Making a solid from 2 parallel surfaces

Help again please!

I have drawn the 2 parallel ends of an object as surfaces, but now want to add extrusions INTO the surface. I have been told that I should have modelled a solid instead. Is there a way to make the “gap” solid?

Ian .

It sounds like you just have to draw your gap curve and extrude that, then trim and join. Post your model so that someone can take a look.

That’ll be my next question then. How do I do that?

Post your model so we can better understand what you have.


A better explanation.


It’s very hard to see what the actual situation is there.
You can post the 3d model in the same way as you did with the images.
From the looks, it can be as simple as just running the join command.

Fuel tank model.ini (105.8 KB)

Are you able to access this?

I can access this file, yes, but it contains your Rhino settings - not the 3D model. When you save your file to disk, you should have a *.3dm file somewhere. That’s the one I’m after. :wink:

COPY FUEL TANK 2.3dm (1.3 MB)

I think I’ve managed to upload correctly. As you can see from the photo, the 4 shapes on the side of the tank should be indentations. Somehow I managed to do the rectangular one, but cannot for the life of me, remember what I did.

No doubt that when I come to add the rectangular box on the bottom, I will make the same mistake. I think I need to know how to “solidify” several surfaces together.

Help please!


Yes :smile:

First off, there are lots of duplicate surfaces in this file. A lot can be selected with the SelDup command and those can just be deleted. Additionally there are others where sub-surfaces are duplicated and those should be deleted as well. You also have two bad objects - use SelBadObjects and delete them.

Then, your top and bottom (as oriented in the file - not in the picture) sheet of metal have no thickness. You will need to decide the thickness of these and put another surface there (you could take one of the duplicates and move them to the correct place).

When that is done, you can use surfaces to split each other where they intersect so that they can be joined into a polysurface.

In the attached file, I’ve put things on separate layers to easier get an overview of the situation. The edge surfaces of the flanges should be simple planars surfaces - I don’t know how you made those but they are overly complex. They don’t join up with the outer vertical walls either and can easily be remade from the vertical outer and inner surfaces (just use the EdgeSrf command and select one edge of one of the outer and one edge of the corresponding inner surface).

To be able to join the cut that is selected in the image above with the top surface, select the top surface and call the Split command. Then select the polysurface that is selected in the picture; delete the new inner part and join the rest.

Give it a shot and let us know how it goes…

@pascal, on layer Default, there is a surface that gets selected with the SelBadObjects but, when selected, calling What asks for the selection of objects. If, on the other hand, the object is selected and you click the Details button in the Properties panel, you get the requested info:

Invalid polysurface.
Edge Tally:
Render mesh: none present
Analysis mesh: none present

Shouldn't What be able to do this?

Partly successful, I think.:slight_smile:

  1. I got rid of all the duplicate surfaces, but am absolutely flummoxed as to how they got there in the first place.:confounded:

  2. Ditto the bad objects. Although something was deleted I have no idea what, as nothing was “highlighted” when I did this. I could only tell something had been deleted by the edit history.

  3. As I had already deleted the duplicated surfaces, I joined the top and bottom by drawing polylines between corresponding points on each surface, and then making the whole lot a solid. Having said that, when I click on any of the surfaces, I would expect the whole thing to be highlighted, rather than the individual surfaces?

  4. The edge surfaces of the flanges should be simple planars surfaces - I don’t know how you made those but they are overly complex. They don’t join up with the outer vertical walls either… I wasn’t aware of the EdgeSrf command and so created the edges by using the “Surface from planar curves” command. I have since deleted all these and did as you suggested. I also added a fillet to both edges of the top of the flanges. When doing so however, even though I had “Extend=No and Trim=Yes” activated, the surfaces wouldn’t trim. I had to go back and trim them all individually.:sleepy:

  5. I have failed miserably with the required action for the cut shown in the image. The action fails every time I try?

2 other questions;

A) Is there an explanatory facility that describes what each command does? I need to explore them as I have no doubt that I will end up doing everything in a roundabout, long winded way.

B) Rather than type in EdgeSrf, for example, numerous times, is there a facility to carry out the same action numerous times. In the 2D CAD programme I use, (cheap and cheerful), double clicking on the command, lets you do that.

I really appreciate your time and effort in helping me out.

Many thanks and regards,


FUEL TANK 3.3dm (1.2 MB)

Hi again,
1 - this file is again full of almost-duplicates. You have to try and look over your shoulder at each single step that you take so that you can find out how that happens.

2 - bad objects that you don’t see: yes, that happens. Do you have -CheckNewObjects set to Yes? Note the hyphen in front of the command - that will bypass the dialog box and actually give you information about the current status.

3 - Yes, when you join surfaces, the entire thing should be selected. It’s impossible to tell where you were at that point in time from the file you sent so I cannot say anything sensible about that :wink:

4 - The new top surfaces of the flanges look good now. Since your surfaces are not joined, I am guessing that you used the FilletSrf command to get those filleted. That command requires a bit of manual labor, yes. In simple cases (I would almost say most cases but not everyone agrees on that), you can use FilletEdge on surfaces that are joined at the edge(s) to be filleted.

5 - No, not really. The thing is that there is a duplicate surface there that is not trimmed that is preventing you from seeing the good result.

A - The help file?
Be sure to let @margaret know if something is unclear here.

B - When the command finishes and you want to run it again, you can just hit Enter or Space.

  • Keep at it!

I deleted the whole thing and started again. Before starting I skimmed through the help index, jotting down a few commands that I thought I would use fairly frequently, whilst also giving a bit of forethought to what I was trying to model, and how best to go about it.
Keeping in mind the issues pointed out above, and “constantly checking over my shoulder”, it turned out to be rather painless, although time consuming, but that will improve with practice.

Many thanks again,:laughing::laughing::laughing:


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