That is correct. The concern is more about assuring all wiring and switches are operating as expected. Also, want to try to isolate where a fault would be located in case of a problem. Thanks to all for the input.

For details on "supervised wiring" I suggest a study of fire detection and alarming systems. The schemes for verifying the integrity of wiring and switches are quite mature, because fire protection systems are life critical.
Certainly this is a lot more complicated than LEDs displaying the operation of a switch, but, as I have explained to some clients,
"it is quite challenging to sense the difference between Zero and Nothing." That is where the supervising portion comes into the picture.
Indicators on a PLC input module will show if a switch has opened or closed, but they will not tell you if the switch will be able to signal that it is open or closed. THAT is where "supervised" arrangements come into play. The good news is that all of that has already been researched and developed for fire safety/alarm industry.

 

This thread seems to have gone way over the top,
All the OP wants to know if he has an error before pushing the start button.!
This issue seems to be surrounding the N.C. axis home/zero switches, which in any CNC machine, industrial or DIY any malfunction will normally prevent the M/C from moving and display as such on the operator panel.

 

If the switch is open because of where the position is and is supposed to close when the initial move is completed, how is the CNC controller going to detect the end to know when to stop if the wire to the switch has failed and is open circuit??? Now if the design requires that the switch at the end of the travel open when that end is reached, then yes, motion would hopefully be inhibited if the wire failed. But we are not given that information, i don't think.

 

My intent was to set up indicators on the panel of the control box to ensure each switch (NC) was closed before running a milling job.

If the Homing SW's are wired NC then the only time they should show open is when triggered, of course. i.e. at home position, and/or O.T. if used in a double function, as Mach3 uses for e.g.

 

OK, If and only if, all of the switches are N.C. Closed in the "at rest, ready to start" position, then duplicating the arrangement shown in post #17 can be OK. BUT that will not verify that any switch in the open position is connected. So verification prior to the very first start, possibly by manually triggering switches that are open, is advisable. BUT that must be done without any motors enabled, in case some switch operation would cause motion. THis would ONLY be needed prior to the very first power-on, or any switch repair/ replacement actions.

 

I have never seen this much concern as to whether CNC home limit switches are working prior to using the M/C??
What about all or any of the other functions?
In just about all the CNC M/C I have come across or have been involved with, as a rule, any malfunction is recognized by the control and it will either 'hang' at that point, or display a msg of some kind.
There is usually some diagnostic screen which will display status of the I/O in order to trouble shoot the problem.
Even the simple DIY PC based CNC such as Mach3/4 will do this to a fair practical extent.

 

Standard practice is to use hard-wired limit switches at the ends to stop the machine when actuated.

The home sensor is located close to the backward end limit switch.


The homing sequence is initiated after power-up. The axis moves back and stops when the home position sensor is sensed. It then creeps forward and stops again, when the incremental encoder marker pulse is received, to register the 'home' position.

 

Just to clarify that.
On a DIY CNC the normal action is to move in rapid, until the Home LS is seen, after an immediate stop, the axis will move at creep speed until it comes off the LS.
Home is then registered .
In the case of some DIY systems, the Home SW then is used as overtravel. Mach3 etc.
In a professional system, as mentioned, there are two sets of switches on each axis, one for home the other for OT.
But the home switch is only used to stop the axis when homing, the axis then creeps off the LS and registers home when it sees the encoder Z(ero) pulse.
If the operator attempts to run a program without first zeroing the M/C, it will normally result in no movement of the axis, and an appropriate msge.