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Numerical control (NC) is a form of programmable automation in which the mechanical actions of a machine tool or other equipment are controlled by a program containing coded alphanumeric data. The alphanumeric data represent relative positions between a work head and a work part as well as other instructions needed to operate the machine.
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Chapter Contents 7.1 Fundamentals of NC Technology 7.1.1 Basic Components of an NC System 7.1.2 NC Coordinate Systems 7.1.3 Motion Control Systems 7.2 Computers and Numerical Control 7.2.1 The CNC Machine Control Unit 7.2.2 CNC Software 7.2.3 Distributed Numerical Control 7.3 Applications of NC 7.3.1 Machine Tool Applications 7.3.2 Other NC Applications 7.3.3 Advantages and Disadvantages of NC 7.4 Analysis of Positioning Systems 7.4.1 Open-Loop Positioning Systems 7.4.2 Closed-Loop Positioning Systems 7.4.3 Precision in Positioning Systems 7.5 NC Part Programming 7.5.1 Manual Part Programming 7.5.2 Computer-Assisted Part Programming 7.5.3 CAD/CAM Part Programming 7.5.4 Manual Data Input Appendix 7A: Coding for Manual Part Programming
150 Chap. 7 / Computer Numerical Control
Numerical control (NC) is a form of programmable automation in which the mechani- cal actions of a machine tool or other equipment are controlled by a program containing coded alphanumeric data. The alphanumeric data represent relative positions between a work head and a work part as well as other instructions needed to operate the machine. The work head is a cutting tool or other processing apparatus, and the work part is the object being processed. When the current job is completed, the program of instructions can be changed to process a new job. The capability to change the program makes NC suitable for low and medium production. It is much easier to write new programs than to make major alterations in the processing equipment. Numerical control can be applied to a wide variety of processes. The applications divide into two categories: (1) machine tool applications, such as drilling, milling, turning, and other metal working; and (2) other applications, such as assembly, rapid prototyp- ing, and inspection. The common operating feature of NC in all of these applications is control of the work head movement relative to the work part. The concept for NC dates from the late 1940s. The first NC machine was developed in 1952 (Historical Note 7.1).
The development of NC owes much to the U.S. Air Force and the early aerospace industry. The first work in the area of NC is attributed to John Parsons and his associate Frank Stulen at Parsons Corporation in Traverse City, Michigan. Parsons was a contractor for the Air Force during the 1940s and had experimented with the concept of using coordinate posi- tion data contained on punched cards to define and machine the surface contours of airfoil shapes. He had named his system the Cardamatic milling machine, since the numerical data was stored on punched cards. Parsons and his colleagues presented the idea to the Wright- Patterson Air Force Base in 1948. The initial Air Force contract was awarded to Parsons in June 1949. A subcontract was awarded by Parsons in July 1949 to the Servomechanism Laboratories at the Massachusetts Institute of Technology to (1) perform a systems engi- neering study on machine tool controls and (2) develop a prototype machine tool based on the Cardamatic principle. Research commenced on the basis of this subcontract, which con- tinued until April 1951, when a contract was signed by MIT and the Air Force to complete the development work. Early in the project, it became clear that the required data transfer rates between the controller and the machine tool could not be achieved using punched cards, so it was pro- posed to use either punched paper tape or magnetic tape to store the numerical data. These and other technical details of the control system for machine tool control had been defined by June 1950. The name numerical control was adopted in March 1951 based on a contest sponsored by John Parsons among “MIT personnel working on the project.” The first NC machine was developed by retrofitting a Cincinnati Milling Machine Company vertical Hydro-Tel milling machine (a 24@in * 60 @in conventional tracer mill) that had been donated by the Air Force from surplus equipment. The controller combined analog and digital com- ponents, consisted of 292 vacuum tubes, and occupied a floor area greater than the machine tool itself. The prototype successfully performed simultaneous control of three-axis motion based on coordinate-axis data on punched binary tape. This experimental machine was in operation by March 1952. A patent for the machine tool system entitled Numerical Control Servo System was filed in August 1952, and awarded in December 1962. Inventors were listed as Jay Forrester, William Pease, James McDonough, and Alfred Susskind, all Servomechanisms Lab staff
152 Chap. 7 / Computer Numerical Control
Ross’s work at MIT became a focal point for NC programming, and a project was initi- ated to develop a two-dimensional version of APT, with nine aircraft companies plus IBM Corporation participating in the joint effort and MIT as project coordinator. The 2D-APT sys- tem was ready for field evaluation at plants of participating companies in April 1958. Testing, debugging, and refining the programming system took approximately three years. In 1961, the Illinois Institute of Technology Research Institute (IITRI) was selected to become responsible for long-range maintenance and upgrading of APT. In 1962, IITRI announced the completion of APT-III, a commercial version of APT for three-dimensional part programming. In 1974, APT was accepted as the U.S. standard for programming NC metal cutting machine tools. In 1978, it was accepted by the ISO as the international standard. Numerical control technology was in its second decade before computers were employed to actually control machine tool motions. In the mid-1960s, the concept of direct numerical control (DNC) was developed, in which individual machine tools were controlled by a mainframe computer located remotely from the machines. The computer bypassed the punched tape reader, instead transmitting instructions to the machine control unit (MCU) in real time, one block at a time. The first prototype system was demonstrated in 1966 [4]. Two companies that pioneered the development of DNC were General Electric Company and Cincinnati Milling Machine Company (which changed its name to Cincinnati Milacron in 1970). Several DNC systems were demonstrated at the National Machine Tool Show in 1970. Mainframe computers represented the state of the technology in the mid-1960s. There were no personal computers or microcomputers at that time. But the trend in computer tech- nology was toward the use of integrated circuits of increasing levels of integration, which resulted in dramatic increases in computational performance at the same time that the size and cost of the computer were reduced. At the beginning of the 1970s, the economics were right for using a dedicated computer as the MCU. This application came to be known as computer numerical control (CNC). At first, minicomputers were used as the controllers; subsequently, microcomputers were used as the performance/size trend continued.
7.1 Fundamentals oF nC teChnology
This section identifies the basic components of an NC system. Then, NC coordinate sys- tems in common use and types of motion controls are described.
7.1.1 Basic Components of an nC system
An NC system consists of three basic components: (1) a part program of instructions, (2) a machine control unit, and (3) processing equipment. The general relationship among the three components is illustrated in Figure 7.1. The part program is the set of detailed step-by-step commands that direct the actions of the processing equipment. In machine tool applications, the person who pre- pares the program is called a part programmer. In these applications, the individual commands refer to positions of a cutting tool relative to the worktable on which the work part is fixtured. Additional instructions are usually included, such as spindle speed, feed rate, cutting tool selection, and other functions. The program is coded on a suitable medium for submission to the machine control unit. For many years, the common medium was 1-in wide punched tape, using a standard format that could be in- terpreted by the machine control unit. Today, punched tape has largely been replaced by newer storage technologies in modern machine shops. These technologies include magnetic tape, diskettes, and electronic transfer of part programs from a computer.
Sec. 7.1 / Fundamentals of NC Technology 153
In modern NC technology, the machine control unit (MCU) is a microcomputer and related control hardware that stores the program of instructions and executes it by converting each command into mechanical actions of the processing equipment, one com- mand at a time. The related hardware of the MCU includes components to interface with the processing equipment and feedback control elements. The MCU also includes one or more reading devices for entering part programs into memory. Software residing in the MCU includes control system software, calculation algorithms, and translation software to convert the NC part program into a usable format for the MCU. Because the MCU is a computer, the term computer numerical control (CNC) is used to distinguish this type of NC from its technological ancestors that were based entirely on hardwired electronics. Today, virtually all new MCUs are based on computer technology. The third basic component of an NC system is the processing equipment that per- forms the actual productive work (e.g., machining). It accomplishes the processing steps to transform the starting workpiece into a completed part. Its operation is directed by the MCU, which in turn is driven by instructions contained in the part program. In the most common example of NC, machining, the processing equipment consists of the worktable and spindle as well as the motors and controls to drive them.
7.1.2 nC Coordinate systems
To program the NC processing equipment, a part programmer must define a standard axis system by which the position of the work head relative to the work part can be speci- fied. There are two axis systems used in NC, one for flat and prismatic work parts and the other for rotational parts. Both systems are based on the Cartesian coordinates. The axis system for flat and block-like parts consists of the three linear axes ( x , y , z ) in the Cartesian coordinate system, plus three rotational axes ( a , b , c ), as shown in Figure 7.2(a). In most machine tool applications, the x - and y -axes are used to move and position the worktable to which the part is attached, and the z -axis is used to con- trol the vertical position of the cutting tool. Such a positioning scheme is adequate for simple NC applications such as drilling and punching of flat sheet metal. Programming these machine tools consists of little more than specifying a sequence of x – y coordinates. The a -, b -, and c -rotational axes specify angular positions about the x -, y -, and z -axes, respectively. To distinguish positive from negative angles, the right-hand rule is used: Using the right hand with the thumb pointing in the positive linear axis direction ( + x , + y , or + z ), the fingers of the hand are curled in the positive rotational direction. The rotational axes can be used for one or both of the following: (1) orientation of the work part to present different surfaces for machining or (2) orientation of the tool or work head at some angle relative to the part. These additional axes permit machining of
Program (^) control unitMachine
Processing equipment
Figure 7.1 Basic components of an NC system.
Sec. 7.1 / Fundamentals of NC Technology 155
(2) continuous path. Point-to-point systems, also called positioning systems , move the worktable to a programmed location without regard for the path taken to get to that loca- tion. Once the move has been completed, some processing action is accomplished by the work head at the location, such as drilling or punching a hole. Thus, the program consists of a series of point locations at which operations are performed, as depicted in Figure 7.3. Continuous path systems are capable of continuous simultaneous control of two or more axes. This provides control of the tool trajectory relative to the work part. In this case, the tool performs the process while the worktable is moving, thus enabling the sys- tem to generate angular surfaces, two-dimensional curves, or three-dimensional contours in the work part. This control mode is required in many milling and turning operations. A simple two-dimensional profile milling operation is shown in Figure 7.4 to illustrate continuous path control. When continuous path control is utilized to move the tool paral- lel to only one of the major axes of the machine tool worktable, this is called straight-cut NC. When continuous path control is used for simultaneous control of two or more axes in machining operations, the term contouring is used.
Tool path
Tool starting point
1 3
2
Work part y
x
Figure 7.3 Point-to-point (positioning) control in NC. At each x – y position, table movement stops to perform the hole-drilling operation.
Tool starting point
Work part
Tool path
Tool profile
y
x
Figure 7.4 Continuous path (contouring) control in NC ( x – y plane only). Note that cutting tool path must be offset from the part outline by a distance equal to its radius.
156 Chap. 7 / Computer Numerical Control
Interpolation Methods. One of the important aspects of contouring is interpolation. The paths that a contouring-type NC system is required to generate often consist of circular arcs and other smooth nonlinear shapes. Some of these shapes can be defined mathemati- cally by relatively simple geometric formulas (e.g., the equation for a circle is x^2 + y^2 = R^2 , where R = the radius of the circle and the center of the circle is at the origin), whereas others cannot be mathematically defined except by approximation. In any case, a funda- mental problem in generating these shapes using NC equipment is that they are continuous, whereas NC is digital. To cut along a circular path, the circle must be divided into a series of straight line segments that approximate the curve. The tool is commanded to machine each line segment in succession so that the machined surface closely matches the desired shape. The maximum error between the nominal (desired) surface and the actual (machined) sur- face can be controlled by the lengths of the individual line segments, as shown in Figure 7.5.
Straight line segment approximation
Straight line segment approximation
Straight line segment approximation
Actual curve
Actual curve
Actual curve
Inside tolerance
Inside tolerance limit
Outside tolerance
Tolerance band
Outside tolerance limit
(a)
(b)
(c)
Figure 7.5 Approximation of a curved path in NC by a series of straight line segments. The accuracy of the approximation is controlled by the maximum deviation (called the tolerance) between the nominal (desired) curve and the straight line segments that are machined by the NC system. In (a), the tolerance is defined on only the inside of the nominal curve. In (b), the tolerance is defined on only the outside of the desired curve. In (c), the tolerance is defined on both the inside and outside of the desired curve.
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7.2 Computers and numeriCal Control
Since the introduction of NC in 1952, there have been dramatic advances in digi- tal computer technology. The physical size and cost of a digital computer have been significantly reduced at the same time that its computational capabilities have been substantially increased. The makers of NC equipment incorporated these advances in computer technology into their products, starting with large mainframe computers in the 1960s and followed by minicomputers in the 1970s and microcom- puters in the 1980s. Today, NC means computer numerical control (CNC), which is defined as an NC system whose MCU consists of a dedicated microcomputer rather than a hardwired controller. The latest computer controllers for CNC feature high- speed processors, large memories, solid-state memory, improved servos, and bus architectures [12]. Computer NC systems include additional features beyond what is feasible with con- ventional hardwired NC. A list of many of these features is compiled in Table 7.2.
7.2.1 the CnC Machine Control Unit
The MCU is the hardware that distinguishes CNC from conventional NC. The gen- eral configuration of the MCU in a CNC system is illustrated in Figure 7.7. The MCU consists of the following components and subsystems: (1) central processing unit, (2) memory, (3) I/O interface, (4) controls for machine tool axes and spindle speed, and (5) sequence controls for other machine tool functions. These subsystems are inter- connected by means of a system bus, which communicates data and signals among the components of the network.
10 20 30 40 50
(20, 20)
(40, 50)
30
Current 20 tool position
Next tool position
0
10
20
30
40
50
y
x
Figure 7.6 Absolute versus incremental positioning. The work head is presently at point (20, 20) and is to be moved to point (40, 50). In absolute positioning, the move is specified by x = 40, y = 50; whereas in incremental positioning, the move is specified by x = 20, y = 30.
Sec. 7.2 / Computers and Numerical Control 159
taBle 7.2 Features of Computer Numerical Control that Distinguish It from Conventional NC
Storage of more than one part program. With improvements in storage technology, newer CNC controllers have sufficient capacity to store multiple programs. Controller manufacturers generally offer one or more memory expansions as options to the MCU. Program editing at the machine tool. CNC permits a part program to be edited while it resides in the MCU computer memory. Hence, a program can be tested and corrected entirely at the machine site. Editing also permits cutting conditions in the machining cycle to be optimized. After the program has been corrected and optimized, the revised version can be stored for future use. Fixed cycles and programming subroutines. The increased memory capacity and the ability to program the control computer provide the opportunity to store frequently used machining cycles as macros that can be called by the part program. Instead of writing the full instructions for the particular cycle into every pro- gram, a programmer includes a call statement in the part program to indicate that the macro cycle should be executed. These cycles often require that certain parameters be defined, for example, a bolt hole circle, in which the diameter of the bolt circle, the spacing of the bolt holes, and other parameters must be specified. Adaptive control. In this feature, the MCU measures and analyzes machining variables, such as spindle torque, power, and tool-tip temperature, and adjusts cutting speed and/or feed rate to maximize machin- ing performance. Benefits include reduced cycle time and improved surface finish. Interpolation. Some of the interpolation schemes described in Table 7.1 are normally executed on a CNC system because of the computational requirements. Linear and circular interpolations are sometimes hardwired into the control unit, but helical, parabolic, and cubic interpolations are usually executed by a stored program algorithm. Positioning features for setup. Setting up the machine tool for a given work part involves installing and align- ing a fixture on the machine tool table. This must be accomplished so that the machine axes are established with respect to the work part. The alignment task can be facilitated using certain features made possible by software options in a CNC system, such as position set. With position set, the operator is not required to locate the fixture on the machine table with extreme accuracy. Instead, the machine tool axes are refer- enced to the location of the fixture using a target point or set of target points on the work or fixture. Acceleration and deceleration calculations. This feature is applicable when the cutter moves at high feed rates. It is designed to avoid tool marks on the work surface that would be generated due to machine tool dynam- ics when the cutter path changes abruptly. Instead, the feed rate is smoothly decelerated in anticipation of a tool path change and then accelerated back up to the programmed feed rate after the direction change. Communications interface. With the trend toward interfacing and networking in plants today, modern CNC controllers are equipped with a standard communications interface to link the machine to other computers and computer-driven devices. This is useful for applications such as (1) downloading part programs from a central data file; (2) collecting operational data such as workpiece counts, cycle times, and machine utiliza- tion; and (3) interfacing with peripheral equipment, such as robots that load and unload parts. Diagnostics. Many modern CNC systems possess a diagnostics capability that monitors certain aspects of the machine tool to detect malfunctions or signs of impending malfunctions or to diagnose system breakdowns.
Memory
Machine tool controls
Sequence controls
Input/output interface
System bus
Central processing unit (CPU)
Figure 7.7 Configuration of CNC machine control unit.
Sec. 7.2 / Computers and Numerical Control 161
must be converted to a form and power level suited to the particular position control sys- tems used to drive the machine axes. Positioning systems can be classified as open loop or closed loop, and different hardware components are required in each case. A more detailed discussion of these hardware elements is presented in Section 7.4, together with an analysis of how they operate to achieve position and feed rate control. Some of the hardware components are resident in the MCU. Depending on the type of machine tool, the spindle is used to drive either (1) the workpiece, as in turning, or (2) a rotating cutter, as in milling and drilling. Spindle speed is a programmed parameter. Components for spindle speed control in the MCU usually consist of a drive control circuit and a feedback sensor interface.
sequence Controls for other Machine tool Functions. In addition to control of table position, feed rate, and spindle speed, several additional functions are accom- plished under part program control. These auxiliary functions generally involve on/off (binary) actuations, interlocks, and discrete numerical data. The functions include cutting fluid control, fixture clamping, emergency warnings, and interlock communications for robot loading and unloading of the machine tool.
7.2.2 CnC software
The NC computer operates by means of software. There are three types of software pro- grams used in CNC systems: (1) operating system software, (2) machine interface soft- ware, and (3) application software. The principal function of the operating system software is to interpret the NC part programs and generate the corresponding control signals to drive the machine tool axes. It is installed by the controller manufacturer and is stored in ROM in the MCU. The operating system software consists of the following: (1) an editor, which permits the machine operator to input and edit NC part programs and perform other file management functions; (2) a control program, which decodes the part program instructions, performs interpolation and acceleration/deceleration calculations, and accomplishes other related functions to produce the coordinate control signals for each axis; and (3) an executive program, which manages the execution of the CNC software as well as the I/O operations of the MCU. The operating system software also includes any diagnostic routines that are available in the CNC system. Machine interface software is used to operate the communication link between the CPU and the machine tool to accomplish the CNC auxiliary functions. The I/O signals as- sociated with the auxiliary functions are sometimes implemented by means of a program- mable logic controller interfaced to the MCU, so the machine interface software is often written in the form of ladder logic diagrams (Section 9.2). Finally, the application software consists of the NC part programs that are written for machining (or other) applications in the user’s plant. The topic of part programming is postponed to Section 7.5.
7.2.3 Distributed numerical Control
Historical Note 7.1 describes several ways in which digital computers have been used to implement NC. This section describes two approaches: (1) direct numerical control and (2) distributed numerical control.
162 Chap. 7 / Computer Numerical Control
Direct numerical control (DNC) was the first attempt to use a digital computer to control NC machines. It was in the late 1960s, before the advent of CNC. As initially imple- mented, direct numerical control involved the control of a number of machine tools by a single (mainframe) computer through direct connection and in real time. Instead of using a punched tape reader to enter the part program into the MCU, the program was transmitted to the MCU directly from the computer, one block of instructions at a time. An instruction block provides the commands for one complete move of the machine tool, including loca- tion coordinates, speeds, feeds, and other data (Section 7.5.1). This mode of operation was referred to by the term behind the tape reader (BTR). The DNC computer provided instruc- tion blocks to the machine tool on demand; when a machine needed control commands, they were communicated to it immediately. As each block was executed by the machine, the next block was transmitted. As far as the machine tool was concerned, the operation was no different from that of a conventional NC controller. In theory, DNC relieved the NC system of its least reliable components: the punched tape and tape reader. The general configuration of a DNC system is depicted in Figure 7.8. The system consisted of four components: (1) central computer, (2) bulk memory at the central com- puter site, (3) set of controlled machines, and (4) telecommunications lines to connect the machines to the central computer. In operation, the computer called the required part pro- gram from bulk memory and sent it (one block at a time) to the designated machine tool. This procedure was replicated for all machine tools under direct control of the computer. In addition to transmitting data to the machines, the central computer also received data back from the machines to indicate operating performance in the shop (e.g., number of machining cycles completed, machine utilization, and breakdowns). A central objective of DNC was to achieve two-way communication between the machines and the central computer. As the installed base of CNC machines grew during the 1970s and 1980s, a new form of DNC emerged, called distributed numerical control (DNC). The configuration of the new DNC is very similar to that shown in Figure 7.8 except that the central computer is connected to MCUs, which are themselves computers; basically this is a distributed con- trol system (Section 5.3.3). Complete part programs are sent to the machine tools, not
Machine tool
Central computer
Bulk memory NC programs
Telecommunication lines
Tape reader
BTR BTR
MCU MCU MCU MCU
Figure 7.8 General configuration of a DNC system. Connection to MCU is behind the tape reader. Key: BTR = behind the tape reader, MCU = machine control unit.
164 Chap. 7 / Computer Numerical Control
There are four common types of machining operations: (a) turning, (b) drilling, (c) milling, and (d) grinding, shown in Figure 7.9. Each of the machining operations is carried out at a certain combination of speed, feed, and depth of cut, collectively called the cut- ting conditions. The terminology varies somewhat for grinding. These cutting conditions are illustrated in Figure 7.9 for turning, drilling, and milling. Consider milling. The cutting speed is the velocity of the milling cutter relative to the work surface, m/min (ft/min). This is usually programmed into the machine as a spindle rotation speed, rev/min. Cutting speed can be converted into spindle rotation speed by means of the equation
N =
v p D
where N = spindle rotation speed, rev/min; v = cutting speed, m/min (ft/min); and D = milling cutter diameter, m (ft). In milling, the feed usually means the size of the chip formed by each tooth in the milling cutter, often referred to as the chip load per tooth. This must normally be programmed into the NC machine as the feed rate (the travel rate of the machine tool table). Therefore, feed must be converted to feed rate as fr = Nn (^) t f (7.2)
Speed
Speed Drill bit
Workpiece
Work speed
Grinding wheel Wheel speed
Feed
(a) (b)
(c) (d)
Feed
Cutter speed Depth
New surface Work part Chip
Depth
Cutting tool Feed
Workpiece Workpiece
Figure 7.9 The four common machining operations are (a) turning, (b) drilling, (c) peripheral milling, and (d) surface grinding.
Sec. 7.3 / Applications of NC 165
where fr = feed rate, mm/min (in/min); N = spindle rotational speed, rev/min; n (^) t = number of teeth on the milling cutter; and f = feed, mm/tooth (in/tooth). For a turn- ing operation, feed is defined as the lateral movement of the cutting tool per revolution of the workpiece, mm/rev (in/rev). Depth of cut is the distance the tool penetrates below the original surface of the work, mm (in). For drilling, depth of cut refers to the depth of the hole. These are the parameters that must be controlled during the operation of an NC machine through motion or position commands in the part program. Each of the four machining processes is traditionally carried out on a machine tool designed to perform that process. Turning is performed on a lathe, drilling is done on a drill press, milling on a milling machine, and so on. The following is a list of the common material-removal CNC machine tools along with their typical features:
Numerical control has had a profound influence on the design and operation of machine tools. One of the effects is that the proportion of time spent by the machine cut- ting metal is significantly greater than with manually operated machines. This causes certain components such as the spindle, drive gears, and feed screws to wear more rapidly. These components must be designed to last longer on NC machines. Secondly, the addition of the electronic control unit has increased the cost of the machine, requiring higher equipment utilization. Instead of running the machine during only one shift, which is the typical sched- ule with manually operated machines, NC machines are often operated during two or even three shifts to obtain the required economic payback. Third, the increasing cost of labor has altered the relative roles of the human operator and the machine tool. Instead of being the highly skilled worker who controlled every aspect of part production, the NC machine op- erator performs only part loading and unloading, tool-changing, chip clearing, and the like. With these reduced responsibilities, one operator can often run two or three NC machines. The functions performed by the machine tool have also changed. NC machines are designed to be highly automatic and capable of combining several operations in one setup that formerly required several different machines. They are also designed to reduce the time consumed by the noncutting elements in the operation cycle, such as changing tools
Sec. 7.3 / Applications of NC 167
Although these characteristics pertain mainly to machining, they are adaptable to other production applications as well.
nC for other Metalworking processes. NC machine tools have been devel- oped for other metalworking processes besides machining. These machines include the following:
168 Chap. 7 / Computer Numerical Control
7.3.2 other nC applications
The operating principle of NC has a host of other applications besides metalworking. Some of the machines with NC-type controls that position a work head relative to an object being processed are the following:
7.3.3 advantages and Disadvantages of nC
When the production application satisfies the characteristics identified in Section 7.3.1, NC yields many advantages over manual production methods. These advantages translate into economic savings for the user company. However, NC involves more sophisticated technol- ogy than conventional methods, and there are costs that must be considered to apply the technology effectively. This section examines the advantages and disadvantages of NC.