[FR] Hole support dividers in classic Technic beams

[VanessaE]

When making a classic Technic beam, i.e. a 1 x N full-height brick with horizontal holes and hollow studs, one thing that's a bit of a problem is the horizontal holes.

Like with a "real" Technic beam, the holes in your model have an inset shoulder around them on both sides. The model renders and slices just fine, giving the perimeters a sort of "pinching" near the bottom of the hole. This leads to a problem -- the insets intersect with the first solid infill layer that closes-off the bottom part of the beam:

That "pinching" shape confuses the slicer's bridging algorithm, causing it to print the innermost perimeter over thin air. With nothing under them, these parts of the perimeters droop down. That leaves nothing for the infill to attach to, so it falls down altogether:

This in turn interferes with the studs from any brick the beam is snapped onto (the width of the fallen-in area is greater than the spacing between studs).

There are three ways to solve this problem:

1. use fewer perimeters in the slicing settings so that the infill never starts over air. The slicer really should detect this condition on its own and adapt, but until it can, this requires remembering to change a setting before printing a beam, and some folks (like me) can barely remember how to turn the printer on some days... :wink:
2. Clean-up the drooping with your handy-dandy X-Acto knife. In my opinion, the risk of cutting oneself is rather high when dealing with this kind of clean-up.
3. Put support walls under the holes, so that the fall-prone lines have something to sit on.

In my opinion, that last option is the best.

Holes in a Technic beam are always offset from the studs (except in a 1x1 brick with a single hole), so those walls would be between the studs as well.

As it happens, a "real" Technic beam has little pegs below the holes, which are sized to fit into the hollow studs of another beam, but printed pegs that small would almost certainly be too weak (not to mention, they won't help this problem much).

Since there is about 3.1 mm of empty space between adjacent studs, putting walls under the holes shouldn't interfere with compatibility with other bricks, and anything up to 2.7 mm or so thick should be safe for even the most horribly-calibrated printer (we don't care if these walls won't grip a stud). :smile:

I might also suggest beveling/tapering the tops of those walls, to increase the thickness of the plastic below the holes, as long as that won't interfere with the studs from another brick.

Here's a mock-up showing 2 mm thick walls, with a roughly 45° taper starting 2 mm up from the bottom of the brick:

[FWeynschenk]

I've been wanting some sort of geometry in 1x wide pieces, I like this solution. Will put it on the todo! It's probably going to take a while, kinda busy with work right now.

[VanessaE]

Note that I refreshed the mockup to show the tapering I mentioned just seconds after your comment. :)

[VanessaE]

It just occurred to me that if you move the upper surface that caps-off the bottom part of the beam, down by 0.2 mm, it will be below the edges of those hole insets, and thus should end up with straight perimeters like a normal brick has, eliminating the need for those walls I suggested. That distance should still keep the surface high enough to not interfere with other bricks' studs.

That said, the walls are still a good idea on both Technic beams and regular bricks, even with that surface moved down, as they would increase the part's surface area on the print bed (better adhesion). Their presence would also increase the outer walls' resistance to bending outward from stud pressure (particularly on longer parts).

[VanessaE]

While I was working on other LEGO stuff, I modeled a test object that is in part, a classic Technic studded beam, and out of habit, I gave it a standard 3.2 mm diameter peg below the hole, as found on real 1xN bricks and plates....

I didn't intend for that peg to be useful (call it OCD :smile: ), but just for grins, I tested how it would behave when fitting a classic Technic beam onto it... At it turns out, a peg really WILL work below a hole! It's not weak at all, and wasn't hard to print like I expected, but I should note that I'm using high-quality filament (Atomic metallic silver PETG at the moment).

So it looks like walls aren't needed -- pegs should work fine.. You'd just want to let the user adjust the diameter (somehow, 3.2 mm worked fine for me). Of course you may want to give the user the option of either pegs or walls.