do this and what materials to use.
I have wanted to extend my somewhat modest collection of replaceable tip carbide tools for some time, but funds do not permit buying them new.
I got a set of 10 X 11mm Sandvik tips (TNMG 11 03 08-QM) from fleebay (a bit more 'spensive than Darren's)

and some lengths of 10mm keysteel from the source that Boggy recommended - very reasonably priced.
The keysteel machines beautifully - excellent value.
The screw hole in the carbide tips is only 2mm and I felt it too weak to drill/tap the keysteel for this, so after milling out for the tip, I "pinned" the tip hard against the milled step with a flush stainless steel 2mm peg and fabricated a step clamp from a scrap of old black steel I had in the bin-box, using an M4 skt. head bolt. I machined a thin step on the back end of the clamp step so as to fit in a slot filed on the keysteel to stop the clamp swivelling when doing up the bolt.
The result is not pretty to look at, but as a sort of "trial" it works a treat.
I tried it on a length of rusty old 50mm bolt from a scrapyard - it cut cleanly with no judder - provided the speed was kept up (800rpm).
Thankyou for the info