Artifacts with boolean modifier Here is a car brake disc Im working on:

so I decided to use boolean mdifier to make the samll holes(one side),but it just caused the other half to disappear:
 
after some experiment，it seems that boolean mod doesnt support unclosed mesh or seperated mesh,and also cant treat closed mesh as holow instead of solid,and when applys to smooth surface it looks like this:



only way I thought to make hole on cured smooth surface without too much subdivision is to use edge split,and then I find out edge split doesnt support vertex gruop which is kind of inconvenient.
So is it possible to ask developer to add these feather or is there better ways?
I expand my comments above, to post some example image. I still think it's a matter of mesh density:

To better visually describe I exagerated size of the diff meshes, but this is the overall setup:

 

this is how a subdiv level of 2 on a standard UV sphere, in boolean diff with a cube, enough subdivided and extruded, then hidden. Still, you see some artifact on the hole edge.

I tried a gif animation showing the effect of changing settings, but due to color banding it gets lost... so here is a rendered comparison with subdiv 0,1,2, and 4 (enough in this example)



As said above I can't show you here, but even "moving around" the sphere doesn't show artifacts (or they are much more subtle) in the viewport, and the same goes in the render.

I don't know how better you can solve this, but using much higher resolution on the edges of the subctraction, at least, will help blender to find a "smooth" setup for normals of geometry around that area...