I tend to prefer to minimise safety risks when I’m using my CNC, and that includes making sure I don’t injure myself touching fast-rotating cutting bits. Obviously it’s unlikely that the motor will start up when I’m removing or attaching a bit, although it’s a little more possible when the G-Code file includes pauses for tool changing. But, to be completely sure nothing can go wrong, I made the 3D printed switch box below to allow me to isolate the spindle motor from its electrical supply. It prints as two parts which need to be glued together to fit the top of a 20mm extrusion, like in the photo above. The hole in the front is made to house a standard 18x11mm rocker switch. Click here to go to Thingiverse to get the STL files for your 3D printer.
Once the two parts of the box are stuck together, with the switch inserted, the positive wire from the spindle motor controller to the motor itself, which should be red, needs to be cut. The position where you do that needs to be planned so that the wires can be run to the box and the full movement of the motor along the CNC’s x-axis isn’t compromised. Personally I found the best place is near the controller board as on my CNC the red wire ran straight past the box position. You can use standard spade connectors crimped onto the ends of the wire, after stripping some insulation from the end. Below is a photo of the back of my switch box so you can see what I mean.