# Corner gets skewed by subdivision surface

First of all I'd like to specify that I'm new to Blender and I can do only very basic stuff.

That said, I'm trying to model a sink and I need it to have corners with a given radius. I started by creating only a quarter of the final model so that I can then use the mirror modifier.

I modeled the profile of the sink and then added some loop cuts to round up the mesh using the subdivision surface modifier.

I placed two loop cuts near the corner, placing them at a specific distance from the edge so that they have the same distance from that edge, thinking that in this way the subdivision surface will generate an arc with a radius of that specific distance.

This is what I have so far:

This is what I get after adding mirror and subdivision surface modifiers:

As you can see the corner loop cuts near the corner have moved and now the mesh is not forming a perfect arc on the corner but it is somewhat skewed. I think this is due to the subdivision surface changing those loop cuts' positions. I tried toying around with the modifier's settings but nothing changed.

How can I achieve what I'm looking for?

This effect is a natural consequence of the Catmull-Clark algorithm, because the centers of adjacent faces,(and their sizes,) are used to calculate new edge-points.

If you're relying on Catmull-Clark to create your curve, and you want it to remain symmetrical about the corner, you will have not only to put in holding-loops for the corner itself, but at least one more loop to ensure the centers of the next faces along are also at an equal distance from the corner.

No, Subsurf modifier doesn't work this way.

Bevel modifier can make perfect round corners:

But of course, it has more limitations, and it not smooths model like subsurf can.

To make it work better you should move corner vertex to position, where middle of the edges touches imaginary radius of chamfer (before doing this, disable On Cage option of Subdivision Surface modifier):

Put circle behind the model like I did, to make it in more precision way.

With On Cage enabled:

The issue is bad topology. SubDiv requires a specific type of topology, else it will break, or create artifacts like that (in the image, there's also another problem, the loop is not supported on the bottom part, which collapses)

Here you can clearly see that the Ngon is too dense, and unnecessary. One of the optimal setup for this case is this topology is this

You can also see that the first image is also bad because the number of faces is an odd number, which in subdiv, it's better to avoid. I'm no expert by any means, but this is the ideal setup. As you can see, the final result is much more cleaner once it's applied.

Hope this clears things out!

I ended up ditching the original idea I had and started again by extruding a circle object, creating the necessary faces and then using the mirror and subdibision surface modifiers.

The end result looks like this, which is almost perfect for my needs:

On the corner now I have these strange rounded "bubbles" which I don't know hot to fix (if it's even possible)