We present an algorithm for computing the skeleton points of a trimmed Non-Uniform Rational B-Spline (NURBS) model, by finding the centre and the radius of maximal spheres contained within the modelled object. This information can then be visualised as a colour coded thickness map on the object's surface, allowing intuitive interpretation by designers and engineers. By making use of surface normal information the task of finding maximal spheres can be reduced to a simple minimisation problem, avoiding the calculation of Delaunay tetrahedra or Voronoi diagrams. Also our method allows the skeleton points to be evaluated one point at a time, so computation can be focused on a localised region of the model. The simplicity of this method makes implementation straightforward allowing it to be rapidly integrated into existing CAD/CAM packages.