Imagine the curvature of the convex surface:

A bi-focal prescription with astigmatism: the convex surface has the degree and angle of curvature varying from not only top to bottom but also left to right.
Now a spec requiring that there can be no reversal of Z axis because it could cause a 1 micron dimple at the peak of the curve at point at which the tool changes direction of cutting. I.e. If one were perform conventional 4 Axis machining, the tool (on Z axis) would have to change the direction of its Z movement at the apex. Even worse, some one could try indexing it sideways causing an unacceptable dimple at the apex. Others made all these mistakes and failed! How we solved it: We mounted the mold at a 45° angle and moved the 5th axis simultaneously with the rest of the 4 Axes of the machine. Result: a perfectly machined mold. In fact, we had even hard milled it to avoid deformity if further heat-treated. Due to the confidential nature of our work, we regret that we are unable to publish more examples. We would be happy to discuss the general nature of some of the exclusive work we had performed for our customers with “unusual and specialized” needs. contact us.