3d Modeling: How to change the angle of a 'wavy' edge?

Roy Klein Gebbinck

I have created a ring by subtracting an inner cylinder from an outer cylinder. To create a 'wavy' edge I have subtracted a low pyramid from the bottom of my ring and rounded the sharp corners with _DMFILLET. I am happy with the sinuous curve that has resulted, but would like to change the angle of the wavy edge. All around the edge I want to add the sections marked in red on the image. So I want to keep the sinuous curve on the outside of the ring, but the edge should always have a 90 degree angle with the center axis of the ring. Can this be done in BricsCAD?

Hans De Backer

Have you tried to sweep the red triangle along the wavy edge and then union the resulting solid with the ring?

Roy Klein Gebbinck

@Hans De Backer:

Have you tried to sweep the red triangle along the wavy edge and then union the resulting solid with the ring?

Not yet. I'll try that later today. Thanks.

Roy Klein Gebbinck

I have tried using the _SWEEP command, but the result is not quit what I want. The yellow profile entity does fit inside the magenta result of the sweep action. I do not know how to correct this. (I have used a rectangle instead of a triangle because the angle of the 'wavy' edge is not constant.) I have also tried SWEEPX and LOFTX but these commands do not allow the selection of subentities of 3D solids as paths/guides.

Ring_Wavy_Edge_02.png

Ring_Wavy_Edge_02.dwg

Knut Hohenberg

Hi Roy,
I think BricsCAD can create the geometry you have in mind, but you would have to follow another path to get there:
1.) create a circular spline on top of the perimeter circle, with a knot each 45 degrees (you could place more knots if you want more control on the wave shape, but let's stick to 8 for simplicity)
2.) move every second knot the desired wave height upwards (I used 0.5 units, the green curve in the attached drawing)
3.) draw a first rectangle to define the bottom surface of the final shape: perpendicular to your circle, with only the base side matching your desired geometry, and the other sides larger than what you want as result (the one in light blue)
4.) SWEEP the first rectangle along the spline, using the base point option, and selecting the point where its base intersects the circle/spline (the result is on layer 1)
5.) draw a second rectangle that will define the other surfaces of your shape (the one in orange)
6.) REVOLVE the second rectangle around the central axis (the result is on layer 2)
7.) INTERSECT the two solids to produce the final shape

If you do not care for exactness, you could also skip steps 5-7 by drawing the first rectangle defining the bottom as well as the side surfaces and simply slice the result of the sweep with a horizontal plane (the vertical surfaces would not be exact cylinders in this case).

Ring_Wavy_Edge_03.dwg

Knut Hohenberg

Forgot to attach a screenshot:

wavy_ring.png

Roy Klein Gebbinck

Knut, that is a very good explanation. I did not know that a spline can be manipulated in 3D. So I have learned something new here. Thank you very much.

I still don't understand why my first attempt, following Hans's advice, did not work.
Why does the profile twist? And why does the created 3D solid end up outside the circular path when the profile is completely inside? (To avoid this I had to choose a different base point than the intersection between the path and the profile entity.

The last problem also occurs for circles and arcs. See the attached image (path and triangular profile in white). Only if I flip the normal from (0 0 1) to (0 0 -1) is the behavior as, I think, it should be.

Ring_Wavy_Edge_03.png

Roy Klein Gebbinck

... In my previous post I am referring to the normal of the circle or arc.

Knut Hohenberg

Hi Roy,
the twisting that occurs when you translate a profile along a 3d-path is kind of inevitable, I think. There are ways to compensate for the rotation (as the sweep command now features), but their working is way over my head. When I wrote a simple (mesh based) sweep command decades ago, I ended up with two modes, a correct 3-d one that resulted in twisting, and some kind of cheating 2.5d mode that kept the profile neatly aligned (see dcl screenshot below)...

And yes, the fact that the  sweep command aligns the profile's normal with the path often means you have to flip the normal in order to get the desired result... in my example, I at first had drawn the blue rectangle with the wrong normal, and had to rotate it 180° to correct for that.

sweep.png

Roy Klein Gebbinck

Thank you for the clarification Knut.

Sakko

Hi,
the option Twist and its suboption Bank control the twisting of the sweep profile along a 3D sweep path.
In the screenshot the results of sweeping in Autocad 2012 and Bricscad 14.2.17 are displayed.
The Bank option functions only in AutoCAD from the ACAD documentation....Banking specifies whether or not the curve(s) being swept will bank naturally (rotate) along a 3D sweep path 3D polyline, 3D spline, or helix)...
In Bricscad the native Sweep command functions correctly with BANK = YES
and the SWEEPX command (my plugin) ignores the setting BANK = NO and always uses BANK = YES.
Both problems in Bricscad are due to internal API inconsistencies.

SweepBank.png

SweepBank.dwg