Introduction:

Modern manufacturing systems are advanced automation systems. The numerical control (NC) and computer numerical control (CNC) machines are an integral part of the automation systems. CNC machining is capable of meeting the tightest tolerances, and producing the most accurate, precise products over and over again. Machining: Process of removing metal with the assistance of mechanical machines. Numerical Control (NC) : NC is defined as control of machine slide movements and various functions by means of letters, numbers and symbols. Numerical Control: Governing the machines automatically through a set of instructions (letters, numbers, symbols). Computer Numerical Control: Process by which an operator can write, adjust and implement instructions using a computer comfort. Most read 5 Most read.

• Letters, numbers and symbols forming some sequence is called Part Program.
• The part program is arranged in the form of blocks of information. Each block contains the alphanumerical data required to produce one segment of the work piece.
• The part program is translated into the appropriate electrical signals for input to motors that run the machine.

What is NC:

NC (numerical control) machine tools are the machine tool, of which the various functions are controlled by letters numbers and symbols. The NC machine tool runs on a program fed to it; without human operator. The NC program consist of a set of instruction or statement for controlling the motion of the drives of the machine tools as well as the motion of the cutting tool.

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 prototyping, 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.

NC Machine Function:

• Starting and stopping of the machine tool spindle
• Controlling the spindle speed
• Positioning the tool at the desired location and guiding it along the desired path by automatic control of the motion of slides
• Controlling the feed rate
• Changing the tools.

History of CNC:

• The first NC machines were built in the 1940s and 1950s by Prof. John T Parson.
• CNC machine came into existence after evolution of computer around 1980.
• Modern CNC Machine are improving further as the technology is changing with a variety of functions according to applications.
• In the early 1950s the Massachusetts Institute of Technology developed a more advanced vacuum tube computer called Whirlwind.

What Is CNC:

Cnc stand for computer numerical controller this is technology that automate machine tools through computer programing. In CNC (Computer Numerical Control) machines, a dedicated computer is used to perform the most of basic NC machine functions. CNC (Computer Numerical Control) machine is a NC machine which uses a dedicated computer as the machine control unit. The entire program is entered and stored in computer memory. The machining cycle for each component is controlled by the program contained in the computer memory. The stored part program listing can be used for future production also.

DIFFERENCE BETWEEN NC and CNC:

Numerical Control Machine:

• The program is fed to the machine through magnetic tapes or other such media.
• The original NC machines were essentially basic machine tools which were modified to have motors movement along the axes.
• An NC machine is numerically controlled but has no memory storage and is run off of the “tape” each time the machine cycles.

Computer Numerical Controlled machine:

• The machines are interfaced with computers.
• This makes them more versatile in the sense that, suppose a change in dimension of a part is required.
• A CNC machine has memory storage and the program can be stored in its control.
• DNC: Number of machines are controlled by a central computer.

Advantages of NC :

Nonproductive time is reduced: NC reduces the proportion of time the machine is not cutting metal. This is achieved through fewer setups, less setup time, reduced workpiece handling time, and automatic tool changes on some NC machines, all of which translate into labor cost savings and lower elapsed times to produce parts.

Greater accuracy and repeatability: Compared with manual production methods, NC reduces or eliminates variations due to operator skill differences, fatigue, and other factors attributed to inherent human variabilities. Parts are made closer to nominal dimensions, and there is less dimensional variation among parts in the batch.

More complex part geometries are possible: NC technology has extended the range of possible part geometries beyond what is practical with manual machining methods. This is an advantage for product design in several ways: (1) More functional features can be designed into a single part, thus reducing the total number of parts in the product and the associated cost of assembly, (2) mathematically defined surfaces can be fabricated with high precision, and (3) the limits within which the designer’s imagination can wander to create new part and product geometries are expanded.

Shorter manufacturing lead times: Jobs can be set up more quickly and fewer setups are required per part when NC is used. This results in shorter elapsed time between order release and completion.

Operator skill requirements are reduced: Workers need fewer skills to operate an NC machine than to operate a conventional machine tool. Tending an NC machine tool usually consists only of loading and unloading parts and periodically changing tools. The machining cycle is carried out under program control. Performing a comparable machining cycle on a conventional machine requires much more participation by the operator and a higher level of training and skill.

Disadvantages of NC:

Higher investment cost: An NC machine tool has a higher first cost than a comparable conventional machine tool. There are several reasons why: (1) NC machines include CNC controls and electronics hardware; (2) software development costs of the CNC controls manufacturer must be included in the cost of the machine; (3) more reliable
mechanical components are generally used in NC machines; and (4) NC machine tools often possess additional features not included on conventional machines, such as automatic tool changers and part changers.

Higher maintenance effort: In general, NC equipment requires more maintenance than conventional equipment, which translates to higher maintenance and repair costs. This is due largely to the computer and other electronics that are included in a modern NC system. The maintenance staff must include personnel who are trained
in maintaining and repairing this type of equipment.

Part programming: NC equipment must be programmed. To be fair, it should be mentioned that process planning must be accomplished for any part, whether or not it is produced by NC. However, NC part programming is a special preparation step in batch production that is absent in conventional machine shop operations.

Higher utilization of NC equipment: To maximize the economic benefits of NC, some companies operate multiple shifts. This might mean adding one or two extra shifts to the plant’s normal operations, with the requirement for supervision and other staff support.

Machining Operations:

The four common machining operations are: (a) Turning(b) Drilling,(c) Peripheral Milling, And (d)Surface Grinding.

CNC Lathe machine 
CNC Milling machine 
CNC Drilling machine 
CNC Grinding machine 
CNC Laser cutting machine 
Water jet cutting machine 
Electro discharge machine

The following is a list of the common material-removal CNC machine tools along with their typical features:

NC lathe: either horizontal or vertical axis. Turning requires two-axis, continuous path control, either to produce a straight cylindrical geometry (straight turning) or to create a profile (contour turning).

NC boring mill: horizontal or vertical spindle. Boring is similar to turning, except that an internal cylinder is created instead of an external cylinder. The operation requires continuous path, two-axis control.

NC drill press: This machine uses point-to-point control of a work head (spindle containing the drill bit) and two axis (x–y) control of a worktable. Some NC drill presses have turrets containing six or eight drill bits. The turret position is programmed under NC control, allowing different drill bits to be applied to the same
work part during the machine cycle without requiring the machine operator to manually change the tool.

NC milling machine: A milling machine requires continuous path control to perform straight cut or contouring operations. 
NC cylindrical grinder: This machine operates like a turning machine, except that the tool is a grinding wheel. It has continuous path two-axis control, similar to an NC lathe.

The CNC Machine Control Unit:

The MCU is the hardware that distinguishes CNC from conventional NC. The general configuration of the MCU in a CNC system is illustrated in Figure 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 interconnected by means of a system bus, which communicates data and signals among the components of the network.

Central Processing Unit: The central processing unit (CPU) is the brain of the MCU. It manages the other components in the MCU based on software contained in main memory. The CPU can be divided into three sections: (1) control section, (2) arithmetic logic unit, and (3) immediate access memory. The control section retrieves commands and
data from memory and generates signals to activate other components in the MCU. In short, it sequences, coordinates, and regulates the activities of the MCU computer. The arithmetic-logic unit (ALU) consists of the circuitry to perform various calculations (addition, subtraction, multiplication), counting, and logical functions required by software residing in memory. The immediate access memory provides a temporary storage for data being
processed by the CPU. It is connected to main memory by means of the system data bus.

Memory: The immediate access memory in the CPU is not intended for storing CNC software. A much greater storage capacity is required for the various programs and data needed to operate the CNC system. As with most other computer systems, CNC memory can be divided into two categories: (1) main memory and (2) secondary memory. Main memory consists of ROM (read-only memory) and RAM (random access memory) devices. Operating system software and machine interface programs are generally stored in ROM. These programs are usually installed by the manufacturer of the MCU. NC part programs are stored in RAM devices. Current programs in RAM can be erased and replaced by new programs as jobs are changed.

Input/Output Interface: The I/O interface provides communication between the various components of the CNC system, other computer systems, and the machine operator. As its name suggests, the I/O interface transmits and receives data and signals to and from external devices.

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 specified. 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 (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 control 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 complex work part geometries. Machine tools with rotational axis capability generally have either four or five axes: three linear axes plus one or two rotational axes.

The coordinate axes for a rotational NC system are illustrated in Figure (b). These systems are associated with NC lathes and turning machines. Although the workpiece rotates, this is not one of the controlled axes on most turning machines. Consequently, the y-axis is not used. The path of the cutting tool relative to the rotating workpiece is
defined in the x–z plane, where the x-axis is the radial location of the tool and the z-axis is parallel to the axis of rotation of the part.

Motion Control Systems:

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 location. 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.

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 system 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

Hardware of CNC:

The drive units of the carriages in NC machine tools are generally the screw & the nut mechanism. There are different types of screws and nuts used on NC machine tools which provide low wear, higher efficiency, low friction and better reliability.

The Automatic Tool Changer:

An Automatic Tool Changer is equipment that reduces cycle times by automatically changing tools between cuts. Automatic tool changers are differentiated by tool-to-tool time and the number of tools they can hold. CNC tool changers allow a machine to perform more than one function without requiring an operator to change the tooling. A CNC tool changer can quickly change the end effectors without the requirement of multiple robots. Tool changers can be a manual tool changers or automatic tool changers. A CNC tool changer full fills the requirement of multiple tooling for a wide variety of machine tools