We get a lot of questions on the best methods for fastening different 3D printed components. When prototyping hardware products, for example, you often need to make assembled structures that are more complicated than a single 3D printed component, like an electronics enclosure or robotic assembly.

Other times, you may need to print components too large for the build envelope of a 3D printer, so you’ll want to consider methods for permanently or intermittently assembling printed parts together.

One of the ways to assemble 3D parts is with snap fit components, but another great method is to use screw threads.

There are a lot of different ways to implement threads into your 3D printed part so we’re going to cover the pros and cons of the most common methods as well as specific installation steps to help get you started.

Threaded Inserts

Our go-to method, and the one we recommend most often, is using threaded inserts due to its easy installation and high quality feel.

Pros

Fast, easy and clean

Unlimited assembly/disassembly

Production quality

Cons

More expensive

Requires increased wall thickness

Materials and Tools

Brass Inserts

Soldering iron

Exacto knife

Installation Steps

Place your insert into the associated hole it will be pushed into Take your heated soldering iron, place it in the middle of the insert and apply a small amount of pressure As the insert starts heating up you’ll see it sink into the hole Once the insert looks to be flush with the part surface, use your exacto blade to check and trim away any extra material

Here’s a helpful video to guide you through these steps.

Self Threading Screws

An alternative method to threaded inserts is to use self threading screws. It’s the easiest and cheapest way to go if you want something quick and dirty, so if this is your first prototype or you’re using a low resolution material like PLA, self threading screws are a great option.

Pros

Easy installation

Minimal design requirements

Cheap

Cons

Brittle material may break (ie. verowhite)

Limited assembly/disassembly

Low strength

Materials and Tools

Installation Steps

It’s as easy as a screwdriver and a screw… just twist :)

Designing Threads into a 3D Model

When you’re designing a part that requires very large threads, the best method is to design the threads into the 3D model itself.

Pros

Can design custom threads

Good when inserts aren’t available (ie. M50 thread)

Works well with brittle materials

Cons

Threads will wear down over time

Difficult to model accurately

Requires high resolution prints

Materials and Tools

None (CAD only)

Installation Steps

Ensure your threads are modeled accurately (use equation-driven features in your model — here’s a helpful resource for modelling in inches, and a resource for modelling in millimeters) Print the component using a high resolution materials (such as verowhite) For internal threads, use a tap to ‘finish’ the threads. If you don’t have a tap available, try using a machine screw. For external threads, use a steel nut sized to your thread dimension and use this to finish the threads on your component. Be sure to tap completely through the component for through-holes — this will ensure excess material is removed from the tapped feature and prepare the part for fastening. For blind holes, be sure you’ve tapped deep enough for assembly and make sure to clean any excess material (trying to torque fasteners into components with debris could damage your features).

When using this method to add threads to a printed part, be sure to maintain perpendicular alignment to the threaded feature; It’s worth it to take extra care to avoid cross-threading, which could result in permanent damage to the part.

Cutting Threads with a Tap

This is the most traditional method for using threads. In subtractive manufacturing, once a CNC places holes where threads will go, a drill tap is used to create the desired threads in each hole.

When prototyping with 3D prints, you can use the same method with a hand drill tap to create threads in your plastic prototype.

Pros

Better assembly/disassembly than self threading screws

Cons

Low strength

Plastic threads wear down over time

Time intensive

Materials and Tools

Drill tap

Screws

Installation Steps

Tap the threaded feature with a corresponding drill tap to cut usable threads Be careful not to damage the component when applying torque

Captured Hex Nuts

Another commonly implemented strategy for fastening components together is to create printed-in pockets for capturing hex nuts.

Pros

Low cost solution

Good holding force

Easy install

Cons

Only works on outside surfaces

Extra material is required to capture the nut

Design constraints

Materials and Tools

Hex nut

Screws

How to use this method

Measure the size of your fastener -- here’s a good resource to help you out. Add a bit of tolerance (0.005” - 0.010”) to the hole size to accommodate any dimensional error. Before press-fitting the nut into the pocket, apply a bit of glue on the but to adhere it to the inside pocket surface. Otherwise as you apply torque to the nut, it may be pulled from the pocket

Pro Tip: For more information on fitment and tolerance, pick up a copy of Machinery’s Handbook

Mechanical Considerations

Here are three questions to help you consider which method is best for your project:

Will you need to disassemble and reassemble the components? What are your strength requirements or holding forces? What are the geometric or spatial constraints inherent in the design of your parts?

Additionally, here are three important considerations to keep in mind as you design your fastening features:

Bending along the axis parallel to printing should be generally avoided since printed components are structurally much weaker in this direction. Be mindful of the allowable stress and strain of your materials when adding assembly features. Double check your CAD model before adding fastening features. ie. if you’re adding a hex nut, check the height of the hex nut you’re using and If you’re using a threaded insert, check the pitch of the insert you’re installing.

With these ideas in mind, you should have no trouble creating larger, more complex assemblies with your 3D prints and for additional questions send us a note at team@ficitv.com.

Is there a method we didn’t mention that works well for you? Let us know so we can add it to the list!