PRESENTATION ON CNC MACHINES

Ch. Devi Lal Memorial Govt. Engg. College Presentation On CNC MachinesPresented To:- Presented By:- Mr. Pawan Kamboj Chetan Gosain (Lect. In Mechanical Department) Roll No:-10512 4th Year, 7thSem.

Aim

To Discuss The CNC Machines.

CONTENTS1. WORKING PRINCIPLE OF CNC2. CONSTRUCTIONAL FEATURES OF CNC3. KEYS OF CNC4. TYPES OF CNC5. SAFETY CHECKING ON CNC 6. PROGRAMMING OF CNC7. FUNDEMENTALS OF CNC PROGRAM8. PROGRAM STRUCTURE9. EXAMPLE OF CNC PROGRAMMING10. CO-ORDINATE SYSTEM11. ADVANTAGES OF CNC MACHINES

INTRODUCTIONComputer numerical control machine is a numerical control system that utilizes a dedicated, stored program computer to perform some or all of the basic numerical control functions. The external appearance of a CNC machine is very similar to that of a conventional NC machines.

PART PROGRAM~> PUNCHED TAPE READER OR ANY OTHER SIMILAR DEVICE~> STORED IN COMPUTER PROGRAM.

WORKING PRINCIPLE OF CNC By controlling the relative movements between the tool and the work piece geometrical shapes are machined. Control of these relative movements through coded letters numbers is known as Numerical Control of machine tools.

Constructional features of CNC machine tools CNC machine tools differ from conventional machine tool in a number of respects. The special constructional features of CNC machine include: 1.)Rigid structural design. 2.)Infinitely variables spindle and feed drive. 3.)Slide ways fitted with plastic inserts. 4.)Linear bearings for axis drive. 5.)Automatic tool changers.

KEYS OF CNCMACHINE STRUCTURE

To meet the requirements of high efficiency the numerically controlled machine tools should have a structure that is correctly designed to withstand normal weight distribution. BECAUSE The higher cutting speeds and feeds in CNC machine results in rapid acceleration and deceleration of the slides and the machines are subjected to fluctuating and variables forces during the machining operation. Also the machine structure design should be such that removal of chips do not on the slide ways.Linear motion bearingsThe sliding friction due to direct metal to metal contact between the slides and slide ways is replaced with rolling friction by the use of antifriction ball or bearings. Rolling element when applied to reciprocating motion has followed advantages. Reduce the drive power to be used. Stick slip problem is completely eliminated.Require only small amount of lubricant. Absorb the machine vibration.

Hydrostatic slide ways Hydrostatic slide ~> Oil pumped~>Cavities or Pockets~>Carriage on slides contact with Slide way with continous supply of thin film of fluid. The hydrostatic slide way provides almost a frictionless condition for the movement of the slide. For efficient operation it is very important that the fluid and slide ways are kept clean. Also the hydrostatic slide ways need a very large surface area to provide adequate support.Measuring systemsIn CNC machines the electronic measuring systems are used on each controlled axis and to monitor movement and compare the position of the slide and the spindle. To Measuring system used in the CNC machines are of two types To measure the position of the slide (linear measurement ) measure the spindle speed (rotary measurement)A linear scale is used as a linear measuring device which is used very often. In this method linear displacement is measured directly at the slide. The measuring device is fixed on to the moving machine element, which detect the actual distance traveled by the machine slide.One of the rotary measuring devices is an incremental rotary encoder.Incremental measurement means measurement by counting. The out put signal of incremental rotary encoders is fed to an electronic counter which provides the corresponding values.

Work holding device and toolingThe CNC machines are capable of performing a number of operations using different tools, on different faces of the component in a single setting. This requires that the component should be accessible from different sides without changing of clamps or repositioning of components. The multidirectional cutting forces to reduce the clamping and unclamping time.A work holding devices for a CNC application should have the following function. It should restrict the linear and rotary motion of the component. The component should not distort or deflect due to cutting forces process. It should facilitate quick loading and unloading of the component. It should not interfere with the cutting tools. It should provide easy removal of chips. It should be safe.

Automatic tool changersThe CNC machines are designed to perform a number of operations in a single setting of the work-piece. To reduce the down time in change over from one operation to the next, the CNC machines are equipped with automatic tool changing facility.

WORKING PRINCIPLE OF CNC MACHINES

WORKING PRINCIPLE OF CNC MACHINES

Types of CNC machines

TYPES OF CNC MACHINESThere are mainly two types of CNC machine :

(1) Machining centers There are very important types of CNC machine and are multi-functioning machine equipped with automatic tool changes and are capable to carrying out, milling, and drilling, reaming, taping, boring, counter boring.In this type of machines there are three types of axis X axis, Y axis, Z axis. Here the tools are rotated clockwise or anticlockwise the works still fixed.

(2)Turning centers/ CNC lathes CNC lathes have only two axes X and Z, X axis for longitudinal movement and Z axis for crosswise movement. Tools are arranged in turret with a number of stations some lathes have rotating tools in the turret to facilitate off-axis machining such as drilling, milling etc These machines have additional controller for spindle rotation such machines are known as turning centers

Safety check on the CNC machine Check the main pressure of the hydraulic and pneumatic system . Check the clamping pressure of the hydraulic and pneumatic system. Check the sliding movement . Check the tool position with respect to the program Check the program control levels try to cut job in a single block. Do not use compressed air for cleaning. Check the coolant supply by pressing appropriate switch manually. Check the speed over the ride position.

PROGRAMMING OF CNC MACHINESCNC programming is a term that refers to the methods for generating the instruction that drive the CNC machine tool. Part program is a set of instructions which instructs the machine tool about the processing step to be performed for the manufacture of a component. There are two distinct techniques adopted for creating CNC programs. i.)Manual programming ii.)Computer assisted programming

Manual programming1). Positioning A dimensional instruction that move and position the tools or work pieces to produces the required work2.) Motion The relative velocity of the tool with respect to the work piece is to be controlled, in both spindle drive and positioning drive 3.) Switching function: - The auxiliary function of the machine such as coolant, tool changes and program stop. The following should take into account when writing a part program. The machine features and capacity. The size if the component. The material to be machined. Machine co-ordinates. The sequence of operation. The tooling to be used. Component holding and location.

Computer Aided part programming CNC machines of complex 3 dimensional components require thousands of instructions and with out computer assistance it would be impractical to write part programs for the production of such components. NC part programming languages and the processors and post processors that translate the language instruction into NC programs. There are several NC languages in use. Some of them are v APT v ADAPT v EXAPT v UNIPAT v SPLIT v PROMPT

FUNDAMENTALS OF PART PROGRAMMING

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 programmer has to do the following while write a program Study the drawing of component thoroughly. Identify the type of material to be used. Determine the specification and functions of machine to be used. Decide the dimensioning units metric or inch. Determine the cutting parameters for the job/tool combination. Establish the sequence of machining operation.

PROGRAM STRUCTUREIn manual part programming the programmer has to determine the machining parameters and the sequence of operations to be performed. Based upon this sequence, the tool path is calculated and program is written. The basic unit of a program input to the control is called a block. The term block structure of format refers to the rules for writing a program block.A block contains any or all of the following. Sequence or block number ( N) Preparatory function(G) Dimensional information (XYZ etc) Decimal point (.) Feed rate(F) Spindle speed (S) Tool number (T) Tool offset function(D) Miscellaneous function(M) End of block(EOB/*)

Example of CNC Programming

What Must Be Done To Drill A Hole On A CNC Vertical Milling Machine

Top ViewFront ViewTool Home1.) X & Y Rapid To Hole PositionTop ViewFront View2.) Z Axis Rapid Move Just Above Hole3.) Turn On Coolant4.) Turn On Spindle.100Top ViewFront View5.) Z Axis Feed Move to Drill HoleTop ViewFront View6.) Rapid Z Axis Move Out Of HoleTop ViewFront View9.) X&Y Axis Rapid Move Home7.) Turn Off Spindle8.) Turn Off CoolantTop ViewFront ViewTool At HomeO0001N005 G54 G90 S600 M03N010 G00 X1.0 Y1.0N015 G43 H01 Z.1 M08N020 G01 Z-.75 F3.5N030 G91 G28 X0 Y0 Z0N035 M30N025 G00 Z.1 M09Heres The CNC Program!Top ViewFront ViewTool At HomeO0001O0001 Number Assigned to this programTop ViewFront ViewTool At HomeO0001N005 G54 G90 S600 M03N005 Sequence NumberG54 Fixture OffsetG90 Absolute Programming ModeS600 Spindle Speed set to 600 RPMM03 Spindle on in a Clockwise DirectionTop ViewFront ViewO0001N005 G54 G90 S600 M03N010 G00 X1.0 Y1.0G00Rapid MotionX1.0X Coordinate 1.0 in. from ZeroY1.0Y Coordinate 1.0 in. from ZeroTop ViewFront ViewO0001N005 G54 G90 S600 M03N010 G00 X1.0 Y1.0N015 G43 H01 Z.1 M08G43Tool Length CompensationH01Specifies Tool length compensationZ.1 Z Coordinate .1 in. from ZeroM08Flood Coolant OnTop ViewFront ViewO0001N005 G54 G90 S600 M03N010 G00 X1.0 Y1.0N015 G43 H01 Z.1 M08N020 G01 Z-.75 F3.5G01Straight Line Cutting MotionZ-.75Z Coordinate -.75 in. from ZeroF3.5Feed Rate set to 3.5 in./min.Top ViewFront ViewO0001N005 G54 G90 S600 M03N010 G00 X1.0 Y1.0N015 G43 H01 Z.1 M08N020 G01 Z-.75 F3.5G00Rapid MotionZ.1Z Coordinate .1 in. from ZeroM09Coolant OffN025 G00 Z.1 M09Top ViewFront ViewO0001N005 G54 G90 S600 M03N010 G00 X1.0 Y1.0N015 G43 H01 Z.1 M08N020 G01 Z-.75 F3.5N030 G91 G28 X0 Y0 Z0G91Incremental Programming ModeG28Zero Return CommandX0, Y0, Z0 X,Y,& Z Coordinates at ZeroN025 G00 Z.1 M09Top ViewFront ViewO0001N005 G54 G90 S600 M03N010 G00 X1.0 Y1.0N015 G43 H01 Z.1 M08N020 G01 Z-.75 F3.5N035 M30N030 G91 G28 X0 Y0 Z0N025 G00 Z.1 M09M30End of Program

Function

MeaningG00Point to point positioningG01Leaner interpolationG02Circular interpolationG03Circular interpolation (Anti clockwise)G70Dimensioning in inchG71Dimensioning in mmG90Absolute dimensioningG91Incremental dimensioningG96Constant surface speedG97Spindle speedPreparatory function (G-words )They are the commands, which prepare the machine for different modes of movement like positioning, contouring, thread cutting etc and for presetting the machine.FunctionMeaningM00Program stopM01Optional stopMO5Spindle stopMO6Tool turret indexM09Coolant offM10Clamp work pieceM11Release work pieceM30End of programMiscellaneous functionThe miscellaneous function word is used to specify certain miscellaneous or auxiliary functions which do not relate to the dimensional movements of the machine. The miscellaneous functions may be spindle start, spindle stop, coolant ON/OFF, etc.METHODS OF LISTING THE COORDINATES OF POINTSThe coordinate systems are used to define and control the position of the tool in relation to the work piece. Each system has its own applications and the coordinate systems may be used indecently or may be mixed, within a CNC part program according to the machining requirements of the component. The co-ordinate systems used in CNC machines are as follows.a) Absolute co-ordinatesb) Incremental co-ordinates

Absolute co-ordinatesIn the absolute co-ordinate system the co-ordinates of a point are always referred with reference to the same datum. The datum position in X axis and Y-axis and Z axis are defined by the program before starting the operation of the machine.A major advantage of this system is that it is very easy to check and correct a program written is made in the value of any dimension in a particular block.

Incremental co-ordinateIn the incremental co-ordinate system the co-ordinates of the any point are calculated with reference to the previous point i.e., the point at which the cutting tool is positioned is taken as a datum point for calculating the co-ordinates of the next point to which movement is to be made.It is difficult to check a part program written in incremental dimension in a particular block, it will affect subsequent blocks.

Advantages of Computer Numerical Control Machines Accuracy and repeatability is very high as compared to the NC machines. Reduced scrap and rework. Reduced inspection time. Reduced material handling . Easy of inter changeability of machine parts . Complicated shapes and curves are easily manufactured with quality assurance and better production. Reduced skilled operators can work in CNC machines. It can work 24 hours a day because the resting times are very low. Time taken to complete the works is minimum as compared to manual machines. Program editing at the machine site.