Ticket #4018 - Projecting an ellipse on an inclined plane

[berka]

I'm working with a model that requires me to constrain to projections of an ellipse to faces in different planes.
I've confirmed with some Googling that the projections are ellipses as well. However, FreeCAD projects these as B-Splines. This is in Sketcher, as external geometry. That's not all that helpful.

I can easily make an ellipse as construction geometry on the Sketcher plane. I have the end points; I just need third point on the original geometry: e.g. the end point of the minor axis would be a good addition. (I may need the center as well, but that problem hasn't come up in my cases yet.)

Question: What is a good way to allow me to get the minor axis end point on my Sketcher plane as external geometry?
The best I can come up with is: break up the original ellipse segments into two segments each. It's just going to be unnecessarily cumbersome all over my parts.

An example demonstrating the issue is attached.

Overall Part:

Screen Shot 2019-06-08 at 06.43.55.png (32.37 KiB) Viewed 788 times

External Geometry as spline:

Screen Shot 2019-06-08 at 06.44.58.png (13.82 KiB) Viewed 788 times

My ellipse construction geometry to work around: I need that third point:

Screen Shot 2019-06-08 at 06.47.51.png (28.74 KiB) Viewed 788 times

OS: macOS 10.14
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.18.16117 (Git)
Build type: Release
Branch: (HEAD detached at 0.18.2)
Hash: dbb4cc6415bac848a294f03b80f65e888d531742
Python version: 3.6.7
Qt version: 5.6.2
Coin version: 4.0.0a
OCC version: 7.3.0
Locale: English/UnitedStates (en_US)

[meme2704]

Hi
You have to try to find a straight segment to avoid a projection in B-spline
In this case make a cross section "provisional"

[berka]

In this case make a cross section "provisional"

Thank you. This sounds promising. Can you elaborate on how?
The "clipping plane" view option is greyed out in Sketcher. The only other related thing I found was "Sketcher->View Section" which is the wrong plane.

[GlouGlou]

Use a datumplane and datumpoint with proximity attachment mode between the sketch and edge. Then use the point for external geometry and hook the ellipse on it.

[berka]

Use a datumplane and datumpoint with proximity attachment mode between the sketch and edge. Then use the point for external geometry and hook the ellipse on it.

Thanks. That works beautifully since I know and can define the line along the top of my part.

Of course, I took two steps forward and hit another snag. I wanted to use the same idea for line/plane intersection. The plane is offset from the face a few mm.

Screen Shot 2019-06-10 at 06.18.45.png (47.2 KiB) Viewed 689 times

The Line/Plane proximity point fails with:

Code: Select all

PositionBySupport: AttachEnginePoint::calculateAttachedPlacement: proximity calculation failed.

I've seen this before, and It's changed the way I model. Finding intersections of two planes or line to plane seem like pretty simple operations I should be able to do. Does anyone know how to achieve that?

[GlouGlou]

the proximity between datumline and datumplane works.

[berka]

Odd. It's never worked for me.

Screen Shot 2019-06-10 at 09.00.36.png (111.11 KiB) Viewed 667 times

[berka]

If there are no objections, I'll start a bug report on this. It's possible it's a platform-dependent issue (MacOS).

[chrisb]

I will try to reproduce on the Mac. What are the exact steps?

[berka]

I will try to reproduce on the Mac. What are the exact steps?

You can try this file:

ellipseprojection.FCStd

FCStd file from earlier in conversation, extended to show the plane/line intersection issue.

(52.91 KiB) Downloaded 20 times

1) Create a datum point
2) Select "DatumPlane Offset Incline" as a reference
3) Select "DatumLine Ridge" as a second reference
4) Try "Proximity Point 1" as Attachment mode.

The order of references doesn't matter. Proximity point 2 also does the same.

Alternately, you can start from a new file:
1) Create body
2) Create a Datum Plane (I used detached)
3) Create a Datum Line
4) Create a Datum point and try proximity point as above.