Brass threaded inserts significantly transformthe look, feel and use of your 3D printed parts.

But how much better do they perform in comparisonto 3D printed or conventionally cut threads? Let’s find out more! Guten Tag everybody, I’m Stefan and welcometo CNC Kitchen.

Threaded parts are usually used in boltedconnections when you want to connect two or more parts.

In comparison to a glued connection, threadedconnections are non-permanent allow you loosen the connection again for example for disassembly.

Such a connection can be established withparts that have through-holes and a bolt or nut.

Instead of using nuts, the threads can alsodirectly be integrated in the part itself in order to reduce the part count or if spacedoesn’t allow the use of a nut.

Such integrated threads can be realized inmany different ways in your 3D prints.

Starting from directly screwing into a slightlyundersized hole over cutting threads with a tap or modeling and printing the threadsitself, putting threaded insets into your parts is a way to add quite some value tothe look and feel of a 3D print.

These threaded insets come in many differentsizes and shapes but mainly consist out of a body that is threaded on the internal diameterand has knurling patterns on the outside.

The shape of the knurling and the groovesare not for esthetical purposes but determine the resistance of the insert against beingpulled out or fail due to the applied torque.

The insets are usually mounted into the partby heating them up with a soldering iron and then slowly pushing them into the part sothat the molten plastic molds around the brass part.

Grooves help against pull out and verticalknurling prevent torque out.

The specific shape depends on your application.

In order to find out how they perform in comparisonto more conventional ways of adding threads to parts I ran a big study with around 50specimens using my test equipment to answer this question as objectively as possible.

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In the upcoming tests I’ll compare 4 differentmethods of threads in a 3D printed part.

I’ve chosen M5 threads for this analysiswhich is a very common size I often use.

Other thread sizes should behave comparable,though probably not perfectly the same.

If you usually use other thread sizes letme know in the comments.

First, I’ll directly screw a bolt into anundersized hole.

Second, I’ll tap the hole.

For both direct and tap method I modeled theminor diameter of the thread in CAD, so in this case 4.

5mm.

Third, I’ll physically model the threadsin Fusion 360 as I have already shown in depth in a previous video.

I didn’t use any offsets and was quite surprisedthat threads even as small as M3 worked perfectly without any rework necessary in both horizontaland vertical printing orientation.

If you want to try this out with your ownprinter and material, I linked my testpart down in the description.

Lastly I’ll put threaded inserts into thetest samples with a 200°C soldering iron.

For this hole I used the diameter of the groovesthat my insets had which might have been slightly too small since there was a bit of flesh remainingafter the installation.

In order to make the results comparable allthreads will have the same length, in my case 8mm.

All samples were printed on my Original Prusai3 Mk2.

5 in Prusament PLA using 0.

15mm layer height and 4 perimeters to add some strengthto the threads.

I will be testing the pull-out and torque-outstrength of the different threads.

The pull-out tests will be performed withsmall test disks on my Universal Test Machine where we will see how and when the bolt isbeing pulled out of the threads it was screwed into.

For the torque-out test I’ll tighten thebolt until the threads fail and measure the failure torque with this torque wrench thatwas delivered with my bike.

It’s not terribly accurate but will stillgive good comparison values.

I’ll also test each thread variant in bothvertical and horizontal printing orientation with 3 samples each for statistics which resultsin almost 50 individual test all in all.

So lets take a look at the pull-out test.

In hindsight I was quite happy that I didn’tuse bigger threads, since my Universal Test Machine got quite to it’s limits duringthe tests.

So, the pull-out strength for cut, modeledand no threads were very similar and the samples failed just shy of 2000N.

All parts besides the modeled threads thatwere printed horizontally failed with the whole threaded section shearing out of thepart.

The threads themselves weren’t the weakpoint but more the material around.

Still with around 200 kilos of failure loadthey are quite strong and all methods seem feasible for adding threads to your parts.

The samples with the threaded insets performedeven better and the highest failure loads were almost 3000N.

Here again the samples failed because theplastic gave way and not the insert ripped out.

The horizontal specimen showed the highestfailure load because in this orientation the most amount of supporting material is printed.

The inserts in general performed better becausethe shear area where the parts failed is simply bigger due to the bigger diameter.

In summary we have seen that if you aren’tprinting with 100% infill, the threads don’t seem to be the weak point of most designs.

The Torque-Out tests might show the real strengthof the threads even better because if you tighten a bolt the tightening torque willbe converted by the threads into an axial load minus some friction, that is in our casedirectly reacted at the bolt head.

So only the threaded section gets loaded.

In order to reduce the influence of frictionas much as possible and only take a look at the pretension I’ve oiled all threads beforethe test and used an oiled washer under the head to not give any design a specific advantage.

The plastic threads again all performed kindof similar with no huge difference besides again the modeled threads that were printedhorizontally.

With the 0.

4mm nozzle, they just didn’tprint precisely enough to add maximum strength.

All threads failed at a tightening torquebetween 3 and 4Nm which isn’t too much but still not too far form the recommended tighteningtorque of regular M5 bolts which is in the range of 6Nm.

In this test the threaded insert could reallyshow where it shines and the samples averaged a failure torque of around 10Nm which is 3xthe load of the plastic threads.

I really expected in this case that the insetis torqued out of the plastic but actually before that happed the brass inset itselffailed which is really remarkable.

This nicely shows that if you have a strongbase part and really need a good threaded connection the inserts are the way to go! Since PLA is quite a strong and rigid materialthis might have been the reason why the plastic often didn’t fail and I’m thinking abouttrying similar tests with other materials in the future.

What do you think? Another benefit of the insert might come intoplay if you have a connection that is not only tightened once but rather more regularlyused.

Plastic threads will wear out way more quicklythan the inserts so in these cases the metal alternative is again the way to go.

As already mentioned in the beginning threadedinserts are not the only way to add metal threads to your part and I honestly have tosay that this has been the first time I really used them.

I also linked a couple of really interestingarticles down below if you want to dive deeper into the proper use of the brass inserts.

The tests have shown that indeed inserts doperform the best.

Still threads in plastic might often be okayfor many applications.

The only option I just wouldn’t recommendis to screw directly into the plastic because that adds a lot of hoop stress around yourhole that can split your part.

For such an application there are specificplastic screws that nicely cut into the material with minimal radial load.

Even though the modeled threads didn’t performthe best they are in my opinion still a feasible way to bolt parts together.

The modeled threads do add a lot of trianglesto your part so the model size will increase significantly but printing time, at leastin my case, ended up basically the same.

I also wanted to share one last trick whichmight not be something that everyone knows.

In order to add strength to the threads it’sa good idea to increase the shell thickness of your 3D prints.

Unfortunately, that might increase the materialuse and the print time of your part significantly because the settings are applied globally.

Similar to my Smart Infill procedure you canuse modifier meshes in Slic3r or Cura at the locations of the inserts and only very locallychange the number of perimeters.

I for example created cylinders directly inFusion 360 at the locations of my threads, exported them as a separate body and importedthem into Slic3r PE as a modifier mesh.

Unfortunately, Slic3r doesn’t leave a closedouter shell and creates seams where the modifier mesh penetrates the main part.

CURA works way better in this regard and nicelyonly reinforces the areas you intended without being visible from the outside.

CURA even allows you to create the modifiermeshes in the slicer so you can even apply them to already existing parts.

Finally I’d really like to get your opinionon that topic.

Please tell me down below how you usuallytackle threads in your designs.

Do you add inserts? Did you have any other experience than theresults from my tests? I’ve only tested PLA so far.

Do you think the results differ with otherpolymers? What other topics would you like to see meinvestigate in the future? Please leave a comment.

Thanks for watching everyone, I really hopeyou enjoyed the video.

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Auf wiedersehen and I hope to see you in thenext one!.