Can I ʼCapʼ a cylinder that has open ends in Rhinoceros 3D?

ByCADbrief Staff

Short Answer

Yes — in Rhino 3D, you can cap a cylinder with open planar ends using the Cap command, which is the fastest and most common professional method. It works best when the cylinder is an open polysurface with flat, closed edge loops at each end. It will not cap non-planar openings.

What You Need to Know Before

Warning: If the cylinder’s open edges are slightly distorted, unjoined, or not perfectly planar, Rhino may fail to create caps even though the model looks closed on screen. Imported geometry from STEP/IGES files is a common source of this problem.

How to Cap Open Cylinder Ends in Rhino 3D

  • Command: Cap

  • Shortcut: Cap

  • Quick Steps:

    1. Select the open cylinder in the viewport, or use the Properties panel to confirm it is an open polysurface.
    2. Type Cap in the command line and press Enter.
    3. Press Enter again to accept the result, then verify in Properties that the object is now a closed polysurface.

Variables & Settings

  • Key Setting: Planar opening required

  • Expert Setting: Cap only works when the open boundary is planar and forms a closed edge loop. If the edge is warped or has tiny gaps, use ShowEdges to find naked edges before trying again.

Why it Fails

  • Cause 1 (Geometry): The cylinder opening is not truly planar, or the edge loop has gaps/overlaps that prevent Rhino from creating a surface cap.
  • Cause 2 (layers/Locks): The object is on a locked layer or the sub-objects needed for repair are locked, preventing edits before capping.
  • Cause 3 (Command/Logic): Cap only closes planar holes; it will not create end surfaces for curved, irregular, or partially broken openings.

Quick Fix & Best Practice

  • Quick Fix: Run ShowEdges to locate naked edges, then use Join or rebuild the boundary before running Cap again.
  • Manager’s Verdict: Use Cap for standard cylinders and other simple planar openings because it is fast and reliable. Avoid it for imported or damaged geometry until you confirm the edges are clean and planar.

FAQ

Can I cap only one end of a cylinder in Rhino?
Yes, if only one end is open and planar, Cap will close that opening.

Why does Cap not work on my imported cylinder?
The boundary is usually unjoined, slightly non-planar, or contains bad edges.

How do I check if the cylinder is fully closed after capping?
Select it and confirm in Properties that it is a closed polysurface.

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