choosing proximity sensors?

[Neale]

Bert - I don't think that the sensor needs to be on the moving part - I can imagine a rotating blade on the end of the ballscrew with a fixed sensor next to it.

Andrew - the term probably comes from the kind of operating disk used with optical sensors - imagine a disk with part cut out that blocks or unblocks the light path as it rotates, hence chopper. In this case, you could use something very similar but with the rotating disk triggering a proximity switch.

Nick - I think that you need to wire the switches in parallel. As long as they are both the same type, you could use NC or NO with appropriate change to the detection logic. The only downside apart from a very small increase in wiring complexity is that you lose the "fail-safe" capability of a single NC switch. There are some failure modes that could let the machine overrun the end position.

It's an interesting technique, though - I did consider something similar when i was building my machine (although I didn't spot the simple way of doing it!) that involved switching to the "cam" sensor when the usual sensor triggered, However, on testing, I found that a single sensor in the conventional "pass-by" position was more than adequate, so I didn't bother exploring. But that's on a router rather than mill, so accuracy requirements are not quite so high.