MMU3 vs. Cart Coin Calibrators

The last time around, I used Cart Coins to verify platform alignment (a.k.a. “leveling”) on the Makergear M2. The Prusa MK4 does mesh probing to ensure accurate alignment, so these new Cart Coins exercised the MMU3 and gave me some giveaways for a recent dinner:

The design, such as it is, mashes a PNG found on the InterWebs with a few go-fast stripes added in LightBurn to balance the layout inside a circle:

The motivations for LightBurn:

• It’s convenient
• TroCraft Eco is within 0.1 mm of the proper thickness
• Laser-cut coins proceed with great speed

Normally you’d export the finished layout as an SVG, but OpenSCAD ignores “holes” within shapes, so I exported it as a PNG to serve as a binary height map:

• Import the PNG into OpenSCAD using surface()
• Resize it to 20 mm wide and 1.7 mm tall
• Knock it out of a 24 mm OD × 1.6 mm tall cylinder (which is why the extra 0.1 mm)
• Add the PNG again as a separate 1.6 mm object to refill the hole

Whereupon out pops a solid model:

Export that as a 3mf file to keep the two objects aligned, import it into PrusaSlicer, then get multi-material on it:

There’s a fourth group with different colors in hiding. I printed 12 identical coins at a time, mostly so I could keep track of what was happening, and it ended well enough.

The black coins with the translucent retina-burn orange cart look surprisingly good.

But this is way faster:

They’re the size of a US quarter, because that’s what unlocks shopping carts around here. Feel free to tweak the parameters for your locale.

The OpenSCAD source code is almost a one-liner:

difference() {
    cylinder(d=24.0,h=1.6);

    resize([20.0,0,1.7],auto=true)
        linear_extrude(height=1,convexity=10)
           projection(cut=true)
            surface("/mnt/bulkdata/Project Files/Prusa Mk4/Models/Cart Coin/Cart Coin layout.png",
                    center=true,invert=true);
}

    color("Black")
        resize([20.0,0,1.6],auto=true)
            linear_extrude(height=1,convexity=10)
               projection(cut=true)
                surface("/mnt/bulkdata/Project Files/Prusa Mk4/Models/Cart Coin/Cart Coin layout.png",
                        center=true,invert=true);

Use them responsibly, OK?