I've posted this on CNC Zone and BobCad/Cam Support Forum too in the hope of the best chance of finding someone who's encountered the problem before and might have found a solution -

I spent the best part of a day fiddling with my model assuming there was a problem with the it but then tried 3-Axis options on the problem surfaces and they all worked, potentially proving the model was OK so I started looking at the posibility that it was a software problem.
Has anyone else encountered a problem in 4- Axis Standard (and possibly found a solution) where BC works around any areas of a part below the centre line, leaving large chunks of stock untouched?
With some parts it's simply a case of using larger stock and moving the part to avoid any surfaces crossing the A-Axis centre of rotation and just making more chips. I'm currently working on something nominally S-shaped making it impossible to move both low sections above centre.

Paths generated -

I've experimented with moving the part around and verified that I can move it up or down far enough to get one or other face right. It's possible to vary the ignored areas slightly by choosing cutting around or along the A-Axis, but there are no feature settings that make any difference to the avoidance of any areas below the rotational axis.

Completed simulation -


Any help would be appreciated, even a confirmation that I need to get my cheque book out again ;-)
Regards,
Nick

As a test I constructed a very basic single throw crank and this fails in the same way -



I'm guessing it's a fault as I've not found mention anywhere in the literature that 4-Axis Standard will not cut below the axis of rotation, it might be cheque book time again!

A very kind chap on cnczone helped out on the issue of rotary cuts below the rotary axis.

Although the somewhat cryptic Help entry gives no hint of this it’s the Base Point which needs to be set to define an axis within the part, paths can then be generated down to this Base Point. (Base Point isn’t even a point, it’s an alternate axis for the part parallel to the defined rotary axis.)
 If your rotary axis is in X then use Y and Z to define an alternate axis parallel to the rotary axis and use this to generate paths for the problem section.

Parts with multiple sections where one axis will not suffice can be processed as several sections, each with it’s own appropriately set Base Point (It’s not a point it’s an axis).

For my S shaped part I needed 3 Base Points with the job split into 3 sections along the axis,
 
Best Regards,

Nick

Double post removed and image of paths generated using 3 Base Point settings with the job split into 3 sections along the x-axis

The paths in the thin section have a "Figure Eight" shaped distortion which is not fixed by variation of any parameters rendering the rotary function usesless for this part :-(