Print a hinge that's already assembled (0.35 mm is the secret)

Can you 3D print a hinge that comes off the bed already assembled, and already moving? No screws, no glue, no metal pin. Let's design one, and find out what makes it work.

Normally a hinge is three parts: two leaves and a pin. You print the leaves, hunt for a pin that fits, and assemble. And if the hole comes out a little too tight, you start over.

But first, pause and guess. If both parts are printed inside each other, how big should the gap between them be? A: 0.1 millimeter. B: 0.35. Or C: a full millimeter?

It's B. About a third of a millimeter. Let's see why.

Print-in-place skips the whole assembly job. Both leaves are printed inside each other, in a single job. Watch what happens when I pull them apart. They were never touching.

The orange leaf carries the pin, and three of the knuckles. The teal leaf has the other two knuckles, with holes, threaded onto that pin. Five knuckles in total, alternating.

So the whole design comes down to one number: the gap. Zoom in, and there it is. Every surface stays 0.35 millimeters away from its neighbour.

Around the pin, and between the knuckles. And I didn't just type that number. The script measures it on the finished model, and refuses to export if it's off.

Why does that number matter so much? Because molten plastic never lands exactly where the slicer says. Every line squishes out a little, and sags by a fraction of a millimeter.

Make the gap too small, and that extra plastic bridges it. The two parts weld together into one solid brick.

Make it too big, and it works, but it rattles like a cheap toy.

On a well-tuned printer with a 0.4 millimeter nozzle, 0.3 to 0.4 usually works. Every printer is a little different, so print a small test first. And PETG likes to stick to itself, so give it a bit more room.

Three design rules make the difference. First: the same gap everywhere, not just around the pin. Second: a pin of at least three millimeters, so it won't snap when you twist it loose.

Third: cut each leaf back around the other leaf's knuckles. Look here: the orange plate stops short of the teal knuckle, with the same gap.

Now, printing. Lay it flat on the bed, exactly like this. The pin spans across the teal knuckles like a tiny bridge, so you don't need any supports.

But watch your first layer. The bottom layers get pressed into the bed and bulge out a little. That bulge can close the gap right at the bottom of the knuckles.

In your slicer, turn on elephant foot compensation. A small value, around 0.1 to 0.2 millimeters, is a good start.

Fresh off the plate, it can feel a bit stiff. Hold one leaf, give the other a firm twist, and you'll feel it break loose. From then on, it swings freely.

So, in short: keep a gap of 0.3 to 0.4 millimeters everywhere, print it flat without supports, and test your own printer first.

This hinge is free to download, and the gap is a parameter, so you can tune it to your printer. Tell me in the comments which gap works for you. I'm Spoolbert, see you in the next one.