CNC 3018XL: Adding Run-Hold Switches

Although the bCNC GUI has conspicuous Run / Hold buttons, it’s easier to poke a physical switch when you really really need a pause in the action or have finished a (manual) tool change. Rather than the separate button box I built for the frameless MPCNC, I designed a chunky switch holder for the CNC 3018XL’s gantry plate:

The original 15 mm screws were just slightly too short, so those are 20 mm stainless SHCS with washers.

The switches come from a long-ago surplus deal and have internal green and red LEDs. Their transparent cap shows what might be white plastic underneath:

I think you could pry the cap off and tuck a printed legend inside, but appropriate coloration should suffice:

Making yellow from red and green LEDs always seems like magic; in these buttons, red + green produces a creamy white. Separately, the light looks like what you get from red & green LEDs.

The solid model shows off the recesses around the LED caps, making their tops flush with the surface to prevent inadvertent pokery:

The smaller square holes through the block may require a bit of filing, particularly in the slightly rounded corners common to 3D printing, to get a firm press fit on the switch body. The model now has slightly larger holes which may require a dab of epoxy.

A multi-pack of RepRap-style printer wiring produced the cable, intended for a stepper motor and complete with a 4-pin Dupont socket housing installed on one end. I chopped the housing down to three pins, tucked the fourth wire into a single-pin housing, and plugged them into the CAMtool V3.3 board:

The CAMtool schematic matches the default GRBL pinout, which comes as no surprise:

The color code, such as it is:

• Black = common
• Red = +5 V
• Green = Run / Start (to match the LED)
• Blue = Hold (because it’s the only color left)

The cable goes into 4 mm spiral wrap for protection & neatness, with the end hot-melt glued into the block:

The model now includes the wiring channel between the two switches, which is so obviously necessary I can’t imagine why I didn’t include it. The recess on the top edge clears the leadscrew sticking slightly out of the gantry plate.

The LEDs require ballast resistors: 120 Ω for red and 100 Ω for green, producing about 15 mA in each LED. Those are 1/8 W film resistors; I briefly considered SMD resistors, but came to my senses just in time.

A layer of black duct tape finishes the bottom sufficiently for my simple needs.

Note: the CAMtool board doesn’t have enough +5 V pins, so add a row of +5 V pins just below the standard header. If you’ve been following along, you needed them when you installed the home switches:

A doodle giving relevant dimensions and layouts:

I originally planned to mount the switches on the other gantry plate and sketched them accordingly, but (fortunately) realized the stepper motor was in the way before actually printing anything.

The OpenSCAD source code as a GitHub Gist:

It seems bCNC doesn’t update its “Restart Spindle” message after a tool change when you poke the green button (instead of the GUI button), but that’s definitely in the nature of fine tuning.