a method of trimming plastic covering a mouth opening of a blow - molded universal urinal involves rigidly holding the urinal in position opposite a trimming machine . the machine mechanically moves a knife mounted thereon in an oval pattern in one plane while simultaneously moving the knife in an alternately concave and convex pattern in a second plane orthogonal to the first plane . the knife is mounted on a platform which is movable in the second plane independently of the movement of the knife . as the platform is moved toward the urinal mouth , the knife is moved in the described pattern , thus smoothly trimming the urinal mouth .