Is there a specific command to ʼcloseʼ a flat open cylinder in Rhino?

ByCADbrief Staff

Short Answer

Yes — in Rhino 3D, the most common way to close a flat open cylinder is with Cap. This command adds planar end surfaces to an open polysurface when the cylinder’s opening is perfectly flat and closed at the edge. It only works if the boundary is planar and the cylinder is already a valid open solid-like form.

What You Need to Know Before

Warning: Cap fails silently if the cylinder edge is not truly planar, or if there are tiny gaps, duplicated edges, or naked edges in the profile. This is common after importing STEP/IGES geometry or trimming surfaces manually.

How-to

  • Command: Cap

  • Shortcut: cap

  • Quick Steps:

    1. Select the open cylinder in the viewport or from the right-side selection panel.
    2. Type Cap in the command line and press Enter.
    3. Check that Rhino creates planar end faces and confirms the object as a closed polysurface in Properties > Details.

  • Use the fastest professional method: run Cap directly on the open polysurface.

  • Real setting: make sure the opening is planar; Cap only works on planar holes.

Variables & Settings

  • Key Setting: Planarity of the opening edge

  • Expert Setting: If the cylinder end is slightly warped or not perfectly flat, Cap will not create a surface. In that case, use PlanarSrf on the boundary curve first, then Join.

Why it Fails

  • Cause 1 (Geometry): the cylinder opening is not perfectly planar, or the edge loop has small gaps or naked edges.
  • Cause 2 (layers/Locks): the object or related curves may be on a locked layer, preventing edits or joins.
  • Cause 3 (Command/Logic): Cap only closes planar openings; it does not patch curved, irregular, or broken edge conditions.

Quick Fix & Best Practice

  • Quick Fix: Run ShowEdges to find naked edges, repair gaps, then use Cap again. If needed, create the end with PlanarSrf and finish with Join.
  • Manager’s Verdict: Use Cap first because it is the fastest and cleanest Rhino workflow for flat cylinder ends. Avoid it when imported geometry is messy or when the opening is not truly planar.

FAQ

Can Cap close both ends of a cylinder at once?
Yes, if both openings are planar and part of the same selected open polysurface.

What if Cap does nothing in Rhino?
The boundary is usually non-planar, broken, or not part of a valid open polysurface.

Can I close a curved opening with Cap?
No, Cap only works on flat planar openings.

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