CAD tip # 13847:

The mathematical definition of a symmetric hyperbolic paraboloid is:

Z = A * X^2 - A * Y^2.

then in LISP:

(defun pringle (u v)(list u v (- (* 0.5 u u)(* 0.5 v v))))

Plot this function using 3DPlot over the X and Y coordinate interval (-1 to 1), with a step size of 0.1:

(3dplot pringle -1 1 0.1 -1 1 0.1)

And to make the shape of the chip perfect, trim the resulting surface to a circular plan-shape. To do this, we must first convert the initial shape from the original polyline mesh to a 3D mesh (MESHSMOOTH) and then to a 3D surface (CONVTOSURFACE). This can then be trimmed with an auxiliary circle (SURFTRIM).

See the video demonstration of this procedure:

And see the finished DWG file Pringle.DWG available for download from the Block Library.