Why do subdivides after a knife cut leave undivided ngons

Hi, can anyone tell why I always see the following undivided ngons after a knife cut to an existing mesh followed by a couple of subdivides? It appears to be because the cut generates an Ngon in the first place, but is there a way to avoid it? It’s laborious to go back and try to fix before or after the subdivides. Or is it better to do subdivides before any knife cutting of edges? I’m preparing for 3D printing, so solid surface smoothness will be important, particularly since I’m trying to model smooth flesh that shouldn’t appear blocky or irregular.

I’m starting with a mid poly human solid model from TurboSquid and am trying to get a smooth flesh mesh as the result. Also trying to make a Tshirt for her simply by cutting out appropriate part of the body and thicken a little so as to look like a Tshirt.


Perhaps redo the post in a way that people can comment and suggest things that benefits what you’re actually doing, instead of providing a keyhole view to a house build and asking questions you can and have answered yourself.

Because this souds/smells/feels like a workflow question that you’ve already decided should be done in a certain way, and what the current post is about is one step in it. There’s 3D printing mentioned, which is important, but I don’t feel like asking few dozen questions to get to the actual point.

Thanks for the advice. I’m starting with a TurboSquid human model, and trying to arrive at a smooth flesh model for 3d print as the result. I also want to give the naked model some clothes and I thought I’d cut out a shirt-shaped part of the body and thicken that for a Tshirt. Any workflow advice? Am I going about it all wrong? I can’t imagine what kinds of questions you might have in mind.


You can fix that bad edge topology first like this, as an example, by removing those difficult to handle ngons. Then I would suggest not using subdivide, but subdivision surface modifier.

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Problem description and following questions doesn’t give you answers, it gives you guesses or questions at best, no replies at worst.

What is needed for troubleshooting? Looking into the same information you have in front of you, first replicating the problem, finding the cause, and then telling it to you with words/visuals/video/workfile when there’s a file that enables it.

What is needed for figuring out options for a modeling workflow? The same information you need to start modeling. What is being modeled, what for, the requirements, references. There’s no one female model that exists, even t-shirts have detail, styles, and are flexible. 3D prints are made in different sizes and the printers are different, which affects the amount of detail you can put in. We also have bigger resolution screens than 275x407px when modeling. So on and forth.

You have a model and want to put a t-shirt on it, are you doing it wrong by cutting it? Can’t answer that nor suggest alternatives.

JA12, if you can’t answer to these questions (you obviously can’t), don’t write anything. This is a simple problem, cutting with knife in arbitrary manner creates ngons and other bad topology. Either you make better cuts or fix those areas later.

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Yes, it’s so simple the op answered it himself. If you could read, I was talking about workflow options, and there are other ways to model than making such arbitrary cuts. But you’re right, shouldn’t answer such posts.

Also, those aren’t the only options. As op mentioned, subdividing first and making cuts later is also very valid option (there are multiple ways to make cuts). Since it’s for 3D printing, it doesn’t matter what the structure is after the cut, if the only requirement is to have a smooth surface. But can’t know if that is a valid approach for cutting it because there’s no information where op is at his workflow, if there are further edits required, and what those are.

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Thanks Krice. That helps. I see that subdivision surface, in addition to the other advantages of modifiers, also smooths (Catmull-Clark anyway) as well as subdivides.