Threads are deceptively tricky to get right. They are everywhere, in almost every assembled product, yet handling them well in a model takes far more care than most people expect, especially for printed parts where a directly modelled thread can turn out weak, coarse and prone to stripping after only a handful of uses.
Our CAD team makes a deliberate, considered choice on every threaded feature rather than reaching for a default. The best approach genuinely depends on the material, the manufacturing process and how often the joint is likely to be undone and remade over the life of the part, and weighing those factors up front saves a lot of trouble later.
To model a thread or not
You can model a thread directly into a part, and for machined components or large coarse threads that often works perfectly well and looks clean. For small printed threads, though, the layer resolution of the process makes them fragile and prone to stripping, so a thread that screws together nicely the first time may fail after being undone only a few times.
In those cases it is almost always better to design a plain hole and add the thread strength some other way, rather than relying on fine plastic threads that simply will not last under repeated use. Recognising when a modelled thread is the wrong tool for the job is one of the quiet skills that keeps printed assemblies working reliably for years rather than weeks.
Better options for printed parts
Heat set threaded inserts are the go to solution for printed enclosures and assemblies that need a durable thread. You design a plain hole sized to suit the insert, then press it in with a soldering iron, which melts the surrounding plastic around the knurled metal body and leaves a strong steel thread that comfortably survives repeated assembly and disassembly.
Captive nuts trapped in a pocket, and self tapping screws driven into a solid boss, are other reliable choices depending on the situation. Each one needs the hole and the surrounding boss sized correctly in the model to work properly, and that is exactly the kind of detail we build in for you so the fastener does its job for the long term.
Designing for service and rework
How often a joint will be opened should shape the fastener choice from the very start. A panel that is opened once a year for a battery change asks far more of a thread than one that is sealed for life at the factory. For anything serviced regularly, a metal insert or a captive nut is well worth the small extra cost, because plastic threads simply wear out under repeated tightening.
It is also worth thinking about the tools and access the person doing the work will actually have. Leaving room above a boss for a screwdriver, choosing a single common head type across the whole product, and keeping fasteners reachable without dismantling half the assembly all make service quick and pleasant rather than a fight. We model that access in from the outset and prototype the joints through prototype development, so the part that ships can be opened and reassembled cleanly time after time without anything stripping or rounding off.
Getting hole sizes right
Whichever route you take, the hole has to suit it precisely, and the correct size is rarely the same as the nominal thread diameter you might first reach for. Sizing holes by feel or by copying a metal part is a common way to end up with inserts that spin loose or screws that split the boss, so it pays to follow the right figures.
- Size insert holes to the manufacturer recommended diameter.
- Allow for shrinkage on printed holes that take fasteners.
- Add clearance for screws that pass through, not into, a part.
- Use generous bosses so self tapping screws have material to bite.