When the surface of a casting, such as a steam cylinder, a propeller blade and hub, etc., is wholly or in its main features a surface of revolution, or may be generated by the revolution of a line about a fixed axis, the mould for such a casting may be formed almost entirely by the use of sweeps in place of patterns. Such parts of the casting as are not surfaces of revolution are moulded from patterns used in conjunction with the sweeps.
Sweeps are made up of pieces of pine board cut to such a profile that they will form the surfaces desired when revolved on a vertical spindle which is the axis of revolution of the casting to be made. The generating edge of the sweep forms a surface in plastic sand on a facing of brick work gradually built up to the contour of this edge as the sweep is moved back and forth by the moulder.
Sweeps are used in making very large moulds built of bricks, known as loam moulds, and not for small moulds made of sand in boxes or flasks. Several sweeps are required to make a complete set. The use of sweeps will be shown in a description of making a loam mould in the next chapter.