sweep

A sweep is a wooden or metal template, typically curved or profiled, that is drawn or rolled across the surface of loam in a moulding pit to shape and smooth a mould cavity. The operator holds or pivots the sweep against a fixed fulcrum or guide, and the profile of the sweep determines the final contour of the casting. In loam moulding, where large castings like bells, wheels, or steam engine cylinders are formed, the sweep replaces the rigid flask used in smaller sand moulding operations.

The sweep itself is the inverse of the casting profile needed. If you are making a large cylindrical pipe, your sweep will be a flat board with a semicircular or segmental curve cut into its edge. The sweep is drawn across the loam surface, which has been rammed and packed firmly into a pit or on a moulding floor. Each pass of the sweep removes excess loam and leaves behind the cavity in the shape of the intended casting. The pivot point is usually an iron rod or bar set at the center of the mould.

A simple straight sweep is a board with a single profile cut along one edge. Compound sweeps have multiple shaped surfaces or curves to form complex sections in a single pass. The support structure holding the sweep, called the sweep frame or sweep rig, can be as basic as a wooden arm pivoted on a fixed post, or as elaborate as a large rotating jib with counterweights. The sweep is typically made from hardwood such as beech or ash, though metal sweeps are used where durability or precision is critical. The working surface is often faced with metal straps or bound with iron to resist wear from repeated passes over damp loam.

The operator controls the speed and pressure of the sweep pass, and loam must be consistently packed and moistened to the right hardness; too soft and the cavity collapses, too hard and the sweep chatters or tears the surface. Large sweeps for heavy castings may require two or three operators to manage. Once the basic cavity is swept out, the mould is refined by hand finishing with smaller tools and templates before the core and pattern elements are set and the moulding is closed.

The term 'sweep' also refers to the curving or arcing motion itself, the long axis along which the template travels. In modern practice, loam moulding and sweeps are used less often, having been displaced by mechanized flask moulding and CNC-driven processes for many work classes. However, sweeps remain standard in foundries producing very large single castings, architectural bronze work, and items where the cost of pattern tooling cannot be justified.