Part Interfaces
A crucial consideration for any design is how it connects to surrounding parts in an assembly. With 3D printing, your options for part interfaces are wildly diverse, thanks to the geometric complexity the technology allows for no additional cost. In this guide, we've compiled some common ways to create part interfaces with 3D printing. Note that this cannot possibly be a comprehensive list; the list of possible configurations for interfaces is infinite. However, there are some common methods which can be adapted and built upon to perfectly complement your design.
A great example of a final product, 3D printed. This is the Multi Material Unit 2S from Prusa Research. Note the PTFE tubes lining the part, the threaded insert for the pneumatic tube, and nuts added in slots to provide robust threads.
Holes
Never drill holes into 3D printed parts during post-processing. Model them in to the part, and not just by using hole callouts; these have to actually be a part of the mesh. The reason why is because 3D prints are typically semi-hollow, and drilling into the part will not yield a strong hole as a modeled one would. As a part of prototyping, drilling holes can be a great way to quickly test new features without starting a new print, but you should never rely on them for your final print.
Holes are discussed at greater length in our page on tolerances, where we discuss how to design for different types of fit.
Inserts
3D printing in plastic typically means that threaded holes lack the strength that is common with traditionally machined parts. One simple, effective way to get strong threads in your part is by using threaded inserts. For example, this PTFE coupler part I designed requires that a pneumatic fitting be attached to a 3D printed part. The M5 threads involved are far too small to be 3D printed robustly, so the part has been designed with a hole for a brass insert to be added via soldering iron in post-processing.
In this specific example, the M5 threaded inserts cost $18 for 40 fittings. For less than 50¢ apiece, these inserts have turned a cheap 3D printed plastic part (<40¢ of plastic) into one that has lasted multiple years of use on a 3D printer. This is an excellent example of an end-use part, quickly and cheaply printed for less than $5 total.
We recommend finding the measurements of the threaded insert and running a few test prints. For this particular print, the outer diameter was measured as 6.50mm, so I ran a test print with a few holes of different diameters. I then added an insert to each hole, and used a machine screw to manually test the pull strength of the insert. The best result came out at 6.80mm diameter. We recommend performing the same test before committing to a larger print. For more information on inserts, Stefan of CNC Kitchen did an excellent analysis on his blog.
Nuts
Using machine screws and nuts are a great way to hold your parts together. This is by far my most frequently used method, as it is cheap, robust, and doesn't require you to throw away the hardware in case the print needs a redesign. It also is incredibly quick and easy to implement: just design a slot where the nut can be added to the part. You're done--easy, robust, removable hardware added to a 3D print!
In these two examples, threads are needed on a flat face. A simple hex slot is added. In the second image, support material is avoided using bridge-based overhang prevention, a cool technique to reduce time wasted in post-processing.
For this situation, threads were needed inside this part. Another hex slot is cut into the part, this time with a path cut so the nut can be inserted during assembly.
Zip Ties
Zip ties are super cheap, and ridiculously simple to design for. They can range from simple cable management for electronics enclosures to hardware for final products--seriously. When used properly, they can be an excellent low- to mid-strength fastener for a wide range of applications. Simply cut a groove into the interior of the part as shown.
Both parts shown above use zip ties as fasteners. The left and center design show an arc-shaped slot geometry used for the zip tie path, while the right uses a rectangular slot through the entire part. This fastener method is more than enough for a <50 micron spec Y axis on the 3D printer shown, a Prusa i3 MK2S, when designed properly.