I was thinking of making the pin components for the knurling tool out of 8mm A/F hexagon bar so the heads could be used with a spanner for tightening. The pins provide the pivots for the arms and also the pivots for the knurling wheels. However it appears that metrix hex bar is quite rare in the world of model engineering suppliers, most of them prefer to provide bar in various imperial sizes.
I do however have some round bar of the right kind of size that can be turned to my requirements. So I came up with the following three ideas for the head of the pin, I could also resort to using a bolt but I over time the screw thread would wear the tool so I’m not keen on that so would prefer to make custom pins.
The first design is a simple slot to be turned with a flat head screwdriver. The second is two holes that can be turned with a special tool known as a spanner head or snake eye and is common as a security screw in the USA. Finally my favourite option which is to machine or file two flats on the round bar so that it can be turned with a simple spanner. I also intend to put a lock nut on the opposite end so I will make a nut to match this design.