Emergency Stop

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Emergency Stop buttons are used to stop all motion in an emergency, for example if a finger, hair, jewelry, etc becomes entangled in the machine. Industrially, E-Stop buttons are typically linked to the circuit breaker that powers the unit. For the shapeoko, there are two ways in which an E-Stop button can be implemented.

A/C Breaker

This method involves placing the E-Stop button between the wall power and the spindle and Arduino. This is more reliable (however I am unsure of its long-term effects on the steppers, PSU, and electronics). This would most easily be accomplished with a PowerSwitch Tail II (can toggle 120V 15A resistive loads) to switch the Power supply/Arduino off, as well as the spindle.

Digital Breaker

This method sends a signal to the Arduino to gracefully cause all stepper operations to cease, and (if equipped with a relay or PowerSwitch Tail), also stop the spindle.

Thoughts on the matter: I think the safest alternative would be a digital relay (for AC) to enable spindle power, which is held 'enabled' when the estop switch is in 'run' mode and supplying dc power to the grbl shield. In this way, hitting the estop will kill power to the shield (and steppers) and disable the relay supplying spindle power.

Power Strip

"A 6 outlet strip with a reset switch on it. I plug in my 24v power supply, and my dremel to the strip. So if something bonkers happens I just hit the switch and it kills power to both. Simple, yet effective."[1] See the commercial option below for a power strip intended for such use.

Part Sources

Commercial Option

An interesting alternative is: http://www.wti.com/p-83-ips-800-ce-internet-power-switch-dual-10a-240v-8c13.aspx which would afford one the option of monitoring a machine remotely, and being able to disable the power.[3]

Forum Examples

Lengthy discussion of safety considerations and implications: http://www.shapeoko.com/forum/viewtopic.php?f=37&t=6188&p=48340#p48340

Emergency stop button for SO3? http://www.shapeoko.com/forum/viewtopic.php?f=7&t=6468