Smooth shading problem

Question

When I try to smooth shade this object it leaves these straight marks, I cleaned up the mesh as seen in 2nd picture and only have bevel modifier enabled, I want the edges to be more round, what should I do to fix this?

Answer 1

Before anything else, you have non-manifold geometry and unapplied, negative, non-uniform scale. You should fix the non-manifold geometry (which will impact both shading and modifiers), then apply the scale, then recalculate normals. These aren't particularly the source of your problems, so I won't go into them, but the first thing I did was fix those problems.

Let's look at it, for a second, without the bevel, to see what's going on. I'll throw a wireframe material on it, enable the display of vertex normals, and select a single face:

It's one big gigantic ngon! In order to interpolate vertex data, like vertex normals, across the face(s), Blender needs to triangulate the faces. In the case of ngons, it's effectively unpredictable how Blender will triangulate it (it uses an algorithm, but nobody makes ngons in full awareness of how they're going to get triangulated; if it matters, they just don't use ngons.) We can see how Blender actually does triangulate it with the wireframe material. Each one of those triangles will get the normals of its three vertices interpolated along them smoothly. But there's tons of different triangles, with tons of different sizes, in tons of different orientations, so the normals aren't changing smoothly. Let's triangulate and look at one face it made:

This is interpolating from one vertex whose normal points up, another whose normal points to the right and down, and one whose normal points up and to the right. There is nowhere on this triangulated face that is going to be flat: the entire thing has a bias to the right.

A bevel modifier can improve things (it's still not the perfect fix, but it'll make any problems much smaller.) But you've been messing with your bevel settings, probably without really knowing what they do, hoping you can get to the right place by trial and error:

You're bevelling vertices, which won't do anything to fix your ngons; and you're bevelling with width percent, when your faces have very different sizes. (You're also using 40 segments, which isn't technically wrong, but I'm not going to be using that many.) When we do things differently, the mesh looks much better:

I'm bevelling it an absolute amount (a very small number: it's a distance, measured in object space) and I'm bevelling edges.

There are still problems, because those big ngons still exist, and their vertex normals still don't point flat, so we'll see the triangulation. But the normals point more flat than they did before. And as we increase the segment count, they'll get flatter and flatter. Here's an example of a problem area with 3 segments, and we can see how that big ngon is still affecting things:

Like I said, it will improve with a higher segment count. But what would improve it better would be to stop using a gigantic ngon and change the topology. Something like this would work well:

The faces are cut to create evenly sized, convex quads (mostly.) The loop cuts through the center follow the lines of the mesh and hold the vertex normals. There are some triangles but that triangle is planar with all its neighbors, so it's fine. With this kind of topology, we don't need to bevel to have smooth normals, but if we do bevel, it'll bevel beautifully, at any segment count.