Separation functional are computed for a class of manufactured object that include cylindrical and rectangular artifacts, bored tubes and parallel surfaces with variable diameter. The perfect forms applied in this analysis are spatial tubes generated from planar regular and convex polygons. The construction of the tube separation functional involves the computation of an "inner envelope" which is functionally dependent on several parameters. The inner envelope is a new construction which allows the visualization of the separation functional for a three-dimensional symmetrical artifact. Examples are used to illustrate the separation functionals associated with various manufactured artifacts. © 1997 Elsevier Science Ltd.