After the machine parts have been cleaned, the remaining operations consist of drilling holes, changing rough and uneven surfaces into smooth and plane surfaces, and so on. This work is done by chipping or turning the metal. For this purpose, tools of very hard steel, attached to one of the different metal-working machines, are used. The most common machines, and the ones generally found in all well-equipped machine-shops, are lathes, planers, milling machines, drills, power hammers, and grinding machines. In addition, many special devices or modifications of the above named machines are used for making standard parts.