Abstract: A mandrel assembly is disclosed for use in a portable lathe apparatus for machining the end face of a hollow workpiece. The mandrel supports an orbiting cutting tool which machines the end face and defines a cutting plane. The angle of the longitudinal axis of the mandrel and, consequently the cutting plane which is perpendicular thereto, is angularly adjustable with respect to the central axis of the workpiece. This enables the formation of a miter cut on the end of the workpiece. Complimentary hemispherical surfaces having common centers located at the juncture of the cutting plane and the central axis of the workpiece allow the mandrel to be angularly adjusted in any direction.