I decided to modify the original arbour I made to take some slitting saws that I have.
For some reason I decided to test the run out when I fitted this back onto the lathe in the 5C collet. With my dial indicator I couldn't see any movement, I checked, fiddled and checked again. Nothing

It couldn't be nothing as I didn't even index the part to the collet. So I changed indicators from 0.001" res to one with 0.0001 res.
Now it moved, by a whopping 0.0002". Ha, pure luck I'd put the part back in the collet at exactly the same position I thought.
So I turned the part in the collet out of pure interest and measured again .... barely 0.0001 ... I give up .....

Anyway, the boss was turned to a very tight 1" to match the slitting saw.

And the other cutter I have with a 1" bore ..

Next I need to chop the end off, face, drill and tap 10mm for an end cap.
But before I do I thought I'd throw this out to some suggestions.
I have 5 identical slitting saw blades, they are 3" dia carbide x 0.0245" thick. I seem to remember Bogs getting some spacers and fitting several saw blades at a time to make an engine cooling fin cutter. Can I ask what your dimensions were?
Second question, the other cutter is 3" x 1/4" tipped with a keyway. Should I cut a keyway on the arbor to match or should I be able to get away without it
