# Extrude cut fail. Bug?

**URL:** <https://discourse.shapr3d.com/t/extrude-cut-fail-bug/18080>\
**Category:** Modeling Questions\
**Created:** [March 17, 2022, 9:41am UTC](https://discourse.shapr3d.com/t/extrude-cut-fail-bug/18080 "2022-03-17T09:41:58Z")\
**Posts on this page:** 6\
**Page:** 1

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**Author:** ![colinmok](https://avatars.discourse-cdn.com/v4/letter/c/ec9cab/32.png) [@colinmok](https://discourse.shapr3d.com/u/colinmok)\
**Post date:** [March 17, 2022, 9:41am UTC](https://discourse.shapr3d.com/t/extrude-cut-fail-bug/18080/1 "2022-03-17T09:41:58Z")

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Hi everyone, I am not able to make a extrude cut like the object in the lower right corner. As long as the object on the left has a square inner cut, I can only make a triangular cut on either side but not both. Any idea why?

Thank you in advance!

 ![image](https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/d/ddf7e2bc0fcc9113bbf0c588ff723f21a6cb07d0.jpeg)

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**Author:** ![PEC](https://avatars.discourse-cdn.com/v4/letter/p/67e7ee/32.png) [@PEC](https://discourse.shapr3d.com/u/PEC)\
**Post date:** [March 17, 2022, 11:49am UTC](https://discourse.shapr3d.com/t/extrude-cut-fail-bug/18080/2 "2022-03-17T11:49:07Z")

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Hi,  
I think the reason is that the extrusion algorithm “thinks” that the double extrusion would create a non manifold geometry.

> **[Figure 4.1 2-manifolds and non-manifolds: the surface of a) a sphere;...](https://www.researchgate.net/figure/2-manifolds-and-non-manifolds-the-surface-of-a-a-sphere-b-a-cube-c-a-torus-is-a_fig10_265219734)**
>
> Download scientific diagram | 1 2-manifolds and non-manifolds: the surface of a) a sphere; b) a cube; c) a torus is a 2manifold; two cubes joined by d) a vertex; e) an edge; f) a face form a non-manifold object. from publication: Modelling and...

The 2 cubes in fig e) are non-manifold object, so it has to be 2 separates objects.

If you look at the short video below, you will notice that the extrusion behavior is very different when you extrude one triangle or both triangles:

- in the fist case, extrusion occurs immediately as the result is always a manifold geometry
- in the second case, extrusion starts by creating 2 bodies until it could totally cut the initial body and create 2 bodies, so it is still manifold geometries all the time.

[https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/8/8f3c441e75c85b90213a10e2c0f14d55c261dade.mp4](https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/8/8f3c441e75c85b90213a10e2c0f14d55c261dade.mp4)

When you extrude the close wall shape, I think the tools would like to create 2 separate bodies but cannot because the initial shape is a close wall shape.

The final shape be can built successfully with the union of 3 shapes, but I’m not 100% certain that the resulting shape is really manifold.

[https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/c/cd13b319c5eba2f9a7923feb7d968b7cf0871788.mp4](https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/c/cd13b319c5eba2f9a7923feb7d968b7cf0871788.mp4)

So it is clearly a border case, which is not trivial for the algorithm to handle.

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<div class="post-metadata">

**Author:** ![colinmok](https://avatars.discourse-cdn.com/v4/letter/c/ec9cab/32.png) [@colinmok](https://discourse.shapr3d.com/u/colinmok)\
**Post date:** [March 17, 2022, 12:13pm UTC](https://discourse.shapr3d.com/t/extrude-cut-fail-bug/18080/3 "2022-03-17T12:13:22Z")

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Hi,

Thank you very much for the prompt response!! I really appreciate it!  
Do you think that this should be considered a bug that needs to be fixed? I think it is pretty trivial but apparently it is not trivial at all to the application.

While the workaround that you have provided solves the problem, I actually need to have the same kind of extrude cut on all sides of the closed wall structure so workflow wise it would be to troublesome.

I hope that Shapr 3D could release an update that fixes this problem.

Thank you so so much again for helping me out! I really appreciate it!!

you sir have a good day!

Best.

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**Author:** ![Koko\_Shapr3D](https://sea2.discourse-cdn.com/flex002/user_avatar/discourse.shapr3d.com/koko_shapr3d/32/1703_2.png) [@Koko\_Shapr3D](https://discourse.shapr3d.com/u/Koko_Shapr3D)\
**Post date:** [March 17, 2022, 1:45pm UTC](https://discourse.shapr3d.com/t/extrude-cut-fail-bug/18080/4 "2022-03-17T13:45:52Z")

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Hey! Unfortunately, the modeling kernel (Parasolid) does not support non-manifold bodies. We might implement an automatic workaround option, but not in the near future.

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**Author:** ![PEC](https://avatars.discourse-cdn.com/v4/letter/p/67e7ee/32.png) [@PEC](https://discourse.shapr3d.com/u/PEC)\
**Post date:** [March 17, 2022, 3:09pm UTC](https://discourse.shapr3d.com/t/extrude-cut-fail-bug/18080/5 "2022-03-17T15:09:26Z")

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Second thought, I wonder if the bug is not to be able to union the 3 parts, as the resulting body is actually not manifold.  
If you try to extrude from the sketch below, it fails with an explicit message : **non-manifold body**.

 ![CleanShot 2022-03-17 at 15.38.42@2x](https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/5/5f394b1966457033bb79e061292132971b35f346.jpeg)  
Be aware that creating the body with the union of 3 bodies just “fools” the algorithm, but in fact, the resulting body can’t be manufactured.  
So if it has to be airtight, add a small wall to make the connexion, or if not, add a small gap (even 0.01mm width is fine and won’t be noticeable). Doing so, you will create valid geometry and will save troubles later on.  
 ![image](https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/8/8cd820861d585025297ba4de1a72920ed81107c0.jpeg)  
 ![image](https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/a/a10651e8207eb628374e900992808f2cb3e4f79c.jpeg)

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<div class="post-metadata">

**Author:** ![PEC](https://avatars.discourse-cdn.com/v4/letter/p/67e7ee/32.png) [@PEC](https://discourse.shapr3d.com/u/PEC)\
**Post date:** [March 18, 2022, 5:24pm UTC](https://discourse.shapr3d.com/t/extrude-cut-fail-bug/18080/6 "2022-03-18T17:24:22Z")

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I have to correct myself:

> [@PEC](#):
>
> Second thought, I wonder if the bug is not to be able to union the 3 parts, as the resulting body is actually not manifold.
> 
> […] creating the body with the union of 3 bodies just “fools” the algorithm

Well, the answer is **no, this is not a bug** as it is extremely well implemented actually and union of the shapes does not fool the algorithm at all.  
If you zoom to the suspicious edge, you have the answer : Shapr3D automatically implement a small arc of circle and generates actually 4 separated edges and a small portion of material actually joining the left and right part of the body.

> [@PEC](#):
>
> So if it has to be airtight, add a small wall to make the connexion […] (even 0.01mm width is fine and won’t be noticeable)

… well, this is so true I didn’t notice it myself 🤣 but Shaprd3D does it automatically for us 👍.

 ![CleanShot 2022-03-18 at 17.57.49@2x](https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/7/7c4b6cd208bfd94f4b2662c31f9d7eb0e3d9e7f9.jpeg)  
 ![CleanShot 2022-03-18 at 18.02.39@2x](https://us1.discourse-cdn.com/flex002/uploads/shapr3d/original/2X/6/6555b0e59877d141c0b05a93cdb86a40b2f2f82b.jpeg)