This guide outlines the process and best practices for importing and using control geometry in Rhino + RhinoCAM to program the Center’s Shopbot CNC router.

what is control geometry?

Control geometry simply refers to the the geometry that is referenced by a machining operation in RhinoCAM. For example, in simplified terms, If you want to cut a circle on the CNC router, you would draw a circle in Rhino then use that circle in RhinoCAM to program the cutting operation; this circle is the control geometry. Different types of operations in RhinoCAM, namely 2-1/2-axis operations vs. 3-axis operations, require different types of control geometry. This guide attempts to outlines those differences as well as best practices.

how do I generate control geometry?

Control geometry simply needs to be vector-based geometry. It can be created directly in Rhino or imported from another CAD software such as AutoCAD or SketchUp.

differences between 2-1/2-axis and 3-axis operations

2-1/2-axis operations 3 axis operations
  • the tool moves in X + Y directions, while the Z-axis is set to a fixed level
  • The tool can move simultaneously in all 3 axes

2-axis operation geometry

2-axis operations
2-axis operations can be programmed using 2D linework
open vs closed curves

Generally, whenever you are trying to cut inside or outside of a boundary, it is best practice to work with closed curves.

This has the following benefits:

  • reliability: it ensures control geometry is continuous and does not contain gaps.
  • convenience: rather than selecting individual line-segments, you can simply select a continuous curve

if a closed shape, is made up of individual curves joined end to end, they can be joined using the join command in Rhino.

3-axis operation geometry

3-axis operations

3-axis operations require a surface to be used as control geometry