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Cylinder has small weird bumps when using Subdivision Surface enter image description here

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    $\begingroup$ On the top of your cylinder you have a face with 32 vertices. The optimal would be four vertices per face. One option would be to delete the top face and then select the top edge and extrude it inward. $\endgroup$ – Dontwalk Jun 30 '17 at 15:27
  • $\begingroup$ Aslo related: blender.stackexchange.com/questions/734/… $\endgroup$ – cegaton Jun 30 '17 at 15:49
  • $\begingroup$ You have a big N-gon at the top... again, n-gons do not subdivide well... $\endgroup$ – cegaton Jun 30 '17 at 16:24
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This is how subdivision works with faces that are created by that many vertices. You'll see that on other parts of the cylinder, particularly the quads, that this effect does not happen. It's less of a problem, and more of just how the modifier works. To fix this I suggest two things:

Delete only the face on top and extrude the edges left behind inward a couple times. enter image description here

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Keep the face on top, but create an edge crease along the top of the cylinder, (and use Auto Smooth if needed) enter image description here

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Creating an edge crease is done by hitting Shift + E, and you'll have to scroll your mouse out to get the full effect. The Auto Smooth is located in the Data tab and controls which parts of the mesh are shaded smooth or not depending on their angle respective to another/other face(s).

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