At NK Labs, we frequently make use of 3D printing for proving concepts and early prototyping. Cases often arise where we need to connect two of these parts together with bolts. However, not every means of incorporating threads is equal. There are often tradeoffs, and some methods are better in some circumstances than others. Here is a quick overview of some common methods for threading 3D-printed parts.
Tapped Threads
Perhaps the most obvious approach, tapped threads can sometimes work well depending on the type of printer, the tap diameter and pitch, and the required strength. Overall, there are more robust options that we would recommend instead of tapping threads in 3D printed parts, but this method can be sufficient for certain applications.
We have had better luck tapping parts made on the Markforged FDM printer than those from our Formlabs Form 2 SLA printers. As a general rule, threads with a larger pitch are better than fine threads. Additionally, printed parts are weaker than conventional plastic parts, so their threads cannot be expected to withstand the same load as the threads on, for example, an injection molded part of the same size. With this in mind, tapped threads are best done by printing an undersized tap hole, then manually drilling out the hole to the proper tap hole size, and then tapping. Printing an undersized hole and drilling it out is useful as the printer may not print a sufficiently accurate, circular hole of the proper size.
Screw-to-Expand Inserts
These inserts can work, so long as the bore they are pressed into has a substantial wall thickness, but they are not ideal. They frequently end up clamping down on the bolt, making installation and removal more challenging and frustrating. This method also benefits from under-sizing holes and then drilling them out to the proper diameter.
Heat-Set Inserts
Heat-set inserts are similar to screw-to-expand inserts, but they always maintain a rigid shape, which allows them to be easier to install a screw. They work by heating the surrounding plastic material and allowing the plastic to conform around the insert’s textured sidewalls. This embeds the insert securely within the printed part. For the majority of applications, heat-set inserts are our favorite method to add threads to 3D prints.
If possible, inserts with flanges installed on the rear face of a part are even better. These inserts are installed on the underside of the part which needs threading. The flange distributes the load being applied by the tightened bolt, leading to a stronger connection between the mating parts.
Cutouts for Nuts
One of our favorite and most reliable methods for adding threads into a printed part is simply by designing a hexagonal cutout into the part for a standard nut to fit in. This does a good job of distributing load, and using a steel nut further improves strength over other methods. The geometry captures the nut so you do not have to use a wrench or socket to tighten/loosen it. Nuts are also readily-available, tried-and-true parts which go nicely with the concept of rapid prototyping.
Printed Threads
In the vast majority of cases, we do not recommend printing conventional threads directly on the part as they are too fine of details to print well, and they are prone to failure. They can also use a lot of resources while modeling, which can cause files to lag and perform poorly. This method can be appropriate if the threads are very large and coarse.
Other Methods to Connect Printed Parts
Self-Tapping Screws
These can work well if the parts only need to be connected once. Repeated installation and removal will damage the newly-threaded part.
Magnets
Can easily be glued, pressed, or otherwise captured in printed parts. These are great for parts that need to be disconnected frequently.
Snap Fits
Like other plastic parts, printed parts can have snaps built into them. Typically, these are better for a one-time installation as repeated removal can damage the snap feature.
NK Labs, LLC is a product development engineering firm in Cambridge, MA. We have broad experience in the design of consumer electronics, robotics, and other innovative products. Please consider us for your next engineering design project!