Can someone help a newbie with the shell command please?

kuifje_tintin

Hi everyone, 

i am new to onshape. I am trying to design a hull of a model sailing boat (for 3d printing) on the basis of an imported STL file. I have created the hull as a solid like this:

• Trace 3d splines over the STL (the STL is only half of the hull)
• Create a surface by lofting over the 3d splines
• Mirror the hull surface.
• Create the deck and the stern.
• Create an enclosure.

Now i try to create a shell, but it doesn't work as i expect. When i remove only the top surface, i am only able to create very thin shells (0.01mm). When I also remove the stern i can also create shells of around 5 mm but not much thinner or thicker.

I read on the forums that a common cause for shells to fail is intersecting geometries. I tried the method of splitting the part and moving the split to see where it is failing, but i can't see anything wrong.

My model is here:

https://cad.onshape.com/documents/3131c85f0fd3d7809ac8f727/w/322214c9ebfb3b5e800f398d/e/2e77eba269d0b582758770fe

Any help would be greatly appreciated.

Erik

kuifje_tintin

Hi Neil and Mahir,

thanks for the tips! This is a good exercise!

So i tried both suggestions:

• Here I "cut" some tiny bits of the stern and the keel (didn't need to remove the bow) and now it seems to work so that's good. hmm I have to say i'm not really comfortable with such unpredictable behaviour but i don't have any 3D Cad experience. Especially since i don't see anything strange. Also i am now left with pieces of hull that are thicker than necessary adding extra weight to the hull. But it's not too bad.
• Here I tried to model half the hull, but i don't manage to enclose the half hull. Could you try to see what it is i'm doing wrong here?

mahir

I played around with your model, and I think the issue is with the sharp point. The surface goes pretty quickly from a soft curve to a knife edge. Because of this there is a pretty extreme curvature in that area. Neither the Shell nor the Thicken command will behave very well when trying to offset a small radius towards its center. I think you'd have better luck shelling everything except the sharp tip, and then adding a solid tip later. It's not a finished solution, but here's an example.

https://cad.onshape.com/documents/4587481b3c3175ea0e060a57/w/d2883eaca5e69534240b26db/e/300dc1ee301f3d43ef5dc882

NeilCooke

Better technique may be to model half the hull and shell or thicken. Then mirror. 

mahir

@NeilCooke, I tried modeling half. Even then, this geometry isn't very amenable to shelling or thickening inward. I didn't try offset, but I imagine the results would be similar. 

kuifje_tintin

Hi Neill, Mahir,

thanks for helping out, i'll try both ways and let you know!

Come to think of it, if the sharp curve at the bow is the problem, i can also soften that a bit, see if it helps. But cutting ofs a small piece of the bow and make that solid is definitely also an option. I'll have a go at it!


Regards,

Erik

kuifje_tintin

Hi Neil and Mahir,

thanks for the tips! This is a good exercise!

So i tried both suggestions:

• Here I "cut" some tiny bits of the stern and the keel (didn't need to remove the bow) and now it seems to work so that's good. hmm I have to say i'm not really comfortable with such unpredictable behaviour but i don't have any 3D Cad experience. Especially since i don't see anything strange. Also i am now left with pieces of hull that are thicker than necessary adding extra weight to the hull. But it's not too bad.
• Here I tried to model half the hull, but i don't manage to enclose the half hull. Could you try to see what it is i'm doing wrong here?