I’m looking to wire up an E-stop and relay control for my router and Jackpot. In my head this is pretty straightforward: wire an electrical plug into the e-stop, and then the e-stop into an outlet with one plug ‘switched’ by the relay. I drew a diagram to illustrate:
Does this make sense? Have I committed some grave electrical error?
I am NOT an electrician, but as far as I remember I did it similarly. I connected an E-stop that kills everything and then another one, the “to Jackpot” that turns the spindle on/off. I think I might have seen the diagram somewhere on the Tube… 
For low power stuff like this it’s probably not too bad, but typically you wouldn’t switch the power with the E-stop mech switch directly, you’d use it to control a separate contactor or use it with a safety relay that can combine NO and NC circuits, as well as provide a separate reset button. That’s often because you don’t necessarily want the power to return immediately upon resetting the E-stop so that you’re not relying on the mechanical latching of the E-stop and there’s less chance of unexpected motor restart etc.
That also means that if you daisy chain multiple E-stop switches you’re not running your main power circuit around unnecessarily. Given the relatively low current single phase we’re dealing with here then that’s also no big deal, really.
That’s all getting a little overly complicated for a single device, though, and in reality as long as you’ve got a good idea of what you’ll do if the E-stop fails to work (grab the plug and yank hard) then you’re probably fine.
Another alternative is to use the E-stop with a couple of normally closed mechs, one to cut power to the router relay, the other to pull a GPIO on the jackpot to 0V to use as an immediate disable. That way you’re using hardware to disable the router, which is the most critical part, and software to do the less critical/dangerous part of disabling the motion system. Relying on software for emergency behaviour is usually not a great idea but in this case I’d say it’s warranted because the risks present in just the motion system itself are incredibly minimal, mostly pinch/entrapment hazards with very little force.
Personally, I just used a switched multi-board where I can individually cut power to the router, the motion system or everything altogether. I don’t see there being that much extra value provided by a latching mushroom button E-stop given that most of the failures are relatively slow such as fire or a ‘shut down after a failure’ like a tool breaks or machine skips steps and not really a ‘panic safety of life’ type thing. The main advantage I could see is if you are expecting others to be in a position where they may need to shut the machine down and you need an obvious option to do so.
Another thing to consider in case you haven’t yet is a fire extinguisher and, ideally, some form of shield in case of flying tool shards 
You should be able to pause first and home the z axis i thinkt that is the most important thing if you managed to get the error before you damage your work.
If by any means you move within the webui without having home your z axis it will continue to get into the buffered toolpath, thats how i have saved countless works.
