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Adding Threads to Your 3D Printed Parts
Screw threads are everywhere in functional design. They hold electronics enclosures together, attach brackets to walls, secure lids to containers, and create adjustable mechanisms. When you're designing 3D printed parts, adding reliable threads can be the difference between a prototype and a finished product.
The challenge is that 3D printing—especially FDM—wasn't designed to produce the fine geometry of screw threads. But with the right approach, you can absolutely create threaded connections that work reliably. Here's everything you need to know about adding threads to your 3D printed parts.
Three Approaches to Threaded Parts
Before diving into specifics, understand that you have three main options, each with distinct advantages:
1. Print the threads directly — Model the thread geometry into your design and print it as part of the object. Best for larger threads (M6/¼"-20 and up) and low-stress applications.
2. Tap or cut threads after printing — Print a pilot hole, then use a thread tap to cut metal-quality threads into the plastic. Works well for smaller threads where printing would lack precision.
3. Use threaded inserts — Design a pocket or boss in your part, then press or heat-set a metal insert that provides durable, wear-resistant threads. The gold standard for repeated assembly and high-stress applications.
Let's examine each approach in detail.
Printing Threads Directly
Directly printing threads is the most straightforward approach—no additional hardware or post-processing required. But it comes with important limitations.
When Printed Threads Work
Printed threads are appropriate when:
- Thread size is M6 (or ¼"-20) or larger
- The threaded connection won't be assembled and disassembled repeatedly
- Loads are moderate (hand-tight, not power-tool tight)
- You're creating mating plastic parts (plastic-to-plastic threads)
Thread Profile Matters
Not all thread profiles are created equal for 3D printing. The standard 60° V-thread (like metric or UNC threads) works, but trapezoidal or ACME thread profiles actually print better. Their flatter, more squared-off geometry is more stable during layer-by-layer printing and results in stronger, smoother-operating connections.
That said, V-threads are fine for most applications, and they have the advantage of compatibility with standard hardware.
Print Orientation is Critical
Here's the non-negotiable rule: print threads along the Z-axis whenever possible.
When threads are printed vertically (with the thread axis perpendicular to the build plate), each layer contributes to the thread profile. The result is relatively smooth threads with consistent geometry.
When threads are printed horizontally (thread axis parallel to the build plate), the layer lines create a staircase effect across the thread flanks. Even with supports, horizontal threads print poorly and won't mate smoothly. For horizontal threaded holes, use one of the post-processing methods described later.
Layer Height for Threads
Layer height directly impacts thread quality. The rule of thumb: use a layer height between 1/4 and 1/2 of the thread pitch.
| Thread Size | Pitch | Recommended Layer Height | |-------------|-------|--------------------------| | M6 | 1.0mm | 0.2mm - 0.25mm | | M8 | 1.25mm | 0.2mm - 0.3mm | | M10 | 1.5mm | 0.2mm - 0.3mm | | M12 | 1.75mm | 0.2mm - 0.4mm |
For threads smaller than M6, the pitch gets too fine for FDM to handle reliably. At 0.2mm layer height, an M4 thread with 0.7mm pitch only gets 3-4 layers per thread cycle—not enough resolution for clean engagement.
Tolerances for Printed Threads
This is where most designs fail. You cannot print a theoretical M10 hole and expect a standard M10 bolt to thread in. The "squish" from FDM extrusion closes small gaps, and threads that should have clearance end up binding.
The fix: Offset your thread geometry.
For internal threads (holes/nuts): Oversize by 0.2-0.4mm in diameter. An M10 internal thread should be modeled as M10.2 to M10.4.
For external threads (bolts/screws): Undersize by 0.1-0.2mm in diameter. An M10 external thread should be modeled as M9.8 to M9.9.
For mating printed parts (printed bolt into printed nut): Shrink the external thread by 0.2-0.3mm and leave the internal thread at nominal size.
These are starting points. Every printer and filament combination behaves slightly differently, so print a test piece before committing to your final part.
Design Details That Help
Add a lead-in chamfer: Put a 45° chamfer on the leading edge of screws and the opening of nuts. This guide makes it dramatically easier to start the threads and removes the fragile starting point of the helix that often prints poorly.
Avoid cosmetic threads: Many CAD programs offer "cosmetic" thread options that look correct on screen but don't export actual geometry. Ensure your threads are true 3D features, not visual textures.
Use generous radii: Sharp internal corners are stress concentrators and print poorly. Add fillets where threads meet other features.
Tapping Threads After Printing
When you need threads smaller than M6, or when horizontal orientation is unavoidable, cutting threads with a tap after printing often produces better results than trying to print them directly.
How It Works
- Design your part with a pilot hole sized for the tap
- Print the part normally
- Use a hand tap (designed for plastics, ideally) to cut the threads
Plastic taps cut more gradually than metal taps and are less likely to crack the part. Standard metal taps work too, but go slowly and back out frequently to clear chips.
Pilot Hole Sizes
For FDM prints, use the standard tap drill size as your starting point, then add 0.1-0.2mm to account for hole shrinkage during printing.
| Thread | Standard Tap Drill | FDM Pilot Hole | |--------|-------------------|----------------| | M3 | 2.5mm | 2.6-2.7mm | | M4 | 3.3mm | 3.4-3.5mm | | M5 | 4.2mm | 4.3-4.4mm | | M6 | 5.0mm | 5.1-5.2mm |
Material Considerations
Tapping works best in materials that are somewhat ductile:
- PETG: Excellent for tapping. Cuts clean threads that hold up well.
- PLA: Works but can be brittle. Go slowly to avoid cracking.
- ABS: Good option, though softer than PETG.
- Nylon: Ideal material for tapped threads—cuts beautifully and handles repeated use.
At Mandarin3D, we typically recommend PETG for parts that will receive tapped threads. It's forgiving during the tapping process and produces durable results.
Heat-Set Threaded Inserts: The Professional Solution
For applications that demand durability, repeated assembly cycles, or higher loads, heat-set inserts are the answer. These brass or steel inserts provide metal threads in a plastic part, combining the design freedom of 3D printing with the mechanical performance of traditional fasteners.
How Heat-Set Inserts Work
Heat-set inserts are installed using a soldering iron or specialized installation tip. The heat softens the surrounding plastic, allowing the insert's knurled exterior to sink into the part. As the plastic cools, it solidifies around the knurls, locking the insert in place with excellent pull-out resistance.
Designing for Heat-Set Inserts
The insert manufacturer specifies a hole diameter for installation. For FDM prints, use that diameter or add 0.1mm if you're getting excessive bulging during installation.
Boss design matters. The plastic around the insert needs enough material to grip the knurls and absorb the heat without deforming the outer surface of your part. General rule: the boss diameter should be at least 2× the insert outer diameter.
Depth considerations: Design the hole slightly deeper than the insert length (add 0.5-1mm). This gives the insert room to fully seat and accounts for any plastic that flows during installation.
When to Use Inserts vs. Other Methods
| Use Case | Best Method | |----------|-------------| | Prototype, single assembly | Printed threads | | Smaller threads (M2-M4) | Tapping or inserts | | Repeated assembly/disassembly | Heat-set inserts | | High-load applications | Heat-set inserts | | Field-serviceable products | Heat-set inserts | | Decorative or low-stress | Printed threads | | Cost-sensitive high volume | Tapping |
Insert Sizing
Common insert sizes for 3D printed parts:
| Insert Thread | Insert OD | Recommended Hole | |---------------|-----------|------------------| | M2.5 | 3.5mm | 3.5-3.6mm | | M3 | 4.0mm | 4.0-4.1mm | | M4 | 5.6mm | 5.6-5.7mm | | M5 | 6.4mm | 6.4-6.5mm |
Heat-set inserts are inexpensive (often less than $0.10 each) and transform 3D printed parts into durable, professional-quality assemblies. They're the standard approach for any commercial product using 3D printed components.
Self-Tapping Screws: The Quick and Simple Option
For the simplest possible threaded connection, self-tapping screws cut their own threads as you drive them in. No post-processing, no inserts, no design changes beyond a pilot hole.
Self-tapping screws work best with ductile materials like PETG, nylon, or TPU. Brittle materials like PLA may crack during installation.
Pilot hole sizing for self-tappers: Use a hole diameter approximately 85-90% of the screw's major diameter. For a #6 screw (3.5mm major diameter), use a 3.0-3.2mm hole.
The downside: self-tapping connections weaken with each assembly cycle as the threads get cut again. They're best for parts assembled once or very rarely.
Material Selection for Threaded Parts
Your material choice significantly impacts thread performance:
PLA: Rigid and precise, but brittle. Threads can crack under stress or during assembly. Fine for prototypes and decorative parts. Not recommended for functional threaded connections that will see repeated use.
PETG: The sweet spot for most threaded applications. Strong enough to hold up, flexible enough not to crack, and prints well on our BambuLab printers. This is our go-to recommendation for functional threaded parts.
ABS: Tougher than PLA with better heat resistance. Good for threaded parts in demanding environments, though it requires more print setup than PETG.
Nylon: Excellent for threaded connections. High fatigue resistance means threads survive repeated assembly cycles. The downside is that nylon is hygroscopic (absorbs moisture) and more challenging to print.
Test Before You Commit
Every combination of printer, material, and settings produces slightly different results. Before printing a complex part with critical threaded features, print a simple test piece.
A basic M10 or M12 nut and bolt test takes five minutes to print and tells you exactly what tolerances your setup needs. Keep notes on what works—those numbers become your starting point for future designs.
Ready to Add Threads to Your Design?
Whether you're printing threads directly, planning to tap them, or incorporating heat-set inserts, the key is matching your approach to your application. Simple prototypes? Print the threads. Durable products? Use inserts. Need precision in a small size? Tap after printing.
If you're working on a design with threaded connections and want to ensure it prints correctly the first time, upload your model to Mandarin3D. We review every file and can recommend the best approach for your specific application. Our BambuLab P1S printers produce excellent results with PETG—the material we most often recommend for functional threaded parts.
Have questions about which method is right for your project? Reach out at orders@mandarin3d.com. We're happy to discuss your design and suggest the best path forward.
