2. introduction to cnc - npkauto cnc.pdf-introduction to cnc machines-© rohan desai- automobile...

-INTRODUCTION TO CNC MACHINES-

© Rohan Desai- Automobile Dept. - New Polytechnic, Kolhapur.

2. INTRODUCTION TO CNC

Q. Define NC Machines.

A method of automation, in which various functions and processing of machine toolsare controlled by letters and symbols.

The general objective of NC technology is to reduce production cost by reducingproduction time. This is achieved by reducing non-production time on the number ofset-ups, set-up time, work piece handling time, tool change time and lead time etc.

Q. Give advantages and application of NC Machines.

The fixtures are reduced. Accuracy is improved due to reduction of human error. These are best suited for small batch production. Inspection time is reduced.

Applications: Milling, Lathe, Grinding, boring and drilling machines.

Q. What are the disadvantages of the NC system?

1. High initial cost.

2. Higher maintenance cost.

3. Higher tooling cost.

4. Need for a controlled environment.

5. Higher personnel costs.

Q. What are the basic components of NC system? Or what is workingprinciple of NC machines?

There are three components of NC systems.

Program of instructions MCU- Machine control unit Machine tool

Programme of instructions:

It is the set of directions which tell the machine tool what to do and in whatsequence. The part program is written in coded form and contains all the



information needed for machining the component. The part program is fed to theMCU through some input medium.

1.Punched card

2.Magnetic tape and

3.Paper tape

MCU:

It consists of the electronics and hardware that read and interpret the program ofinstructions and convert it into mechanical actions of the machine tool. Programmereader is a device used to read the coded instructions. Types of readers are

1. Card reader

2. Punched tape reader

3. Magnetic tape reader

Machine tool:

In a numerically controlled machine all the movements of the tool and the machinetable are done automatically with the help of electric motors. The functions liketool change, machine ON/OFF and coolant ON/OFF are controlled through partprogram.



Q. Define CNC.

CNC or Computer Numerical Control is an NC system using a dedicatedmicrocomputer as the machine control unit.

The presence of a microprocessor, RAM, ROM, input and output devices haveraised the level of automation in NC systems.

Q. Give advantages and disadvantages of CNC machines.

Advantages:

Reduced lead time.

Elimination of operator’s errors.

Lower labour cost.

Flexibility in changes of component design.

Reduced inspection.

Disadvantages:

Higher investment cost

Higher maintenance cost.

Higher personnel costs.

Planned support facility is required.

Q. What are the basic components (elements) of CNC system? Or what isworking principle of CNC machines?

A CNC machine also has a tape reader or any other input media for entry of the partprogram. CNC uses the part program in a different manner though there is similaritybetween NC and CNC. In CNC, entire program is first fed to the inbuilt computermemory. Once the program is stored, the machine cycle is then executed by theprogram. Software with control algorithms converts the part program instructionsinto actions by the machine tool.



This is done by generating pulses for each axis from the controller. Each pulseproduces one small unit of motion (SUM). The slide travel is thus decided by thenumber of pulses. In a closed loop system, the pulses are fed to a reference. Thefeed back device also sends the signal to the reference. These two signals arecompared and necessary action is controlled.

Q. What is DNC?

Direct numerical control (DNC) is defined as a system which integrates a number ofmachines by controlling it through a central computer by direct connection.

The central computer is designed to provide instructions to each machine tool ondemand. The central computer also receives data back from the machines. Thus,there is a two way information exchange between the central computer and each ofthe machine tool.



Q. what are the functions of DNC System? What are its types?

1. To use the central computer for storing and editing programs for all machinetools connected to it.

2. To transfer the stored programs to the connected machines on demand.

3. To post process part programs written in higher level languages like APT.

4. To integrate CAD with CAM by having a common database.

5. To provide a link between a central computer and various plant computersconnected through modems and networks even though the plants may beplaced several kilometers away from each other.

Types of DNC system

1. Behind the Tape Reader (BTR) system

2. Specialised MCU



Q. Give advantages and disadvantages of DNC system.

Advantages:

1. DNC eliminates the use of the tape reader, which is certainly the weakestcomponent in the NC system.

2. Time sharing by the central control makes it possible to keep a close controlover the complete machine shop.

3. The huge memory of DNC allows it to store a large amount of part programsfor subsequent use. It also receives the memories of NC control unit.

4. Presence of a central bulk memory allows the same program to be run ondifferent machines at the same time without duplicating it at individual places.

Disadvantages:

1. DNC uses a central control and in an event of computer failure, the completeactivities of the machine shop would come to a standstill.

2. DNC is expensive and its use is practical in areas where high automation isrequired.

Q. Classify the CNC machines

CNC are classified in two ways,

1. According to number of axis

2. According to CNC control system

1. According to number of axis:

Generally, there are three major axes on CNC machines. In the programs theseaxes are denoted as X, Y and Z. Each axis moves in right angles to each other. U, Vand W are parallel to X, Y and Z respectively. A,B and C are three rotating axes ofthe major axes.

a. 2 Axis machine: In this machine only two main (major) axes are usedfor tool movement. Example- Turning centres and Chucker.

b. 3 Axis machine: In this machine three major axes are used for toolmovement. Example- VMC.



c. 4 Axis machine: for the tool movement, three major and either rotatingor parallel axis are used. Example- VMC.

Now days, in industries 5 Axis, 6 Axis and 7 Axis machines are also used.

2. According to CNC control system: (Control on feed)

Tool on CNC machines moves either in rapid or in feed. It moves in feed duringcutting operation otherwise it moves rapidly. Machine is classified according to feedgiven to tool in three ways:

a. Point To Point control system:

In this control system, tool moves in feed in only one direction (only in one axis)while tool moves rapidly in another directions (axes). Example- Drilling, reaming,tapping, punch press and jig boring.

b. Straight cut control:

In this system, tool can move in feed in any two (directions) axes and it can moverapidly in all directions but tool follows straight path with the feed in one direction ata time. Example- Milling, step turning, slot cutting, key way cutting and groovecutting.



c. Contouring control system:

In this system, all axes can be moved in feed and rapid mode. Due to this anycritical and complex surfaces can be cut easily on such machines. Example- Inclinedor curvature slot cutting and helical groove cutting.

Q. which types of coordinate systems is used in CNC machines?

The part program on CNC machine depends upon a tool path which is preparedaccording to operations on workpiece (job). It is necessary to locate a work zeroposition on the job to prepare a tool path. With reference to work zero position thecoordinates of all points on a tool path are determined.



To determine the coordinates, graphical method is used. On the graph, the point ofintersection of two axes termed as an original point (coordinates of original point arezero). All axes of job intersect at work zero point and considering this point as anoriginal point, coordinates of remaining points are determined. Use of proper signsplay significant role for determining these coordinates. To determine thecoordinates, two systems are used.

Absolute Coordinates system

Incremental Coordinate System

Absolute Coordinates system:



In this system, work zero point is considered as an original point and coordinates ofall points are determined accordingly. The distances between work zero point andany other point along the each axes are the coordinates of a particular point.

Incremental Coordinate System:

In this system, work zero is considered as an original point only for determining thecoordinates of first point, after that coordinates of all points are determined byconsidering the previous point as an original point.

Q. what is axes identification? Or write short note on axis designation.

The first axis to be identified is the Z axis. This is then followed by the X and Y axesrespectively.

Z axis and motion:

Location: The Z axis motion is either along the spindle axis or parallel to thespindle axis. In the case of machine without a spindle such as shapers andplaners, it is identified as the one perpendicular to the work holding surface,which may or may not be passing through the control point (e.g. the cuttingtool tip in case of shaper).

Direction: The tool moving away from the work holding surface is designatedas positive Z direction. This means during machining tool moves in negative Zdirection.



X axis and motion:

Location: It is perpendicular to the Z axis and should be horizontal andparallel to the work holding surface wherever possible.

Direction: When looking from the principal spindle to the column, the positiveX is to the right. For turning machines it is radial and parallel to the crossslide.

Y axis and motion: It is perpendicular to both X and Z axes and the direction isidentified by the right hand Cartesian coordinate system.

Rotary motions: A, B and C define the primary rotary motions.

Location: These motions are located about the axis parallel to X, Y and Zrespectively.

Direction: Positive A, B and C are in the directions which advance right-hand screwsin the positive X, Y and Z directions respectively.



Q. what do you mean by reference positions?

There are three reference positions on CNC machine,

1. Machine zero point

2. Work zero point

3. Tool home position

1. Machine zero point: At this point coordinates of all axes are zero. Tool moveswith respect to this point and position of all axes can be seen on computerscreen. Machine zero point is decided by manufacturer of machine.

2. Work zero point: For preparing a program, first tool path is preparedaccording to operation sequence and then coordinates of all points aredetermined. These coordinates are determined by considering an originalpoint on the job where all the axes intersect and coordinates of that point arezero. This original point is known as work zero point.



3. Tool home position: Tool is placed away from work zero point as well asmachine zero point for sake of safety of tool, job and machine. The tool ischanged only at home position.

Q. Write the procedure for developing the part program.

There are two methods of part programming: manual part program and computerassisted part programming.

Manual part programming: To prepare a part program using the manualmethod,

1. The programmer writes the machining instructions on a special form called apart programming manuscript. The manuscript is a listing of the relative tooland workpiece location.

2. The NC tape is prepared directly from the manuscript.

3. Define the axis coordinates in relation to the work part.

4. Define safe (target point) point and origin point (work zero).

5. The tape is inserted to read the first block in to the system.

6. The functions like machining, tool changing, spindle ON/OFF, coolantON/OFF, program stop and tape rewinding are carried out as per theprogram.

Computer- assisted part programming: This method is useful for most criticaland complex parts. The part programmer and the computer are main tools in thismethod.

1. The part programmer first defines the work part geometry.

2. He specifies the operation sequence and tool path.

3. The computer interprets the list of part programming instructions, performsthe necessary calculations to convert this into a detailed set of machine toolmotion commands, and then controls a tape punch device to prepare thetape.

4. The tape is verified for accuracy.

5. The NC system machines (makes) the part according to the instructions ontape.



Q. What is program configuration (format)?

Program is the main part on the CNC machine. English letters and numbers areused to prepare these programs. These alphabets (letters) are termed as Address.A Word is formed by making pair of an address and numbers. e.g. X52.6, G00, M30etc. When these words are arranged in a line to perform the specific operations, theline is known as block. Every instruction or block is ended with ‘EOB’ (end of block).e.g. G01 X42 Z-45 F80 EOB

Program format:

Program number

Block 1

Block 2

Block 3

-

-

Block n

Program end

Q. what is block format?

N_ G_ X_ Y_ Z_ R_ F_ S_ T_ M_ EOB

N: Every block has given number according to tool path. Numbers from 1 to 9999can be used. e.g. N05

G: For preparatory functions i.e. for all type of tool movement, codes obtained byaddress G are used. G code is followed by coordinates of X, Y and Z axes. e.g. G00

X, Y, and Z: These addresses are used to represent the distances travelled by toolwith respect to axes. e.g. X20, Y50 and Z-20.

R: Radius for curvature is given by address R it is also used to give parameters. e.g.R20 or R4=56.

F: This address is used to give feed. It can be given as mm/min or mm/rev. e.g.F80 or F0.8



S: To give spindle speed, this address is used. e.g. S500

T: The tools in the magazine or in turret head are numbered. Address T is followedby tool number in the turret head or tool magazine. e.g. T05

M: For miscellaneous functions or all activities except tool movements M codes areused. e.g. M05

EOB: This sign is used to represent end of block.

Q. list the ISO codes used for preparatory and miscellaneous functions.

G codes M codes

G00- Rapid travel (lathe/milling) M00- Program stop

G01- Linear interpolation (lathe/milling) M01- Optional program stop

G02- Clockwise circular interpolation(lathe/milling)

M02- End of program

G03- Counter clockwise circularinterpolation (lathe/milling)

M30- End of program and reset

G70- Input in inches (lathe/milling) M03- Spindle ON clockwise

G71- Metric input (lathe/milling) M04- Spindle ON anticlockwise

G74- Return to home position(lathe/milling)

M05- Spindle stop

G54 to G59- work offset codes 1 to 6selection (lathe/milling)

M06- Tool change

G90- Absolute coordinate system(lathe/milling)

M07- High pressure coolant ON

G91- Incremental coordinate system(lathe/milling)

M08- Coolant ON

G92- Thread cycle (lathe) M09- Coolant OFF

G84- Tapping cycle (milling) M10- clamp

G94/ G95- Feed in mm/min or mm/rev M11- declamp (unclamp)

2. introduction to cnc - npkauto cnc.pdf-introduction to cnc machines-© rohan desai- automobile...

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