Many people with a 3D printer are familiar with the well-known ‘brass threaded inserts’. These copper tubes can be melted into a 3D-printed object and provide sturdy threads. And of course, I have also used these little things myself with great pleasure and success for the mounting holes of the covers of my printed electronics enclosures.
However, when I was later looking for a sturdy mounting method for a printed gear wheel on a metal shaft, I tried using these tubes in a different way.The result exceeded my expectations!
The photos below show the most common application of these copper inserts. By melting them in place, which can be done carefully with a soldering iron, you create a mounting point with a sturdy screw thread. I have regularly used these bushings myself to provide printed enclosures with a removable cover.
One of my hobbies is printing parts that can be used with fischertechnik. Over time, I have designed and printed quite a few wheels, spoked wheels, and gears. A recurring problem is attaching them to the 4 mm metal axles.
If the wheel is closed in the middle and large enough, the normal fischertechnik clamp hub can of course be used for this. But with a decorative spoked wheel or small-diameter gear, this hub is not usable. In my search for alternative fastenings for self-printed wheels, I also experimented with a co-printed socket in which I then tapped M2.5 thread. The idea was that an Allen screw could then be tightened sufficiently to attach the wheel to an axle without slipping. I printed the socket solid with 100% infill to make the thread as strong as possible.
The problem, however, is that you must not tighten the locking screw too much. Plastic threads cannot withstand much force, and once ‘over-tightened’, the wheel is basically useless. For gears where the torque increases and which need to be securely attached to the metal axle, this is therefore not a viable solution.
I had previously used metal flanges in larger gears, but the mounting plate (with a diameter of 21.5 mm) is obviously much too large for the small gear which I wanted to print now. So I started looking for possible alternatives that could be incorporated into or onto the 3D-printed wheel. I did find some copper shafts that were actually intended for a different application and could possibly be made suitable with some pre-processing, but they were not really a usable, cost-effective solution I was looking for.
While I was searching and puzzling, I thought of the well-known brass inserts. After all, the main requirement is that the screw must run in metal thread instead of 3D-printed PLA or PETG material for a good, tight attachment to the axle. And, of course, these inserts could fully meet that requirement.
I decided to give it a try and designed a socket for the wheel into which the familiar copper threaded sockets could be easily melted. The result exceeded my expectations. The Allen screw can be tightened very firmly, allowing the wheel to be mounted very tightly on the axle. This is much stronger than the thread that is tapped directly into the 3D-printed plastic.
I had not previously seen this application of the inserts in the many YouTube videos about these screw thread sockets, and it exceeded my expectations. For my first trials, I used M3 inserts, but of course, there is still plenty of room to experiment with other diameters and multiple attachment points in the shaft. It already seems to me to be a very useful, unexpected application that I will definitely use more often!