By “crunching” I mean that something tries to occupy the same space as the head/gantry as it’s moving. Might be a piece of wood/plastic/whatever that’s warped upward while being cut. Might be somebody’s hand or face. Either would be bad, which is why I think an auto-to-zero could cause trouble.
Meanwhile, I see from this thread that the focal length of the final lens is 50mm, or about 2". Which means that even with head and window there should be more than an inch of clearance, probably more like 1-1/2". Which makes me feel better about reaching in/under.