Moire Vernier Radius Gauge

I couldn't remember initially where I'd gotten the idea to use the moire effect for motion amplification in this way, but I think a lot of it came from this clock project: https://www.thingiverse.com/thing:5173948

This is a radius gauge; it measures the diagonal cut off from a 90 degree corner by a quarter-circle, and scales that by the appropriate factor to read out the radius of the curve. I was inspired to make this after reddit user VVJ21 shared a model similar to this for chamfers, and a model for measuring radius, but that used two racks and a pinion gear to amplify the motion of the probe to make it readable. I was struck that the chamfer mechanism was simpler, smaller, and more elegant, and asked if that mechanism would work for the radius gauge. He pointed out that the scaling factor involved would require marks too fine for a 3d printer to fit them.  So I thought, well, a vernier scale would solve this, but I think a lot of people don't know how to read a vernier scale. Then I thought, I wonder if you could make a direct-read version of a vernier scale using a moire effect between the sliding parts. This is the result of that experiment.

You read the tens place of the measurement from the upper scale. So, if it's between 0 and 1, the tens place is a 0; if it's between 2 and 3, the tens place is a 2. You read the ones place of the measurement from the lower slots; the slot that shows a line is the reading. If the line shows in two slots, the reading is between the those two values. It's not the most accurate tool, but it's accurate to about +- 1mm.

I made a brief video that illustrates how the lower line appears to move much faster than the slider itself:  https://i.imgur.com/jnDpWgj.mp4

This is a larger gallery of images of the device, including a (very similar) video, but using a slightly earlier version of the design:  https://imgur.com/gallery/moire-vernier-radius-gauge-design-3d-printing-ajy0GBg

I think the tool is moderately useful for checking the radius of fillets for 3d design, but I think it's mostly interesting as an illustration of the idea behind both moire effects and the vernier scale, both of which I think are a little bit underappreciated.  :)

I may come up with a revised form of the thumb slider on this, maybe one that mounts on the side, to make it a little bit easier to use, but as this kind of blew up when I posted my early prototype, I thought I'd better get a version up for people to start playing with.

The “slider” and “housing” parts are multi-object prints; I printed them using the “single extruder multimaterial” settings in Prusa slicer, with two extruders. It only requires three filament swaps (start with the line color loaded). But someone asked if I could make one that wasn't multimaterial, so I added the “single color with indented marks” versions for that purpose; on these the lines and numbers and the arrow will just be indented, so you can color them in with an ultra fine point sharpie or something similar. I do think the multi material version is pretty manageable even with a single color printer, but options are always nice.

Tips for printing: I printed this in two contrasting colors of PETG, using Prusa Slicer's “Single Extruder Multi Material” setting. There's info about this online, and I don't think my settings are the best; I had trouble with it dragging leaking material across the printed layer. For what it's worth, though, my “tool change” gcode is this:

{if layer_num >= 0}
G91  ; relative positioning
G0 Z5 ; raise toolhead 5mm
G90 ; back to absolute positioning
G0 X0 Y0 ; go to front/left
M600 ; change to filament for extruder {next_extruder + 1}
{endif}

Raising it 5mm before going to the home position avoided crossing the print before the tool change, but still crossed it afterwards because it lowers itself back down to resume printing, so it's definitely not perfect. You can research this aspect yourself.

I also had trouble with less-than-perfect bed adhesion; since it's drawing single-width extrusion lines spaced well apart before switching colors to fill in the gaps, you need basically perfect adhesion. I ended up raising the bed temperature by 10 degrees and the extrusion temperature up to 245 for the first layer just to make sure it all stuck. I would imagine similar adjustments would help with PLA, but at lower absolute temperatures.

Support material: For the most part, this doesn't require support material, but it does print better if you enforce supports for the tiny segment that the probe prong sticks through. If you forget, though, you can just clean it up afterwards with a small file.  I fixed the orientation of the thumb stick, but it does require some supports so it'll print well. Organic / tree supports work well for that.