Cutting a hole in a rounded edge

I use an off the shelf plastic box for one of my electronic projects. Rather than having to cut holes in the box with a dremel, I decided to download the 3d model, add the holes in SU, and 3d print it.

One of the cut-outs I need is along an edge of the box. The edge is rounded, and I haven’t been able to cut out through that edge and keep what SU thinks is a valid solid.

I tried using a solid rectangular object and subtracting it from the box solid, but I’m left with surface borders I can’t figure out how to fix. I left the rectangular box in the model in case using a Solid Tools function will actually work.

I’ve also tried drawing lines on the rounded edge to try and create a face that I could push back to open up the corner. While tedious, I can do that on the outside of the corner. But I haven’t been able to do it on the inside of corner.

Surely there’s an easy way to do this?

box.skp (1.2 MB)

Try using the Dave method.
As you see here I only needed to scale it up by 10 for solid subtract to work.
Dave meth


Ah, should have tried that!


Also, I’ve scaled things up by a 1000x and back down again to address other issues. But, I discovered (I think) that scale up and scale back can loose some model integrity. Maybe because of length snapping being enabled? Does that seem right?

Length snapping should only affect edges you draw via the GUI. It should not affect scaling. Can you share an example in which scaling “can lose some model integrity”?

It’s been a while, but best I can recall was that I scaled something up by 1000x to work around some SU weirdness. When I scaled back down, some edges that needed to be critically aligned with other objects were off a bit. I did undos and repeated at maximum precision and no length snapping, and everything worked out ok.

But, I didn’t carefully reproduce the problem to make sure I hadn’t done something stupid–that’s why I asked the question. But, I do the scale up, scale back thing often If I see it again, I’ll post.


Actually, when I do the subtract, I get a surface border, even at 1000x.

But, I can export the model to .stl and 3d print it, so I don’t really care. But, if I had to do more cut-out’s I’d have to figure out how to repair the surface border to get a valid SU solid.

Did you check to see if SU though your object after subtraction was a solid?

The issue could be due to all your up and down scaling. The component is scaled down in your model, so it is sort of fighting with itself. Right click on it and choose Scale Definition, then try again.

BTW using the Dave Method as I demoed means you don’t need to scale anything down.

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That is a good tip–make a component, a copy, then edit the scaled copy. That way you can skip the scaling back down step. That’s handy!

But, to be clear, you get surface borders after doing the subtract. At 10x, Solid Inspector complains about 12 surface borders.

When I use the Dave Method I always use 1000x scale factor.

I must be doing something wrong then.

Here you go, with your model, nothing else done but scale definition.
You should make the block larger so it doesn’t fight with the flat/curve transition.
Scale definition

Pay attention to what Box wrote about extending the cutting shape. Don’t be stingy with it.


Did you create this model from scratch in SketchUp, or did it start as an import from some other model type? I ask because there are some strange features that I wouldn’t expect in a model built entirely in SketchUp.

In particular, how did the triangulation come to be erratic and unsymmetric at the corners?

I expect it’s from an imported .stl file. Personally I would just model the box from scratch in SketchUp to avoid the awful geometry that comes from the .stl file. This is a typical problem with imported .stl files, though.

That’s exactly what I was getting at.

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Yes, Box. I was able to fix the surface errors. I just wanted to confirm that SU won’t necessarily give you a solid object when subtracting one from another, regardless of the scale used.

Yes, as I mentioned in the original post, I downloaded it. It wouldn’t be worth the effort for me to model it from scratch…

Sketchup considers it a valid solid after import, and as I mentioned there’s no issue with printing the exported stl.

Personally I would just model the box from scratch in SketchUp to avoid the awful geometry that comes from the .stl file

Why? Because SU can’t handle it? Why else would I care about “awful geometry”?

No. Clearly SketchUp can handle that awful geometry. If it were modeled from scratch the geometry could be much cleaner and easier to work with. I guess it depends on what you want to spend your time doing, though. My preference is to create clean models that are easy to work with from the beginning than to spend time going through fixing things.

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The geometry you see when you unsoften the model, in other words all the awful extra lines and triangles, is what your 3d printer will see. While Sketchup can do its graphic magic and make it look smooth a printer just prints what it is given.
Good 3d prints are made from good geometry. You may not care for a small box, but it is best to start as you mean to proceed.

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I modeled your box from scratch. Much cleaner geometry and at no time was I struggling to make it a solid and printable. It was considered solid from the moment it was 3D.

Looks like it’ll make a nice print, too.