Why can I not make a thinner shell?

So I am trying to make this (which I have done, but now I am trying to see how I could do it better).


So I create a sketch, and a Bézier curve of half of the tear drop shop which is about 16mm tall, and then revolve it 180°, which is fine, and easy, but then when I use the Shell command, it won’t let me do it any thinner than 1mm.

But why is it invalid? Even if the shell was 0.001mm, the object would still be valid.

So why do I get this failure?


(Version 5.590.0 (6756) / macOS)

Are you typing a value into the input field, or using the arrow to set the shell distance?

If using the arrows, have you tried to zoom in further before grabbing the arrow? It sometimes tries to snap to the whole number if you’re zoomed out too far.

Looks like this is something with your method. On the video below I’m trying to reproduce your problem but failed.

I think the problem is in the 180° rotation and using the shell command on the half body.
If you rotate it like Xdrakosha fully 360° the shell command works fine. You cut cut the body in half after you made a shell.

Interesting. How did you do the sketch? I used a Bezier and then joined the ends with a line.

You’re right. If I rote it only 180 degree it starts acting weird.

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Another workaround is draw wall with needed thickness in sketch.

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This is a very interesting question and the answer will probably surprise you :grinning: :
the correct behavior is when the shell tool refuses to do the operation.

You are not doing exactly the same thing:

  • @peternlewis is revolving 180° and shell the inside of the body, which does not work because the requested operation is actually impossible
    *@Xdrakosha is revolving 360° and shell the outside of the body, which works but should not for exactly the same reason

You are not requesting exactly what you think you do:

  • What you really want to do is to offset the spline of your profile by a certain amount (0.4mm for instance) and then revolve it around the axis. By the way, it is the correct way of doing it if you want a sharp summit, as @Xdrakosha proposed.

  • What you actually request using the shell tool (with the same 0.4mm) is to have everywhere on the body a skin’s depth of exactly 0.4mm, which is impossible at the tip of the body with a sharp summit.

You cannot have a sharp edge on (1) and (2) and have the same width everywhere.
If you have a sharp edge on (2), then the outer surface must follow the red circle of center (2) and radius 0.4mm in the example.

If you have a round edge on (1), then it is valid to shell inward if the skin’s depth is less than the curvature of the tip. To say it simpler, if the tip is rounded by a 1mm radius circle, then the maximum shell value inward will be 1mm.

But even if it is is mathematically valid to shell outward by any value in case of a sharp summit (because the tool could create the tip as a portion of a sphere of radius equal to shell’s width), Shapr3D allows it only when the summit’s tangent is perpendicular to the rotation axis, hence when the summit is rounded. This is actually the only bug if any :wink:.

I hope it answers your question, and to be honest, I’m the first surprised by it :joy:


I’m not very good at math but was thinking about the same. Unfortunately couldn’t explain this as you are :slight_smile:

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Thanks, that is very helpful.

So what is the best way to do this? I tried it, but it took several goes and none of them were particularly obvious.

If I create just the bezier then offsetting results in the pointy part moving across the centre line.

If I add the line and close the curve, then the inset is smaller than the original and thus does not extend all the way to the centre line.

I got the result by extending a horizontal line from the Bezier, then insetting the Bezier and the line, then adding the vertical line parts and trimming everything to close the curve. But that seems tedious.


Did you see my video above?

Yes, but I can’t see what you do at 19 seconds in to close and clean the shape…?

Nothing. It is already closed. All I did is offset the spline.

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Ahh, I see, the difference is closing the curve, but then selecting just the curve. Thanks.

So this made a lot of sense. Until I started thinking about it. If that was the case, wouldn’t it mean you could not shell a cube? Or any object with a sharp corner?

So I tried that, and I could shell a cube to any thickness. So then I tried just using a triangle, and revolving that 180°, and that worked fine. So then I went back and remade my sketch again from scratch, and this time it did work fine. So now I am just completely confused as to what the difference was the first couple times I tried to do this.

Ahh well, I clearly have a lot to learn.

I definitely appreciate the help, this forum is a great resource.

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You are perfectly right, each time there is a sharp corner, the shell tool has to “guess” what to do at this corners because it is not possible to keep a constant skin width at such a summit.

This is what surprised my yesterday because I had never thought about that before, and it is not that trivial in the end.

When it is straight and simple geometry, mostly made out of lines and circles, the algorithm handle it the way we expect it, because, regardless of the maths, we need to be able to shell usual bodies :grinning:.

When it is more complicated, using spline, the tool does what it can, and when it fails, we can help it by adding rounded edge for instance at the sharp summit of your shape, or by changing the shape of the spline near the summit.

Video below with rounded summit
(the last two control points are on a line perpendicular to the revolve axis)

Negative shell = outward : the tool keeps the skin’s width constant by creating the large radius summit

Positive shell = inward : the tool has to compromise with the request at the summit; the rounded edge there helps a lot (from my experience).

Again not that trivial when you deep dive into it, and I’m sure that we’ve just scrapped the surface.