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Introduction to Computer Numerical Control (CNC)
Contents
3.1 Introduction to CNC Technology
3.3 Coordinate Systems and Reference Points
3.4 The Ten Steps of CNC Programming
3.5 Advantages and Disadvantages of CNC
3.6 When to Use CNC Technology
3.7 Summary
3.8 Key Words
3.9 Review Questions
3.10 Bibliography
Objective
The objective of this chapter is to provide a thorough understanding of the terminology and basic operating concepts of computer numerical control (CNC) technology.
3.1 Introduction to CNC Technology
Computer numerical control (CNC) technology is, in the simplest of terms, the automation of traditional manual machining processes by electrical and computer technology. In traditional “manual” machining, a machinist (or operator) decides upon and directs the motion of a tool relative to the workpiece, thus creating the desired shape of a finished workpiece. In CNC technology, a computer controller plays the role of the machinist, so to speak, directing the motion of the tool by following a stored sequence of coded machine commands or directions called a program of instructions, or more traditionally, a part program. A sample CNC program of instructions is shown in Figure 3-0. The program directs the motion of the tool relative to the part and contains commands that control all essential machine functions, such as tool choice, spindle rotation speed, tool feed rate, and other functions. The program of instruction, written in a language that is understood by the CNC controller, is often called a G-code program because its commands are alphanumeric codes beginning with the letter “G.” This is evident in Figure 3-0.
Figure 3-0 Sample G-code program
3.1.1 Manual Machining and Numerical Control Technology
Manual machining is still used in industry for low volume applications, maintenance, and repair. In manual machining, mechanical technology in the form of slides, gears, belts, and feed screws implements a tool’s movement relative to a workpiece. A typical manual vertical milling machine is shown in Figure 3-1.
Figure 3-1 Manual vertical milling machine
A part is milled or machined by fastening the workpiece to the machine and moving the workpiece into the rotating cutter, held by the spindle, at a specific feed rate and depth of cut. Spindle rotational speed and direction is often controlled with gears or belts and pulleys. The workpiece is fastened to the machine with some type of fixturing. In Figure 3-1 the fixturing is a simple vise. The vise, in turn, is fastened to the mill table. The mill table, and hence the workpiece, can then be moved in three directions relative and perpendicular to the spindle.
The Cartesian coordinate system supplies the layout of the directions in which the mill table can be moved. Again, as shown in Figure 3-1, the mill table can move longitudinally across the front of the machine. This is shown as the x direction in the figure. The table can also be moved at a right angle to the x direction, into the machine, designated as the y direction. The third direction is along the spindle axis, and is shown as the z direction. Linear bearings, called slides, or ways, both short for “slipways,” guide the movement of the table along each axis.
Figure 3-2 shows a manual vertical milling machine with exploded views of the x-and y-axis slides and lead screws. The table is moved along a specific axis by turning the appropriate hand crank. This in turn drives the lead screw (or feed screw), which pushes or pulls the table along the slides. Figure 3-3 shows a closer view of the slides. Note that dovetail slides are used to constrain motion perpendicular to the sliding direction. The feed screws for each axis can also be powered by the machine and moved at specific speeds or rates.
Figure 3-3 shows such a machine, with the x-axis equipped with a power feed. Typically, in manual milling, powered table movement occurs in only one direction at a time. Standard operations for manual vertical mills are slot cutting, planing, and hole drilling. Movement that occurs between any pair of axes during the cutting operation is not very accurate and is difficult to accomplish. Cutting complex surfaces may require movement in the direction of all three axes. However, such an operation is not possible on a traditional manual mill; numerical control technology was developed to specifically address this limitation.
During the 1940s a contractor to the U.S. Air Force by the name of John Parsons began experimenting with methods to produce more accurate inspection templates for helicopter blades. The inspection templates were a complex airfoil shape. Machining these shapes accurately was a challenge. Parsons’ method involved calculating points along the airfoil’s shape and then, using two operators (one for each axis), manually moving the machine tool to each of these points. Because the calculations were so complex, Parsons used a punch card tabulating machine to perform the calculations. The punch cards would be fed into a card reader at the machine, which would read the data, then pass the information on to a machine controller, which in turn directed the motion of each of the machine axes.
Figure 3-2 Exploded view of manual vertical milling machine
