Thing-O-Matic: Z-Minimum Platform Height Switch

Z-minimum switch - left view
Z-minimum switch - left view

Getting a good bond between the build platform and the first extrusion layer depends on the nozzle height above the platform: too high and it doesn’t stick, too low and the excess plastic ruffles up to ruin the next layer. I’ve been setting the height manually, but that’s tedious, fraught with error, and goes best with a cool platform that may change size when it heats up. Measuring the Outline or Skirt extrusion provides after-the-fact information about build platform alignment, but doesn’t ensure that the current print will work.

The best way to do this is to measure the actual height of the nozzle above the build platform immediately before starting the extrusion, with everything at operating temperature, then set the Z axis so that Z=0.0 puts the nozzle on the platform. Nophead put a height measurement station beside the build platform and I’ve built a tool length probe for my Sherline, so I’m not breaking new ground here.

The catch is that the switch must sit flat on a 120 °C platform, withstand being poked with a 220 °C nozzle, repel ABS, and trip with better than 0.1 mm repeatability. I don’t know that what I’ve done here meets all those criteria, but it’s a first step along the way.

The top picture shows a surplus SMD pushbutton switch with a metal actuator button mounted on a small steel strip. The gray epoxy blob to the front secures a brass tube that protrudes from the bottom into the middle socket head cap screw along the right edge of the platform.

[Update: The surplus place was likely Electronic Goldmine, but it seems they have no more. Sorry. Maybe glue a metal disk atop a plastic switch?]

Drilling thin sheet metal
Drilling thin sheet metal

The steel strip was an RF shield from a junked wireless network card. I bent the edge in a small sheet-metal brake to keep the whole thing rigid and, somewhat to my surprise, the strip remained as flat as I can measure throughout the adventure.

Drilling a good hole through sheet metal is easier when you clamp it between two sacrificial sheets and drill through the whole stack. That keeps the metal from warping and gives you a nice, circular hole; otherwise, you get a rumpled sheet with a triangular hole that’s good for nothing. You can see the bent edge sticking up on the left; the drill center is 4 mm from that side.

The brass tube in the hole and the bent edge constrain the switch to a known position relative to the underlying HBP. The tube fits snugly in the center bolt’s hex socket to set the XY position and the bent edge keeps the whole affair parallel to Y. That allows the upper plate to shift slightly in XY while the switch remains in the same location relative to the TOM’s XY home switches (the bent edge allows a bit of slop in X for the top plate’s hole tolerance)

The Z height, of course, depends only on the altitude and thickness of the top plate, which is exactly what’s being measured relative to the nozzle.

I built it in two stages: epoxy the brass tube, then mount the switch and a second tube as a strain relief around the wire. A layer of Kapton tape insulates the SMD switch terminals from the steel strip; the epoxy sticks well enough to the tape for my present purposes.

Attaching Z-min switch and cable
Attaching Z-min switch and cable

None of the dimensions are critical, although having the whole assembly narrow enough to stay out of the build area seems like a Good Idea. The center line of the platform bolts sits 4 mm inward of the plate edge, to give you an idea of the scale.

The right-side bulldog clamp holds it securely in place, with the wire from the switch threaded beside the platform and wrapped around a solderless lug on the front corner:

Z-minimum switch on build plate
Z-minimum switch on build plate

I think a tiny neodymium magnet embedded in the top plate would work just as well, although it’d tend to suck steel swarf out of the rest of the shop. This is not the right place for a random speck of grit!

The wire connects to a 24 inch CD-ROM audio cable fresh from the usual eBay supplier. I snipped off the end and added a resistor to resemble an MBI mechanical endstop switch: 10 KΩ to +5 V, switch connects the resistor to ground. No LED, alas.

Next, a dab of G-Code to poke the nozzle into the switch…