The design intent was two-fold:
To have useful Imperial & Metric models for checking thread-size on loose screws, and to have readily on hand design guides and visual aids when designing for heat inserts and for gauging thickness of materials.
These models use heat inserts made for plastic. I purchased mine through Mcmaster Carr but you may be able to find them through another supplier. I am not affiliated with Mcmaster in any way but love using their website for things like this, as they have always supplied me with quality products, and always have the design data to use their products right on the website. I have included a PDF example of one of the inserts design data.
Heat inserts are used when you need a permanent brass thread in plastic models. They are small inserts set inside tapered holes designed into your part. The inserts are then heated using a standard soldering iron tip which melts the surrounding plastic. The user then guides the insert with the soldering iron as it sinks to the preferred depth and the plastic melts in around the textured insert forever locking it in place.
The numbers on the 3d printed models are:
TOP - Thread size of heat insert
BOTTOM - Minimum thickness needed for that insert and height of that step
LEFT - small diameter for the insert tapered hole
RIGHT - larger diameter for the insert tapered hole
(Note: I did not purchase all the heat insert models available from that supplier, just the shorter ones I felt I needed. That is why only one side of the pictured models have inserts and the other side is still showing tapered holes.)
Edit: 05/21/16 13:56 CST
Added in Metric_Rev1 model, due to forgetting to change the Inventor export stl settings from imperial to metric. Thus the first version will need to be scaled up by a factor of 25.4 or else it will be miniscule on your print bed. Simplify3d will recognize this scaling factor and ask if you want to convert to metric and it works fine. If your slicer does not allow scaling or conversion, use the Rev1 model. Thanks for the catch reibuehl. :)