Incorporating Screw Threads Into FDM Parts
Screw threads are one of the most common components used for fastening parts in an assembly or attaching pipe tubing together. Here are five ways to incorporate screw threads into Fused Deposition Modeling parts, followed by some useful applications.
Threads directly from the FDM machine
Depending on the pitch and helical profile, it may be possible to design your desired thread right into the CAD file. Typically, large pitched threads on parts used for functional prototyping or fit testing can come right from the FDM machine, as long as the forces they experience are relatively low.
Drilling and tapping
Taps can be created in FDM parts by drilling and tapping a hole either directly into the center of the raster fill, or into a designated area made just for this post-process. I suggest bulking up the drilling area so it mimics a solid block of plastic; threads will hold together better throughout the layers.
Heli-coil inserts provide a sturdy alternative to the methods above. They’re great at dispersing the extra stress brought on by the standard tapered pipe thread and provide a good medium to seal other standard pipe components to when sealing is important.
Heat stakes or ultrasonic weld inserts
These inserts come in various shapes and sizes, and with features that protect against pull-out or spin-out failures. Using energy, these inserts are melted into the plastic, which forms a solid block around the insert, adding strength and durability. These are the preferred methods of creating screw threads in FDM parts.
Press-fit inserts are easy to install and are good for tensile loads. They can withstand some torsional loads, but tend to spin out at high torsional stresses. The first three methods are great ways to increase functionality of FDM parts, especially for rapid prototyping. They can help analyze designs and determine the points of largest stress.
For digital manufacturing, the benefits of inserts become apparent. Since some high-performance inserts include a taper and require a customized bit for production, FDM becomes a great bridge-to-tooling and low-volume-production asset.
When a part requires a particular thread and a few types of inserts must be tested before going to production, the ability to adjust the pilot hole on the fly becomes valuable. Instead of making multiple specialized bits to test many inserts, create the pilot hole directly on the FDM machine and test with values that mimic those of injection molded parts. This will prove cost-effective after multiple design iterations.
It’s worth noting that the study of how inserts perform in FDM parts is ongoing. As more and more users across industries experiment with inserts, I’d love to hear about your experiences.