Sketch on body face flipping problem

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Cekuhnen
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Joined: Sat Jul 10, 2021 11:37 pm

Re: Sketch on body face flipping problem

Post by Cekuhnen »

sure here:

The arc flip is interesting:

https://drive.google.com/file/d/1DPqytQ ... d_HEA/view
Screen Shot 2021-07-31 at 3.26.52 PM.png
Screen Shot 2021-07-31 at 3.26.52 PM.png (50.56 KiB) Viewed 707 times
Screen Shot 2021-07-31 at 3.26.41 PM.png
Screen Shot 2021-07-31 at 3.26.41 PM.png (41.09 KiB) Viewed 707 times
Attachments
Trim Surfaces Raw.FCStd
(10.82 KiB) Downloaded 20 times
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GeneFC
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Re: Sketch on body face flipping problem

Post by GeneFC »

Cekuhnen wrote: Sat Jul 31, 2021 7:28 pmsure here:
I tried multiple ways to duplicate the so-called flip, following your video, and I could not. I am using a very recent version of FC, 20.25306.

Then I realized you were using version 0.19. When I tried that version I was able to make the *sweep* flip.

Neither sketch actually flipped in any case, including the release version 0.19. The sweep flipped, but that is an entirely different problem.

When a sketch flips it will be completely obvious while one is still working in the sketcher. I can make it happen at will if I greatly scale up or down the existing elements in a sketch. I try not to do that. :o

Gene
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Cekuhnen
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Re: Sketch on body face flipping problem

Post by Cekuhnen »

GeneFC wrote: Sat Jul 31, 2021 9:57 pm Neither sketch actually flipped in any case, including the release version 0.19. The sweep flipped, but that is an entirely different problem.
Very interesting - could you elaborate?

I should have stated that I was using 0.19.

But I have this with 0.2 too and also RealThunder

See here:
https://drive.google.com/file/d/1ExDsVV ... 1M7Lt/view

It seems the moment the sweep path top point when switching from going to the right side going to the left side is the moment
the second sketch flips
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Cekuhnen
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Re: Sketch on body face flipping problem

Post by Cekuhnen »

Here I remember a similar issue with sketch elements flipping on its own - but I do use projection of faces

https://forum.freecadweb.org/viewtopic. ... 57#p520257

When I have a moment I will do the screen cast
Have the baby on my arm now
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Re: Sketch on body face flipping problem

Post by chrisb »

Flipping sketches are well known and discussed in the forum every now and then. They can occur when a solution for solving the constraints is not unique. It is a solver issue, which is ok, though unexpected.

What you describe in the OP is something different where the attachment mechanism is involved. Again it is ok, because all required conditions for the attachment are fufilled.
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Re: Sketch on body face flipping problem

Post by GeneFC »

Cekuhnen wrote: Sat Jul 31, 2021 11:20 pm Very interesting - could you elaborate?
The distinction might be fleeting, but there is a major difference between the classic "sketch flipping" and what you are seeing. Sketch flipping is an old problem, and it has been quite actively addressed and mostly controlled.

Here is a set of before and after screen shots to show what I am saying.

All of this is in version 0.19.

Before (as loaded)
Main Window Before.png
Main Window Before.png (19.39 KiB) Viewed 617 times
Sketch003 Before.png
Sketch003 Before.png (5.46 KiB) Viewed 617 times
Sketch004 Before.png
Sketch004 Before.png (1.56 KiB) Viewed 617 times

After moving Sketch003 a bit
Main Window After.png
Main Window After.png (23.25 KiB) Viewed 617 times
Sketch003 After.png
Sketch003 After.png (4.65 KiB) Viewed 617 times
Sketch004 After.png
Sketch004 After.png (1.55 KiB) Viewed 617 times

As can be seen, Sketch004 did not change from a frown to a smile. It looks unchanged. However, the *attachment* of Sketch004 to Sketch003 as part of the Sweep operation did indeed "flip".

I am in no way arguing this is good, but it is a different problem than sketch flipping. As I noted above, the attachment problem appears to be fixed in version 0.20.

For this test:

OS: Windows 7 Version 6.1 (Build 7601: SP 1)
Word size of OS: 64-bit
Word size of FreeCAD: 64-bit
Version: 0.19.24267 (Git)
Build type: Release
Branch: master
Hash: b2ca86d8d72b636011a73394bf9bcdedb3b109b7
Python version: 3.8.6+
Qt version: 5.15.1
Coin version: 4.0.1
OCC version: 7.5.0
Locale: English/United States (en_US)

Gene
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Cekuhnen
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Re: Sketch on body face flipping problem

Post by Cekuhnen »

Got it - thank you for the explanation and pointers.

Here is a recording of the chaos I can see inside my parametric side stand.
NOTE: I apply a little the workflow I am simply used to, work with professionally, and teach - with Fusion360
and I was curious to see how this could be applied to FC and RealThunders build.


https://drive.google.com/file/d/1tI_6VO ... hsyij/view
Attachments
Side Table2.FCStd
(309.58 KiB) Downloaded 15 times
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Re: Sketch on body face flipping problem

Post by chrisb »

GeneFC wrote: Sun Aug 01, 2021 12:52 am As I noted above, the attachment problem appears to be fixed in version 0.20.
I can reproduce the flipping attachment in 0.20.
OS: macOS 10.16
Word size of FreeCAD: 64-bit
Version: 0.20.25311 (Git)
Build type: Release
Branch: master
Hash: c5524bb58a7126fa2242d6137eb82ee9d54e82d4
Python version: 3.9.6
Qt version: 5.12.9
Coin version: 4.0.0
OCC version: 7.5.2
Locale: C/Default (C)
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GeneFC
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Re: Sketch on body face flipping problem

Post by GeneFC »

Here is a very simple-minded exhibit of what is commonly called sketch flipping.

Incomplete shape:

Start.PNG
Start.PNG (3.71 KiB) Viewed 528 times

Select lower right open ends and use coincident constraint. What was wanted:

Expected.PNG
Expected.PNG (3.63 KiB) Viewed 528 times


What actually happened:

Actual.PNG
Actual.PNG (3.43 KiB) Viewed 528 times


The upper right line segment has "flipped".

Some cases are more complex or subtle, of course, but this is the general concept of sketch flipping.

Gene
GeneFC
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Re: Sketch on body face flipping problem

Post by GeneFC »

I played with this example a bit more and came up with an even more curious observation.

The attachment details and model shape seem to depend on the orientation of the end of the line in Sketch003. When the end of Sketch003 is exactly vertical there is a different result from what was shown previously.

Vertical end.PNG
Vertical end.PNG (5.23 KiB) Viewed 508 times
Intermediate case.PNG
Intermediate case.PNG (21.68 KiB) Viewed 508 times

The attachment goes through some sort of transition state.

Gene
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