Thread forming screws vs bolts question

I’m wondering if people have considered using thread-forming screws for connecting plastic parts on the Primo? I think that there are some major advantages to this approach.

An example (although an expensive one) would be these McMaster ones: #99512A675

From the theoretical side, these will be more resistant to overtightening than a bolt/nut connection, since the total surface engagement area is about 1.5x larger for a 3/8" thick part. Additionally, they are very resistant to working their way loose, which is why thread-forming screws are used in industry when connecting parts to plastic components. Even with the current recesses for the lock nuts reducing thread engagement length, parts should be at least as strong.

Practically speaking, I’ve done a little testing, with very positive results. The No. 10 thread-forming screws are at least as rigid as the 5/16 bolts, without having to worry about over-tightening nearly as much. Additionally, you don’t have to mess with embedded lock nuts. Obviously, this wouldn’t work for the bearing connection bolts.

One other major advantage to this (at least for me) is that getting a really clean/concentric hole on the print becomes less of an issue.

The only real issue with this that I can see is that the current parts have cutouts for the lock nuts, which reduces the engagement length for the threads. Ideally, one would print without the hexagonal recesses.

I’m curious what people think? I’ll try and post some results from pullout / torqueout tests when I have a chance.


Whats the price difference on screw vs nut/bolt? I know Ryan works hard to reduce part count, and if they work well, you’re effectively printing the nut, right? It’s just actually part of the part.

If its sufficient to hold the parts together with enough force to be rigid and stay put, AND it costs the same or less, then it sounds like the right kind of thing for the mpcnc.

It depends on how many you’re willing to buy :slight_smile:

For a package of 50 1" No. 10 screws, it’s about $7-$14 depending on what exactly you get (torx vs. phillips for example). So price is similar to a 5/16 bolt, but without the need to buy locknuts (well, except for the bearings). It’s a tradeoff, because you increase the BOM, while reducing the total part count.

To me, the advantages are more assembly and mechanical strength/longevity.

They would need to be 8mm or 5/16" otherwise you would also need a new axle. The current bolts are giant just so We don’t have to buy super expensive axles or bearing shafts.

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Yeah, I wouldn’t use these for the bearing axles, only for connecting plastic parts (corner top to bottom, bottom to leg lock, etc). The main reason why you would use these instead of bolts is just that they are more mistake resistant, stronger, and reduce the total part count.

You would probably need wider corners to make more meat to grab onto, we rely on the nut surface area a lot.

Prints split pretty easy, much better in compression than tension.

Durr, I just realized that I was messing up the m5 bolts and the 5/16 bolt :upside_down_face:. Which explains why my math wasn’t working out for this. It’s the M3 and M5 bolts that can be replaced, not the M8 or 5/16" ones. Hopefully this question makes more sense now.

I think this is the key. Thread forming puts incredible pressure on the material, and I think print delamination would be prevalent. In a molded or even sintered 3D print, the outlook would be much better.


I’m always learning stuff from y’all.

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Thread forming screws in plastic only work once or twice. After that they chew up the plastic too much.

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