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FYI I am very new to Blender.

I need to: Create a cylinder with a cavity - just like a cup.

What did I try:

  1. Created a cylinder.
  2. Created a cylinder within it.
  3. Used "Boolean" "difference" to create the cavity.
  4. Smooth result using "Subdivision modifier".

The problem: the "Subdivision modifier" creates a very weird result (see images).

Original cylinder

After adding subdivision modifier

Two questions:

  1. Why this is happening?
  2. How should this be done properly?
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The subdivision modifier does not work well with n-gons (faces with more than four vertices). In fact, you should if possible only use quads (faces with four vertices/edges).

After your operation there are four ngons (the fourth is on the bottom of the cylinder).
ngons

Delete the ngons. X
delete

To fill the gap between the outer and inner wall at the top, use the Bridge Edge Loops operation from the Edge Menu. CtrlE
bridge edge loops

For the circular shape it is enough to add a supporting edge loop before merging AltM the vertices at the center. The resulting triangle should only be on planar surface and surrounded by the supporting edge loops.
merge

You can also fill such holes with quads using the Grid Fill operation, though it may be overkill in this situation where your surface is planar.
grid fill

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  • $\begingroup$ Thank you @Leander, I certainly learned something new about subdivision. My problem was that I am so new I couldn't figure out how to "add a supporting edge loop" - what is the command for that?? :-/ $\endgroup$ – Roy Apr 22 at 11:59
  • $\begingroup$ Ctrl R for loop cuts, only works if there are existing faces with edge loops. E for extrude. $\endgroup$ – Leander Apr 24 at 21:10
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No need to use Subdiv modifier which gives you more details but not necessarily smooth shading (unless you have high enough subdivision which is overkill).

How about starting from a cylinder, inset, extrude down, shade smooth and auto-smooth?

enter image description here

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  • $\begingroup$ many thanks, I think I will go with your suggestion - you really helped me! . Is your advice still valid if I add that this object is going to be 3D printed? Is this kind of smoothing only a "cosmetic" one or will it apply to an actual body as well? $\endgroup$ – Roy Apr 22 at 11:57
  • $\begingroup$ The smooth shading is only a rendering effect where the renderer interpolates the normal to the surface between vertices. It does NOT modify the geometry of your object. In other words, whether you apply smooth shading or not will have no influence on a 3D print $\endgroup$ – Bruno Apr 22 at 12:38

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