3D Printed Calipers
Accurate to -/+ 2.5 thousandths -
STL files on Thingiverse - https://www.thingiverse.com/thing:187267
Optimized for regular FDM printer - https://www.thingiverse.com/thing:1271651
Note: this was printed on a Stratasys Objet Eden stereolithography (SLA) (laser-cured resin) printer - so may be better than normal printers can do.
- SLA - which is what was used to print the calipers - has 140 micron spot size, and layer heights of 25 to 100 microns. 
- FFF - Lulzbot Taz - 50 micron layer height - 300 micron spot size if 0.3 mm nozzle - 
- FFF - Ultimaker - 20 micron layer height - 250 micron spot size (assumes .25 mm nozzle) 
- In principle, one can print with 0.15 mm nozzles. Smallest from E3D is 0.15 mm. Smallest nozzles can be very difficult to calibrate (bed adhesion?) 
- E3D claims accuracies as good as SLS/SLA - Here is howto from E3D with 0.15 mm - ABS works well, PLA doesn't due to lower flow rate.
- Thus: the calipers should be doable on D3D