If you're like me, you sometimes get something nice when thrifting, only it's missing its battery cover.
That happened recently when I got a TI-84 Plus graphing calculator. These are the kind that many high school kids are pushed to buy. It was in really good condition. It had its overall cover, and didn't have scratches on the LCD screen. There wasn't even any battery corrosion on the terminals (which is kind of standard for things you get when thrifting).
But, it was missing its battery cover.
I went to Thingiverse to see if I could print one, and spent a chunk of time and plastic doing so. However, it didn't really fit correctly. Perhaps it was for the TI-84, not the Plus. Furthermore, it wasn't really a good design for printing, because you'd have to print it sideways. Printing it flat, which is the fast way to do it, would result in having layers under stress.
And, sure enough, after printing it and trying to get it to fit, I broke off one of the two pins, and the latching mechanism.
Instead, I decided to try to laser cut something for it. After much trial and error, I found that a good final combination would be a laser-cut cover, laser-cut hinge pins, and a 3d-printed latch.
Here's the calculator in all its glory.
Lacking the original battery cover, but having the 3d-printed plastic, I did some measurements and was pleased to find that the cover's thickness was close to the thickness of my Michael's basswood pieces (1.8 to 1.9mm). In the end, the basswood cover doesn't protrude much at all.
I got basic X-Y dimensions for the cover from the 3d-printed piece. I put the calculator on the flatbed scanner to figure out the curvature at its hinge end, and designed it in Fusion, and exported the shape to SVG.
I then put in some notches for hinge ells. The notches were made intentionally around 1.65mm. That way, I could design the hinge pieces so they'd fit snugly. The flatbed-scanned image made it possible to see where the holes were, and how big they were.
On the latch end, I tried various designs, but ended up making a notch to hold the 3d-printed latch. I did experiment along the way, and found that Loctite Super Glue (CA) actually bonds PET-G well to the basswood. But I chose to use a latch design that would wrap around the edge of the notch.
This is what the profile of the hinges looks like. It's teeny when laser cut, but holds on well once it's all glued. I made extras in case any didn't cut properly or got lost.
This is what the latch looks like. One one side, there's a 1.8mm gap (larger, actually, to account for shrinkage) that holds to the notch area. The other side has a 1.5mm gap where the latch clicks onto the calculator frame.
Here is a closer view of the hinge pins, glued to the cover.
And here is a picture of the ugly, 3d-printed latch. I don't have my settings right for printing this well, but it worked out ok. I printed it vertically, meaning that the curvy profile sits flat on the print bed, and it rises up 18mm or so. I used PET-G to print it for flexibility, and used a 2.0mm brim to keep it steady.
I haven't glued it at all in this picture. It's just friction-/compression-fit onto the wood.
And here's the end result. The panel snicks onto the back properly with a satisfying click. It took about a dozen to get this right, with most of those going to re-prints of the hinge, since it's really hard to take any measurements for it. I also had a warpy piece of basswood near the end of the project, so I just laser-cut a new one after checking to make sure the wood was mostly flat.
(Side view to show how it doesn't protrude from the calculator bottom.)
