Caliper Battery

caliper-batteryI had been using an inexpensive digital caliper to take measurements feeding into my Onshape CAD projects. It has proved sufficiently precise for my hobbyist level work but I’d definitely recommend paying for higher quality caliper for professional level work.

One annoying aspect of this caliper is that pushing the on/off button doesn’t really turn it on/off. It’s more akin to on/standby, where the display turns off but some part of the electronics are still on. This is clearly visible by turning the caliper “off” then moving the caliper – it detects the movement and displays comes back to life showing the new reading, which is impossible if the device actually turned off.

The consequence of this feature is that the device is constantly draining the little LR44 battery, which lasts only a few weeks no matter how little the caliper is used.

I decided to solve this problem with a bigger battery. I had set my eyes on the AA battery, which has significantly more capacity and far less expensive than a LR44. When the dimensions didn’t work out, I downsized to AAA battery size.

I didn’t want to make any permanent changes to the original caliper, so no drilling, gluing, or soldering. This meant that I had to:

  1. Find an attachment point: I settled on the thumb wheel, which is held in place by a plastic hook screwed into the main assembly. My project will displace this hook, taking over the thumb wheel retention duty. This allowed a solid connection conveying movement force parallel to the axis of caliper movement. Unfortunately, it doesn’t help hold things in place perpendicular to the axis of caliper movement, which led to…
  2. Grasp the rail: The battery tray needed to grasp the rail both above and below the rail in order to remain aligned to the slide at all times.
  3. Clear the rail end: The tray necessarily reduce the travel range and thus the maximum value I could read on the caliper. An early draft reduced the usable length by the length of the battery, which was unacceptable. Redesigning the battery case reduced the loss to roughly 1cm of range.
  4. Emulate a LR44: Since I didn’t want to solder, something would have to pretend to be a LR44 battery. This took the form of a cylinder with strategically placed wires exposed to make contact with the battery terminals inside the caliper.

AAA batteries are far more plentiful and far less costly than LR44 batteries. The reduced measurement range hasn’t proven to be terribly annoying. Certainly far less annoying than replacing an expensive LR44 battery all the time!

caliper-aaa

This was the most geometrically complex shape I’ve created to date. The dimension requirements were to hold the thumb wheel in place, grasp the rail tightly enough yet still allow sliding motion, and present the cylinder pretending to be a LR44 battery. It took quite a few iterations to get all the pieces positioned relative to each other.

It was also too complex of a shape to be printed directly on the flat bed of a 3D printer. All of my previous projects avoided any need for printed supports by creative positioning, but I couldn’t circumvent the need this time. The support requirements were complex and the automated support generation in Cura proved insufficient. I needed a way to adjust the support to fit the requirements of the project and the capabilities of my specific 3D printer.

Eventually I gave up on Cura and switched to Simplify3D as my slicing software, mostly for the ability to customize the generated supports.

That’s a story for another post.

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