INTRODUCTION TO COMPUTER NUMERICAL CONTROL …

1 1 CHAPTERONEINTRODUCTION TO COMPUTERNUMERICAL CHAPTER OBJECTIVESAt the conclusion of this chapter you will be able what COMPUTER NUMERICAL CONTROL (CNC) is and what basic componentscomprise CNC the objectives, advantages, and special requirements concerning CNC the different media used to input and store CNC the two different punched-tape formats used with CNC INTRODUCTIONThe basic concepts of NUMERICAL CONTROL (NC) and COMPUTER NUMERICAL CONTROL (CNC)technology are discussed. Traditional NC and contemporary CNC hardware configura-tions are described.

2 The important benefits to be derived from CNC operations are listedand explained. The different types of media used for storage and input of CNC programsare then explored. The reader is introduced to different formats for punched tape, andmachining centers with automatic tool changers, the latest development in CNC, NUMERICAL CONTROL DEFINITION, ITS CONCEPTS AND ADVANTAGESNC has been used in industry for more than 40 years. Simply put, NC is a method ofautomatically operating a manufacturing machine based on a code of letters, numbers, andspecial characters.

3 A complete set of coded instructions for executing an operation is calleda program. The program is translated into corresponding electrical signals for input tomotors that run the machine. NC machines can be programmed manually. If a COMPUTER isused to create a program, the process is known as COMPUTER -aided programming. Theapproach taken in this text will be in the form of manual , NC systems have been composed of the following components:Tape punch:converts written instructions into a corresponding hole pattern. The holepattern is punched into tape which is passed through the tape punch.

4 Much older unitsused a typewriter device called a Flexowriter, and later devices included a microcom-puter coupled with a tape punch reader:reads the hole pattern on the tape and converts the pattern to a corre-sponding electrical signal :receives the electrical signal code from the tape reader and subsequentlycauses the NC machine to machine:responds to programmed signals from the controller. Accordingly, themachine executes the required motions to manufacture a part (spindle rotationon/off, table and or spindle movement along programmed axis directions, etc.)

5 See Figure 7/20/12 7:19 PM Page 12 CHAPTER DEFINITION OF COMPUTER NUMERICAL CONTROL AND ITS COMPONENTSA CNC machine is an NC machine with the added feature of an onboard COMPUTER . Theonboard COMPUTER is often referred to as the machine CONTROL unit or MCU. CONTROL unitsfor NC machines are usually hardwired, which means that all machine functions are con-trolled by the physical electronic elements that are built into the controller. The onboardcomputer, on the other hand, is soft wired, which means the machine functions areencoded into the COMPUTER at the time of manufacture, and they will not be erased whenthe CNC machine is turned off.

6 COMPUTER memory that holds such information is knownas ROM or read-only memory. The MCU usually has an alphanumeric keyboard for director manual data input (MDI) of part programs. Such programs are stored in RAM or therandom-access memory portion of the COMPUTER . They can be played back, edited, andprocessed by the CONTROL . All programs residing in RAM, however, are lost when the CNCmachine is turned off. These programs can be saved on auxiliary storage devices such aspunched tape, magnetic tape, or magnetic disk. Newer MCU units have graphics screensthat can display not only the CNC program but the cutter paths generated and any errors inthe components found in many CNC systems are shown in Figure CONTROL unit:generates, stores, and processes CNC programs.

7 The MCU alsocontains the machine motion controller in the form of an executive software Figure machine:responds to programmed signals from the MCU and manufacturesthe of traditional NC systems offer some advantages over manual production CONTROL of tool motions under optimum cutting part quality and tooling costs, tool wear, and job setup time to manufacture production planning and placement of machining operations in the hands ADVANTAGES OF CNC COMPARED WITH NCCNC opens up new possibilities and advantages not offered by older NC in the hardware necessary to add a machine function.

8 New functions can beprogrammed into the MCU as 7/20/12 7:19 PM Page 2 INTRODUCTION to COMPUTER NUMERICAL CONTROL Manufacturing3more modern approachuses PC's and workstationsOR NC program generatedfor part manufacture2traditionalMachine CONTROL unit(MCU)displayed(optional)Programmedpa th of cutter isProgram is entered into MCU via LAN RS-232 cable, diskettes, CD's or is typed at MCU's NC machineinstructions sentNC machinemanufacturedTool changer6 Part is435 Part description(dimensions,manufacturing notes)1manuscriptFIGURE of modern CNC modern machine CONTROL unit.

9 (Photo courtesy of Dejan ) CNC program can be written, stored, and executed directly at the CNC portion of an entered CNC program can be played back and edited at will. Toolmotions can be electronically displayed upon different CNC programs can be stored in the CNC machines can be linked together to a main COMPUTER . Programs writtenvia the main COMPUTER can be downloaded to any CNC machine in the network. Thisis known as direct NUMERICAL CONTROL or DNC. See Figure NUMERICAL 7/20/12 7:20 PM Page 34 CHAPTER SPECIAL REQUIREMENTS FOR UTILIZING CNCCNC machines can dramatically boost productivity.

10 The CNC manager, however, can onlyensure such gains by first addressing several critical issues, such as the capital must be allocated for purchasing quality CNC equipment must be maintained on a regular basis by obtaining a full-servicecontract or by hiring an in-house must be thoroughly trained in the operation of CNC machines. In particu-lar, many jobs require setups for machining parts to comply with tolerances of formand function (GD&T). production planning must be studied because the hourly cost of operation ofa CNC machine is usually higher than that for conventional NUMERICAL FINANCIAL REWARDS OF CNC INVESTMENTI nvestors are encouraged to look to the CNC machine tool as a production solution withthe following savings in direct labor.