There are two categories of event here and I think it’s important to distinguish them:
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Feed hold: Use this when you want to politely ask the machine to stop. If it’s in the middle of a job and you’d like to clean away the chips if it’s not too much trouble, you use the feed hold.
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E-stop: Use this when you’re making the machine stop whether it likes it or not. If someone foolishly stuck a limb into the machine while it was running and the spindle is currently in the process of removing some fingers, you use the E-stop. If the machine is about to cause a fire, you use the E-stop. If the machine is running but its controls are not responding, you use the E-stop.
In my opinion, the E-stop is essential, the feed hold is optional.
There was a big thread on E-stop a little while ago which I think is a good start. The important thing is that when something goes wrong, the E-stop can be used instantly, without thinking, and it should do whatever needs to be done to ensure the machine cannot continue to operate.