Accurate measuring tools are essential for successful 3D printing of anything that mates with anything else. While these calipers aren't as accurate as the real McCoy, they are handy when your Mitutoyo's are hiding elsewhere. They have both external and internal measuring jaws for versatility.

The main reading is taken off of the scale on the body (in 2mm increments), and to that, you add a secondary reading from the screw dial (0 - 2mm in 0.05mm increments). Realistically, it's probably only accurate to about 0.1mm, but that's still way better than a ruler.

The measuring jaw is moved by rotating the dial/nut. The secondary reading is where the fiducial line on the movable slider lines up with the marks on the dial. But because the threads have some backlash, you need separate fiducial lines for external (screwing in) vs. internal (screwing out) measurements.

To set the external fiducial line, screw the calipers all the way shut so that the jaws are touching (snug but not tight). The 0.0 line on the dial should end up somewhere towards the upper half of the blank space on the slider. Use an ultrafine-tip permanent marker to mark the location of the 0.0 line, as shown in the last photo. For the internal fiducial line, hold the jaws shut with one hand while trying the unscrew the dial. The play in the threads will allow it to move about 30 degrees before it stops. Mark another line at the new position of the 0.0 line.

Printing
I printed this with plain PLA and 0.2mm layers. Use a 0.4mm nozzle or smaller. All parts print without supports in the orientations shown. A carbon fiber filled filament might be an even better choice if you have some. And for increased visibility of the main scale, try doing a color change for the top layer of the body.

Assembly
First slide the dial onto the slider with the scale facing the jaws.

Next, flex the clip piece over the back end of the slider so that the little internal tabs fit into the slots in the slider. (At some point you may want to put a dot of superglue on the clip to keep it from popping off, but leave it as-is until you know everything is working.)

Slide the slider onto the two legs of the body and start screwing the dial along the body to advance the slider all the way to the zero position. The threads will be a little rough initially, but working the dial back & forth will loosen things up.

Finally place the end cap over the ends of the legs of the body and tap it into place. It should be a pretty snug fit. (If not, more superglue.)

After the threads have loosened up, apply a little lubricant to the threads to make the motion completely smooth. Any rough spots will make it difficult to judge exactly when the jaws have closed the object you are measuring.

Lastly, if you've got a steady hand, you can color in the raised markings on the dial to make it easier to read.