Basics of Model Design for 3D printing

Fine Details
The smallest feature on a model you can make is double the size of the thread width. The thread width is calculated by finding out the diameter of the 3D printed nozzle. Typically printers have a 0.4mm or 0.5mm nozzle. This meaning that the smallest circle that the printer can draw is the size of two thread widths. E.g. A 0.4mm nozzle produces a 0.8mm circle.
Walls along the X and Y axis should not be thinner than 0.4mm, otherwise nothing will print. Even then the wall will be very unstable. It is only possible to print parts less that 0.4mm on the x axis of high spec printers.

Tolerances
It is advisable to do your own tests to get the optimum fit for interlocking parts.
General approximations:
0.2mm offset for a tight fit, for press fits or connectors
0.4mm offset for loose fits, such as hinges.

Factor in Support Material
Any part of an object which overhangs by more than 45degrees will require support material underneath of it. Support material can damage the outside of your model and is time consuming to remove. Try to design pieces to require as little support as possible. You can split up pieces of a print to be reassembled later on.
Using a lot of support material wastes money as it is removed and binned afterwards. Plus the product will take longer to print. On top of this you will also spend a lot of time cleaning off the support material.
Support also adds extra complexity to a model which results in a higher risk of errors when printing.

Design to Not Use Support Material
Take note of bridges in designs. Imagine you are trying to print the letter H. The middle section stretching between the two edges is the bridge. Typically, if a bridge is under 36mm you will get drooping of 0-0.5mm.
Longer bridges of 36-60 will have drooping of 0.5-2mm.
However these are only rough estimates and depend on you printer, material and print settings.
If you look at bridges in more complicated prints you can factor in the distance apart to try and prevent the need for support material.
Say you were trying to print the letter T, he top of the letter will undoubtedly create errors in the drawing as there is nothing for the material to hold onto. Yet if you changed the orientation of this print, so that the letter was lay down you would no longer have this problem. You don’t always need to make changes to your model to improve the results.

Orientate the Model for Best Resolution
As the prints resolution along the x and y axis are predetermined by the thread width you can only get finer detail along the Z axis. Meaning that finer details should be facing up towards the printer head to be able to print in layers finer than 0.4mm.

Questions to ask yourself
Which side will be seen? Are you making multiples? Will it be easy to clean? Will you be able to remove support?

Fixing Digital Models
Some software might not render your file to a high enough quality. This typically results in tiny holes in the design where parts of the model have not connected together properly. You can also get overlapping walls which can cause problems when it comes to printing.
There is free software on the market which allow you to check and repair your model for errors and holes. Examples of this are, Meshlab, Meshmixer, and Netfabb Basic. These will either check or repair your file for errors or holes in the design.

Stress Areas
Consider adding a chamfer onto the points where different planes meet. For example if you are printing a model of a house, the 90degree angle where the wall and floor meet needs a small chamfer where they meet. This will relieve stress at the point where they join. If dropped this is where they are most likely to break.