Dread staring at hundreds of tiny boxes at your local hardware store? Learn the basics of how to successfully design and 3D print screws and threads!
First things first: What’s the difference between a screw and a thread?
A screw is a fastening element used to form a joint that can be later dismantled, while a thread is the main fastening feature of a screw. That said, threads are not only used for screws; they also exist on pipes, linear drives, worm gears, and many other devices.
The common common feature among all threads is the way they are formed. Every thread is a continuous helical groove of a specific cross-section produced on the exterior or interior of a cylindrical surface.
In most cases, the cross-section, or form, is triangular or trapezoidal. Triangle thread forms are mostly used for fasteners (screws), while trapezoid thread forms, a variation of square thread forms, are used for power transmission and linear drives on lead screws. To make things simpler, this article discusses only triangular-shaped threads, but everything applies to both types.
(Image: Common thread forms include square, triangular, and trapezoidal. Source: Anime_Edu – Civil Engineering Videos / YouTube)
A further level of categorization distinguishes metric threads from inch threads. The former are mostly used in Europe and Asia, while the latter are used in America and UK. To the untrained eye, they look same, but a difference exists in the shape of the triangle and the pitch of the helix curve.
In the following, we’ll take a look at the basics of designing and 3D printing screws and threads.
Before starting to design threads, there are a few terms and concepts you should be familiar with.
External or internal thread: An external or male thread extends from a cylindrical surface. An internal or female thread is the exact negative of an external thread, meaning it is carved into a negative cylindrical surface. Bolts, for example, employ external threads, while nuts use internal threads.
Thread axis: The line running trough the center of the cylinder on which the thread is formed.
Root: The bottom of the groove running around the thread body.
Crest: The highest point of the thread profile.
Major diameter: The diameter of the cylinder that encircles the crest of the external thread or the root of the internal thread. This cylinder is concentric to the thread axis.
Minor diameter: The diameter of the cylinder that encircles the root of the thread in an external thread or the crest in an internal thread. This cylinder is concentric to the thread axis and the major diameter. The minor diameter is also known as the drill size diameter when referring to inner threads.
Pitch: The distance between equivalent points on adjacent threads. For example, the distance between two neighboring crests of a triangular thread.
Metric threads: The ‘M’ designation of a metric thread indicates the nominal outer diameter of a thread in millimeters. For example, the M5 thread has a nominal outer diameter of 5 mm. In an external thread, the nominal outer diameter is equivalent to the major diameter. In an internal thread, the nominal outer diameter can be determined by measuring the minor diameter and consulting a metric thread table.
Inch threads: Inch threads are designated using a number of standards, including the Unified Thread Standard (UTS), which primarily names standard thread sizes with numbers (e.g. #4). The two most important measurements in the UTS are the major or minor diameter in external or internal threads, respectively, and the threads per inch (TPI).
Here, we demonstrate the process of designing external and internal threads using Fusion 360, which provides a simplified thread generation function.
Other CAD programs have tools of varying degrees of similarity. The important thing is to understand the basics, as presented in the previous section. With this knowledge, it should be possible to use any capable modeling tool, manipulating models and providing the necessary values to generate the desired threads.
Let’s start with the external thread of a bolt.
And, that’s it. You have your external thread! To make it a proper bolt, you’ll have to attach it to a head of your liking.
Now let’s design the nut with an internal thread.
There you go. Your first threads are ready to print!
It may seem like a simple thing to do, but printing threads isn’t always easy, especially if you want small diameters.
Suppose you’re using 0.4-mm nozzle and a 0.2-mm layer height. With this setup, the smallest pitch you’ll be able to print will likely be around 0.5 mm (give or take 0.1 mm). Such a pitch is good for an M3 thread and isn’t a big problem if you’re trying to print an internal thread in a relatively large part. That’s because your thread will have enough time to cool down while the nozzle is elsewhere.
Things get interesting if you need an external thread on a screw or a bolt, for example. In this case, there’s nowhere else for your nozzle to go, meaning you’ll probably need some extra cooling. Test your printer before you decide to print many thin external threads.
In general, it’s a good idea to try printing a thread test. This is the best way to test your 3D printer’s capabilities.
The following are some general guidelines for setting up your printer for 3D printing threads.
Even if your first test isn’t successful, there’s still hope! Here are some final words of wisdom:
Feature image source: Pinshape / CreativeTools
License: The text of "3D Printing Threads and Screws – All You Need to Know" by All3DP is licensed under a Creative Commons Attribution 4.0 International License.
Subscribe to updates from All3DP
You are subscribed to updates from All3DP
You can’t subscribe to updates from All3DP. Learn more…