CAM Lecture

7/31/2019 CAM Lecture

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CAMMANUFACTURING ENGINEERING

Dr. Maqsood Ahmed KhanAssociate Professor & Co Chairman, IMD

COURSE PLAN

Topic Lectures

Conventional Numerical Control 02

NC Part Programming 03

Computer Numerical Control 02

Midterm Exam 01

Group Technology and FMS 03

Process Planning 02

CAD/CAM Software 03

Software: CATIA V5 (R18) ; Matlab R2008a

Marks Distribution:

1. Final theory paper = 60 Marks2. Sessional Marks

i. Attendance = 10

ii. Midterm Exam = 15

iii. Project = 15

40 Marks


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BOOKS

CAD/CAM

1. CAD/CAM is a term which means computer-aided designand computer-aided manufacturing.

2. It is the technology concerned with the use of digitalcomputers to perform certain functions in design andproduction.

3. It is a bridge between design and manufacturing.

Definition-CAM

Computer-aided manufacturing (CAM) can be defined as theuse of computer systems to plan, manage, and control the

operations of a manufacturing plant through either direct orindirect computer interface with the plants productionresources.


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MANUFACTURING SUPPORTAPPLICATIONS:

(INDIRECT

INTERFACE

)These are the indirect applications in which the computer is

used in support of the production operations in the plant, but

there is no direct interface between the computer and the

manufacturing process.

MANUFACTURING SUPPORTAPPLICATIONS:

(INDIRECT INTERFACE)Some examples of CAM for manufacturing support are

1. Numerical control part programming by computers

2. Computer-automated process planning

3. Computer-generated work standard

4. Production scheduling

5. Material requirement planning

6. Shop floor control (detailed management of activities and

the flow of materials inside the plant - including the

workers, materials, machines and time)


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THE PRODUCT CYCLE AND CAD/CAM

Product Cycle: Various activities and functions that must be

accomplished in the design and manufacture of a product is termed

as the product cycle.

Figure-1 Product Cycle without CAD/CAM

Figure-1 Product Cycle without CAD/CAM


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Figure-1 Product Cycle with CAD/CAM

Product Cycle with CAD/CAM


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CONVENTIONAL NUMERICAL CONTROL

Definition

Programmable automation in which the mechanical actions of a

machine tool are controlled by a program containing coded

alphanumeric data that represents relative positions between a work

head (e.g., cutting tool) and a work part.


Motivation

To manufacture complex curved geometries in 2D or 3D was

extremely expensive by mechanical means (which usually would

require complex jigs to control the cutter motions)

Machining components with repeatable accuracy

Unmanned machining operations

NC technology has been applied to a wide variety of operations,

including machining, assembly, inspection, sheet metal press-

working, and spot welding.


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History Late 1940s

John T. Parsons (contractor of USAF) introduced a method of using punchedcard containing coordinate points of complex three dimensional profiles tocontrol a machine tool. The machine was directed to move in smallincrements, thus generating the desire surface of a helicopter blade.

1948

Parsons demonstrated his first concept to the U.S. Air Force.

1951

MIT was involved in the project.

1952

MIT demonstrated first prototype of vertical NC milling machine (punchedtape input).

1955After refinements commercially available NC machines were displayed atNational Machine Tool Show

BASIC COMPONENTS OF AN NC SYSTEM

A typical NC system consists of the following three elements

1. Program of instruction

2. Machine control unit (MCU)

3. Machine tool or other controlled process


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Program of Instruction:

The program of instructions is the detailed step-by-step set of

directions which tell the machine tool what to do.

It is coded in alphanumerical and symbolic form on some type of

input medium that can be interpreted by the controller unit. The

most common input medium today is 1-inch wide punched tape.


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There are two other methods of input to the NC system:

1. Manual data input (MDI)

2. Direct link with the computer (DNC)

Controller Unit

The second basic component of the NC system is the controller unit.

This consists of the electronics and hardware that read and interpret

the program of instructions and convert it into mechanical actions of

the machine.


Typical elements of a conventional NC controller unit

Tape reader

Data Buffer

Signal output channels to the machine tool

Feed-back channels from the machine tool

Sequence controls

Machine tool

The third basic component of an NC system is the machine tool.


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THE NC PROCEDURE

Process Planning

Part Programming

1. Manual Part programming

2. Computer-assisted part programming

Tape preparation

Tape verification

Production

NC COORDINATE SYSTEM

In order for the part programmer to plan the sequence of positions and

movements of the cutting tool relative to the work-piece, it is necessary to

establish a standard axis system.

All the machine tools make use of Cartesian coordinate system for the

sake of simplicity.

The familiar right hand coordinate system is used for designating the

axes

In addition to the three linear axes, milling machine may have the

capacity to control one or more rotational axes.


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FIXED ZERO AND FLOATING ZERO

The Programmer must determine the position of the tool relative to

the origin (zero point) of the coordinate system.

NC machines have two methods for specifying the zero point.

Fixed zero

Floating zero

Fixed zero

The origin is always located at the same position on the machine table.

Usually, the position is the lower left-hand corner of the table and all

tool locations are defined by positive x & y coordinates.


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Floating zero

This is more common feature on modern NC machines, which allows

the machine operator to set the zero point at any position on the

machine table.

The part programmer decides the location of zero point

The decision is based on part programmers convenience

The location of zero point is communicated to the machine operator

At the beginning of the job, the operator moves the tool manually to

the target point

The target point is some convenient place for the operator (corner

point, pre drilled hole etc.)


The target point has been referenced to the zero point by the

programmer.

When the tool has been positioned at the target point, the machine

operator presses a zero button to define the origin point.

M/C coordinate system

Target point


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NC MOTION CONTROL SYSTEM

There are four basic types of motion control systems.

Point-to-point NC

Sometimes it is also called a positioning system. The objective of

machine tool control system is to move the cutting tool to a

predefined location.

Straight-cut NC

Straight cut control systems are capable of moving the cutting

tool parallel to one of the major axis at a controlled rate suitablefor machining.

NC MOTION CONTROL SYSTEM

2-Axis Contouring NC

Contouring is the most complex, the most flexible, and the most

expensive type of machine tool control. It is capable of

performing both PTP and Straight-cut operations. It has

simultaneous control of more than one axis movement of the

machine tool.

3-Axis Contouring NC

It can control three axis simultaneously. Suitable for machining

3D complex profiles encountered in industrial practice such as

aerospace components, mould and dies etc.


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INTERPOLATION METHODS

The selection of appropriate interpolation method is importantfor 2 and 3 axis contouring jobs.

The paths that a contouring-type NC system is required tomachine often consist of circular arcs and other smooth nonlinearshapes.

Some of these shapes can be defined mathematically by relativelysimple geometric formulas.

Whereas others cannot be mathematically defined except byapproximation.

In any case, a fundamental problem in generating these shapesusing NC equipment is that these shapes are continuous,whereas NC is digital.

To solve this problem the path (circle or any other free formcurve) must be divided into a series of straight line segmentsthat approximate the path.


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The tool is commanded to machine each line segment in

succession so that the machined path closely matches the desired

path.

The maximum error between the nominal (desired) surface and

the actual (machined) surface can be controlled by the lengths of

the individual line segments,


If the programmer were required to specify the endpoints for each ofthe line segments (to machine along a curve), the programming taskwould be extremely laborious and likely to errors. Also, the partprogram would be extremely longbecause of the large number ofpoints.

To ease the burden, interpolation routines have been developed thatcalculate the intermediate points to be followed by the cutter togenerate a particular mathematically defined or approximated path.

A number of interpolation methods are available to deal with smoothcontinuous path in contouring.

1. Linear interpolation

2. Circular interpolation

3. Helical interpolation4. Parabolic interpolation

5. Cubic interpolation


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Linear interpolation:

This is the most basic and used method when a straight line pathis to be generated in continuous path NC. The programmerspecifies the beginning point and end point of the straight lineand the feed rate to be used along the straight line. Theinterpolator computes the feed rates for each of the two (or three)axes to achieve the specified feed rate.

Circular interpolation:

This method permits programming of a circular arc by specifyingthe following parameters: (1) the coordinates of the startingpoint, (2) the coordinates of the endpoint, (3) either the center orradius of the arc. The generated tool path consists of a series ofsmall straight line segments calculated by the interpolation

module.


Helical interpolation:

This method combines the circular interpolation scheme for two

axes described above with linear movement of a third axis. This

permits the definition of a helical path in three-dimensional

space. Applications Include the machining of large Internal

threads.

Parabolic and cubic interpolation:

These routines provide approximations of free form curves using

higher order equations. Most applications are in the aerospace

and automotive Industries for free form designs that cannot

accurately and conveniently be approximated by combining

linear and circular interpolations


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ABSOLUTE AND INCREMENTAL POSITIONING

Another option sometimes available to the part programmer is to

use either an absolute system of tool positioning or an

incremental system.

Absolute system

It is always defined with respect to

the zero point.

Incremental system

It is defined with reference to the

previous tool location.

NC PART PROGRAMMING

CHAP # 8


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THE PUNCHED TAPE IN NC

TAPE CODING AND FORMAT

NC tape coding

There are eight regular columns of holes.

There is also a ninth column of holes between the 3rd and

4th regular columns used as sprocket holes.

The coding of the tape is provided by either the presence or

absence of a hole, hence it becomes the binary code.

It uses the base 2 number system, which can represent any

number in the decimal system.

Besides numbers, alphabetical letters and other symbols can

also be coded.

Eight columns provide more than enough binary digits to

define any of the required symbols.


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How Instructions are formed

A complete row makes a character, which represents a letter,

number, or other symbol.

A word is a collection of characters

A block is a collection of words.

A block of words is a complete NC instruction.

To separate blocks, an end-of-block (EOB) symbol is used.

The tape reader feeds the data from the tape into the buffer in

blocks.


NC words

Sequence number (n-word): N10, N20, N100.

Preparatory word (g-word): g00, g01, g02.

Coordinates (x-,y-, and z-words): x+9.4625, y-23.6845

Feed rate (f-word): f30.

Cutting speed (s-word): s2000

Tool selection (t-word): t05

Miscellaneous function (m-word): m03 (start spindle).


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MANUAL PART PROGRAMMING

To prepare a part program using the manual method, the

programmer writes the machining instructions on a special

form called a part programming manuscript.

Manuscripts come in various forms, depending on the

machine tool and tape format to be used.

The manuscript is a listing of the relative tool and work-piece

positions.

It also include other type of data, such as miscellaneous

instructions, and speed/feed specifications.

Except for complex parts with many holes, manual

programming is ideally suited for PTP applications.

On the other side, except for the simple jobs, manual

programming become quite time consuming for contouring

jobs.

EXAMPLE

Suppose that the part to be programmed is a drilling job. The engineering

drawings for the part is presented in the figure. Three holes are to be drilled

at a diameter of0.484-in. The close hole size tolerance requires reaming to

0.5-in dia. Recommended parameters areas follows:

Drill bits are manually changed by the machine operator. The machine has

the floating-zerofeature and absolute positioning.

Speed (rpm) Feed (in/min)

0.484-in dia. drill 592 3.55

0.5-in dia. reamer 382 3.82


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COMPUTER-ASSISTED PART PROGRAMMING

In the more complicated PTP jobs and in contouring applications,

manual part programming becomes an extremely tedious task and

subject to errors.

In these instances it is much more appropriate to use the high-

speed digital computer to assist in the part programming process.

Many part programming language systems have been developed

to perform automatically most of the calculations.

In computer assisted part programming the machining

instructions are written English-like statements of the NC

programming language.

These statements are processed by the computer to prepare the

tape.

The computer automatically punches the tape in the proper tape

format for the particular machine.

THE PART PROGRAMMERS JOBThe part programmers responsibility in computer-assisted part

programming consists of two basic steps:

Defining the work-part geometry

Specifying the operation sequence and tool path


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DEFINING THE WORK-PART GEOMETRY

No matter how complicated the work-part may appear, it is

composed of basic geometric elements, points, straight lines,

planes, circles, cylinders, and other mathematically defined

surfaces.

It is the part programmers task to identify the elements out

of which the part is composed.

Each geometric element must be identified and the

dimensions and location of the element explicitly defined.

SPECIFYING THE OPERATION SEQUENCE AND TOOL PATH After defining the work-part geometry, the programmer must

next construct the path that the cutter will follow to machine

the part.

It involves a detailed step-by-step sequence of cutter moves.

The moves are made along the geometry elements, which

have already been defined.

The programmer can use the various motion commands to

create these movements.


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THE COMPUTERS JOB

The computers job in computer-assisted part programming

consists of the following

1. Input translation

2. Arithmetic calculations

3. Cutter offset computation

4. Postprocessor

THE COMPUTERS JOBInput Translation

The part programmer enters the program written in APT or

other language

The input translation component converts the coded

instructions into computer-usable form for further processing.

1. Syntax check of the input code to identify errors in format,

punctuation, spelling etc.

2. Assigning sequence number to each APT statement.

3. Converting geometry elements into suitable form for

computer processing

4. Generating intermediate file called PROFIL that is utilized in

subsequent arithmetic calculations


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THE COMPUTERS JOB

Arithmetic Calculations

It is a set of subroutines for solving the mathematics required

to generate the part surface and generate tool-path.

The arithmetic calculations are performed on the PROFIL file.

It frees the programmer from the time-consuming and error-

prone geometry and trigonometry calculations.

The output of this module is CLFILE.

Cutter offset computation

The second task of the part programmer is to construct the toolpath. However, the actual tool path is different from the part

outline.

The actual tool path is achieved by offsetting the path from the

desired part surface equal to the radius of the cutter.

THE COMPUTERS JOBPost-Processor

NC machine systems are different, they have different features

and capabilities

High-level programming languages are not intended for only

one machine tool type. They are designed to be general

purpose.

The final task of the computer in computer-assisted part

programming is post-processing, in which the CLFILE file is

converted into low-level code that can be interpreted by the NC

controller.

The output of post-processing is a part program consisting of

G-codes, x-, y-, and z-coordinates, S,F,M, and other functions in

word address format.

A unique post-processor must be written fro each machine tool

system.


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NC PART PROGRAMMING LANGUAGES APT (Automatically Programmed Tool)

ADAPT (Adaption of APT)

EXAPT (Extended subset of APT)

UNIAPT

SPLIT (Sundstrand Processing Language Internally Translated)

COMPACT II

PROMPT

CINTURN II

The most widely used NC part programming language is APT.


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APT LANGUAGES

APT is a three dimensional system that can be used to control

up to 5-axes.

Our discussion is limited to the more familiar axes, x, y, and z,

and excluding the rotational coordinates.

APT can be used to control a variety of different machining

operations, but we will cover only drilling and milling

applications.

There are four types of statements in APT language:

1. Geometric Statements

2. Motion Statements

3. Post-processor statements

4. Auxiliary Statements

GEOMETRIC STATEMENTS These define the geometric elements that comprise the work-part.

The general form of an APT geometry statement is

Symbol = geometry type / descriptive data

Symbol: To identify the geometric element

Geometry type: To identify the type of geometric element (should beAPT vocabulary word)

Descriptive data: To define the geometric element precisely

Example:

P1 = POINT / 5.0, 4.0, 0.0


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L3 = LINE / P3, P4

L4 = LINE / P5, PARLEL, L3

PL1 = PLANE / P1, P4, P5

PL2 = PLANE / P2, PARLEL, PL1

C1 = CIRCLE / CENTER, P1, RADIUS, 5.0

EXAMPLE


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P0 = POINT / 0, -1.0, 0

P1 = POINT / 6.0, 1.125, 0

P2 = POINT / 0, 0, 0

P3 = POINT / 6.0, 0, 0

P4 = POINT / 1.75, 4.5, 0

L1 = LINE / P2, P3

C1 = CIRCLE / CENTER, P1, RADIUS, 1.125

L2 = LINE / P4, LEFT, TANTO, C1

L3 = LINE / P2, P4

PL1 = PLANE / P2, P3, P4

MOTION STATEMENTS The general form of motion statement is

motion command/ descriptive data

Example:

GOTO/P1

At the beginning of the motion statements, the tool must be given a

starting point.

The part programmer keys into this starting position with thefollowing statement

FROM/TARG


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POINT-TO-POINT MOTIONS

There are only two basic PTP motion commands:

1. GOTO

2. GODLTA

The GOTO statement instructs the tool to go to a particular point

location.

GOTO/2.0, 7.0, 0.0

The GODLTA command specifies an incremental move for the tool.

Example: GODLTA/ 2.0, 7.0, 0.0

GODLTA command is useful in drilling and related operations

EXAMPLE

P1=POINT/1.0, 2.0, 0

P2 = POINT/1.0, 1.0, 0

P3 = POINT/3.5, 1.5, 0

P0 = POINT/-1.0, 3.0, 2.0

FROM/P0

GOTO/P1

GODLTA/0, 0, -1.0

GODLTA/0, 0, +1.0

GOTO/P2

GODLTA/0, 0, -1.0

GODLTA/0, 0, +1.0

GOTO/P3

GODLTA/0, 0, -1.0

GODLTA/0, 0, +1.0GOTO/P0

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