View Full Version : Limit switch problems

10-09-2016, 11:53 AM
Hi. Hope I'm posting in the correct place and apologies if I don't explain problem as I should.
I have a Sherline mill with Routout boards running Mach 3. There are 2 limit switches on each axis. Everything works fine except top (home switch) on z-axis. Switches are NC (normally closed).
It isn't a faulty switch and seems to be connected properly to the board however nothing seems to detect it. The bottom z axis switch works fine. I can upload any pics, config files etc that may help.
Would love fault finding suggestions. David

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John S
10-09-2016, 12:18 PM
Mach 3 can only detect axis switches on limits. It doesn't know whick one, plus or minus, so disconnect the wiring from the top switch and daisy chain the two switches together and try that.

10-09-2016, 01:09 PM
Thanks. Will try that as soon as home and post back.

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10-09-2016, 01:14 PM
All your limit switches should ideally be NC and like John says only need to use 1 input for two switches. Infact you only need one input for ALL the switches if NC because can wire them in series (Daisy Chain)

Home Switches tend to be NO but can be NC if required.
Have you watched the LED in diagnostics while tripping the switch manually.? It could be that it's working but the polarity is wrong so mach not seeing any change. If LED changes then flip the Active Low in Ports & pins.

But to test the input for your Z axis (or any axis) just touch jump wire between input (+) and Gnd. Watch the diagnostic screen for the LED changing.
This will confirm if it's Wirng/Switch or the Actual Input on BOB.

If works ok then test the switch and if that works start tracing the wires looking for bad/lose connections.

Be methodical and you'll find it quick.

10-09-2016, 04:58 PM
Thank you both. It seems there was a break in one wire to one of the switches. So as suggested I daisy-chained both z axis switches together. I think I had developed brain ache by then though as I had wired them in parallel! Then did it properly and 100% now. Thanks again.

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10-09-2016, 08:30 PM
bottom Z axis ? why would you want a limit switch on there ?

ideally you are correct in having NC switches, this is so that if the wire is damaged and breaks the machine will stop
if the wire breaks on a NO switch , the machine is non the wiser and carries on , prolly resulting in a crash :-S

im suprised your machine ran with a break in the wire ... it shouldnt have allowed you to

10-09-2016, 08:39 PM
It had limit switches on z x and y when I got it.

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10-09-2016, 08:42 PM
So didn't bother to remove them. I suspect some other wiring issue and has somewhere had a workaround in the past. Rewiring the switches seems to have sorted it out.

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John S
10-09-2016, 09:04 PM
Magic, thanks for the update.

Clive S
11-09-2016, 12:02 AM
im suprised your machine ran with a break in the wire ... it shouldnt have allowed you toI think he said he had them wired in parallel