Top surface of 3d object mysteriously elongated in the X when machining

[danielcoyle]

With Rhino 3.0 I machine wood helmet shells. Taught/teaching myself. The shells have an inner and outer surface which is uniform, about .2 inches. I rough and finish cut the inside of the helmets from a large cube and then flip it over and run additional roughing and finishing files to cut the top surface. In the image you can see a helmet where the seemingly X- shifted toolpath has resulted in shaving the front too thin (actually creating a large hole) and leaves the back of the helmet much thicker than intended (you should be able to see the hole I drilled in it through the front hole. It is approximately .4 inches thick at that hole when it should be .2.

I have tried everything I can think of to figure out why this is happening but no luck. Any thoughts? I tried to attach an example file here but it was too large apparently.

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[Zorbit]

I think we need more detail as to how you are physically flipping the job over. Did you create the two machining jobs by mirroring the internal or external surfaces along the Z axis ? Or by rotating one of the surfaces ?

[yham]

How do you reference inner surface to outer surface? Do you have some reference pins?