When building complex or large models, you’re often required to print parts separately and connect them. One of the most effective ways to do this is using screw threads.
Connecting parts together allows you to create models larger than your printer’s build volume. You’re also able to make components using multiple different materials and colours. Epoxies and glues work in some cases, but aren’t compatible with all materials. They also form a permanent bond that can not be modified afterwards. Using screws to connect your parts can in some cases create a stronger bond than glues or epoxies and allows parts to be adjusted later. In this post, we’ll be going over the four easiest and most effective techniques for adding screw threads to your parts.
How to Add Screw Threads to Your Parts
One of the most effective methods for adding threads is to create a pocket for a nut to slide into. Allow for at least 2mm between the top of your part and the pocket for the nut. This allows you to apply high amounts of torque to your part before breaking.
To design your pocket, sketch out the nut you’ll be using in CAD. Keep in mind, you’ll want to give 0.3mm – 0.5mm of tolerance to ensure the nut slides smoothly. Half of your sketch will remain the same and the second half will be cut and extended to the outer wall of your part.
You’ll want to avoid using supports in the nut pocket as they can be a challenge to remove. To do this, fill in the hole just above the nut pocket with a depth that equals one layer of your print (If using 0.2mm layers, make this extrusion 0.2mm deep). This will allow you to bridge the void more effectively and your screw will easily penetrate through the one filled in layer.
Thread Cutting Screws
This is one of the easiest techniques for combining parts as it requires very few modifications to your design. These tap the necessary thread into your design and provide a more permanent fixture.
To use these, you’ll want to measure the minimum diameter and maximum diameter of your thread cutting screw. The minimum diameter is most easily measured at the tip and the maximum diameter can be measured at any of the threads. The screw will cut the necessary channel into your design. Keep in mind, you’ll want to get the correct thread cutting screw for your material. Different thread cutting screws exist for metal and plastic applications.
This can be one of the more challenging techniques as it requires that you measure the specification for your screw and design that thread into your part. Most thread profiles are intricate and have steep overhang angles upwards of 80 degrees. FDM printers will struggle with resolving these details so this tends to be a more viable technique for SLA. Check out Formlabs’ post for more details for adding screw threads to SLA printed parts. Most of their tips also apply to adding screw threads to FDM parts.
Your process for designing threads will vary based on which software you’re using. In general, you’ll need to create a sketch for the cross section of your screw and then create a helix for the cross section to be cut along.
This technique is uniquely accessible to FDM as most compatible materials are designed to melt at relatively low temperatures. Create an extrusion for your nut but, for this technique design it about 0.5mm smaller than the nut itself. Position the nut atop the extrusion and break out your soldering iron. You’ll heat the nut with the soldering iron until it melts into your part.
The bond this technique creates is sometimes not as strong than those created using the other methods. Metal nuts tend to have smooth exteriors which doesn’t give the molten plastic much to adhere to. Sanding the exterior of your nut will help to improve the roughness and can improve the bond between the nut and your part.
Which technique you use will depend on your specific geometry and how permanent you want the bond between your parts to be. Nut pockets tend to be a favorite because they’re relatively easy to design into your parts, provide high strength, and can be adjusted afterwards.
You can grab the model used for this tutorial here! Each corner uses one of the different mentioned fastening techniques so you can try them out for yourself.