Metal cutting may take the form of a number of production and manufacturing processes. Metal cutting operations have been performed as far back as the Greek and Roman era. The skills of the metal worker during those times were kept secret and held tightly to the chest. The concepts and designs for metal cutting machines were founded in the apparatus and mechanical schemes of early wood cutting lathes. Real-world metal cutting operations are made in three dimensions, described by the term oblique cutting, where the cutting edge is inclined at an angle to the direction of cut, thus facilitating a sideways curl to the chip. That third dimension complicates the theoretical modeling process, so a simplification was made to make the analysis only two dimensional, described by the term orthogonal machining, where the cutting edge is at right angles to the direction of cut.