MPCNC: Autolevel Probe, Tactile Switch Edition

So I intended to shrink the Autolevel probe with 1/8 inch drill rod and a tactile membrane switch:

MPCNC - Simple Z probe - pogo tactile switch

MPCNC – Simple Z probe – pogo tactile switch

Unfortunately, it didn’t work nearly as well as I expected, because the switch membrane requires slightly less than the 180 g of pressure that pushes the P100 pogo pin entirely into its housing, leaving no overtravel worth mentioning. The membrane switch mechanism itself has much less than 1 mm of overtravel after the dome snaps, which left me with an uncomfortable feeling of impending doom.

I managed to figure that out before completely assembling the thing, saving me a bit of time.

The end of the pogo pin initially sported a dot of epoxy to spread the load over the switch dome:

Pogo pin with epoxy switch-pusher drop

Pogo pin with epoxy switch-pusher drop

I dismantled the pogo pin to see whether I could substitute a more forceful spring how it worked. As expected, a teeny spring drives the probe up against a trio of indentations in the brass housing. I didn’t expect the probe to have such an intricate shape, but it’s obvious in retrospect.

The OpenSCAD code for the housing required minimal tweakage from the larger version, so it’s not worth immortalizing.



  1. #1 by Jason Doege on 2018-02-23 - 09:31

    Funny thing about pogo pins and force. I used to work for an automated test-equipment startup and we were building a tester that had 512 signals plus associated grounds. It turns out that 1024 x 180g is a lot of g! All bearing against a PCB. This, of course required strong stiffening frames and rather robust clamping mechanisms that nevertheless could be operated by your typical pasty lab tech (me) or test floor operator.

    It’s funny how forces start to add up in mechanical assemblies when you aren’t paying attention. Another time I was working on building plates to cover holes in a vacuum chamber for an e-beam microscope. The e-beam chamber had a large rectangular top and the e-gun fired up at a hole in that top that the plate would seal up and had to be quite close to the microprocessors we would put through that hole (it also could tell what charge was on the tiny traces through electron back-scatter). Anyway, having built a new plate to spec for a new CPU and providing all the connectivity through it and socketing and such, we found the vacuum being compromised because the gun kept crashing into the CPU which should not have been possible, mathematically. I slept on it and woke up with an epiphany which was the possibility that Schlumberger hadn’t taken into account deflection due to atmospheric pressure. The top of the chamber was 14″x14″ which, when you multiply it up with atmospheric pressure, amounted to an elephant standing on top of it. I put a straight edge across the top and measured something like .2″ deflection which was more than enough to cause contact inside. It’s actually pretty amazing that anything ever worked on that thing. My subsequent conversation with Schlumberger was amusing.

    • #2 by Ed on 2018-02-23 - 19:31

      A raging herd of pinpricks: color me impressed.

      I forwarded your story to our Young Engineer: sometime she’ll be tasked with the same thing and, if she can skip the mistake, it’ll be All Good. Thanks!

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