Horizontal hole shape option for classic Technic beams/bricks

[VanessaE]

I've been doing a lot of puttering-around with modeling and printing Technic parts, and one thing I've run into a lot is that horizontal holes are difficult to get juuuuuuust right.

The problem? Bridging droop. Even with high-end filament, decent cooling, and a very well-tuned slicer and printer, bridging lines across the top layer of a horizontal hole will always droop a tiny bit, and with LEGO's tight precision requirements, there's basically no room for this kind of error.

But I found a trick, and it's actually a really old one: make horizontal holes "teardrop" shaped, with roughly 45-degree sides, like back in the old days (I guess this is how the green RepRap teardrop logo came to be).

Except, make one change: there is so little room to work with between the top surface of a beam and the top of the hole, so one has to "truncate" the peak, i.e. cut it flat somewhere below the apex, so that the bridging droop has somewhere to go that won't interfere with axles/pins.

Here's a wireframe view of a test beam/brick I made as part of the aforementioned "puttering-around", as an example:

Here you can see that the peak is cut flat at 3.05 mm above the center of the hole, putting it about 0.95 mm below the top surface of the brick.

Of course one need not get that precise, :stuck_out_tongue: but I did a lot of trial and error before I found the sweet spot, and I wasn't being all that precise with the final sizes (just scaling up and down by rough amounts), plus I was using 0.2 mm layer height in my test prints, so the model could just as well have had the flat spot at an even 1 mm from the top of the brick (e.g. 3.2 mm from hole center).

You can also see that the "shoulder" surrounding the hole is flattened a couple tenths above that.

Here's an actual part modeled with those same measurements, printed with 0.2 mm layers at ~35 mm/s, with a (real) standard friction pin in one hole for reference.

Yes, that's a Technic type 2 turntable. :smile:

Axles turn smoothly with relatively little "catch" on the layers, and since they completely avoid the bridging, the real diameter of the hole is all that's at play.

I suggest only one new user-adjustable parameter is needed: the distance of the truncated flat spots from the hole center, for the hole itself, since the sides should normally be close to 45°, and the width of the flat spot would vary automatically with distance from center and with hole diameter.

If you're feeling adventurous, I suppose you could also let the user set the angle of the peak's sides (the better a printer's cooling, the less-steep those sides can be be, and the less vertical space the bridging droop will occupy at the top).

I'm not actually convinced that the shoulder around the hole needs to be peaked, let alone as deeply as the hole... this may need more testing, but it may be sufficient to just tolerate whatever droop there is, since the shoulder diameter need not be as precise as the hole.

[FWeynschenk]

some good ideas, i'll look into it when i have some more time after the holidays