How do I make a D shaped swept cut along the top surface - It needs to adjust if I am to alter how tapered the bar is.
I.e. if i were to make the bike crank straight the middle portion would change length due to global parameters and therefore the sweep needs to adjust to this
If I am imagining the right type of feature, I think you'll want to use a sweep feature to make the cut.
The path would essentially be an arc. The profile will be your "D" shape.
It it not clear how you want to change the length or width, but you have two options.
For a "D" shaped profile, you could change how deeply it cuts into the arm, this will change its width as well.
You could also do a more advanced sweep feature with guide curves to further refine the shape.
Sweeps and lofts are very closely related, so setting this up as a loft could work, but I'd put some thought into how the final shape will be machined in the real world. Nobody wants to program freeform surfaces for a shape like this. Running a ball end mill along a simple path would make for an easier setup.