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. 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.

2 Simply put, NC is a method ofautomatically operating a manufacturing machine based on a code of letters, numbers, andspecial characters. 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. 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.

3 Accordingly, themachine executes the required motions to manufacture a part (spindle rotationon/off, table and or spindle movement along programmed axis directions, etc.).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. COMPUTER memory that holds such information is knownas ROM or read-only memory.

4 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. 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.

5 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.(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 .

6 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. 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.

7 One CNC machine s output is commonly equivalent to that ofseveral conventional in operator training in shop supervisory due to tighter, more predictable production in real estate, since fewer CNC machines are in power consumption, since CNC machines produce parts with a minimumof motor idle from improved cost estimation and due to the elimination of construction of precision jigs, the reduced need forspecial fixtures, and reduced maintenance and storage costs of these in tool engineering/design and documentation. The CNC s machining capa-bility eliminates the need for special form tools, special boring bars, special threadcutters, inspection time due to the CNC machine s ability to produce parts withsuperior accuracy and repeatibility. In many cases, only spot-checking of critical areasis necessary without loss of machine DNC systems can also be networked to form a large distributive numericalcontrol system.

8 Refer to Figure CNC program can be input from flash or floppy disks or downloaded from localarea 7/20/12 7:20 PM Page 4 INTRODUCTION to COMPUTER NUMERICAL CONTROL Manufacturing5 Using Payback Period to Estimate Investment EfficiencyThe payback period calculation estimates the number of years required to recover the netcost of the CNC machine Return on Investment (ROI) to Estimate InvestmentEfficiencyThe ROI calculation predicts what percent of the net cost of the CNC will be recoveredeach year. The ROI calculation accounts for the useful life of the CNC machine the investment figures in Table for implementing a new CNC machinetool, determine the payback period and the annual return on investment. The CNC isconservatively estimated to have a useful life of 12 calculation estimates that the net cost of the CNC will be recovered in calculation estimates that the investor can expect 57% of the net cost of the CNC(or ) to be recovered each year if the CNC machine suseful life is 12 *$95,250=$54,293 ROI=.

9 57 ROI= 63,100-95,250/1295,250 Payback Period= years Payback Period= 95,250-95,250*.163,100-63,100*.46+10,900 *.46 ROI= Average Yearly Savings-Net Cost of CNC/Years of LifeNet Cost of CNCP ayback Period= Net Cost of CNC-Net Cost of CNC*Tax CreditSavings-Savings*Tax Rate+Yearly Depreciation of CNC*Tax RateTABLE 1-1 Initial investment($)One-time savings in tooling($)Net cost of CNC($)Average yearly savings($)Tax credit(10%)Tax rate(46%)Yearly depreciation of CNC($)130,25035,00095,25063, , CNC MACHINING CENTERS AND TURNING CENTERSM achining centers are the latest development in CNC technology. These systems comeequipped with automatic tool changers capable of changing 90 or more tools. Many are alsofitted with movable rectangular worktables called pallets. The pallets are used to automati-cally load and unload workpieces. At a single setup, machining centers can perform suchoperations as milling, drilling, tapping, boring, counterboring, and so on.

10 Additionally, byutilizing indexing heads, some centers are capable of executing these tasks on many differentfaces of a part and at specified angles. Machining centers save production time and cost 7/20/12 7:20 PM Page 56 CHAPTER ONEFIGURE vertical spindle machining center.(Photo courtesy of Hardinge Inc.)FIGURE horizontal spindle machining center with an automatic tool changer andtwo pallet work changers.(Photo courtesy of metalpix/Alamy)reducing the need for moving a part from one machine to another. Two types of machiningcenters are shown in Figures and more complete discussion of the important features found on machining centers isdeferred until Chapter centers with increased capacity tool changers are also making a strong appear-ance in modern production shops. These CNC machines are capable of executing manydifferent types of lathe cutting operations simultaneously on a rotating part.