From the LoCoRo documentation …
3D printers are great tools but they are not 100% accurate. Each printer will have small variances. These variances have little effect on esthetic parts but even 0.1mm can prevent parts from mechanically interconnecting. The tolerances are especially important when mating a 3D printed part with a commercially manufactured part. Examples included matching the mounting hole of a Raspberry Pi, fitting a servo horn into a wheel, or attaching an RC servo to a chassis.
The LoCoRo project provides the source CAD design files so users can make changes – whether to add features or adjust for the variances of their particular printer.
The interlocking block test model – available by clicking the image – may be used to help calibrate the 3D printer slicing software. The model consists of two parts which – when printed accurately – interlock snuggly. Each half is 20mm x 30mm x 10mm. The three rails are 5mm wide while the two rails are 7.5mm wide.
If the printed parts do not fit together, there are two options. First, attempt to adjust settings in the slicer software. As a final solution, make adjustments to the model. If the latter solution is used, similar adjustments will be required for other models.
Tip: Once the interlocking block fit, write the slicer adjustments directly onto the blocks and save them for future reference.