If you’ve been researching Rhino 3D and other CAD software, the chances are you’ve come across a lot of pieces talking about NURBS. Standing for Non-Uniform Rational Basis-Spline, NURBS is essentially an intuitive way for working with curved shapes with high precision. It also allows you to analyse curvatures, volume distribution and section lines in your design software.

Breaking down NURBS

You know what NURBS stands for, but what does it mean?
• Non-uniform- the portions of a curve, affected by individual control points, aren’t necessarily evenly distributed along the curve.
• Rational- the pull of each control can be adjusted.
• Basis-Spline- functions define how much control each point will influence the curve at any parameter value.

NURBS curves and surfaces are largely controlled by two factors:

• number of control points
• degree

NURBS curves and surfaces all have control points that act like magnets, pulling the curve to themselves. Rhino is a great piece of design software as all you need to do to draw a curve, is plot the point exactly where you want them. Then, if you wish to change the shape of the curve, you simply move these control points.

The degree of a curve determines the influence each control point has the curve. To give a curve less local influence and a higher global influence, you need a higher degree curve.

NURBS surfaces

The best and easiest way to think of a NURBS surface is like a rubber sheet that can be almost infinitely deformed.

For example, you can collapse one edge into a single point by placing all the control points of the edge at the same place.

Modelling tips

When you’re playing around with NURBS, there are two tips we find really help you create models easily and simply:

1. Use as little data as possible to describe the shape

This will leave you with a smoother model, whilst giving you greater control of curvatures. You will also find the model a lot easier to edit.

• Only introduce new points if they describe new features in the curves.
• New points or curves should only be introduced if they describe new features in surfaces.
• Avoid using multiple small patches if one large surface can describe the shape

2. Wherever possible, use oversized surfaces with four corners

• Rather than collapsing edges or squeezing edges into continuous lines, it is better to have surfaces with four corners.
• You can always trim any surplus surface to create your final shape.

However, if you need precision for the dimensions of the edge itself, you’re better to use the true surface edge as the final boundary. You will also need to take this approach if you need to match the surface edge to another surface.