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Chapter 1 Introduction of CNC

1.1 Narrate the concept of NC. 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 codedalphanumeric data. 1.2 Narrate the concept of CNC. A CNC machine is an NC machine with the added feature of an onboard computer. The

onboard computer is often referred to as the machine control unit or MCU. Control units for NCmachines are usually hardwired, which means that all machine functions are controlled by the

physical electronic elements that are built into the controller. 1.3 What are the components of CNC machine tool, and the function of each component?

Structure of CNC machine tool

CNC machine is composed the following parts. (1) CNC device

The CNC device is the kernel of the CNC system. Its function is to handle the input part

machining program or operation command, then output control command to the appropriate

executive parts and finish the work which the parts machining program and operation need. (2) Servo Unit, drive device and measure device

Servo unit and drive device include spindle servo drive device, spindle motor, feed servodrive device and feed motor. Measure device means position and speed measure device, it is a

necessary device to finish the spindle control, closed-loop for the feed speed and for the feedposition. (3) Control panel

Control panel, called operation panel, is a tool used for mutual information between the

operator and the CNC machine. (4) Control medium and program input and output equipment

The control medium is an agent to record the part machining program and it is also the

medium to set up contraction between man and machine. Program input and output equipment

are the devices by which the information exchange can be done between the CNC systemandexternal equipment. Its effect is to input the part machining program recorded on the control

medium into the CNC system and to store or record the debugged part machining programon the

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appropriate medium with the output device. (5) Machine body

The object of CNC system is the executive part to fulfill the machining parts. It is composedof the main motion parts, feed motion parts, bearing rack and special device, automatic platformchange system, automatic tool changer system and accessory device. 1.4 What is the Point-to-point control, Contouring control?

Point-to-point system, also called Positing control systems, moves the worktable to a

programmed location without regard for the path taken to get to the location. The contouring facility enables a CNC machine to follow any path at any prescribed feedrate. The contouring control system, also called continuous path control systems, manages the

simultaneous motion of the cutting tool in two, three, four, or five axes (the fourth and fifth axes

are angular orientations)by interpolating the proper path between prescribed points. 1.5 What is the open-loop control, Half-closed-loop control and Closed-loop control?

Open-loop systems have no access to the real time data about the performance of the systemand therefore no immediate corrective action can be taken in case of system disturbance. A semi-closed-loop control CNC system uses feedback measurements to ensure that the

worktable is moved to the desired position. It is characterized as a system that the indirect

feedback monitors the output of servomotor. In a close loop system, feed back devices closely monitor the output and any disturbance will be

corrected in the first instance. 1.6 What are the advantages and disadvantages of CNC?

CNC opens up new possibilities and advantages not offered by older NC machines. (1) Reduction in the hardware necessary to add a machine function. New functions can be

programmed into the MCU as software. (2) The CNC program can be written, stored, and executed directly at the CNC machine. (3) Any portion of an entered CNC program can be played back and edited at will. Tool

motions can be electronically displayed upon playback. (4) Many different CNC programs can be stored in the MCU. (5) Several CNC machines can be linked together to a main computer. Programs written via

the main computer can be downloaded to any CNC machine in the network. This is known as

direct numerical control or DNC. (6) Several DNC systems can also be networked to form a large distributive numerical

control system. (7) The CNC program can be input from flash or floppy disks or downloaded fromlocal

area networks. CNC machines can dramatically boost productivity. The CNC manager, however, can onlyensure such gains by first addressing several critical issues, such as the following:

(1) Sufficient capital must be allocated for purchasing quality CNC equipment. (2) CNC equipment must be maintained on a regular basis by obtaining a full-service

contract or by hiring an in-house technician.