Practical 3D Printing: Threaded Features

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Threaded Features

05.17

00:00	Threaded features are commonly used in the design of automotive parts for all kinds of fittings and connections.
00:06	There are a lot of crossovers between parts that could be produced via subtractive manufacturing methods like machining and additive manufacturing, often times with parts that could be produced with either method, although usually more suited to one than the other.
00:21	Generally, speaking though, machining processes are much better at creating threaded features.
00:27	That's not to say that it's impossible with 3D printing.
00:30	To be clear, we wouldn't typically print the actual threads.
00:33	Most threads are much too detailed for FDM 3D printing, even at the finest layer heights.
00:39	Some very large threads with forms designed for plastics can be printed nicely if we need a simple way of connecting two parts together.
00:48	But the strength and precision still isn't anywhere near what we'd typically be used to with machining processes.
00:54	You might be wondering if it's possible to use taps or dies to cut and form threads into 3D printed thermoplastics, and it absolutely is.
01:03	Plastic is extremely easy to cut and form compared to metals.
01:07	Think about how easy it is to tap threads into aluminium compared to steel.
01:11	Most of our 3D printed thermoplastics are much easier again.
01:15	We just need to make sure that there's enough material to cut the thread into.
01:20	Recalling our discussions on shell thickness.
01:23	The shell needs to be thick enough to cut the thread into without breaking through into the infill.
01:28	This said, in recalling our first statement about not printing threads, we can still get good results if we chase printed threads with a tap.
01:36	From here we can use conventional hardware or other threaded 3D printed parts to make connections.
01:42	While it's much easier to form the thread compared to metals, the downside as you'd expect is it's much easier to strip the thread from over-talking or the hardware pulling out.
01:53	For more strength, precision and also durability, we need to turn to threaded inserts or nut pockets.
02:00	Let's look at nut pockets first.
02:02	These are simply a pocket in the back or even through the side of our part with the intention to house a nut.
02:08	The sides of the pocket should be triangular or hexagonal with just enough clearance to allow the nut to be installed easily, but then hold the nut and prevent it from rotating when the hardware is tightened.
02:20	If we overtigthen the hardware, the pocket would typically fail first before stripping the thread.
02:26	And the pull out strength would depend on the wall thickness and the infill density of the pocket.
02:31	Generally, though, compared to forming the threads into the plastics themselves, this will be much stronger.
02:37	In some cases, the nut can also be glued in place using a super glue or CA glue, being careful of course not to get glue into the threads.
02:46	The alternative way to make a nut completely captive is to encapsulate it inside of the print.
02:51	For this to be possible, the print just needs to be paused on the layer just before the nut would be covered.
02:57	At this point, the nut can be inserted into the pocket, the printing resumed, and the nut will then be covered by the following layers.
03:04	Some printers and slicers will have a feature that allows us to do this, while some other printers need a more manual approach.
03:12	And of course, we want to avoid printing supports into the pocket as well.
03:16	This can be a challenging technique to get right in some cases, because essentially we're printing over the nut, which isn't perfectly flat and obviously has a hole in the middle.
03:25	So, we'll typically use bridging across the gap and some trial and error could be required to get the results dialled in.
03:32	The alternative option is a threaded insert.
03:35	As you might have guessed, these are metal, usually brass, inserts with threaded holes that are able to be pressed into our parts.
03:43	The outer face of the insert has a tapered leading edge, which is slightly smaller than the hole in the 3D printed part that'll be used for the insert.
03:50	This will allow us to be able to press into the hole.
03:51	The rest of the outside is knurled, so once the part is installed, it has more grip on the part to prevent it from turning or pulling out.
03:59	The other factor that helps with this is that the inserts are heated as they're pressed into the holes in our 3D printed parts.
04:06	And this essentially melts or softens the plastic around them, allowing them to be pressed into the hole smaller than the outer diameter.
04:14	Once in place, the thermoplastic will cool again and solidify or harden, securing the insert.
04:20	This is typically done with use of a soldering iron, with a special tip just smaller than the inner diameter of the threaded insert.
04:28	The insert is held just at the start of the hole with the tapered section.
04:32	Then the soldering iron can be used to heat the insert and then force it the rest of the way in.
04:38	Just be careful not to use your fingers to help, as the brass insert will heat very quickly and burn.
04:44	In summary, if we want to include threads in our design, there are some clear limitations when it comes to additive manufacturing them with thermoplastics.
04:53	Printing threaded features is possible, and so is cutting and forming threads with dies and taps, but generally the strength, accuracy and durability of these features leaves a lot to be desired.
05:04	For more integrity, we'll often turn to the use of nut pockets for captive hex nuts, or threaded inserts that can be pressed in with heat, locking them in place.