Hi Paul,
I remember only too well the nightmare I had when I first got into mini rotary tools as everyone seemed to use different thread sizes from each other!
With everything from 9/32" x 40tpi (imperial) to M8 x 0.75mm (metric) and even another one somewhere in between it made buying accessories a bit of a problem. (I think M7 but with a different thread size from the others and I'm sure that's the Draper version.)
The only common ground was the neck shaft for when you screw on flexible drive shafts with the imperial being 3/4" - 12UN and the metric equivalent being 19mm x 2mm. As the metric equivalent is so close to the imperial (19.05mm x 2.117mm) they seem to fit each other quite well.
I now have a couple of flex shafts to do everything which fit all my tools being Proxxon, Dremel, Draper and a typical Chinese unit (which unusually has a much higher power motor but far less torque). Each flex shaft has a different chuck one being the typical screw on collet affair and the other being a mini three pin chuck which I much prefer.
The nice thing about using that is the flex shafts are much easier to use but all the rotary tools will fit my many home made accessories such as router head, router table and a mini drill stand. (Nice for those micro holes or PCB work.) Being a lazy git I leave each of the rotary tools in their respective adapters preferring the smoothness of the Proxxon on the flex shaft for hand working.
Why do I have so many rotary tools?
Family members went through a phase of buying them as gifts on Birthdays/Xmas as I'd gifted them wood carvings previously.
That's all gone off the rails for a radio forum hasn't it?
Not so when the carbide heads can make mincemeat out of steel, aluminium etc. and you need to cut the right size hole for the latest radio project or modification.
Hope that helps Paul.
All the best to you and Sarah,
Victor