How strong are threads in 3D prints?

Obviously not as strong as in metal, but stronger than you expect.

This is a 30x30 x20mm deep block with a slightly tight M6 thread printed in.

Using a spring balance and a lever to pull straight up it started to fail at 108kgf. That's nearly 240 pounds or considerably more than I weigh.

How far in is the screw, not too impressive if it is in the full depth of the block but if only 1D thats a bit better.

Interesting, thank you Neil. How was the block held - was it, say, below a plate with an M6 clearance hole in it - or was it in a vice? How did you get axial-only pull? Could you post an image of your test rig please?

Dave

It was inserted by eight threads.

The test rig was basically a 300mm bar with a 6mm hole 25mm from one end so the ratio of forces was 12:1, the block was held in a vice with another bar across the vice to provide a fulcrum level so the bar was pretty much level. I used a spring balance.

I did other experiments with blocks under the vice jaws (not clamped) and 'hook' made from bar so the spring balance could pull up, but the maximum force that way was 75kgf.

I got some very interesting results but they will remain confidential for now!

I've not 3D printed threads, but have tapped them in plain holes. They were plenty strong enough for my application - trial run of my traction engine gear change mechanism.

It would be interesting to see a comparison of printed versus tapped holes, both cut and roll formed.

Andrew

Not sure it will make much difference as it's failure of the fill not the thread.

Neil, I would wonder how much compression of the test sample from the vice would affect things - would be intrigued if repeated with just a pair of bars with, say, 7mm holes - so the only force on the block was on its top surface?

I'm sure this may have had a negative effect by straining the fill.

I think it is important that there's a larger area of unsupported top surface, just straining the thread is unrealistic and will give a misleadingly high result - I haven't had a thread strip before the fill let go. I will make sure there's a roughly 20mm circle clear around the fixing.

I will repeat when I have PLA back in the printer, I'm using ABS at the moment.

If you were designing with 3D printing in mind would you not prefer a screw with a sharp helix around a cylindrical core? This, I presume, is to attach a 3D printed part to something with an existing thread...

So why doe thes thread title refer to strength of threads when you're actually looking at the strength of the surrounding material?

Andrew

If we are getting into sophistry, I'm looking at the strength of 3D printed threads and I have discovered they are stronger than the bulk material.

I was recently working with a product designer using M6 bolts into 3D printed parts. Thinking that the strongest result would be to have a metal - metal thread, Helicoils were being inserted into the plastic parts. Unfortunately, they appeared to self-tap into the holes without needing to used the supplied tap to cut a thread first.

The result was that on inserting the M6 bolt, the coil tried to extend to its design pitch which internally de-laminated the printed part, seriously (though not terminally) weakening it. Not visible from the surface, but recognisable by the creaking sound of the material splitting internally.

Although this was the result of incorrect process, I imagine that simply self-tapping a machine screw could have similar results, especially if the metal thread pitch 'beats' with the layer thickness.

David

(long-time lurker from Nottingham)