CNC Emergency stop button

This is something I didn’t have when I first started building a machine, let’s face it it’s not a priority when you’ve got an idea, some scraps of paper with drawings on and a few motors and linear drives and stuff. You want to get the bits together and see motors and parts moving, right. That’s what impresses people not a stop switch.

You suddenly start moving a stop switch up on your priority list after a mishap or two. The greater the mishap the more you really think you should wire an emergency stop switch to your machine.

I had a few mishaps and knew I should have an emergency stop button but it wasn’t until the spindle plunged itself down into the bed so severely it bent the bit holder not to mention the damage to the bed. You will probably go down the same route until something bad happens. At least it was only damage to the machine.

If I could get you to take one piece of advice out of everything I tell you it would be to wire in an emergency stop button as soon as you get your machine moving and not after a mishap. Lecture over you probably won’t and then you’ll read this again and wish you had!

Which type of emergency stop button?

If you look at some of the designs on the internet, some emergency stop buttons are wired to the controller board and will stop but won’t cut off all power.

A proper emergency stop button will stop everything, the machine from moving, and the spindle from rotating and cut all power. It is the safest way to do it, once you move away from lower powered spindles that get their power from the board and move to something bigger like mains powered routers the spindle is no longer controlled by the board and you have to turn it on and off manually anyway.

It’s probably best to have a number of stop buttons. I know a lot of people will disagree but if you can knock power off to your spindle and to your machine that’s a good start.

If you’ve had to hit the traditional “all power off” emergency switch, where ever you were in the job will be lost and it is possible to wire a “pause the machine” stop button which might be OK in some situations where you can see what is going wrong and think it may be retrievable in terms saving the job.

A simple black push button, it doesn’t look much but it can save a lot of bother. The red and black sockets below are for connecting the probe.

Probably the worst option you can have is stopping the spindle but having the machine continue to run, before I had a stop switch I saw the machine was cutting wrong and turned the cutting spindle off but as the machine was still running and the “stopped spindle” continued to move through the job breaking the cutting tool and damaging the material and pulling the clamps off. It really is amazing the power stepper motors have at moving things.

If I’d stopped the machine and left the spindle running it wouldn’t have damaged anything else.

Pausing the machine is usually the best bet for a stop button as it’s usually the combination of the machine moving and the spindle cutting that’s the problem, if you pause the machine, the spinning spindle won’t cut anything more than it already has.

Wiring a CNC emergency stop button

The best way I have found with my home built CNC machine is to have 2 stop buttons. One is a red “stop” button that pauses the machine. This can be wired into the Arduino UNO feed/hold A1, wire the push button to connect this pin to earth.

The second is the traditional big red emergency stop button. This turns power off to the machine and the spindle.

Why have two stop buttons on a CNC machine?

The reason for having two stop buttons is if you can get by using the “pause” type of stop button, you can sometimes salvage something if you can put right what the reason was why you halted and then resume where you were in the cutting program.

If you have to hit the emergency stop button for safety reasons then where you are in the program will be lost and if you can resume the job you will have to do it from the start.

Along with an emergency stop button the next most important thing to upgrade your machine if you don’t already have them are limit switches. I have an article about them here.