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3D Printing Threads and Screws – All You Need to Know

by Miroslav SarcevicJun 30, 2019

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!

3D Printing Threads and Screws Screws and Threads – What Are They?

Nuts and bolts are typically made with triangle threads.
Nuts and bolts are typically made with triangle threads. Source: Melfast

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.

3D Printing Threads and Screws Things You Should Know

A figure illustrating thread terminology.
A figure illustrating thread terminology. Source: Kelston Actuation

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).

3D Printing Threads and Screws Designing Threads

Threads in Fusion 360.
Threads in Fusion 360. Source: Miroslav Sarcevic / All3DP

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.

External Thread

  1. Draw a circle, where the diameter is the desired thread’s major diameter.
  2. Create a cylinder by extruding the circle to the desired thread’s length.
  3. Go to “Create” and select the “Thread” option.
  4. Select the cylinder you just created. Make sure that the “Modeled” checkbox is checked. Set the thread type and other thread parameters. Hit “OK”.

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.

Internal Thread

  1. Draw a hexagon. For the purposes of this tutorial, just make sure it’s bigger than the thread you want to design.
  2. Extrude it to the desired height.
  3. Make a hole in the center by selecting the “Hole” option in the “Create” menu. The hole diameter should be the desired thread’s major diameter.
  4. Select the internal surface of the hole you just created, go to “Create”, and choose the “Thread” option.
  5. Remember to check the “Modeled” option. Set the thread size and other parameters. Click “OK”.

There you go. Your first threads are ready to print!

3D Printing Threads and Screws How Good is Your Printer?

A 3D printed thread test.
A 3D printed thread test. Source: GoEngineer

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.

3D Printing Threads and Screws Final Thoughts

Cutting a thread with a tap wrench.
Cutting a thread with a tap wrench. Source: 3D Hubs

Even if your first test isn’t successful, there’s still hope! Here are some final words of wisdom:

  • Even if you manage to print a nice-looking external thread smaller than M6 (6 mm in diameter), think twice before using it to carry any weight. Due its small diameter and the nature of 3D printing, this size of thread is best for visual models only. If it has to be a functional part, consider a different design.
  • Internal threads smaller than 4 mm in diameter have small pitch, making them difficult to print. At this size, think about printing a blank hole and cutting the thread with a tap wrench. Either way, it’s always good practice to clean a thread before use, whether it’s 3D printed or cut.
  • Some materials shrink more than others. Before you start printing large parts with threads, make some small samples to check the thread dimensions. You may end up with a tight thread when printing internal threads or a very loose thread when printing external threads.

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.