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  • sir,can i have more info regarding to surface grinding process carried out in cnc by using programe codes.consider to give finishing to zig zag surface.....plz answer my question asap sir....plz...
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Cnc ppt by APC Sir Cnc ppt by APC Sir Presentation Transcript

  • Fundamentals of CNC Technology Mr. A. Panchariya Dept. of mech. Engg. MNIT Jaipur 1
  • Fundamentals of CNC Technology 2
  • Session objectives • On completion of this session, the delegates would have understood about • • • • • The history of NC and CNC machine The components of CNC machine Working of a CNC machine Application and limitations of CNC machine Merits and demerits of adopting CNC 3
  • What is a machine tool? • The main function of a machine tool is to remove metal from a piece of work to give the required shape and size. • This is accomplished by achieving proper relative motion between the work-piece and the tool. 4
  • Conventional Machine Tool • In conventional lathe the job is fixed between centers and made to rotate by powering the spindle. • The tool motion is achieved by operating the hand wheels and the feed rates are manually controlled. • The productivity and quality of the product depends on the operator’s skill and attitude. • These machines are characterized by low speed, feed and metal removal rate. • Producing complex part will be difficult 5
  • Today’s requirement • In, recent times, the main requirement of the machining industry is that the machine tool must be flexible, generally suitable for batch production, capable of carrying out many operations on single setting and high metal removal rate. • The conventional machines don’t meet these requirements; hence the machine tool with new concept was invented 6
  • Numerically Controlled Machine Tools • Numer ically cont r olled (NC) is t he t er m t o descr ibe t he cont r ol of machine movement s and var ious ot her f unct ions by inst r uct ions expr essed as a ser ies of number s, let t er s or symbols and init iat ed in an elect r oniccont r olled syst em. Computer numerical control • Comput er ized numer ical cont r ol (CNC) is t he t er m used when t he cont r ol syst em includes a comput er in t he loop (1970s) 7
  • History of NC machines • 1947-U.S. air force has great need for automated machining of free from surfaces. • Late 1940’s- john person devices method involving drilling holes at locations specified on punch cards. • 1951 - Servo-mechanism Lab at MIT subcontracted to refine system. • 1952 lab demonstrates modified milling machine (first NC machine).MEEM4403 Computer Aided Design Methods. • 1970-computer controlled developed (called computer numerical control – CNC • 1980’s- direct numerical control developed • One mainframe computer controls many machines. 8
  • Central computer 9
  • History of NC machine • 1980’s – Dist r ibut ed numer ical cont r ol (DNC) developed. • Mainf r ame comput er sends pr ogr ams t o PC cont r olling each NC machine. 10
  • Benefits of CNC • Improved automation • The operator invention related to producing work piece can be reduced or eliminated. • Many CNC machine can run attached during their entire machining cycle ,freeing operator to do other tasks. • Reduced operator fatigue • Consistent and predictable machining time for each work piece. • Today’s CNC machines boast almost unbelievable accuracy and repeatability specifications. • CNC machine tools are flexible. Since these machines are run from program, running a different work piece is almost as easy as loading a different program. 11
  • Any limitations of CNC? Relatively few but… • Initial cost and subsequent installation. • Continuous working to meet cost. • Steady flow of work is must. • Operator skill and management. • Lots of preplanning and high caliber maintenance schedule. 12
  • Schematic of an NC machine tool • MCU – machine control unit- controls the motion of the NC machine tool • DPU – data processing unit –reads and interprets the part program; sends immediate commands to CLU. • CLU- control loop unit – reads position sensors and sends control signals to motors. 13
  • Numerical Control 14
  • NC/CNC System Element 15
  • Punched tape used for Transfer of Part Program to NC Machine 16
  • CNC machine Basic Working • Motion Control- The heart of CNC - All forms of CNC equipment have to or more direction of motion called axes. - These axes can be precisely and automatically positioned along their length of travel • The two most common axis types are: - Linear (driven along a straight path) - Rotary (driven along a circular path) 17
  • Conventional Machine - Working • A conventional machine’s slide is moved by an operator turning the hand wheel • Accurate positioning is accomplished by the operator accounting the number of revolutions made on the hand wheel, plus the graduation on the dial 18
  • In CNC Machine 19
  • In CNC Machines • CNC machines allow motions to be actuated by servomotors under control of CNC, and guided by part program. • Generally speaking, the motion type (rapid, linear and circular), the axes to move, the amount of motion and notion rate (feed rate ) are programmable with almost CNC machine tools. 20
  • Types of CNC Machines 21
  • CNC Vertical milling Machine 22
  • CNC vertical Machining centre 23
  • CNC Horizontal milling centre 24
  • Double-column machining center 25
  • CNC Turning Center 26
  • CNC Dual Turret turning center 27
  • Heavy duty CNC Lathe 28
  • CNC measuring Machine 29
  • CNC Fabrication Machine 30
  • Machining Centers • The latest developments in the CNC machine tools are the versatile machining center. • This is a single machine capable of doing a number of operations such as drilling, reaming tapping, milling and boring. • All types of tools are mounted on a drum/ chain or egg box type magazine which are put into the spindle by automatic tool changer (ATC) under the control of tool selection instruction. 31
  • Establishing coordinate system • Establish coordinate systems- Tool path coordinate system must match machine tool coordinate system • Z-direction-: same as tool spindle rotation or work-piece rotation axis. + ve --- tool moves away from work-piece • Y- direction: chosen to give right hand coordinate system Some machine tools have secondary slide motion labeled u, v, and w. • Some machine tool have rotational motion about x, y, and z axes labeled a, b, and c respectively • 2- axis machine allows controlling two motion simultaneously a 3axis machine allows controlling three motion simultaneously ,etc. • 2-axis, 3-axis ,4-axis and 5-axis machines are most popular; 9 axis is also available 32 32
  • Establishing coordinate system 33
  • Electric drives • Drive motor are required to perform the following functions: • To drive the main spindle (spindle drive) • To drive the saddle or carriage(Axis drive) 34
  • Spindle drive • In CNC machines, large variation in cutting speed is required • The cutting may vary from 10 meters per minutes to 1000meters per minutes • The cutting speed are provided by rotation of main spindle with the help of an electric motor through suitable gear mechanism • To obtained optimum cutting speed and feeds, the drive mechanism should be such as to provide infinitely variable speed between the upper and the lower limits. • The infinitely variable speed system used CNC machines employ either electrical motors (AC or DC) or fluid motors 35
  • Drive Motors • The drive may be direct from the motor to machine spindle or indirect, through belt or gear transmission • If belt drive is employed, Toothed Belts are used and if gear drives are employed, is constant meshing type of gear box where gears giving various ratio are usually in constant mesh and are operated by remote controlled electro magnetic or hydraulic clutches • Stepper motor drives have limited use in CNC machines 36
  • Drive Motors 1. 2. 3. 4. 5. 6. 7. 8. 9. AC induction motors are used to drive main spindle directly The AC induction motor are more reliable, easily Maintainable and less costly compared to other electrical motors Speed variation in AC motors can be obtained by pole change method The step-less speed variation can also be obtained by varying the frequency of the help of frequency converters DC motors are being extensively used for step less speed variation of spindle The step-less variation of speed is obtained by varying the D.C. voltage applied to the motor Fluid motor are also being used for driving the spindle Pressurized oil or air supplier by a pump running at constant speed is directed on to the blades of the motor, which are capable of giving very high rotational speed 37
  • Grinding Processes 38
  • Session objectives • By the end of session ,the delegates would have understood: • Principles of grinding • Classification of grinding processes • Grinding wheel specifications • Detail of abrasive wheel parameters • Different wheel forms • Dressing of grinding wheels 39
  • GRINDING Definition: Another material removal process, in which abrasive particles are contained in bonded grinding wheel, that operates at very high surface speed. the grinding wheel is usually in disk shaped and is precisely balanced for high rotational speeds. Process characteristics: • Dimensional accuracy: 0.3 to 0.5 µm • Surface finish: Ra = 0.15 to 1.25 µm • Specific energy for grinding: 50 j/mm3 (other processes: 2 to 5 j/mm3) 40
  • Horizontal Grinding 41
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  • Chip formation • In grinding, the chips are small but are formed by the same basic mechanism of compration and shear. • Burning chips are the sparks observed during grinding with no cutting fluid, because the chips have heat energy to burn or melt I the atmosphere. • The feed and the depth of cut in grinding are small, while the cutting speed is high. grinding may be classified as non-precision or precision, according to the purpose and procedure. 44
  • Non-precision grinding The common form called, snagging and off-hand grinding. Both are done primarily to remove stock that can not be taken of as conveniently by other methods. The work is pressed hard against the wheel or vice versa. The accuracy and surface finish are of secondary importance. Precision grinding It’s concern with producing good surface finished and accurate dimension. 3 types of precision grinding exists External cylindrical grinding internal cylindrical grinding surface grinding 45
  • Grinding wheel • A grinding wheel is made of abrasive grins held together by bond. • These grains cut like teeth when the wheel is revolved at high speed and is brought to bear against a work piece. • The properties of a wheel that determine how it acts are the kind and size of abrasive, how closely the grains packed together and amount of bonding material. 46
  • Grinding wheel specifications • All grinding wheel manufacturers use substantially the same standard wheel making system. • This system uses numbers and letters to specify abrasive type, grit size, grade structure, and bonding material. • However, properties of the wheels are determined to a large extent by the way the wheels are made. • The processes vary from one plant to another, and wheels carrying the same symbols but made by different manufacturers are not necessarily identical. 47
  • Grinding wheel information 48
  • American national standard institute's marking system for standard wheels Prefix-Abrasive type-Abrasive grain size-Grade-structureBond-type-Manufacture record Ex: 51-A-36-L-5-v-23 Prefix : Manufacturer symbol indicating exact kind of abrasive (use optional) Abrasive type: A: aluminum oxide C: silicon carbide B: boron nitride D: diamond Grain size: coarse:8-24,medium:30-60,fine:70-180,very fine:>220 49
  • American national standard institute's marking system for standard wheels(continue….) Grade : ranges from A-Z, where A represent soft , Z represent hard wheel grade. Structure : Scale is numerical. 1: very dense and 15 very open. Bond type : B : Resinoid E : Shellac R : Rubber S : Silicate V : Vitrified Manufacturer record : Manufacture’s private marking to identify the wheel. 50
  • Correctly Mounted Wheel Bench or pedestal mounted grinders 51
  • Abrasive materials • Different abrasive materials are appropriate for grinding different work material. • Abrasive are hard substances used in various forms as tool for grinding and other surface finishing operations. • They are also able to cut materials which are too hard for other tools and give better finishes and hold closer tolerances. 52
  • Common abrasive materials 1. Aluminum oxide (Al2O3) known as Alundum or Aloxide. Various substances may be added to enchance hardness, toughness, etc. plain is Al2O3 white, and used to grind steel ferrous , high strength alloys. 2. Silicon carbide (SiC) known in trade as Carborundum and Crystalon. Harder than Al2O3, but not tough. Used to grind : aluminum, brass, stainless steel, cast iron, certain brittle ceramics 53
  • Common abrasive materials(continue) • Boron Nitride in the forms of single-crystal cubic boron nitride (CBN) and microcrystalline cubic boron nitride (MCBN) under trade names such as Boraszon or Borpax . Used for hard materials such as hardened tool steels and aerospace alloys. • Diamond a pure form of carbon. Used on hard materials such as ceramics cemented carbide and glass. 54
  • Characteristics of abrasives 55
  • Bonding materials To get wide range of properties needed in grinding wheels, abrasive materials bonded by using organic or inorganic materials. Inorganic bonds : 1. Vitrified bond: clay bond melted to a porcelain or glass like consistency. it can be made strong and rigid for heavy grinding and not affected by water ,oil , acids. 2. Silicate bond: essentially water glass hardened by baking. It holds grains more loosely than a vitrified bond gives closer cut . 56
  • Bonding materials and wheel speeds 57
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  • Wheel shape and sizes The principal dimensions that designate the size of grinding wheel are the outside diameter, width, hole diameter. Standard wheel shapes are made in certain sizes only, but variety is large. 59
  • Surface Grinding Operations 60
  • Surface Grinding 61
  • Cylindrical Grinding Operations 62
  • Centerless Grinding 63
  • Grind Wheel Dressing 64
  • Grind Wheel Dressing 65
  • Honing Process • Honing is a low abrading which uses bonded abrasive wheel sticks for removing stock from metallic and non metallic cylindrical surfaces as well as flat surface. • Performed as a final operation to correct the errors that have occurred from the previous machining operations. Objective: Correction of geometrical accuracy – out of roundness, taper, axial distortion Dimensional accuracy 66
  • Session Summary • • • • • • • Principles of grinding Classification of grinding processes Grinding wheel specifications Details of abrasive wheel parameters Different wheel forms Dressing of grinding wheels A brief outline of honing process have been discussed 67
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