Pro tip: You can copy/paste connections from the the Cart Bot svg file and reapply them to your own robot! After I made my first robot, I reused connections frequently in my subsequent designs. Make sure to design your pieces to account for that plus the 0.15mm that'll be cut away from all sides due to the kerf (or whatever the kerf is for your laser cutter). I found that having the tab pieces about 0.2mm wider than the slot results in the best fit.

This means the tab will be 0.2mm wider than the slot, leading to a nice snug fit. The actual tab size will be 15.1mm and the slot will be 14.9mm. When you account for the kerf (+0.3mm to the widths). To ensure a snug fit, the tab is 15.4mm and the slot is 14.6mm. The image above is taken from the Cart Bot svg file. The difference is 0.6mm, which is big enough to prevent the pieces from fitting snugly. So the tab is actually 9.7mm (0.15mm taken from both sides of the width = 0.3mm lost) and the slot is 10.3mm wide. Why? Because the kerf of the laser cutter will remove an extra 0.15mm from all sides.

If you design the tab and slot to both be 10mm, then they'll fit loosely and fall apart. Let's say you want a 10mm tab to fit snugly into a 10mm slot. To get pieces to fit snugly, you need to design the connections so that one part is slightly larger than the other while also accounting for the width that will be lost in the kerf.

Slot machine vector file