Few details separate amateur FDM design from professional FDM design more clearly than how threaded features are handled. Beautifully designed parts routinely fail at the threads: stripped on first installation, pulled out under load, or simply unable to develop the clamp force the design assumed.
There are three main approaches to threading in FDM, each with appropriate use cases and predictable failure modes. Choosing the right approach for the application is half the battle, and specifying it clearly on the drawing is the other half.
Approach 1: tap directly into the print
The simplest option is to print with a slightly undersized hole and tap a metal thread directly into the printed material. It works adequately for low-load applications, infrequent assembly, and larger threads (M5 and above) in engineering grades from the OzFDM catalogue such as PA-GF or PA-CF.
Limitations are real. Thread strength is modest, typically 30 to 50 per cent of an equivalent metallic thread. Durability is limited to five or ten fastener cycles before stripping. Vibration tolerance is poor; threaded interfaces in vibrating environments will loosen and fail rapidly. Use it where the duty matches, and not elsewhere.
Approach 2: heat-set brass inserts
Heat-set inserts are knurled brass cylinders with internal machine threads, installed by heating the insert with a soldering iron and pressing it into a printed boss. The plastic flows around the knurls and locks the insert into the part. This is the standard approach for any threaded interface that will see repeated assembly. Strength frequently exceeds the surrounding plastic, durability runs to hundreds of cycles, and cost is modest at typically AUD 0.10 to 0.50 per insert in volume.
Approach 3: bonded or helicoil inserts
For high-load, high-cycle, or vibration-prone duties, chemically bonded inserts or helicoil-style spring inserts deliver near-metallic thread performance in printed parts. They are more involved and more expensive than heat-set inserts but justify themselves in aerospace, defence and motorsport applications where the threaded interface sits on the critical path of part performance.
A practical decision flow
Spread the load
Through-bolts in load-bearing applications should always include load-spreading washers. A fastener bearing directly on a printed face will dig in under load, and the joint will lose clamp force long before the plastic fails.
Estimate the peak load and the number of assembly cycles expected over the part's life.
If load is light and cycles are few, draft for a tapped thread in a glass-filled grade.
If repeated assembly is expected, draft for heat-set inserts with a boss wall thickness 1.5 to 2 times the insert outside diameter.
If the duty is high load, high cycle or exposed to vibration, draft for bonded or helicoil-style inserts.
Add a 45 degree by 1 mm chamfer at every thread entry to ease fastener engagement and reduce stress concentration.
Specify the chosen approach on the drawing using a callout, not just a thread size.
Specifying threading on drawings
Always specify the threading approach on the drawing. 'M5 thread' is ambiguous: is it tapped, heat-set, or bonded? Each has different design implications, different strength implications and different cost. The supplier can produce any of them, but they need to know which.