Helicoils, Threaded Insets and Embedded Nuts in 3D Prints - Strength & Strength Assessment

Considering the view count of my other two videos on threaded inserts, you seem to be really interested in that topic. Adding threads to your assemblies doesn’t only make then look better; it also increases their functionality by adding strength to the connection and adds a reliable ways to assemble and disassemble them. I got tons of questions and suggestions below the last videos and today I’ll be tackling a couple.

One of the easiest and most inexpensive solutions for adding metal threads to your prints is using regular nuts that you can embed in a couple of different ways in your prints. We’ll be testing a pocket on the side of a part in which we slide the nut and also a pocket below our print in which we firmly seat a nut. Keep in mind that this method can’t always be used if you don’t have access from the back and from the side. Also, in comparison to the other methods, it can sometimes be hard to get the proper fit. If the pocket is too small, installing the nuts can be difficult, if it’s too big, the nuts constantly fall out and drive you crazy. I’m sure you’ve also been at that point! If you want to see different methods of holding parts together being tested, let me know down in the comments.

Böllhoff Helicoil Thread Repair Kit

Another technique are Thread Repair Coils, often referred to under the brand name Helicoils. These little spring-like looking parts are something that shouldn’t be missing in any workshop. If you ever damaged or stripped threads in an expensive component, be it your bike frame or the spark plug thread of your engine, these Helicoils can save your butt. I got myself a whole, original Böllhoff repair kit with inserts of different sizes and lengths and all the tools to use them. This case cost me a pretty penny, but there are also less expensive alternatives from Asia available nowadays. You drill out the old threads or a new pilot hole with the provided drill bit and cut new threads with their tap. Next you simply screw in the wire coil insert and break the tab at the bottom. This will leave you with a new set of threads. You also often see these inserts directly installed from factory in aluminum parts to avoid damaging the soft threads in the first place, reduce the risk of seizing and prevent electrochemical corrosion due to the dissimilar metals. We’ll be adding them to our 3D printed parts so that the bolt that we use to screw parts together doesn’t touch and damage the plastic but will only be in contact with the steel wire insert.

Thread Coil in aluminum bike frame

As a reference, I’ll also be testing just directly screwing into a printed part, with a slightly undersized hole. We’ll also include the winner of the last test, the Ruthex heat seated insert. Since so far, I always tested the different methods in PLA; this time, I’ll print the parts in PETG, to also get an idea of how the strength changes.

Tested methods - Overview

For that, I printed flat test pieces with four perimeters and 100% infill on my Prusas so the poor printers finally were able to get some diversion after printing one and the same faceshield for weeks now. All the threads that I tested were M3 because that size seems to be the most commonly used for our 3D prints.

We’ll be testing two properties, the Torque Out strength and the Pull Out strength. The Torque Out Strength tells us at which tightening torque we start to damage the parts. This is important for two reasons because higher tightening torques are, on the one hand, basically directly proportional to the pretension force in the bolt. On the other hand, higher acceptable torques help us to make our connection more foolproof because we are less likely to overtighten the connection and therefore damage our threads. This is quite important for small M3 screws because they are easily overtightened.

Our sample for testing the torque out strength is a simple bar with five holes for the different connection methods. I’ll be screwing a bolt with a washer under its head for less friction in each thread and then load it with my trusty bike torque wrench to find the maximum bearable torque.

The maximum allowable torque for the bolt screwed directly into the plastic was only 1Nm. At that point the PETG threads started to shear and failed. I was able to remove the bolt after the test. Next I tested the threaded insert that was able to take 3Nm until the insert started to rotate in the plastic. The bolt and the insert were so tightly clamped together that I wasn’t able to remove the bolt anymore because the insert always turned. The Helicoil, unfortunately, also was only able to bear 1Nm of torque before the threads in the plastic sheared. Still, even though this doesn’t sound like a lot but 1Nm torque on an M3 bolt already results in more than 1500N in pretension which is mostly plenty for our 3D printed assemblies.

Pretension Force for an M3 bolt with a friction coefficient of 0.14 at different torques

Here, I was able to remove the bolt after the test. Next came the nut in the side pocket. I expected quite a lot, but I wasn’t able to load it more than 2Nm. At this torque the PETG failed in compression, and the nut dug into the material. I wasn’t able to remove the bolt anymore because it jammed with the nut that just rotated in its pocket. The last test was very similar, only that here, the nut was added from the bottom and not from the side. This part failed due to the same reason and with the same failure mode as the last one and the nut dug into the material at roughly 2Nm.

Torque-Out Strength

Let’s next get to the pull-out test in which we evaluate how much axial load a specific threading method is able to bear until it fails. This is important because it shows us which technique is capable of bearing the most amount of load and can clamp our parts together the tightest. I loaded the samples into special holders on my DIY universal test machine. The test pieces, of which I tested three of each type, are placed in the lower jig, and a small metal plate is in the upper one that serves as support for the bolt that is then screwed into the sample. The Universal Test machine will load all parts at the same speed for more comparable results.

The samples in which I directly screwed the bolt into failed, on average, at 118kg. At peak load the threads in the plastic failed and sheared off, which remains we can even later find in the threads. Interestingly, the threaded insert was not able to bear more load and also ripped out at 119kg. The Helicoils also failed at the same load and sheared the plastic threads on average at 120kg. The samples where we inserted the nut from the side failed significantly earlier, and this connection was only able to take 86kg on average. The nut sheared mostly cleanly through the sample. At last, I tested the parts with the nut inserted from the bottom. Those were the strongest, and they were able to take around 166kg of axial load. Thes parts failed quite violently, and they ripped a huge crater out of the part. So this simple method of adding metal threads to your part really seems to be feasible.

Pull-Out Strength

Typical Force Displacement Plots of the different connection methods

Comparing the PETG results to the previous PLA ones also shows that even though it’s more ductile and doesn’t shatter that fast, it’s not as strong as PLA.

Material Strength Comparison

In order to explain why the bottom pocket performed better than the side pocket and also how we could have predicted the performance of the other 3 methods, let’s do some strength analysis. I’ll go over it briefly, but the formulas are available on my website for your if you want to go though them again. So, the reason why I built my Universal Test Machine in the first place yeas back was, so that I can test dog bone samples and measure the material properties of 3D printing materials. The main material property that we need for our assessment is the tensile strength of the material and for the PETG I used, this is roughly 50MPa. We’ll be taking a look at two different failure scenarios. Whichever comes first should size our part. The first one is shear-out. A shear load is a load when basically two surfaces, real or imagined, slide over each other. In our case, for example the insert or the threads shearing out the plastic. For the inserted nuts, it’s the loaded material above the nut, shearing out of the rest of the part.

Dimensions used for the Shear-Out Calculation

The shear strength of a material is not the same as the tensile strength and is not the most easy to test. I don’t have any values for it, still, we can apply the “von Mises Theorem” which is applicable for mainly ductile materials and calculate an estimated shear strength by dividing the Tensile Strength by the squareroot of 3.

Ultimate Shear Strength estimation by von Mises

Next we measure or calculate the effective shear surface and for example with the threads and inserts, this is the cylinder where the metal meets the plastic. For the nuts, it’s hexagon-shaped. Now we just multiply the shear area with the shear allowable and get the theoretical failure load.

Estimated shear failure calculation

Let’s see how those values compare to the test.

Calculated and Tested Failure Loads

They clearly show which the strongest and weakest connection is and are just all a bit higher. Not spot on but still not too bad. Interestingly, if would have calculated the allowable shear strength with the Tresca Theorem used for really ductile materials, we basically would have been perfectly on the money. But that’s a bit like throwing darts on the wall and then drawing the dart board around it.

Ultimate Shear Strength estimation by Tresca

Results with reduced Strength Allowable (Tresca)

I’m a metal engineer by trade so I on’t have a ton of experience with polymer stress analysis. If you know more about equivalent stress calculation for plastics, please let me know! With this calculation, we also now understand why the screw, that was directly in the plastic was just as strong as the Helicoil. Even though the shear diameter was higher, the resulting shear area was basically the same due to being shorter. Makes sense!

But let’s not forget compressive failure which is only important for our nut connection because they directly press against the PETG. Because I don’t have any measured compressive strength values, I’ll be using the tensile strength which is mostly close enough.The compressive area is the top surface of the nut and by multiplying that with our compressive strength gives us a predicted failure load.

Estimated compressive failure calculation

Interestingly, the one for the sample with the pocket below the part is quite a bit lower than the shear failure and also the final strength we saw in the test.

Calculated Compressive Failure and Comparison to Shear-Out failure

This is probably because due to plastic deformation and the amount of material there was above the nut, loads distributed more evenly and over a bigger area making it significantly stronger. This just shows that even though you can use strength analysis for your parts, predicting the real point of failure can be challenging and requires quite some experience with the material and product. So be aware!

Load shifting due to plastic deformation

There can be several more reasons why the results differ like anisotropic material behavior, inaccurate material properties – remember our shear strength estimation? – and that in general the exact point of failure can sometimes be very hard to predict due to plasticity and non-linear material behavior. Still, we were reasonably close and were able to show the trends nicely. So if you size your parts always apply reasonably high margins of safety, especially for safety-critical applications.

In summary we’ve seen that Thread Repair Coils, at least the type I’ve used and no real alternative to heat seated threaded inserts because they don’t add to pull-out or torque-out strength in comparison to directly screwing into the plastic. Adding nuts is a feasible alternative but can require putting some thoughts into your designs so that you’re able to insert them. Still, they sometimes really annoy me when they constantly fall out during assembly. In my opinion, heat seated inserts are still the king for durable connection that you want to use over and over. But what’s your opinion? Which method do you prefer and why? Discuss down in the comments!