Fundamentals of CNC Technology

Mr. A. PanchariyaDept. of mech. Engg.

MNIT Jaipur 1

Fundamentals of CNC Technology

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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

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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.

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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

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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

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Numerically Controlled Machine Tools

• Numerically controlled (NC) is the term to describe the control of machine movements and various other functions by instructions expressed as a series of numbers, letters or symbols and initiated in an electronic-controlled system.

Computer numerical control• Computerized numerical control (CNC) is

the term used when the control system includes a computer in the loop (1970s)

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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.

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Central computer

History of NC machine

• 1980’s – Distributed numerical control (DNC) developed.

• Mainframe computer sends programs to PC controlling each NC machine.

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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.

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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.

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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.

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Numerical Control

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NC/CNC System Element

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Punched tape used for Transfer of Part Program to NC Machine

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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)

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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

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In CNC Machine

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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.

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Types of CNC Machines

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CNC Vertical milling Machine

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CNC vertical Machining centre

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CNC Horizontal milling centre

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Double-column machining center

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CNC Turning Center

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CNC Dual Turret turning center

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Heavy duty CNC Lathe

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CNC measuring Machine

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CNC Fabrication Machine

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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.

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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 3-

axis 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

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Establishing coordinate system

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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)

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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

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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

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Drive Motors1. AC induction motors are used to drive main spindle directly2. The AC induction motor are more reliable, easily3. Maintainable and less costly compared to other electrical motors4. Speed variation in AC motors can be obtained by pole change method5. The step-less speed variation can also be obtained by varying the

frequency of the help of frequency converters 6. DC motors are being extensively used for step less speed variation of

spindle7. The step-less variation of speed is obtained by varying the D.C. voltage

applied to the motor8. Fluid motor are also being used for driving the spindle 9. 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

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Grinding Processes

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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

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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)

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Horizontal Grinding

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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.

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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

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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.

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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.

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Grinding wheel information

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American national standard institute's marking system for standard wheels

Prefix-Abrasive type-Abrasive grain size-Grade-structure-Bond-type-Manufacture record

Ex: 51-A-36-L-5-v-23Prefix : Manufacturer symbol indicating exact kind of

abrasive (use optional)Abrasive type: A: aluminum oxide

C: silicon carbide B: boron nitride

D: diamondGrain size: coarse:8-24,medium:30-60,fine:70-180,very

fine:>220

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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.

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Correctly Mounted Wheel

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Bench or pedestal mounted grinders

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.

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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

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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.

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Characteristics of abrasives

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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 .

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Bonding materials and wheel speeds

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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.

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Surface Grinding Operations

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Surface Grinding

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Cylindrical Grinding Operations

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Centerless Grinding

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Grind Wheel Dressing

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Grind Wheel Dressing

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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

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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

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