Does teardrop need a bottom corner?

[adrianVmariano]

          There are a lot of clearance guidances published, recommending as much as 0.5 mm for a loose fit. There are going to be horizontal tolerance variations as you print, arising from:

• Variations in the filament diameter. A typical spec you see on amazon is ±0.03 mm, which creates variations in volumetric flow, which in turn creates variations in line width.
• Extrusion multiplier. This is filament-specific and most people don't calibrate it for each new spool they get. When I printed a cube with white silk PLA in vase mode with a line width of 0.45, I got a wall thickness of 0.5-0.54, which is considered an acceptable tolerance variation. But that means the perimeters from that filament are nearly 0.05 mm wider on each side. Generally slight overextrusion like this results in better quality parts.
• Perimeter order. Usually one prints inner perimeters first, so that outer perimeters have something on the side to stick to when printing overhangs. However, because the inner perimeter may be squished a bit wider, the outer perimeter is going to get pushed slightly outward.

Printed vertical variations from the design arise mostly from:

• Using constant increments of layer height. If your design has something 10.27 mm off the build plate, there is no way a 0.15 or 0.2 layer height can match it. I don't know the slicer algorithm, but I expect the slicer would add or omit a layer depending on which action results in the smallest error.
• First layer. Typically the first layer is 0.2 mm unless you consciously set it to something else. The Prusa default printer profiles all have 0.2 mm for the first layer regardless of the layer height you print with. And that first layer, because it is squished onto the build plate, is not guaranteed to be 0.2 mm either.
• In the case of horizontal holes, the edge of the perimeter, due to line width and printing outer perimeters last, may intrude a bit into the hole, and at the bottom of the hole this intrusion can result in an entire layer's worth of vertical clearance loss, which is why it's a good idea to put a 1-layer-deep point at the bottom of a teardrop.

These variations typically combine to cause 0.1 mm clearance horizontally to be a pretty tight fit. I find that 0.2 mm clearance for small holes is pretty snug. And the vertical variations are why the teardrop needs a little point at the bottom.

Which brings me to your question about what's wrong with 3 layers of bridging. More than 3 layers would likely work fine if the layer height in the design equals the layer height being printed. It may not be in exactly the right place due to the first layer thickness offset, but close enough to work. Where it goes awry is when you have the design layer height smaller than the print layer height, in which case you'll get the first layer of bridging and maybe the second, but likely not the third. To compensate for this, when I release a design to the public that has holes, I assume a layer height of 0.3 mm because printing at a smaller layer height is going to hit all the bridges, and may print two layers on one bridge, but that's OK.

Originally posted by @amatulic in #1467 (comment)

[adrianVmariano]

The test fit device I was using is for vertical holes. For me a 0.5mm clearance sounds ridiculously large unless what you want is not a "fit" so much as a roll around. I have not attempted any kind of test fit on horizontal holes. But I have found horizontally printed screw holes to work fine at M3-M5 size without the bottom point and without a huge clearance. But I am not sure what clearance I was using.

If the focus is on tiny holes, though, there's another issue. In OpenSCAD a small hole doesn't have resolution to resolve those points, generally speaking, unless you've gone crazy with your $fa and $fs settings. Or on the flip side, the point will be there automatically because of faceting. Actually it seems like using circum=true might be more important and more useful than adding a bottom corner, especially if the focus is on tiny holes. It appears that screw_hole is not using circum=true, which is maybe a mistake.

[amatulic]

Design guidance I was referring to in #1467 - https://www.hydraresearch3d.com/design-rules#holes-horizontal

In the figure below, "a" is basically the layer height. This is what I always use for horizontal holes: a 40° slope at the top tangent to the circle, not ending in a point but ending at a bridge 1 layer above the top of the hole. And a point at the bottom 1 layer from the bottom of the hole. When I print this for a small hole (like 3 mm or less) it comes out looking like a round hole.

[RAMilewski]

If you're going to break open the code for teardrop anyway:

There are situations where it would be useful to be able to apply the teardrop rounding to the top of a cuboid.

Here's a shell for a prototype USB dongle made by splitting the box and rotating the top half for printing. I would have preferred to use rounding rather than chamfering, but that would require a teardrop rounding on the top to print well.

[adrianVmariano]

Putting teardrop on the top would require some kind of mode for signaling where you want teardrops, and I don't know the cuboid code at all. This has nothing to do with the actual form of teardrops (e.g. the extra corner).

If you do want teardrops on top, you can put them there using the edge masking stuff with attachments if you don't want to do a mirrored cube.