Need Help with Blending Arcs in a 3D Model

Hello, I’m working on a 3D model and have encountered a challenge. I need to create a compound structure where three smaller arcs blend seamlessly into a larger arc, as shown in the attached image. The smaller arcs should appear as a smooth continuation of the larger arc’s surface. I’m struggling with how to merge these arcs smoothly in SketchUp. Any advice or recommended tools/plugins to achieve this would be greatly appreciated!

I’ll embed the picture of the original (red reference) model and another one from the current model I’m drawing (grey model) in conjunction with the reference model, you can see the red dots that I’ve put into the picture, those are some of the geometries I’m trying to create in my model.

BTW! The original model is an architect model and the second one which I’m drawing will be an “Acoustical simulation model” where I run acoustic simulations to get a better picture of how the model will sound in real life before this gets built. The acoustics simulator doesn’t interpret thicknesses of walls which I’m eliminating so you can see in my model that there’s only flat wall layers and almost no thicknesses, it’s a bit unconventional but that’s how we do it when working with acoustics.


No need for plugins !

All you have to do is intersect the small arches with the big one. Best thing to do is to use native solid tools (substract) and solid groups / components, it’d be just a few clicks - see images bellow. But it also works by intersecting and manually deleting unwanted geometry if you don’t need wall thickness


Hey Paul,

Thank you so much for the quick reply and the useful information! I’ll get back on this and see if I can get it done with the info you provided, I’m somewhat a beginner so things tend to take a bit longer than for the experienced user. I’ll get back to you if I might need more help! Have a good one!

you’re welcome,

Here’s a Gif to illustrate the solid tools operation



Wow! That’s a very smooth demonstration! I was able to follow your instructions and I managed to intersect the arcs in a similar way :smiley: thanks again !


This is the way!

Hey Paul, I want to thank you again for the help. I’ve now managed to draw up 97% of the whole church that I’ll be working with in the acoustics simulator. Now it seems like that I’m facing the “Final Boss” in terms of how far my knowledge extends in SketchUp. I’ll embed a picture. Do you know how I can make an Arc which stretches unsymmetrically like this in two different directions towards another arc. The red pictures are from the refrence model and the grey pictures above is my model.

Best regards,



this is not very conventional for a church. I’d probably use curviloft plugin; by fredo6, with the skin contours plugin.
Question : the original model is also a sketchup model so why redesigning it ? if you just want to remove wall thickness you don’t have to start all over again.
can you upload the original model so I can look at this particular detail ? it will help finding the best way to design it

It’s a russian orthodox church. I’m not a specialist on churches but this is the design that was handed to me. Thanks for the suggestion regarding Curviloft, I’ve heard good things about it. I managed to fix the problem by using the Soap Skin and Bubble plugin, so I copy pasted the outlines into my model and got a autoconstructed face built into it with the plugin, very quick and easy actually.

Yeah, well it’s not very apparent in the screenshots but the original model is actually a AutoCad model and I imported it into SketchUp so that I could have it as a reference, measure it and so on. Due to it being a AutoCad model I almost have to work with the whole model as a component, if I explode any geometry and want to hide a single wall or something, the faces break up very unevenly as you can see in the picture. With that being said, reducing wall thicknesses becomes a very tedious job. Please inform me if you know of a practical way to get around this kind of an error.

I did find out later that it did help to just highlight the outlines of the hardest geometries of the church (AutoCad model) and copy paste them over to my model. You have to understand also that I’m very much a beginner, but I did learn a whole lot through this process. I can imagine that when you look at this problem that you would go a more practical way of reconstructing it.

Unfortunately I’m not allowed to share the the whole model due to confidentiality from my work and the owner of the project but I was allowed to share small segments.

Here’s the result.

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Your imported autocad model is triangulated which causes you troubles deleting faces. To get around that, once your geometry is exploded, triple click on it, right click and pick soften / smooth edges and check “soften coplanar”. you can also play on the angle between normal to soften unwanted lines.
Once done you’ll be able to delete the faces without ending with the result you posted.

Another option would be using cleanup³ plugin, by thomthom. it will merge coplanar faces and delete stray edges.

As for the result you got, soap skin and bubble is a way but it gives you an inflated/deflated surface which is kind of wrong and surely will have a bad impact on your acoustics analysis
You can either play on soap skin bubble pressure parameter (through the “Bub” button, imputing negative or positive pressure) or use curviloft as suggested before.

I quickly redrew a similar shape and here are different results to illustrate what I’m saying :

from left to right : first the contours, then the curviloft result using “skin contours”, then the raw skin and bubbles result (close to yours, inflated towards ground), and finally the very same skin and bubble result with a -150 pressure)

@Valli , I agree with Paul that Curviloft is probably going to give you a slightly better result than Soapskin and bubble.
I also want to bring to your attention that you have a lot of reversed faces. Not sure if that will impact your accoustic analasys in any way but it’s always good to keep track of them and reverse them as you go.
Much easyer than doing it at the end of your moddeling and it keeps your model clean and easyer to work with…