Cnc Presentation

Computer Numerical Control Machines

Contents What is NC? Brief History of NC Basic Components of NC System NC ~ CNC Advance Elements in MCU of CNC

System Classification of CNC Machines CNC Part Programming Example of a Part Programming Computer Aided Part Programming Advantages of CNC Disadvantages

What is NC?NC stands for “Numerically Controlled”

The word “NUMERICAL” is defined as the expression of something by “NUMBERS”.

“CONTROL” is defined as the exercise of directing, guiding or restraining power over something.

Numerical Control is a form of programmable automation in which the mechanical actions of an equipment are controlled by alphanumeric data instructions.

The set of data instructions, which is required to produce a part is called as a part program.

The program is translated into appropriate electrical signals for input to motors that run the machine.

Brief History of NC 1947: The concept was credited to John

Parson, who was able to control the position of machine tool, using punch cards, in an attempt to machine helicopter blades.

Layout of Punched Cards NC Machine

Part DrawingEquipment for

part programming

Punch Tape MCU, that converts part programming

into electric signals

Machine Tool

Brief History of NC 1947: The concept was credited to John

Parson, who was able to control the position of machine tool, using punch cards, in an attempt to machine helicopter blades.

1949: US Air Force teamed up with MIT to develop a programmable milling machine.

1952: A three axis milling machine was demonstrated, that used punched cards and electromechanical controller. The term Numerical Control (NC) was originated.

1980s: A digital computer was linked directly to the controller, upgrading NC to CNC.

Why NC machines were developed?

To achieve

Flexibility to manufacture complex or impossible jobs at lesser cost

Improvement in the accuracy and quality of manufactured parts

Automation and repeatability Reduction of non-cutting time in the machining cycle

Controller

Machine Tool

Prototype of NC Milling Machine Developed By MIT

Basic Components of NC System

An NC system consists of three basic components,

a) Program of Instructionsb) Machine Control Unit (MCU)c) Processing Equipment

ProgramMachine Control Unit

Processing Equipment


a) Program of Instructions:

The part program is the set of detailed step-by-step commands that directs the actions of the processing equipment.

The program is coded on a suitable medium for submission to the machine control unit.

1-inch wide punched tape was first used as medium for many years.

Now, the punched tapes has largely been replaced by newer technologies that include magnetic tape, diskettes, and electronic transfer of part program from a computer.


b) Machine Control Unit (MCU): MCU of NC system reads and interprets the

program of instructions and convert them to electrical signals to control the mechanical actions of processing equipment.

It consists of following elements, Program Reader Signal output channel to processing equipment Feedback channel from processing equipment Sequence Controllers


c) Processing Equipment: The third basic component of NC system is

the processing equipment that performs the actual productive work, such as, Machining Processes (Drilling, Milling etc.) Sheet Metal Processes (Punching, Bending etc.) Welding Processes (Spot and arc welding

processes) Thermal cutting Tube bending Assembly work Inspection work (CMM etc.)

NC ~ CNC Since the introduction of NC in 1952, there have

been dramatic advances in digital computer technology.

The significant reduction of size and cost of digital computers and the introduction of microcomputers, urged the makers of NC system to incorporate these advances in their products.

Thus, in 1980s, NC was transformed into CNC. CNC, Computer Numerical Control, is an NC

system whose MCU is based on dedicated microcomputer rather than on a hard-wired controller.

Today’s CNC feature high-speed processors, large memories, solid-state flash memory, improved servos, and bus architectures.

Advance Elements in MCU of CNC System

The MCU is the hardware that distinguishes CNC from conventional NC. MCU consists of following components and sub-systems,

1) Central Processing Unit (CPU)

2) Memory

3) I/O Interface

4) Machine Tool Controls

5) Sequence Controls

MemoryROM – Operating SystemRAM – Part Programs

Central Processing Unit

(CPU)

Input/output InterfaceOperator panelTape reader

Machine Tool ControlsPosition Control Spindle Speed Control

Sequence ControlsCoolantFixture Clamping

System Bus


1) Central Processing Unit (CPU):It is the brain of Machine Control Unit that manages other components of MCU. CPU can be divided into three sections,

a) Control Section: It retrieves commands and data from memory and generates signals to activate other components in MCU.

b) Arithmetic Logic Unit: It performs various calculations, counting and logical functions required by the software.

c) Immediate Access Memory: It provides a temporary storage for data being processed by CPU.


2) Memory:A storage capacity is required to store various programs and data. Memory can be divided into two categories;

a) Main Memory: It consists of ROM and RAM devices. Operating system software and machine interface programs are stored in ROM and part programs are stored in RAM.

b) Secondary Memory: High capacity secondary memory stores large programs and data files that can be transferred to main memory. Hard disks are the common secondary memory storage devices.


3) Input/output Interface: The I/O interface transmits and receives

data and signals to and from external devices.

The operator control panel is the basic interface by which the machine operator communicates to the CNC system.

It may include alphanumeric keypad, keyboard or even a display screen, to indicate current status of program as it is being executed.


4) Machine Tool Controls: These are hardware components that control the

position and velocity of each machine axis as well as the rotational speed of spindle.

The control signal generated must be converted to a form and power level suited to the particular position control system used to drive the machine axes.

5) Sequence Controls: They control additional auxiliary functions such

as coolant control, tool changer, fixture clamping etc.

To avoid overloading of CPU, PLC may also be used to control these functions.

Classification of CNC System

CNC machines can be classified on the basis of the following;

Motion Control System Point-to-point Continuous path

Control Loop System Open Loop Control System Closed Loop Control System

Positioning System Absolute Positioning Incremental Positioning

Classification on the basis of Motion Control System Point-to-point Systems:

It controls the motion of the worktable to programmed location without regard for the path taken to get to that location. Once the move has been completed some processing action is accomplished by the workhead at that location, such as drilling or punching a hole.

Classification on the basis of Motion Control System Continuous path System:

This system is capable of controlling continuous simultaneous control of two or more axes.The tool performs the process while the worktable is moving, thus creating angular surfaces, 2D curves, or 3D contours.This control mode is required in many milling and turning operations.

Classification on the basis of Motion Control System Interpolation Methods: The paths that a contouring type CNC

system is required to generate, consist of circular arcs or non-linear shapes, which are defined by mathematical expressions or by approximations.

To cut along a circular path, the curve must be divided in to a series of straight lines that approximate the curve.

The interpolation module in the MCU performs the calculation and directs the tool along the path.

Classification on the basis of Motion Control System Interpolation Methods:Following are the interpolation methods for various

cases,a) Linear Interpolation: it is used when a straight line path is

to be generated. The programmer specifies the start and end point of the straight line and the feed rate.

b) Circular Interpolation: It is used for circular curves. Start point, end point, radius of arc and direction of cut needs to be specified.

c) Helical Interpolation: It combines the circular interpolation for two axes with the linear movement of the third axis.

d) Parabolic and cubic interpolation: It is used for approximating free form curves using high order equations. Most

applications are in the aerospace and automotive industries.

Classification on the basis of Control Loop System Open Loop Systems:

In an open loop control system, the electrical motor drives the slides as per the input signal or command. There is no monitoring of the actual displacement of the machine slide.

Classification on the basis of Control Loop System Closed Loop Systems:

In a closed loop system the displacement of slide is achieved to very high degree of accuracy by using a monitoring devices. This feedback is compared with the input info & the slide position is regulated to agree with the desired positions.

Classification on the basis of Positioning System Absolute Positioning:

In this positioning system, the workhead locations are always defined with respect to the origin.

Incremental Positioning:In incremental positioning, the next workhead position is defined w.r.t the present location.

A (10, 20)

B (20, 35)

Move from A to B is specified as,Absolute: X = 20 Y = 35Incremental: X = 10 Y = 15

CNC Part Programming CNC part programming contains geometric

data about the part and movement commands of the cutting tool with respect to the work piece.

The machine receives instructions as a sequence of blocks containing commands to set machine parameters such as tool paths, feed, spindle speed, etc.

Preparatory functions (G-Codes) are associated with tool movements.

Miscellaneous functions (M-Codes) are associated with other auxiliary machine functions, such as coolant on/off function, fixture clamping, etc.

Common G-CodesCode FunctionG00 Rapid Movement

G01 Linear Interpolation, controlled feed

G02 Circular Interpolation, clockwise

G03 Circular Interpolation, counter clockwise

G04 Dwell for programmed duration

G17 Select x-y Plane

G18 Select y-z Plane

G19 Select x-z Plane

G70 Inch units

G71 Metric units

G90 Absolute dimensions

G91 Incremental dimensions

Common M-CodesCode FunctionM00 Program Stop

M01 Optional Stop

M02 End of Main Program

M03 Spindle CW

M04 Spindle CCW

M05 Spindle Stop

M06 Tool Change

M07 Flood Coolant ON

M08 Mist Coolant ON

M09 Coolant OFF

M17 End of Subprogram

M30 End of Program

Other Symbols in part ProgrammingO the program numberN the block sequence address letter F the feed address letterS the spindle speed or cutting speed

address letterT the tool address letterX X-axis identification letterY Y-axis identification letterZ Z-axis identification letter

Example of Manual Part Programming

To move the tool to four points using rapid positioning command.

Program: Ooo1N001 T0101;N002 S1000 M03;N004 G00 Z10;N005 G00 X12 Y20;N006 G00 X62 Y20;N007 G00 X62 Y50;N008 G00 X12 Y50;N009 G28 X0 Y0 Z0;N010 M30;

A B

CD

Steps involved in part Programming

1. Read the part drawing2. Make a process flow sheet3. Select cutting parameters4. Mount the raw material block5. Select datum/origin on work piece6. Mount all tools required7. Enter the part program according

to the process flow sheet8. Start cycle

Computer Aided Part ProgrammingCNC part programming can also be done using

CAD/CAM Technology. Develop or obtain the 3D geometric model of

the part, using CAD. Decide which machining operations and

cutter path directions required. Choose the tooling required. Run CAM software to generate the CNC part

program. Download the part program to appropriate

CNC Machine. Verify and edit the program if necessary. Run the program and produce the part.

Advantages of CNC More complex part geometries are possible Greater accuracy, repeatability and flexibility Shorter manufacturing lead times Nonproductive time is reduced Inspection requirements are reduced Simpler fixtures are needed Scrap reduction Operator skill-level requirements are reduced

Disadvantages of CNC Higher investment cost Higher maintenance effort Only feasible for higher utilization.

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