Using a threaded fastener is a popular and effective way to assemble 3D printed parts into one piece. Many threaded fasteners are used in injection molded, traditionally machined and 3d printed parts. With these metal parts, we can assembly and disassembly products many times.
Because most 3d printer owners are not mechanical engineers, we will try to talk that in a simple way.
1.Self-Tapping Screw
Self-Tapping screws are screws that have the ability to cut their own threads while driven into the materials. It creates a tight fit between the threads, and that helps fight vibration loosening.
It is ideal for joining two different types of materials. However, note that it is not the best solution for parts that need to be disassembled and assembled frequently. It may break easily for some materials.
There are several types of Self-Tapping Screws, such as type A, type AB, type B, type BP, we won’t talk more about the types. Because most 3d prints are plastic parts, we can only discuss Self-Tapping screws for plastics.
To use these screws, there should be a pilot hole before installation. We can drill a hole in the desired location. Or we can design a hole on the model to get it directly by 3d printing. You can try to get information from your supplier about the pilot hole size.
The pre-hole diameter should be between the major (thread) diameter of the screw and the minor diameter. You can get some information from this table.
Note that Self-Tapping screw is different from Self-Drilling screw. There don’t need to be a pilot hole for Self-Drilling screws. For 3d printed parts, we don’t suggest Self-Drilling screws. Of course, you can still test that type of screws.
2.Threaded Inserts for plastic-most common threaded fastener
There are some different types of threaded inserts. For example, some are designed for metal, wood, composites or plastic. Most desktop 3d printing materials are plastic or resin. We will just talk about thread insert for plastic. There are 3 types that can be used on 3d printed objects.
2.1 Press-Fit Inserts for plastic
It is the most common type of insert for 3d printed objects. We will only discuss one type of press-fit inserts-expansion type.
You can install the thread insert with hand tools or standard press. There need to be a 3d printed or drilled pre-hole, and the hole size for the insert is provided by the manufacturer. When a mating screw is installed, the sides of these inserts are expanded and force the fins or knurls into the sides of the holes. That holds the thread insert in place. You can use these thread inserts on FDM printer materials or resin.
2.2 Heat-Set Inserts for plastic
Before installing, there should be a 3d printed or drilled hole. The thread insert is heated via thermal conduction and pressed into the hole once the plastic reaches the proper melting temperature. After the heated press tip is retracted, the plastic will become cool and solid, locking the thread insert into place. These thread inserts can work with FDM 3d printing materials, such as ABS, PLA, Nylon, and more. But this method requires temporary melting of the material, it is not recommended for resin.
To install heat-set inserts, we can use a soldering iron simply. Of course, we can purchase some specialized tools too.
2.3 Tapping Inserts for plastic
When installing a tapping insert into a pre-hole which is drilled or 3d printed, the insert cuts its own thread. These cut threads create maximum shear surface area while minimizing induced stress, and have excellent pull-out resistance. It is easy to install the threaded inserts. You can do that with a hand tool, such as a ratchet wrench. It is suitable for thermoset and brittle materials, including FDM 3d printing materials and resin.
2.4 Design rules for threaded inserts
You can get data from the threaded inserts manufacturer. If you cannot get these data, please refer to the following rules.
3.Nut-another common threaded fastener
We can assemble two 3d printed parts together by using a nut. Usually, we need to prevent the nuts from rotating inside the 3d printed parts. So square nuts and hex nuts are the two common nuts that are suitable for this application. And the socket dimension is a key factor.
We can get these nuts easily, especially hex nuts. There are two methods to use these nuts.
3.1 Insert a nut into a socket after printing
It is the most common method to use a nut on 3d printed objects. We need to design a nut socket on the 3d model before printing.
Step 1. Get the dimension of the nut you plan to use. We can get the standard dimensions by searching on the internet. But it is best to measure the nut by yourself, especially for small nut, such as M2, M3, M4, etc.
Step 2. Design a nut socket on the 3d model. The socket should be a little bigger than the nut, but it cannot be too big, otherwise, the nut will rotate inside the socket. We can get dimension e and s after we measure a nut. Then the clearance between nut and socket (dimension c) is a key.
Recommend dimensions:
1. s-e≧0.5 mm. So when you use small nuts, such as M2, M3 nuts, you must measure the nuts to ensure that; In fact, we don’t recommend using small nuts in this way.
2. c≧0.1 mm. If it is too small, it is hard to install the nut. Of course, it depends on the 3d printing material and 3d printer.
3. X=(e-s)/2-c. If dimension X is too small, the nut may rotate when you install bolts. So we need to make dimension c as small as possible when designing.
Step 3. When installing a nut, you can add some superglue to help secure it in place. And in fact, we can install bolts into some cavities too.
3.2 Insert a nut during the 3d printing process
Some FDM printers can be paused in the middle of the building process. This allows users to add some hardware into a pre-designed cavity, and then resume the build. To do this, you need to be very skilled in 3d printing. We won’t talk more here.
4.Tapping Threads
There are two kinds of threading, external thread, and internal thread. For internal threads, usually, they are produced with taps or thread-mills; For external threads, we can manufacture them with threading dies or lathing. We will only talk about internal threads on 3d printed parts here. And for small threads, tapping is the most popular method. We can use a tap wrench to manufacture a thread in a drilled or pre-printed hole. When you buy tapping tools, you will find there are two types, cutting taps and forming taps.
4.1 Cutting taps
Cutting taps are designed to create threads by removing material from the pre-hole. The tools utilize a feature known as chamfer, to achieve a gradual machining motion as the tap enters the pre-hole. It is best to use some coolant or lubricant when tapping thread. Cutting taps can be used with most materials.
4.2 Forming taps
Forming taps generate thread by displacement of the material inside the hole. It won’t remove material. To form the thread successfully, we must try to reduce the friction. So we need to pay more attention to the lubrication when tapping. And forming taps require a slightly larger pre hole than cutting tapping, as the material being threaded flows into and away from the threads of the tap. And after the process is complete, the minor diameter of the hole will become smaller than the pre hole.
Forming taps are limited to be used in aluminum, copper, soft steel and some other materials. For plastic, cutting taps and forming taps are all ok usually. If you cannot use forming taps on your material, just choose cutting taps.
Usually formed threads are stronger than cut threads, and formed taps have a longer life too. But they are interchangeable and gauge identically.
About the diameter of pre-holes, you can get the data on the internet. Note, the hole size is not the drilling tool size. Usually the drilling tool diameter should be a little smaller than the pre-hole diameter, especially when you use a hand tool to drill the hole.
