Need ideas (or someone to make) a widget to couple motor shaft to M8/M10 threaded rod

[HankMcSpank]

....turn down a threaded bar with bearings on each end and the last section tuned to 6.35mm so you can use a flexi coupler to the motor.

The problem with a bearing on both ends of the threaded rod is one of access...I need to get my hands in/out a lot (to wind the copper wire on to the main bobbin etc)...this is why if you look at my last photo (of my 'loosely lashed together up' design), I supported the threaded rod a few centimetres in from the end of the rod with a small bearing ....that keeps everything nice & clear/open for ease of access (& it's also easier to thread the copper wire under an open ended bit of threaded rod)

[Ross77]

The problem with a bearing on both ends of the threaded rod is one of access...

The bearings don't have to be at the very ends. You just need the shaft constrained by two bearings to keep it concentric, then the flexible coupling can do it's job and take out any minor alignment problems. keep the arrangement you have but add another support plate and bearing after the coupling, as you have it at the moment the threaded bar will pivot about the middle bearing and exaggerate any alignment issues.

Have you thought about using a hex bolt and cutting the head off? that way the shank could act as a shaft for the bearings and having a solid round end would be better for forcing on a under sized flex mount.