Introduction of CNC Machine

INTRODUCTION OF CNC MACHINE

INTRODUCTION In order to meet the increasing demand to manufacture complicated components of high accuracy in large quantities, sophisticated technological equi equipm pmen entt and and mach machin iner ery y have have been been deve develo lope ped. d. Prod Produc ucti tion on of thes thesee comp compon onen ents ts call callss for for mach machin inee tool toolss whic which h can can be set set up fair fairly ly rapi rapidl dly y without much attention. The design design and constr construct uction ion of Comput Computer er Numeri Numerical cally ly Contro Controlle lled d (CNC) machines differs greatly from that of conventional machine tools. This difference arises from the requirements of higher performance levels. The CNC machines often employ the various mechatronics elements that have been developed over the years. However, the quality and reliability of these machines depends on the various machine elements and subsystems of the machines. There are some of the important constituents parts and aspects of CNC machines to be considered in their designing, for example Machine structure, Guideways, Feed drives, Spindle and Spindle bearings, Measuring syst system ems, s, Cont Contro rols ls,, Soft Softwa ware re and and Oper Operat ator or inte interf rfac ace, e, Gaug Gaugin ing, g, Tool Tool monitoring. The control of a machine tool by means of stored information through the com computer uter is know known n as Com Compute puterr Num Numeri erical cally Contr ontro olled lled.. The The information stored in the computer can be read by automatic means and converted into electrical signals, which operate the electrically controlled servo systems. Electrically controlled servo systems permits the slides of a machine tool to be driven simultaneously and at the apporopriate feeds and direction so that complex shapes can be cut, often with a single operation and without the need to reorient the workpiece. Computer Numerically Control can be applied to milling machines, Lath Lathee mach machin ines es,, Grin Grindi ding ng mach machin ines es,, Bori Boring ng mach machin ines es,, Flam Flamee cutt cutter ers, s, Drilling machines etc.

GOVERNMENT POLYTECHNIC WASHIM, 2007-2008.

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HISTORICAL DEVELOPMENTS IN CNC MACHINES

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A relatively short time ago, machines were operated by craftsmen who determined the many variables such as speeds, feeds, depth of cut etc. Now much of this work is being assigned to computer and machines that are numerically controlled. Numerical control as applied to machine tools, had it’s beginning in 1949 1949,, when when the the Unit United ed Stat States es Air Air Forc Forcee comb combin ined ed with with Pars Parson onss Corporation to produce contoured surface from instructions in the form of punched tape. Numerical control equipment was substituted for the tracer controls on a three axis Cininnati C ininnati Hydrotel Vertical Planer Mill. A successful feasibility demonstration of continuous contour milling was held in March 1952 followed by a final report; both the Air Force and and Priv Privat atee Indu Indust stry ry bega began n furt furthe herr work work to exte extend nd the the Nume Numeri rica call Cont Contrrol techn echnol olog ogy y deve develloped oped at Massa assach chu usett setts, s, Inst nstitut itutee of Technology, U.S.A. Between 1955 and 1960 only 500 Numerically Controlled Machine were installed in U.S.A. During 1960 and 1964 approximately 4000 more were added. Till the middle of 60’s Russia, West Germany, Japan, and U.K. were for behind U.S.A., in the production of Numerical Control Machine Tools. Japan entered the Numerical Control commercial scene during mid 60’s and by 1971 surpassed the U.S.A. in NC ratio. NC ratio = Production of of NC machines machines tools (units) (units) Total production of machine tools U.S. U.S.S. S.R. R. is also also one one of the the larg larges estt prod produc ucer erss of nc mach machin inee tool toolss mainly for industrial use.


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FEATURES OF CNC MACHINES •

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Rigid machine structure is provided, in order to bear the static load, Dynamic load , Thermal load , Vibration. Guideways are used in machine tools to control the direction or line of action of the carriage or the table on which a tool or workpiece is held , To absorb all the static and dynamic forces. On a CNC machine the function of feed rate drive is to provide motion to the slide as per the motion commands. Since the degree of accuracy requirements are high, the feed drive should having high efficiency and response. Hydrodynamic bearings, Hydrostatic Bearings, Antifriction Bearings are provided to the CNC machine. In order to achieve the accuracy and the quality of the work produced depends directly on the geometrical accuracy, running accuracy and the stiffness of the spindle assembly. On all CNC machines, an electronic measuring system is employed on each controlled axis to monitor the movement and to compare the position of the slide and the spindle with the desired position. CNC controls are the heart of the CNC machines. The early CNC controls were developed for simple applications in turning, machining cent centre ress and and grin grindi ding ng.. But But with with incr increa ease sed d capa capabi bili liti ties es on mode modern rn machine tools such as higher spindle speeds, higher rapid traverses and more number of axes, CNC systems have been developed to meet these needs. Better work piece quality is one of the most important advantages in usin using g a hi-t hi-tec ech h CNC CNC mach machin ine. e. To main mainta tain in qual qualit ity y the the effe effect ct of parameters like tool wear and thermal growth can be eliminated by automatic gauging system. Pres Presen entl tly, y, esta establ blis ishe hed d tool tool moni monito tori ring ng sens sensor orss and and syst system emss are are avai availa labl blee comm commer erci cial ally ly for for inte integr grat atin ing g and and syst system emss are are avai availa labl blee commercially for integrating with CNC machines. Tool monitoring systems enable the introduction of adaptive controls on machines for optimizing the cutting parameters. Feedback is taken from various devices, e.g. Ecoders, Transducers.


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MACHINE STRUCTURE The machine structure is the load carrying and supporting member of the machine tool. All the motors, drive mechanisms and other functional assemblies of machine tools are aligned to each other and rigidity fixed to the the mach machin inee stru struct ctur ure. e. The The mach machin inee stru struct ctur uree is subj subjec ecte ted d to stat static ic and and dynami dynamicc forces forces and it is, theref therefore ore , essent essential ial that that the struct structure ure does does not deform or vibrate beyond the permissible limits under the action of these forc forces es.. All All comp compon onen ents ts of the the mach machin inee must must rema remain in in corr correc ectt rela relati tive ve positions to maintain the geometric accuracy, regardless of the magnitude and direction of these forces. The machine structure configuration is also influenced by the consideration of manufacture, assembly and operation. The basic design factors involved in the design of machine structure are as follows, 1. Static load 2. Dynamic load 3. Thermal load 4. Guideways 5. Feed drive: - 1)Servo motor, 2)Mechanical Transmission Transmission System 6. Spindle / spindle bearings 1)Hydrodynamic 2)Hydrostatic 3)Antifriction 7. Measuring Measuring Systems: - 1)Direct 1)Direct 2)Indirect 8. Controls, Software and user interface 9. Gauging 10.Tool monitoring monitoring systems: systems: - 1)Direct 2)Indirect


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CONFIGURATION OF CNC SYSTEM Fig shows a schematic diagram of the working principle of an NC axis of a CNC machine and the interface of a CNC control. A CNC system basically consists of the following:Central processing unit (CPU). • Servo control unit • Operator control panel • Machine control panel • Other peripheral devices • Programmable Programmable logic controller • Figure gives the typical control configuration of Hinumerik 3100 CNC system.

1. Central Central Proce Processi ssing ng Unit (CPU) (CPU) ::The CPU is the heart of a CNC system. It accepts the info inform rmat atio ion n stor stored ed in the the memo memory ry as part part prog progra ram. m. This This data data is decoded and transformed into specific position control and velocity signals. It also oversees the movement of the control axis or spindle and whenever this does not match with the programmed values, a corrective action as taken. All the compensation required for machine accuries (like lead scre screw w pitc pitch h erro error, r, tool tool wear wear out, out, back backla lash shes es.) .) are are calc calcul ulat ated ed by CPU CPU depending upon the corresponding inputs made available to the system. The same will be taken care of during the generation of control signals for the axis movement. Also, some basic safety checks are built into the system thro throug ugh h this this unit unit and and cont contin inuo uous us nece necess ssar ary y corr correc ecti tive ve acti action onss will will be provided by CPU unit. Whenever the situation goes beyond control of the CPU, it takes the final action of shutting down the system and in turn the machine.


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2. Ser Servo vo cont contro roll unit unit :The decoded position and velocity control signals, generated by the CPU for the axis movement forms the input to the servo control unit. This unit in turn generates suitable signals as command values. The command values are converted by the servo drive units which are interfaced with the axes and the spindle motors. The servo control unit receives the position feedback signals for the actual movement of the machine tool axes from the feedback devices (like linear scales, rotary encoders, revolvers, etc.)

Operator Control Panel :The Operator Control Panel provides control panel provides the user interface to facilitate a two way communication between the user, CNC system and the machine tool. This consists of two parts; Video display unit Keyboard

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4. Machine Machine Control Control Panel Panel :It is the direct interface between the operator and the NC system, enabling the operation of the machine through the CNC system. During program execution, the CNC controls the axis the motion, spindle func functi tion on or tool tool func functi tion on on a mach machin inee tool tool,, depe depend ndin ing g upon upon the the part part program stored in the memory. Prior to the starting of the machining process, machine should first be prepared with some specific takes like, Establishing a correct reference point Loading the system memory with the required part program Loading and checking of tool offsets, zero offsets, etc. • • •


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Other peripheral devices :These include sensor interface, provision for commun communica icatio tion n equipm equipment ent,, progr programm amming ing units, units, printe printer, r, tape tape reader reader interface, etc. 5.

6. Programmable logic Controller:A PLC matches the NC to the machine. PLC’s were basically as replacement for hard wired relay control panels. They were basically introduced as replacement for hard wired relay panels. They deve develo lope ped d to be re-p re-pro rogr gram amme med d with withou outt hard hardwa ware re chan change gess when when requ requir irem ement entss were were alte altere red d and and thus thus are are re-u re-usa sabl ble. e. PLC’ PLC’ss are are now now avai availa lab ble wit with incr increa ease sed d funct unctiions, ons, more memo emory and and large arger r input/output capabilities. In the CPU, all the decisions are made relative to cont contro roll llin ing g a mach machin inee or a proc proces ess. s. The The CPU CPU rece receiv ives es inpu inputt data data,, performs logical decisions based upon stored programs and drives the output s. connection to a computer for hierarchical control are done via the CPU.


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PROGRAMMING FOR CNC MACHINES A program is a set of encoded information giving coordinate values and other details to indicate how a tool should be moved in relation to a work piece in order to achieve a desired machining form. A part program contains all the information for the machining of a component which is input to CNC system. The CNC system provides signals at the correct time and in the correct sequence to the various drive units of the machine.

PROCEDURE OF PART PROGRAMMING Study the relevant component drawing thoroughly. 2) Identify the type of material to be machined. 3) Determine the specifications and functions of machine to be used. 4) Decide the dimension and mode i.e. Metric or inch. 5) Decide the co ordinate system i.e. Absolute or incremental 6) Identify the plane of cutting. 7) Determine the cutting parameters for the job / tool combination. 8) Decide the feed rate programming. 9) Check the tooling required. 10) Establ Establish ish the sequen sequence ce of machin machining ing operat operation ions. s. 11) Identi Identify fy whethe whetherr use of specia speciall featur features es like like subrou subrouti tines, nes, mirro mirror r imaging etc. is required or not. 12) Decide Decide the the mode mode of stor storing ing the the part part progr program am once once it is is comple completed ted.. 1)


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STRUCTURE OF A PART PROGRAMMING

Part program defines a sequence of NC machining operations. The information contained contained in the program can be dimensional or non dimensional like speed, feed, auxiliary functions, etc.. The basic unit of a part program inpu inputt to the the cont contro roll is call called ed a bloc block. k. Each Each bloc block k cont contai ains ns adeq adequa uate te information for the machine to perform a movement and or functions. Block in turn are made up of words and each word consists of a number of char charac acte ters rs.. All All bloc blocks ks are are term termin inat ated ed by the the bloc block k end end char charac acte ter. r. The The maximum block length for each CNC is fixed. A block may contain any or all the following :• • • • • • • • • • •

Optional block skip ( / ). Sequence or block number (N). Preparatory functions (G). Dimensional information information ( X,Y,Z etc. ) Decimal point (.). Feed rate (F). Spindle speed (S). Tool number (T). Tool offset function (D). Miscellaneous functions (M,H, etc. ) End of block (EOB / *)


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BLOCK EXAMPLE N – 1234 G…..X..…Y…..S..…M…..F…..T…..D…..EO G…..X..…Y…..S..…M…..F…..T…..D…..EOB. B.

Where , N ……...= Address of block number. 1234…. = Block number. G ……..= Preparatory function. X…Y…= Transcient information. information. S…...= Spindle speed function. M….= Miscellaneous function. F…..=Feed function. T…..=Tool function. D…..= Compensation function. EOB =End of Block.

Desi Design gnat atio ion n of some some lett letter erss for for cert certai ain n addr addres esse sess had had been been standardized, for e.g. The letter N always designates block number, other letters in standardized list are P,G,M,X,Y,Z,I,J,K,L,U,V,W,A,B,C,F,S,T,EOB. A CNC manufacturer may use some letters which are not standardized for special words, e.g. R and and H used sed in cont contrrols ols manuf anufac acttured ured by Fanu Fanucc or Siem iemens. ns.


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CONTROL SYSTEMS FOR CNC MACHINES Following are the two types of control systems used in the CNC machines 1. Open loop control system. 2. Closed loop control system.

1. Open loop control system:In an ope open loop oop syst system em the machi achine ne slid slides es are are disp isplac laced according to the information loaded from the part program into the control system. Hence there is no measurement of slide position and no feedba feedback ck signal signalss for compar compariso ison n with with the input input signal signal.. The correc correctt move moveme ment nt of slid slidee enti entire rely ly depe depend ndss upon upon the the abil abilit ity y of the the driv drivee systems to move the slide through the required exact distance. The most common method of driving the lead screw is by a stepper motor. The stepper motors are the simplest way for converting detail electrical signals into proportional movement. As there is no check on the slide position, the system accuracy depends upon the motors ability to step through the exact number of steps provided at the input as shown in fig.

2. Closed loop control system:A closed loop system is as shown in fig. sends back a signal to the cont contro roll unit unit from from a meas measur urin ing g devi device ce call called ed as tran transd sduc ucer er.. The The transducer is attached to the slide ways. The signal indicates the actual movement and position of the slides. The control unit continues to adjust the position of the slide until it arrives it’s destination, this system has feedback. Although more costly and complex than open loop system, these system gives more accurate positioning. For this type of system, servomotors are used.


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TYPES OF CNC MACHINES

Following are the some important types of CNC machines,

1. Machining centre. a) Vertical Vertical machinin machining g centr centre. e. b) Turret machining centre. c) Horizont Horizontal al machi machining ning centre. centre. 2. CNC Lathe Machine. 3. CNC Turning Machine. 4. CNC Drilling Machine. 5. CNC Milling Machine. 6. CNC Special Purpose Machine.


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ADVANTAGES AND DISADVANTAGES OF CNC MACHINES

ADVANTAGES • • • • • • • • • •

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Higher flexibility. Increased productivity. productivity. Consistent quality. Reduced scrap rate. Reliable operation. Reduced non-productive time. Reduced manpower. Shorter cycle time. Just in time manufacture. An automatic material handling. Lesser floor space. Increased operational safety. Machining of advanced materials.

DISADVANTAGES •

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CNC machines are more expensive than manually operated machines. Investment in CNC machines can lead to unemployment. Higher maintenance cost. Costlier CNC personnel. Air conditioned place are required for installation of the machines. Unsuitable for long run applications. Training of part programmer is required.


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CNC SYSTEMS AVAILABLE IN INDIA

Various companies are as follows, • •

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FANUC INDIA. HINDUSTAN MACHINE TOOL COMPANY. KIRLOSKAR MACHINE TOOL COMPANY. LAKSHMI ELECTRIC CONTROLS. ELEPRO ENGINEERING. ELECTRONICA. INDIAN PRECISION MACHINE TOOL. SARATHI ENGINEERING WORKS. N.D. AUTO CORPORATION. SIX SIGMA ENGINEERING. BHAGYALAXMI INDUSTRIAL CORPORATION. JEO INTERPRISES. BHATS TOOLING EQUIPMENTS. CHATURVEDI TOOLS AND EQUIPMENTS PVT LTD. CREATIVE CARBON COMPANY LTD. SUPER CUT INDUSTRIES. VEDA TOOLING SYSTEMS PVT. LTD. PRESS TOOLS INDIA LTD.


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CONCLUSION

Deve Devellopm opment ent of Com Comput puter Numer umeriicall cally y Cont Contrrolle olled d machines (CNC) is an outstanding contribution to the manufacturing industries. It has made possible the automation of the the mach machin inin ing g proc proces esse sess with with flex flexib ibil ilit ity y to hand handle le smal smalll to medium batch quantities in part production. At present Indian industries needs CNC machines, which facing competition in global market, due to day by day uncertainties in customer demand requirements in term of huge variety of products with better quality and at lesser time. CNC mach machin ines es are are best best suit suitab able le for for bett better er accu accura racy cy and and less less man man power.

REFERENCES 1.

ADVANCED MANUFACTURING TECHNOLOGY


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By. Prof. Prof. K. Varaprasad Rao.

2. CAD/C CAD/CAM AM/CI /CIM M By. Prof. P. Radhakrishnan.

3. PRODUCTI PRODUCTION ON TECHN TECHNOLOG OLOGY Y By. Prof. R. K. Jain.

4. CNC CNC MACHIN MACHINES ES By. Prof. Prof. R. K. Mehta. 5.

MECHATRONICS By. Prof. N. Ramanuja.


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Introduction of CNC Machine

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