Let's create the toolpaths for machining the backplate. I can't do the machining until next weekend, as I need to wait for the arrival of some 1.5" long 7/16"-14 UNC cap heads to hold the backplate to the (imperial) spindle. The only ones I have are 1" long and only hold on by 1-2 threads. I also lack a decent set of hex drivers, so have ordered a set of Bondhus T handled ball heads.

Firstly, here's the "stock" is what I have currently ie the backplate as received. No need / point actually modelling the counter bored fixing holes.

And here's the required backplate, according the dimensions given in the Vertex drawing. I'll need to check them before cutting any metal. Also, I'm not certain how far the register needs to stick out without fouling the back of the chuck recess. All I know is that it's 65mm and the bore is 55mm. And the bolt PCD is 95mm.

But that's enough to create the toolpaths. Firstly, rough out the smaller nose / register (leaving some meat for a finishing pass), then use a finishing profile to take the register to its final size and face off the main surface. I've dimensioned the register diameter 0.25mm oversize in the model, so that I can measure the final diameter and then measure / recalibrate the X axis to gradually take it down to size - or simply machine it manually.

Right, that will do for now. Next weekend, I should have the backplate machined - and the new chuck should hopefully have arrived - so I should finally be in business.