Machining Fundamentals 392 Copyright Goodheart-Willcox Co., Inc. S ince their development, computer numerical control (CNC) machines have revolutionized machining. CNC machines can perform complex machining operations faster than manual machines with greater accu- racy and ease. This chapter introduces CNC milling and turning machines, including their history, advantages, and disadvantages. This chapter also covers CNC safety and the basics of CNC movement systems. 21.1 History of CNC Computer-controlled machining got its start during the 1950s as numerical control (NC). Programs to control machine tools were created and stored on punched paper tape, Figure 21-1. Programmers punched holes in the tape at specific locations to produce multiple combinations. Each combination represented a separate letter, number, character, or command. Paper tape readers read the rows of punched holes on the tape at a rate of up to 1,000 lines per second and transferred the information to the computer that controlled the motion of the machine tool. When these machines were first introduced, the cost of the machines and the skills needed to operate them out- weighed the potential cost savings, so many companies were slow to develop the technology. However, by the end of the 1950s, the use of NC equipment was beginning to increase. The development of the minicomputer in the 1960s dramatically increased the acceptance and use of NC equip- ment. This technology converted numerical control to com- puter numerical control (CNC). Computer control lowered the cost of the equipment and made it more cost-effective due to the faster processing capability of the minicomputer. Also during the 1960s, programming codes were devel- oped to help standardize CNC programming languages. Until then, each machine manufacturer had implemented its own language. These codes, which are still in use today, are described in Chapter 22, CNC Programming Basics. As computer and machine drive technologies contin- ued to develop from the 1970s through the 1990s, equip- ment became faster and more capable of producing complex shapes, making CNC machining a more cost-effective method, Figure 21-2. Also, the use of computer memory to store programs began to replace the use of punched paper tape. Today, the use of punched paper tape is all but obsolete. 21.2 Advantages and Disadvantages of Using CNC The primary advantage of using CNC equipment is that it eliminates much of the variation caused by human interac- tion during the manual machining process. Even the best machinists cannot perfectly duplicate their actions for every part. A CNC machine is not perfect, but its movements are better controlled than those of a human, which reduces vari- ation in the parts that are produced. Another general advan- tage is the rate at which parts are produced. The faster parts can be machined, the less they cost per unit to manufacture. This allows manufacturers to offer lower prices to customers, making them more competitive in the global marketplace. Figure 21-1. Punched tape. A—Tape code. Note that every level of RS-244 tape has an odd number of punches for parity check. RS-358 tape has an even number. Parity check is a method of automatically checking to reduce the possibility of tape errors caused by a malfunctioning tape punch. B—A roll of punched tape. A B Goodheart-Willcox Publisher Ailisa/Shutterstock.com