Spindle on/off under CNC software control - safety worries!

[JAZZCNC]

I guess ultimately then, it's how much do you trust the relay (or more so, the relay contacts) that controls the spindle startup! (I'd figured there'd be a little more 'conditional' scenarios involved before a spindle could be allowed to start)

Not really the relay contacts because it's a NO relay and when signal drops power to relay coils the contacts break naturally ( Unless faulty in which case it won't stop anyway.!)
It's the control software or more accurate the signal state you can't trust and in extreme case's it's possible it could switch state thru noise interference.!!

You could build into the Macro some conditional checking like checking the state of a input. This could be done by using double throw relay which sets a spare input used has a NOT-READY-NO-GO
This is one of the reasons for using a M6 tool change macro. It's in this macro where you'd put code for things like auto tool changers etc and also for doing safety checks like this if needed.! (could even turn kettle on for you for quick brew between changes. . Lol)

[HankMcSpank]

Not really the relay contacts because it's a NO relay and when signal drops power to relay coils the contacts break naturally ( Unless faulty in which case it won't stop anyway.!)
It's the control software or more accurate the signal state you can't trust and in extreme case's it's possible it could switch state thru noise interference.!!

Exactly, I realise the relay contacts in play will be NO ...but it's the software/circuit that drives the coil I worry about...this is why I figured some condition dependent time delay would be brought into play here on the relay circuit ...therefore if a transient occured (in software or the control board), the spindle wouldn't kick into life straight away ...ultimately, I kinda seek a beep-beep-beep (like bin truck reversing sound), a couple of seconds before the spindle starts ...that way if I hear beep-age, I know to get my hands off the spindle sharpish!

[JAZZCNC]

.ultimately, I kinda seek a beep-beep-beep (like bin truck reversing sound), a couple of seconds before the spindle starts ...that way if I hear beep-age, I know to get my hands off the spindle sharpish!

Sure it's not that difficult to implement if your that concerned but to be honest IME not really needed and something along the vain of my last suggestion would be easy to do and pretty much fail safe.