Fastners for 3D Printing: Part 0 of n

There are a few 3d printers on the market that are capable of producing solid parts with the stiffness and durability needed for making robots.

When building a robot, these parts will often require some sort of fasteners to hold them together. It is possible to build them in such a way to achieve a snap-fit or be glued together and I suppose you could rivet them together, but often for the sake of easy disassembly and repair screws are the preferred solution. There are a few different techniques and options for using screws to hold together 3D printed parts made with fused deposition modeling of plastics like ABS.

• Tapping Threads for Machine Screws
If you print small pilot holes of about 1mm and drill them out to the correct size you can tap them fairly successfully. If you don't drill them out then the hole will usually fall out, try it you will see what I mean. A drop of CA glue after drilling and before tapping will also help increase the strength of the hole and the thread forms. Make sure to let it dry before tapping. The tapped threads will be relatively weak when compared to similar threads in metal but they are often good enough for basic prototypes. Tapping threads is however a time consuming process when done manually, and you will soon tire of this method.
• Brass Inserts
Brass inserts are usually used with injection molded parts but they probably could be glued into a 3D printed part. Unfortunately, in most cases, the additional strength provided by the insert will simply move the failure from the screw threads to the boss. The brass inserts also add an additional cost to the project.
• Nuts
Using nuts may also work depending on your application, however they can be cumbersome to install often requiring two tools at once to tighten. Regular nuts also have a tendency to loosen if a threadlocker is not used.
• Wood Screws and Sheet Metal Screws
After a little bit of trial and error both wood screws and sheet metal screws may make effective fasteners for plastic. Wood screws will probably work better for softer thermoplastics like ABS and sheet metal screws may work in more brittle plastics like acrylic. Sizing the screw holes correctly may be difficult and I have not really had a chance to extensively test either approach yet.
• Plastic Thread Forming Screws
If there are screws for metal and wood then logically there should be screws made specifically for plastic. I have ordered some and will post the results once testing is completed.

Also, have you ever wondered, what is the point of flat washers?