Unwanted Hard Edges

Kyuss

So I've always been bad at fixing unwanted hard edges. I've tried different combinations of edge flow/merged vertices, Googled, scoured YouTube, but I've never been able to get high res models completely smooth. I always end up with some pinching. As you can see in the images, I'm trying to keep the smooth, round cylinder shape, but also have holding edges for the edges/corners where they need to be. I feel like I overthink this and there should be a simple solution. Any tips?

Example below (Model is in smooth preview mode).

Before trying to get rid of unwanted hard edges:



Before smooth preview:


After trying to get rid of unwanted hard edges(Still have a little crease where it needs to be smooth):



Got rid of unwanted edges, merged some verts, but still have a crease:

Axi5

This is a classic. Look at the how do I model this sticky thread and you'll see similar things on almost every page. I.e how to keep a cylinder actually round when cutting into it. 

It's quite simple in this case. Add more edge loops around the cylinder so that it doesn't need those support edges anymore. 

Thanez

You don't need 100 edges on the cylinder.
Your problem is that you didn't use the cylinder's existing geometry to help with the subdivision. You cut existing edgeloops, deleted the unwanted geo and capped the holes, then started to struggle when you forced supporting geo (orange into a cylindrical shape.

The solution is to cut near to, but not into existing geometry, so that existing geo will act as your support loops. You'll still have some triangles or ngons in that intersection, but you'll swiftly eliminate it by adding double edgeloops beneath and above it.

a six-sided cylinder will probably be as low as you can go. The more segments you put in, the prettier your result will be.

Axi5

I know @perna is an advocate for that way of working and I agree it has it's uses but if you're doing a high poly anyway I really don't think it matters that much ultimately. Sure it shaves a bit off performance off a render for VFX or when baking but it's faster to model and simpler to understand if you just add more geo, especially for shapes like this.

The other problem with your solution is that the cut is now not centered in the middle of the top cap, in this case it's not relevant and it may only be a few pixels to care about but if it's something you do need to care about it then there's that. Finally depending on your model it may be easier to reduce the poly count to a low-poly asset one way or the other, typically it's easier to remove full edge loops than have to deal with individual edges sliced in so while there's more to remove it's probably still quicker to do so via the more geo method, also automated reduction tools would find this easier too.

Either technique is valid if you know what you're doing, I'm just defending the more geo option here since it gets scoffed at now and again.

Good response @Thanez!

Thanez

True, which ever technique you use to achieve the intended effect is valid, though there are pros and cons to most of them.
While you've put a lot of thought into this, those thoughts and experiences never get transferred to newcomers when you tell them how to overcome all of their obstacles by simply adding geo. That may not be what you're intending to tell them, but it is what they learn, and that's why I offer the alternative

My cuts aren't centered, but the discrepancy is too small to be noticed unless this is a hero-detail. If that's the case, then maybe more geo is what you'll want anyways. The flip side is that if this guy learns that 'more geo' is the solution to to all his woes, what do you think will happen when he decides to model the rivets on an airplane mesh? That's gonna rek his system.
Adding the supporting edgeloops in it's own modifier on top of the stack lets you delete all of the supporting geo in two clicks. If your HP base mesh equals what you want your lowpoly to be, then you're set except for optimizing the flats.

I don't hate 'more geo', really. Sometimes it's the best solution to a specific problem. My issue with it is that when it's presented, it's  done so without context or explanation.
Thanks for taking the time to argue your stance. I don't disagree with it, just the amount of words it's usually presented in

Axi5

Ahh modifier stacks, something as an ex-max user turned Maya user it's something I really miss and forget for this kind of thing. I really wish adsk would hurry up and give Maya some proper non-destructive workflows!

For OP's case, I'd go with your solution out of these two

Butthair

Crease edges if your in Maya.