3D-Printed Threads or Metal Inserts? Designing Parts for Repeated Assembly

When a printed part is fixed into an assembly with screws, the fastening detail can look like a fairly small CAD decision. That is often true if the screw goes in once and stays there. It becomes more important when a cover is removed for maintenance, a sensor bracket is adjusted, a housing is opened regularly or the same component has to survive repeated assembly.

There are several ways of dealing with the thread. It can be printed directly, added afterwards with a tap, formed by the screw, or provided by a metal insert. The right choice depends less on which method is easiest to model and more on how the joint will actually be used.

A thread that only needs to retain a light cover has a different job from one holding a component that will be removed and refitted throughout the life of the machine.

Directly printed threads have their place

Printing the thread with the part is attractive because there is no additional hardware or installation operation. On processes with enough resolution, a usable thread can come straight from the build.

I would still keep the application fairly modest.

Directly printed threads are generally better suited to lighter-duty applications, particularly where the thread is relatively large and will not be assembled and disassembled repeatedly. Once the fastening point is expected to carry more load or survive regular servicing, a metal thread becomes more attractive.

That does not make a printed thread a poor design. A large cap, adjustment feature or cover that is opened occasionally may be perfectly well served by one. There is little benefit in adding hardware to every threaded feature simply because inserts are available.

The difficulty starts when the printed thread is being asked to behave like a conventional metal fastening point without allowing for the material or process used to make it.

Thread form, surface finish and dimensional accuracy all enter into the fit. The surrounding material still has to carry the load, and wear starts to matter when the screw is removed and refitted.

Tapping the hole is another simple option

Leaving a hole in the printed part and tapping it afterwards can work well when a conventional machine thread is wanted without adding an insert.

It is a familiar operation and may be the quickest route for low quantities. The printed hole can also be finished to size before tapping if the application needs better control than the as-printed bore provides.

The limitation is still the material forming the thread.

Tapping is a practical way of adding a conventional machine thread to a printed polymer part, but the screw is still engaging directly with the polymer. It therefore suits applications where the thread will see limited service better than applications that will be opened and closed repeatedly.

For a component that is assembled once or only opened occasionally, that may be completely acceptable. If a maintenance technician is likely to remove the same screw every month for the next few years, I would want to look more closely at what happens to that thread over time.

There is also a difference between processes and materials. A tough nylon part and a rigid photopolymer should not be treated as though they will respond to tapping in exactly the same way. The fastening detail needs to suit the printed material rather than being copied from a machined version of the component.

Self-tapping screws can remove an operation

A thread-forming or self-tapping screw is useful when the assembly does not justify tapping every hole or fitting a separate insert.

The hole is printed and the screw forms its engagement with the polymer during assembly. This can be a very efficient solution for parts that are put together once and remain assembled, particularly when the material has enough ductility to tolerate the process.

Thread-forming screws work best when the polymer can deform around the thread without cracking. That makes the material important. A tougher thermoplastic can behave very differently from a brittle resin when the screw is driven into the hole.

They are particularly useful for assemblies that are put together once and left alone. Repeated removal gradually makes the condition of the polymer thread more important.

For an internal cover that will probably never be opened, that may be a sensible trade. For a service panel that is removed regularly, saving an insert during production can create an avoidable repair later.

This is why I would normally decide how the part will be serviced before deciding how the screw goes into it.

A metal insert changes the wear surface, not the whole joint

Threaded inserts are useful because the screw runs into metal rather than repeatedly working against the printed polymer. They can give a reusable thread and improve the strength of the fastening point.

There are different ways of installing them, and the printing process makes a difference.

For thermoplastic parts, heat-installed, ultrasonic, press-in and other insert types are available. Heat installation can work well where the surrounding polymer can soften and reform around the insert. A cured thermoset resin does not behave in the same way, so bonded, press-fit or other mechanically suitable insert methods may be required depending on the material and part.

The insert manufacturer's recommended hole geometry and installation conditions should be part of the design work rather than something looked up after the components arrive.

Adding a metal insert improves the threaded interface, but the insert still transfers its load into the printed material around it.

Give the insert enough part to work with

A small brass insert can look insignificant in CAD, so it is tempting to put a hole into an existing wall or boss and assume the insert will provide the required strength. Its performance depends heavily on what surrounds that hole.

Insert performance can be affected by the printed material, hole geometry, wall thickness, the amount of solid material around the insert, infill strategy and the way the part is built. These factors influence how well the insert resists rotation or pull-out once the joint is loaded.

That is useful design information because it moves the decision beyond simply choosing an M4 or M6 insert from a catalogue.

If the boss around it is too light, the insert can pull out or rotate with the screw. If the hole is wrong for the insert, installation and retention suffer. If a process change alters how that area of the part is built, the same piece of hardware may no longer behave in quite the same way.

On an FDM part I would pay particular attention to the solid material around the hole rather than assuming that a high general infill percentage has solved the problem. On powder-bed parts the material and surrounding structure are different again, and the insert system should be chosen and validated for that manufacturing route.

A fastening point is part of the component structure.

Screw torque is not the only load to think about

There is another issue when a bolt is clamping through a polymer component.

The thread needs enough strength to accept the tightening torque, but the joint also relies on maintaining clamp load after assembly. Polymer under sustained compression can relax over time, particularly where temperature and load are significant.

That matters because the initial tightening torque is only part of the joint's job. It also needs to retain enough clamp load while it is in service.

A metal insert improves the threaded connection, but it does not remove that behaviour from the rest of the component.

The screw head or washer may still be clamping onto polymer. The boss can still deform, and the component can still see temperature, vibration and sustained load. In a more demanding joint there may be a need to manage the compression path separately rather than asking the printed polymer to hold a high preload indefinitely.

For many covers, housings and brackets this will never become a serious issue. For a structural fastening point, it deserves checking.

Installation becomes part of the manufacturing route

Once an insert is selected, fitting it is also a production operation.

Hole size, installation depth, alignment and the way heat or force is applied can all affect how well the insert is retained. If heat is used, enough surrounding material needs to soften and reform around the insert without damaging the boss or leaving it poorly aligned.

For a few parts installed by an experienced engineer, this is easy to manage informally. Once the same component is being made repeatedly, I would record the insert and installation method with the rest of the manufacturing definition.

That can be quite simple: the hardware part number, the hole or boss geometry, the installation process and any checks that matter to the assembly.

It also prevents a later revision from replacing one insert with another that happens to have the same internal thread but needs a different hole or installation condition.

The fastening method should follow the service requirement

There is no need to put a metal insert into every printed hole.

For a light-duty thread used occasionally, printing it directly may save time and parts. A tapped hole can be a good workshop solution when the thread will see limited service. A thread-forming screw suits many assemblies that are put together and left alone. A metal insert becomes much more attractive when the thread needs to carry more load, maintain better durability or be opened and closed repeatedly.

Some additive processes can reproduce surprisingly fine thread geometry, but being able to print the shape does not automatically make it the best fastening method. A thread that looks excellent straight out of the machine may still be the wrong choice if the joint will be heavily loaded or serviced repeatedly.

That is also why I would avoid specifying the fastening method at the very end of the design.

If an insert is the likely route, the boss can be designed around it from the beginning. There can be enough material around the hole, access for installation, clearance for the screw and room for any washer or mating component. The part can then be printed with that assembly detail already considered.

Trying to add an insert to a thin wall after the first parts have been made usually leaves fewer good options.

Designing the printed part as an assembly

We have already covered the broader question of selecting polymer additive manufacturing for end-use parts elsewhere on the D2M site. Fastening deserves its own treatment because it is one of the places where the printed component meets the rest of the machine.

A cover still needs to come off. A housing has to close properly after the electronics are serviced. A bracket needs to hold the sensor where it was designed to sit. Those requirements continue long after the print itself has finished.

D2M can review the fastening detail as part of the wider application, including the printed material and process, the geometry around the joint and whether a secondary insert or conventional hardware makes more sense than printing the thread directly.

If the component is going to be opened repeatedly, I would make that decision before locking the CAD. It is much easier to give an insert enough material and access at that stage than to repair a fastening point after the parts have been produced.