How do you assemble 3D-printed parts? Practical tips on threads & screws for 3D printing

Threaded fasteners, such as screws and inserts, are a popular method for securing 3D-printed parts together. Threaded fasteners allow quick assembly and disassembly, granted the threading has been designed and printed properly, and provide robust connectivity so parts don’t come apart easily.

This article covers the optimal threaded fastening techniques to apply when dealing with 3D-printed parts and examines the methodology behind implementing each of them.

To learn more about the different types of threads, especially if you're also interested in CNC machining custom parts, read our extensive guide to threads.

What are threaded fasteners for 3D printing?

As a rule of thumb the minimum wall thickness around a thread should match the diameter of the fastener (e.g. an M5 fastener requires a minimum of 5mm wall thickness around the threaded hole). If the wall thickness is too low, parts can bulge and distort due to the added stress and in some cases (particularly with FDM) delamination or fracturing can occur.

The table below introduces the threaded fastening methods that are best suited to 3D printing.

What are inserts for 3D print assemblies?

The two types of inserts that are best suited for 3D printing are heat-set inserts and tap-in inserts. Inserts provide strong metal-on-metal contact and are very easy to install. An accurate pilot hole is required, so drilling is recommended before installation.

Methodology for heat-set inserts

1. Align the insert with the pre-printed or drilled hole (for hole sizes refer to the insert manufacturer's recommendations)

2. Insert a soldering iron into the insert, heating it and the surrounding material up (avoid overheating and melting the surrounding material)

3. Slowly apply pressure, pushing the insert down into the hole to the desired depth.

Methodology for tap-in inserts 

1. Align the insert with the pre-printed or drilled hole (for hole sizes refer to the insert manufacturer's recommendations)

2. Using a hammer gently tap the insert down into the hole to the desired depth.

What is an embedded nut in 3D printing?

Another method of securing 3D printed components together is to embed a nut into the component via a nut-shaped cavity (often referred to as a nut boss). This method does not require any material removal. Often determining the optimal nut boss dimensions requires several iterations. Printing small test parts to determine the ideal dimensions can save on time and material costs.

Methodology for embedded nuts

1. Measure your nut. If you do not have access to the nut a quick internet search will reveal standard overall dimensions for both metric and imperial nuts.

2. Include the desired nut profile in your CAD model. An iterative process may be required to find the best nut clearance based on printer calibration. As a starting point a 0.2 mm offset around the nut (0.1 mm on each side) should give a loose fit. This also may need to be increased for nuts greater than M12.

3. Select the appropriate cut-out depth (typically just below flush).

4. Including a drop of superglue on the back side of the nut will help secure it in place.

What are self-tapping screws?

Self-tapping screws cut a thread into a pre-drilled hole as they are screwed down. This offers a quick assembly method but is not suited for applications where parts will regularly be assembled/disassembled. Special self-tapping screws for plastic can be used that limit the radial stress on 3D printed holes lowering the likelihood of bulging, delamination or fracturing occurring.

Methodology for self-tapping screws

1. 3D print or drill a hole in the desired screw location. For optimal pilot hole size consult the self-tapping screw provider. A pilot hole size that provides 75% to 80% thread engagement is a good starting point if this information is difficult to come by.

2. Assemble components to be secured ensuring to correctly align all holes where the self-tapping screws will be used.

3. Slowly screw the self-tapping screw down into the hole, ensuring it remains perpendicular to the hole during fastening.

What is thread cutting (or cutting a thread) for 3D printing?

Thread cutting (more commonly known as tapping) involves using a tap wrench to cut a thread in a pre-printed or drilled hole. Threads are regularly cut in 3D printed plastics.

Methodology

1. 3D print or drill a hole in the required location of the thread. For pilot hole (tap drill) sizes that correspond to each thread size refer here

2. Using the correct size tap wrench and ensuring it remains perpendicular to the hole, slowly cut the thread regularly reversing or “backing off” to remove excess material to avoid binding.

3. Avoid forcing the tap wrench as this can lead to fractures or splitting of the 3D printed material.

4. Continue tapping to the desired depth.

Insert your desired threaded fastener before assembly to ensure a clean fit.

3D printing threads for assembly

The process of 3D printing threads eliminates the need for any extra steps post printing and allows parts to quickly be assembled together. Limitations on printer accuracy and resolution will govern the success of a printed thread. Threads smaller than M5 printed via FDM should be avoided with one of the other threaded fastener methods discussed in this article implemented instead.

After printing the threaded fastener should be screwed and removed from the hole several times to clean the printed thread before final assembly.

Practical advice for threads, screws & more assembly methods for 3D printing

• For a more accurate fastener pilot hole consider drilling the hole after printing rather than printing it in the part.

• The minimum wall thickness around a thread should match the diameter of the fastener (e.g. an M5 fastener requires a minimum of 5mm wall thickness around the threaded hole).

• Heat-up or tap-in inserts and embedded nuts are the most popular methods for securing 3D printed components due to their simple installation, connection strength (metal-on-metal) and ease of repeated assembly and disassembly.