Bent tube questions

[vicman52]

I am a model car enthusiast, and have been using CAD to 3D print custom parts. Headers are the one part that really need a 3D sketching ability to design well. So I was excited to try the "Bent tube" solid in Layerling as an option to resorting to Solid Edge, but I did notice something that makes it difficult to use. If the tube is laying parallel to the baseplane, changing the straight length of Segment 1 works as expected, with the start end staying in place and the curved end moving. However if the tube is rotated around the Y axis, increasing the length causes the start point of the tube to shift up on the Z axis, as well as moving the curved end further away. It would be easier to use if the start end was fixed in space so changes to the length would only move the curved end, regardless of the angle off the baseplane.

[henmedia]

@vicman52 thanks for the clear description, and nice to hear the bent tube helps with headers. @plazmabokor could you have a look at this one?

layerling is free, and a small donation helps it stay that way if you'd like to support it.

[plazmabokor]

Hi @vicman52, thanks for the report, and @henmedia thanks for pointing me to it. I looked into it, and it is a real bug.

What I found
If the tube is tilted around a horizontal axis (e.g. rotation Z 30°, or a combination of X and Z), lengthening Segment 1 by 20 mm moves the start of the tube vertically, by about 5 mm in my test. X and Z stay where they are. A flat tube, or one that is only turned around the vertical axis, does not move, which is probably why it is easy to miss.

Why it happens
When a segment parameter changes, bentTubeParameterPatch re-anchors the start point only in X and Z (the comment there says the height is left to the caller). The editor then keeps the lowest point of the body on the same floor (heightPatchKeepingBase). For a tilted tube, the lowest point is not the start, so a longer or shorter segment shifts the whole tube up or down and the start moves with it.

Proposed fix

1. Segment changes (length, bend angle, bend radius, roll) keep the start fixed in all three axes, so only the curved end moves.
2. Profile changes (size, wall, inner/outer profile) keep today's behaviour, where the bottom stays on the plate. Otherwise a thicker flat tube would sink below the plate with every change.
3. If a segment change takes part of the tube below the plate, it is left there. Lifting it would move the start again, which is exactly what we want to avoid. A slicer drops the part onto the bed anyway, and it can be raised in layerling if needed.

Existing designs must not change
The fix would only touch the calculation that runs when a parameter is edited. The mesh generation and the way saved projects are loaded would stay as they are, so a tube that is already made keeps its exact shape and position. I would add a unit test that builds tubes with the old and the new code and checks that the geometry is identical, plus new tests for the vertical start position of tilted, turned and baked tubes.

Questions for you

• Are you happy with this split (segments keep the start, profile keeps the floor)?
• If you see a better solution, please say so. The one hard requirement I would keep is that already-made bodies must not deform or move.

If you give the go-ahead, I can have a PR ready with tests.

[henmedia]

@plazmabokor thanks for digging into it so thoroughly! The split sounds right to me: segment changes keep the start fixed, profile changes keep the floor. And leaving a tube below the plate after a segment change is fine, moving the start would be worse. Please go ahead with the PR.

[vicman52]

Thank you very much for fixing this. The bent tube now works as expected! I looked at Layerling as a "less complex" replacement for Solid Edge or Fusion and I think you have a winner. A "small donation" is on it's way.

[henmedia]

@vicman52 great to hear the bent tube works for your headers now, and thank you very much for the donation! The credit for the fix goes to @plazmabokor, who sorted it out.