Unless you’ve got a shop with a well-stocked hardware bin, it’s a trip to the hardware store when you need a special screw. But [Sanford Prime] has a different approach: he prints his hardware, at least for non-critical applications. Just how much abuse these plastic screws can withstand was an open question, though, until he did a little torque testing to find out.
To run the experiments, [Sanford]’s first stop was Harbor Freight, where he procured their cheapest digital torque adapter. The test fixture was similarly expedient — just a piece of wood with a hole drilled in it and a wrench holding a nut. The screws were FDM printed in PLA, ten in total, each identical in diameter, length, and thread pitch, but with differing wall thicknesses and gyroid infill percentages. Each was threaded into the captive nut and torqued with a 3/8″ ratchet wrench, with indicated torque at fastener failure recorded.
Perhaps unsurprisingly, overall strength was pretty low, amounting to only 11 inch-pounds (1.24 Nm) at the low end. The thicker the walls and the greater the infill percentage, the stronger the screws tended to be. The failures were almost universally in the threaded part of the fastener, with the exception being at the junction between the head and the shank of one screw. Since the screws were all printed vertically with their heads down on the print bed, all the failures were along the plane of printing. This prompted a separate test with a screw printed horizontally, which survived to a relatively whopping 145 in-lb, which is twice what the best of the other test group could manage.
[Sanford Prime] is careful to note that this is a rough experiment, and the results need to be taken with a large pinch of salt. There are plenty of sources of variability, not least of which is the fact that most of the measured torques were below the specified lower calibrated range for the torque tester used. Still, it’s a useful demonstration of the capabilities of 3D-printed threaded fasteners, and their limitations.
Obligatory reminder: plastics creep under load, so a tensioned plastic “bolt” will loosen or bend given some time, even if it appears “strong enough”.
I think many metals creep as well. Not as much as some plastics, but they do nonetheless. Yet, you can adapt to this, using a tensionner (like a spring) I think.
They do but the creep rate for metals is so slow that for most things it’s a non-issue. We had a heavy metal object on a plastic cart from Global Industrial (some kind of GFRP material) and less than a year later we came in to find the top shelf of the cart had creeped and finally failed due to the weight. Crazy to see.
I appreciate bonkers things, but let’s be clear, printing bolts in that orientation, with PLA, is bonkers.
Usually I’d say, sure, why not trying printing [X], you might be pleasantly surprised. But with a bolt, in the highly likely event that it breaks, you’ve also lost the hole it was in. Although, unlike a broken metal bolt, I suppose you might be able to remove it with a heat gun and pliers.
Printing bolts is bonkers anyhow.
Don’t print what you can buy.
After years of disassembling dead devices and buying one or two extra screws on each hardware store trip, I’ve got a halfway decently stocked kit. The other year I finally bought some sorters and sorted out all the machine screws and their matching nuts by size/thread pitch, as well as all the other various fasteners and accessories by type.
Now when I need something, I just grab the appropriate sorter. Hoarding has never been so useful :)
Dad is still using the “big jar” method for machine screws, and as such tends to ask me when he needs one, rather than braving the jar full of randomly sized screws (or similar jar of nuts).
I still have the Big Jar I had when I realized I had to start being more organized. I started sorting it but that got boring quickly, so I immediately stopped and never returned to the project. I’ve slowly been using them by measuring them out to use in one-off 3d printed projects. I just need a rough diameter and some patience to “tap” the thread with the screw.
I suspect there will be a ton of screws left in The Jar when I kick it, but at least some will get a second life.
I’ve used ABS fdm printed bolts of various sizes for years in non-critical low load applications. Much less brittle than PLA. For anything that’s not mostly sitting on my desk I dive into the hardware stash.
It is important to note here that torque is an indirect measurement of what we really need to know. Clamping force, which is a function of bolt stretch is what is really important in most cases. A different thread design and pitch may be necessary to maximize function.
Has anyone tried printing the bolt horizontally or at a 45 degree angle? It would be interesting to see how and where it fails.