Machinists use machine tools, such as lathes, milling machines, and spindles, to produce precision metal parts. Although they may produce large quantities of one part, precision machinists often produce small batches or one-of-a-kind items. They use their knowledge of the working properties of metals and their skill with machine tools to plan and carry out the operations needed to make machined products that meet precise specifications.
Before they machine a part, machinists must carefully plan and prepare the operation. These workers first review blueprints or written specifications for a job. Next, they calculate where to cut or bore into the work piece (the piece of metal that is being shaped), how fast to feed the metal into the machine, and how much metal to remove. They then select tools and materials for the job, plan the sequence of cutting and finishing operations, and mark the metal stock to show where cuts should be made.
After this layout work is completed, machinists perform the necessary machining operations. They position the metal stock on the machine tool—spindle, drill press, lathe, milling machine, or other—set the controls, and make the cuts. During the machining process, they must constantly monitor the feed and speed of the machine. Machinists also ensure that the work piece is being properly lubricated and cooled, because the machining of metal products generates a significant amount of heat. The temperature of the work piece is a key concern because most metals expand when heated; machinists must adjust the size of their cuts relative to the temperature. Some rarer, but increasingly popular, metals, such as titanium, are machined at extremely high temperatures. Machinists also adjust cutting speeds to compensate for harmonic vibrations, which can decrease the accuracy of cuts, particularly on newer high-speed spindles and lathes.
Some machinists, often called production machinists, may produce large quantities of one part, especially parts requiring the use of complex operations and great precision. Production machinists work with complex computer numerically controlled (CNC) cutting machines. Frequently, machinists work with computer-control programmers to determine how the automated equipment will cut a part. The programmer determines the path of the cut, and the machinist determines the type of cutting tool, the speed of the cutting tool, and the feed rate. After the production process is designed, relatively simple and repetitive operations normally are performed by machine setters, operators, and tenders. Other machinists do maintenance work—repairing or making new parts for existing machinery. To repair a broken part, maintenance machinists may refer to blueprints and perform the same machining operations that were needed to create the original part.