Extrude shape to 3D produces strange geometry

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ygoe
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Extrude shape to 3D produces strange geometry

Post by ygoe »

Hello,

I'm just making my first steps with FreeCAD. My current playground is a wood construction that I want to build. It'll be a separator wall that I can stand in my room behind me to serve as a webcam background and that'll be filled with rock wool to improve acoustics.

So I started by creating two sketches, one for the back wall and one for the border. Both shall be layered on top of each other. The back wall is a simple rectangle, extruded to 3 mm thickness. I guess that worked well. But the next layer, a frame of four wood bars, produced strange results. I can't even describe that rendering error. I've attached the source file and a screenshot of it.

I'm working in the Part workbench. The sketches should be fixed and complete. The sketch "Rahmen" (frame) was made from 4 rectangles.

What have I done wrong?

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papyblaise
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Re: Extrude shape to 3D produces strange geometry

Post by papyblaise »

we've said it a hell of a lot of times :!: :mrgreen: :oops: :?: :!: , [edited out by mod]
NO T-PROFILE
do for example the top and the bottom, then the left and the right

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Shalmeneser
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Re: Extrude shape to 3D produces strange geometry

Post by Shalmeneser »

No 3-lines vertex (T-junction) ! The border line of a surface need to be just a line
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Shalmeneser
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Re: Extrude shape to 3D produces strange geometry

Post by Shalmeneser »

2 versions : Part or PartDesign wb.
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Cekuhnen
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Re: Extrude shape to 3D produces strange geometry

Post by Cekuhnen »

ygoe wrote: Thu Jul 22, 2021 12:18 pm My current playground is a wood construction that I want to build.
Hi fellow wood worker :)
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GeneFC
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Re: Extrude shape to 3D produces strange geometry

Post by GeneFC »

This topic has already been fully explained in terms of FreeCAD. However, there is a difference in the experiences of a woodworker and a metalworker. (I do both.)

If I gave the attached sketch to a metal worker the response would be "What do I do with this?

A woodworker might respond "How tall should the strips be?"

Tee-junctions.jpg
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Gene
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Re: Extrude shape to 3D produces strange geometry

Post by Kunda1 »

papyblaise wrote: Thu Jul 22, 2021 2:26 pm we've said it a hell of a lot of times :!: :mrgreen: :oops: :?: :!: , [edited out]
@papyblaise that's inappropriate.
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Cekuhnen
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Re: Extrude shape to 3D produces strange geometry

Post by Cekuhnen »

ygoe

FreeCAD unlike other CAD apps cannot extrude from a sketch profile or sketch element selection.
it only fully extrudes everything in the sketch


If you use the RealThunder build you can make a master sketch and then from it export children
and from those extrude the parts needed.

Look here
https://www.youtube.com/watch?v=8JdwrVEKvIU&t=76s

also good
https://www.youtube.com/watch?v=jfNBfdIpzDQ&t=1s
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ygoe
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Re: Extrude shape to 3D produces strange geometry

Post by ygoe »

Oh dear, first of all thank you for the numerous replies. I'm sorry if this has been asked a thousand times before. Maybe the application isn't sufficiently clear in this regard then. (Just a guess though. I'm not feeling comfortable with the app yet, it doesn't seem to be novice-friendly.) But this all sounds complicated. I'm not really a mechanical guy, but I guess I can imagine composed objects pretty well and now just trying to find my way into CAD. I'd like to use it later for creating 3D print designs of small cases and other parts. Now I tried to use it for some wood project. Until now I simply used pen and paper for things like that.

I don't know where I made a T profile here. Maybe that means something else than I think the term means. Google for "T profile" and those images resemble exactly my interpretation of the term. At least it wasn't my intention to build that.

I understood that a sketch doesn't know what parts are meant to be filled, even though I created the object outlines in a way that should be obvious. It might be because FreeCAD doesn't remember how lines were added to a sketch. In the end there are just lines, no more rectangles. And then it wasn't smart enough to recognise that it doesn't know what it's doing and just renders garbage. This would mean that a sketch isn't all that useful, at least for designs like this.

I opened one of the provided files here but didn't understand how it was created, so unfortunately I couldn't make much use of it other than see that my intended design is actually possible (which I assumed).

The first YouTube video seemed to do something else (at least I don't see similarities here). The second about the Japanese toolbox gave me the impression that a sektch isn't a good solution here. I'll try to restart from scratch by creating 3D parts directly and positioning them in space. From what I've seen about the part manipulation tools, this should be sufficient for the project. The tutorial videos I watched first suggested that it's a good practice to use a sketch and then extrude things in the Part workbench. Now I believe this isn't always true.
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Re: Extrude shape to 3D produces strange geometry

Post by GeneFC »

ygoe wrote: Fri Jul 23, 2021 4:34 pm I don't know where I made a T profile here. Maybe that means something else than I think the term means. Google for "T profile" and those images resemble exactly my interpretation of the term. At least it wasn't my intention to build that.
Look at the sketch I showed a few posts back. There are several Tees, very similar to your sketch. Three line "segments" meet at a single point.

The problem is that FreeCAD has no idea what it should do when three lines intersect at a point. If you are trying to "pad" the sketch into a solid should FreeCAD extrude both sides of the Tee line equally? Should only one side be extruded? Which one?

Some sketches are more complicated, of course, so the problem becomes worse and worse. That is why FreeCAD has strict rules on how sketches are created if those sketches are to be used for pads and pockets. (There are other uses for sketches as well, but that is beyond this topic.)

As a woodworker you will instantly know what to do. However, FreeCAD is used for many purposes, and that Tee structure is ambiguous, even to humans.

You are not the first and unfortunately will not be the last to hit this snag.

Hang in there!

Gene
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