Giant 20' x 3' x 2' router build

[EddyCurrent]

just tried just a steel bolt and it still had the fault

Did it have a hole in the middle like the cap head screw or was it solid metal right across and at least the same diameter as the proximity switch ?

This is from Baumer inductive sensors catalogue.
Baumer

"Maximum cable length A large cable length is a capacitive load for the output circuit and increases the influence of interference. Length should therefore be kept as short as possible."

They don't quote a figure for max length though.

This is why I suggested using single core bits of wire just as a test over the distance, the capacitance is going to be low if they are kept well apart.

Save yourself the grief and fit mechanical switches for limits, you won't have these problems then.