Welding is the process of heating and melting metal parts to join them together permanently. It is used to construct and repair parts of cars, airplanes, ships, and sheet-metal products. Welding is also used to join beams when constructing bridges and buildings. Some welders work at steel mills, railroad shops, and highway departments. Other welders are cutters, using their tools to cut metal into pieces, as required by a blueprint or design, or to cut structures into pieces so they can be discarded.Welders are classified as either skilled or unskilled, depending on their training and the kind of welding they do. Skilled welders work from blueprints and written specifications. They know the welding properties of various kinds of metal. Some welders use computers and robots to join metal pieces. With additional training, they can learn to program the computers. Unskilled and semiskilled welders work on projects in home construction, industry, shipbuilding, and other fields. They often work on assembly lines and do repetitive work that requires no special knowledge of welding properties.