What is a ʼCapʼ for a planar hole in Rhino?

ByCADbrief Staff

Short Answer

A cap for a planar hole in Rhino is a flat surface automatically created to close an open planar edge loop on a polysurface. The most common professional method is the Cap command, which adds planar end faces to open objects. It only works when the opening is truly planar.

What You Need to Know Before

Warning: Cap often fails when the hole edge looks flat but is actually slightly warped or contains tiny naked-edge gaps. In production models, imported geometry and trimmed surfaces are the most common reason the cap will not form cleanly.

How to Cap a Planar Hole in Rhino

  • Command: Cap

  • Shortcut: Cap

  • Quick Steps:

    1. Select the open polysurface or extrusion in the viewport or layers panel.
    2. Type Cap in the command line and press Enter.
    3. Check that Rhino creates a new planar face and confirm the object becomes closed if the opening is valid.

Variables & Settings

  • Key Setting: Planar opening condition

    Expert Setting: Cap only works when the boundary of the hole lies on a single plane. If the edge loop is not planar, use PlanarSrf on the boundary curves first, then Join the new surface back to the object.

Why it Fails

  • Cause 1 (Geometry): The hole boundary is not truly planar, or the edge loop has tiny gaps or overlapping edges.
  • Cause 2 (Layers/Locks): Part of the object or related construction geometry is on a locked layer, preventing proper selection or repair.
  • Cause 3 (Command/Logic): Cap only creates flat planar closures, so it will not patch curved or irregular openings.

Quick Fix & Best Practice

  • Quick Fix: Run ShowEdges to find naked edges, repair the boundary, then use PlanarSrf and Join if Cap does not work.
  • Manager’s Verdict: Use Cap for fast closure of clean planar openings, especially on extrusions and simple solids. Avoid relying on it for imported or messy geometry without checking edge quality first.

FAQ

Can Cap close a curved hole in Rhino?
No, Cap only closes planar openings.

How do I check if the object closed successfully?
Use What or check whether Rhino reports it as a closed polysurface.

What if Cap does nothing?
The opening is usually non-planar or has naked-edge problems.

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