Application Stuff of Cnc Machine

8/16/2019 Application Stuff of Cnc Machine

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(1) : Computer Aided Assembly Planning

(1) Introduction

An assembly is a collection of independent parts. It is important to understand the dependencies between various parts in an assembly to assemble the parts properly. Assembly model includes

the spatial positions and hierarchical relationships among the parts and mating conditions between the parts. ne of the obvious ways to facilitate the assembly process at the design phaseis to simplify the product by reducing the number of different parts to a minimum. In addition to

product simplification! the assembly process can be greatly facilitated by introducing guides and

tapers into the design of various parts. "harp corners usually hinder guiding parts into their

correct positions during assembly. Assembly planning is the chec#ing of both hard and softclashes! of generating assembly se$uences in order to chec# the feasibility of the removal path

and verifying the correct se$uence and the correct fit both dynamically and statically during the

disassembly process.

%igure &'.1 (a) Collection of pieces used to

define an assembly planning problem

%igure &'. (b) Assembly planning determining

a se$uence of motions for assembly

he use of computers for planning the assembly of mechanical products originated in the research

on planning with artificial intelligence. here are many reasons for systemati*ation and thecomputeri*ation of assembly planning! some of which are as follows+

• Industrial designers will benefit from having a tool with which they can $uic#ly assess

their designs for ease of assembly

• Although many e,perienced personals have a s#ill for devising an efficient ways to

assemble a given product! systematic procedures are necessary to guarantee that no good

assembly plan has been over loo#ed. "ometimes! the number of different assembly

alternatives is so large that even s#illful engineer may fail to notice many possibilities.

An automatic generator of assembly se$uence can be an efficient aid to designer. -henever onemodifies the feature of the product! the influence of these modifications can immediately be

chec#ed on the se$uences. %or small batch production! the automatic generation of assembly

se$uences is faster! more reliable and more costeffective than manual generation.

(2) Computer Aided Assembly Planning

Computer aided assembly planning involves automatically determining a se$uence of motions to

assemble a product from its individual parts. he motions can include part motions! grasping

locations! tool access! fi,ture planning! factory layout! and many other issues! all of which have


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comple, geometric components that use computational geometry techni$ues. A number of

technical issues that must be addressed for assembly automation includes following+

(2.1) Representation of assemblies and assembly plans

A computer representation of mechanical assemblies is necessary in order to automate thegeneration of assembly plans. he main issue in this stage is to decide what information about

assemblies is re$uired! and how this information is represented in the computer. An assembly of parts can be represented by the description of its individual components and their relationships in

the assembly. Assembly data base stores the geometric models of individual parts! the spatial

positions and orientations of the parts in the assembly! and the assembly or attachmentrelationships between parts. ne of the widely used methods for representation of assemblies is

based on graph structures. In this scheme! an assembly model is represented by a graph structure

in which each node represents an individual part or a sub assembly. he branches of the graph

represent relationship among parts. %our #inds of relationships e,ist: partof (P)! attachment (A)!constraint (C) and sub assembly ("A).

• he /partof/ relation represents the logical containment of one ob0ect in another. %or

e,ample! the head and shaft of a screw are /partof/ the screw itself.

• here are three types of attachment relationships: rigid! non rigid! and conditional.

o igid attachment occurs when no relative motion is possible between two parts.

o 2on rigid attachment occurs when parts cannot be separated by an arbitrarily large

distance but relative motion between the two parts is possible.

o Conditional attachment is related to parts supported by gravity! but not strictly

attached.

• Constraint relationships represent physical constraint of one part on another.

• "ubassembly relationship indicates that an assembly is merged into a higher assembly.

he graph structure of electric clutch assembly is shown in figure &'.


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%igure &'. 3raph structure of electric clutch assembly

Another method for representation of assembly is location graph of the part which is a relative

property. In this method! a coordinate system is the used to specify location of one part relativeto another. A location in one coordinate system also defines a new coordinate system for thelocated part! with its origin and a,es. ther locations can be defined in terms of this second one.

hus! a chain of locations can be defined such that each location is defined in terms of another

part4s coordinate system. A set of these chains results in a graph referred to here as a location

graph.


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%igure &'.& 5ocation graph of electric clutch assembly

(2.2) Generation of Assembly Seuences and Assembly plans

%or usefulness! an assembly planning system must generate correct assembly plans. %urther! tosolve problems that re$uire optimi*ation! such as selection of best assembly alternative! one must

be able to traverse the space of all candidate solutions. he number of distinct feasible assembly

plans can be large even for assemblies made of a small number of parts therefore complete

enumeration is not possible in most cases real applications. %inding systematic ways to narrowdown alternatives is crucial for the automatic planning of assembly. "ome of the widely used

techni$ues in evaluation of assembly se$uence are discussed below.

Precedence !iagram

he precedence diagram is designed to show all the possible assembly se$uences of a given product. o develop the precedence diagram for a product! each individual assembly operation is

assigned a uni$ue number and it is represented by an appropriate circle with the number

inscribed. he circles are connected by arrows showing the precedence relations. he precedencediagram is usually organi*ed into columns. All the operations that can be carried out first are

placed in the first column! and so on. 6sually! one operation appears in the first column: the

placing the base part on the wor# carrier where whole assembly process occurs.


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%igure &'.& Precedence diagram of electric clutch assembly

"iaison#Seuence Analysis

he liaison method develops all possible assembly se$uences in two steps. %irst it characteri*esthe assembly by a networ# wherein nodes represent parts and lines between nodes represent anymating conditions between parts. hese mating conditions referred to in this method as liaisons.

he networ# itself is #nown as liaison diagram.


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%igure &'.7 5iaison"e$uence of electric clutch assembly

-hen developing a liaison diagram! to note that the liaison count (number of liaisons) l is related

to the part count (number of parts) n by the following ine$uality:


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Precedence Grap$

6nli#e the previous two methods! this method is fully automatic. It is based on the virtual lin#data structure and re$uires the mating conditions as input to automatically generate assembly

se$uences for various assemblies. nce the mating conditions are provided! they are organi*ed

in the form of a mating graph. he parts in an assembly are then structured in a hierarchalassembly tree. hen assembly se$uence is generated with the aid of interference chec#ing. In

this method! the assembly se$uence is referred to as a precedence graph.

%igure &'.7 Precedence graph of electric clutch assembly

(2.%) Integration &it$ CA! programs

A mechanical assembly is a composition of interconnected parts. %re$uently! the parts are being

designed using CA8 programs therefore the shape of each part and geometric information are

already available in computer database. he assembly planning will be more efficient if theseCA8 databases can be directly integrated with programs that generate assembly models.

(2.') Integration &it$ tas and motion planners

-ith the progress of digital electronics! the programmable robots are introduced in

manufacturing. hese robots can be adapted to e,ecute different operations by changing their

internal programs. as# and motion planners that will facilitate robot programming are constantly


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getting developed. -ith a view toward future integration! the output of assembly planners should

compatible with what is re$uired by tas# and motion planners. It is also desirable that assembly planners also ta#e into account the capabilities and limitations of tas# and motion planners.

(%) enefits of Computer Aided Assembly Planning

• Accelerate new product introductions

•

"horten timetoproduction

• ptimi*e production management

• 8ecrease operating costs

• 9nsure overall product and process $uality

• Allow engineers! designers and shop floor personnel to collaborate interactively

() : Computer Aided %unction

( 1 ) Introduction

Inspection or testing is an act of chec#ing materials! parts! components or products at various

stages of manufacturing detecting poor $uality manufactured products for ta#ing corrective

action. Inspection is performed before! during and after manufacturing to ensure that the $ualityof the product is consistent with the accepted design standard. he design standards are defined

by the product designer! and for mechanical components they relate to factors such as dimension!

surface finish and appearance. he ob0ective of any inspection process is either to ta#e actualmeasurements of the values of the specified product characteristics or to chec# whether specific

characteristics meet design standards.

-hen inspection and testing is carried out manually! the sample si*e is often small compared to

si*e of the population. In high production runs! this si*e may be very small which may result in

slipping of defective parts. In principle! the only way to achieve 1; good $uality is to use1; inspection using which only good $uality parts will pass through the inspection procedure.


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-hen similar ob0ects are subse$uently inspected! points from each surface of interest are spatially

averaged to give high accuracy measurements of ob0ect dimensions. he inspection device usesseveral multiple,ed sensors! each composed of a camera and a structured light source! to measure

all sides of the ob0ect in a single pass.

Computers are used in many ways in inspection planning and e,ecution also.

( 2.1 ) Computer controlled inspection euipment

Coordinate =easuring =achine (C==) is a &dimensional measuring device that uses a contact

probe to detect the surface of the ob0ect. he probe is generally a highly sensitive pressuresensing device that is triggered by any contact with a surface. he linear distances moved along

the & a,es are recorded! thus providing the ,! y and * coordinates of the point. C==s are

classified as either vertical or hori*ontal! according to the orientation of the probe with respect to

the measuring table.

%igure &>.1 Coordinate =easuring =achine (C==)

( 2.2 ) Computer aided inspection setup planning

ComputerAided Inspection Planning (CAIP) is the integration bridge between CA8?CA= and

Computer Aided Inspection (CAI). A CAIP system for n=achine =easurement (==) is


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proposed to inspect the complicated mechanical parts efficiently during machining or after

machining. he inspection planning consists of 3lobal Inspection Planning (3IP) and 5ocalInspection Planning (5IP). In the 3IP! the system creates the optimal inspection se$uence of

features in a given part by analy*ing the various feature information. %eature groups are formed

for effective planning! and special feature groups are determined for se$uencing. he integrated

process and inspection plan is generated based on the series of heuristic rules developed. heintegrated inspection planning is able to determine optimum manufacturing se$uence for

inspection and machining processes. %inally! the results are simulated and analy*ed to verify the

effectiveness of the proposed CAIP.

( 2.% ) Computational metrology

Computational metrology deals with fitting and filtering discrete geometric data that are obtained

by measurements made on the parts. "ome basic facts about manufacturing and measurement are

best captured using following two a,ioms.

• A,iom of manufacturing imprecision: All manufacturing processes are inherently

imprecise and produce parts that vary.• A,iom of measurement uncertainty: 2o measurement can be absolutely accurate and with

every measurement there is some finite uncertainty about the measured attribute ormeasured value.

*itting+# %itting is the tas# of associating ideal geometric forms to nonideal forms (such as! for

e,ample! discrete set of points sampled on a manufactured surface). 2ormally! %itting is done for

the following reasons:

• !atum establis$ment+ 8atum is a reference geometric ob0ect of ideal form established

on one or more nonideal geometric forms of a manufactured part. 8atums are used for

relative positioning of geometric ob0ects in parts and assemblies of parts.

• !e,iation assessment+ It is often important to determine how far a manufactured surfacedeviates from its intended ideal geometric form which can be $uantified by fitting.

%itting problems are broadly divided into two categories on the basis of the ob0ective function

which is to be optimi*ed. %irst one is 5east s$uares fitting! which has the ob0ective to find an

ideal geometric ob0ect (a smooth curve or surface) to minimi*e the sum of s$uared deviations ofdata points from desired ob0ect. "econd on is C$ebys$e, fitting! which has the ob0ective to

minimi*e the ma,imum deviation.

*iltering %iltering is the tas# of obtaining scaledependent information from measured data. Ata more mundane level! filtering can be used to remove noise and other unwanted information

from the measured data. In the conte,t of engineering metrology! engineers are interested in use

of filtering mainly for the two applications.• Surface roug$ness+ =any engineering functions depend on roughness or smoothness of

a surface on the piece. 8esigners define bounds on certain roughness parameters to

ensure functionality of parts. hese smallscale variations are subtracted from the surfacemeasurement data before form and other deviations are assessed.

• -anufacturing process diagnosis+ =anufacturing processes leave tool mar#s on

surfaces.


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he computational scheme used for filtering is one of convolution. wo types of

convolutions are in use+

• Con,olution of functions+ %iltering is often implemented as discrete convolution of

functions. In the most popular version! the measured data is convolved with the 3aussianfunction. It has a smoothing effect on the surface data.

• Con,olution of sets+ =orphological filters are implemented using =in#ows#i sums.

hese can be regarded as convolutions where the input set is convolved with a circular or

flat structuring element.

( 2.' ) Computer aided part localiation and S$ape -atc$ing

=any a times in inspection process comparison of shapes of two geometric entities is done.

"hape matching can be used for comparison of shapes of two geometric entities. %or e,ample

comparison of shapes between designed model and molded part. "hape locali*ation is done withthe help of transformations. ransformations can be translation and rotations. "hape matching can

be broadly classified as

• CurveCurve =atching

• "urface"urface =atching

• "olid"olid =atching

• =atching curve! surface or solid with point data set


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%igure &>.: "hape 5ocali*ation and matching of two curves

"hape matching is used for many applications li#e character recognition! ob0ect recognition!

medical imaging! etc. he same concept can be e,tended in the inspection of mechanical parts. In

shape matching! the manufactured part is located with respect to designed part and error ismeasured. In character reorgani*ation! characters are fitted with smooth curves and then matched

with predefined templates.

%igure &>.&: "hape matching

( % ) enefits of CAI

• he CAI process saves both time and money.

• he computer software processes data from a &8 point cloud from a laser scanner!

eliminating the need for slower and more timeconsuming C== measurements.

• Inspecting with C==s re$uires that designers create a 8 drawing in addition to the &8CA8 model of a part. he drawing is used to inspect the part at specific locations to verify

that it matches the design. Pointcloud data is ta#en from a laser scanner or other &8

scanning device. he software provides a graphical comparison of the manufactured part

compared to the CA8 model.

•


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everywhere and not be limited to the specific locations on a drawing.

(&) : everse 9ngineering

(1) Introduction

9ngineering is the profession involved in designing! manufacturing! and maintaining products!

systems! and structures. he whole engineering process can be broadly classified in two groups+

forward engineering and reverse engineering.

%orward engineering is the traditional process of moving from highlevel abstractions and logical

designs to the physical implementation of a system.

%igure &@.1 %orward 9ngineering

he process of duplicating an e,isting component! subassembly! or product! without the aid of

drawings! documentation! or computer model is #nown as reverse engineering.

%igure &@. everse 9ngineering

everse engineering can be mainly viewed as the process of analy*ing a system to identify itscomponents and their interrelationships! to create representations of it in another form or a higher

level of abstraction. An important reason for application of reverse engineering is reduction of

product development times. In the intensely competitive global mar#et! manufacturers areconstantly see#ing new ways to shorten leadtimes to mar#et a new product. %or e,ample!

in0ectionmolding companies must drastically reduce the tool and die development times.


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• "ome bad features of a product need to be designed out. %or e,ample! e,cessive wear

might indicate where a product should be improved

• o strengthen the good features of a product based on longterm usage of the product

• o analy*e the good and bad features of competitors4 product

• o e,plore new avenues to improve product performance and features

• o gain competitive benchmar#ing methods to understand competitor4s products and

develop better products

• he original CA8 model is not sufficient to support modifications or current

manufacturing methods

• o update obsolete materials or anti$uated manufacturing processes with more current!

lesse,pensive technologies

It can be said that reverse engineering begins with the product and wor#s through the design process in the opposite direction to arrive at a product definition statement. In doing so! it

uncovers as much information as possible about the design ideas that were used to produce a

particular product.

(2) Re,erse engineering met$odology

he reverse engineering process can be divided into the following broad steps:

%igure &@.& everse 9ngineering =ethodology


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(2.1) !igitiing or collecting data from p$ysical part

ne of the reverse engineering methods is construction of a CA8 model of the physical parts

whose drawing is not available. his is done by digiti*ing an e,isting prototype which is mainly

creating a computer model and then using it to manufacture the component. he ob0ective of this

method is to generate a &8 mapping of the product in form of a CA8 file. his involves theac$uisition of the product surface data by either contact or non contact methods in form of ! B

and coordinates of large number of points on the product surface. he methods of obtaining the

product surface data can be divided into two broad categories+ Contact method and 2oncontactmethod. he contact method re$uires contact between the component surface and a measuring

tool that is usually a probe or a stylus. he noncontact method uses light as the main tool in

e,tracting the re$uired information. he contact discreti*ation method uses Coordinate=easuring =achines (C==) or electromagnetic digiti*ers or sonic digiti*ers to get the co

ordinates of the desired points on the surface. he noncontact discreti*ation techni$ue uses white

light or laser scanners to scan the &8 ob0ect from which the CA8 model is generated. he choice

of discreti*ation method is based on the speed and performance during digiti*ation and avoidance

of damage to the product.

A C== is a &dimensional measuring device that uses a contact probe to detect the surface of theob0ect. he linear distances moved along the & a,es are recorded! thus providing the ! B and

coordinates of the point. he part to be discreti*ed is placed on the measuring table! and the co

ordinates of a number of points on the surface of the ob0ect are then read. hese points are inputinto a 4geometry data4 file! which can be transferred to a CA8 system to generate the model of the

part. In this way the shape of the ob0ect is captured in the form of a CA8 drawing that can be

manipulated and modified as needed.


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In electromagnetic digiti*ers! the product to be digiti*ed is placed on a table which encloses

electronic e$uipment and a magnetic field source. It creates a magnetic field in the volume ofspace above table. A hand held stylus is used to trace the surface of the part. his stylus houses a

magnetic field sensor that! in con0unction with the electronic unit! detects the position and

orientation of the stylus. he data can be transferred to a computer through a serial port.

In sonic digiti*ers! sound waves are used to calculate the position of a point relative to a reference

point. In this techni$ue! the ob0ect is placed in front of a vertical rectangular board on the corners

of which are mounted four microphone sensors. A free hand held stylus is used to trace thecontours of the ob0ect. -hen a foot or a hand switch is pressed! the stylus emits an ultrasonic

impulse! and! simultaneously four cloc#s are activated. -hen the impulse is detected by a

microphone! the corresponding cloc# is stopped and the times ta#en to reach each of themicrophones recorded. hese time recordings! called slant ranges! are processed by a computer to

calculate the ,! y and * coordinates of the point.

(2.2) -anipulation of t$e collected data to obtain a CA! model

After obtaining the product surface data as a sea of points in space! the ne,t important step is thefitting of geometry to this point data. Darious methods were developed for the fitting of surfaces

to the point data. he surface can be mathematically described as either algebraic or parametric

surfaces. Algebraic surfaces are represented by a polynomial e$uation of the type f(,! y! *) E !and usually represent infinite surfaces. Parametric surfaces on the other hand! are finite surfaces

defined by certain basis functions and control points e.g.


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Step 1+ Point Cloud !ata in Sub Regions

Step 2+ Point Cloud !ata after applying -aimum 0rror -et$od


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Step %+ Surface fitting to Point Cloud !ata


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Step '+ Surface after Cleaning


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Step + Computer mouse after Prototyping

(7) : apid Prototyping

(1)Introduction

ne of the important steps prior to the production of a functional product is building of a

physical prototype. Prototype is a wor#ing model created in order to test various aspectsof a design! illustrate ideas or features and gather early user feedbac#. raditional

prototyping is typically done in a machine shop where most of parts are machined on

lathes and mills. his is a subtractive process! beginning with a solid piece of stoc# andthe machinist carefully removes the material until the desired geometry is achieved. %or

comple, part geometries! this is an e,haustive! time consuming! and e,pensive process. A

host of new shaping techni$ues! usually put under the title Rapid Prototyping ! are beingdeveloped as an alternative to subtractive processes. hese methods are uni$ue in that

they add and bond materials in layers to form ob0ects. hese systems are also #nown by

the names additive fabrication, three dimensional printing, solid freeform fabrication

(SFF), layered manufacturing etc. hese additive technologies offer significant

advantages in many applications compared to classical subtractive fabrication methods

li#e formation of an ob0ect with any geometric comple,ity or intricacy without the need

for elaborate machine setup or final assembly in very short time. his has resulted in their wide use by engineers as a way to reduce time to mar#et in manufacturing! to better

understand and communicate product designs! and to ma#e rapid tooling to manufacture

those products. "urgeons! architects! artists and individuals from many other disciplinesalso routinely use this technology.

(2) -et$odology of Rapid Prototyping (RP)

P in its basic form can be described as the production of three dimensional (&8) parts fromcomputer aided design (CA8) data in a decreased time scale. he basic methodology of all P


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process can be summari*ed as shown in following figure.

%igure &F.1 apid prototyping process chain

(2.1) !e,elopment of a CA! model

he process begins with the generation CA8 model of the desired ob0ect which can be done by

one of the following ways+

• Conversion of an e,isting two dimensional (8) drawing• Importing scanned point data into a CA8 pac#age

• Creating a new part in CA8 in various solid modeling pac#ages

• Altering an e,isting CA8 model

P has traditionally been associated with solid rather than surface modelling but the more recent

trends for organic shapes in product design is increasing the need for free flowing surfaces

generated better in surface modelling.

(2.2) Generation of Standard triangulation language (S3") file

he developed &8 CA8 model is tessellated and converted into "5 files that are re$uired for P

processes. essellation is piecewise appro,imation of surfaces of &8 CA8 model using series of

triangles. "i*e of triangles depends on the chordal error or ma,imum fact deviation. %or betterappro,imation of surface and smaller chordal error! small si*e triangle are used which increase

the "5 file si*e. his tessellated CA8 data generally carry defects li#e gaps! overlaps!

degenerate facets etc which may necessitate the repair software. hese defects are shown in


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figure below. he "5 file connects the surface of the model in an array of triangles and consists

of the ! B and coordinates of the three vertices of each surface triangle! as well as an inde,that describes the orientation of the surface normal.

%igure &F. essellation defects

(2.%) Slicing t$e S3" file

"licing is defined as the creating contours of sections of the geometry at various heights in themultiples of layer thic#ness. nce the "5 file has been generated from the original CA8 data

the ne,t step is to slice the ob0ect to create a slice file ("5I). his necessitates the decisionregarding part deposition orientation and then the tessellated model is sliced. Part orientation

will be showing considerable effect on the surface as shown in the figures.


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%igure &F.& 9ffect of Part deposition rientation

he thic#ness of slices is governed by layer thic#ness that the machine will be building in! thethic#er the layer the larger the steps on the surface of the model when it has been built. After the

"5 file has been sliced to create the "5I files they are merged into a final build file. his

information is saved in standard formats li#e "5C or C5I (Common 5ayer Interface) etc.

(2.') Support Structures

As the parts are going to be built in layers! and there may be areas that could float away or of


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overhang which could distort. herefore! some processes re$uire a base and support structures

to be added to the file which are built as part of the model and later removed.

(2.) -anufacturing

As discussed previously! the P process is additive i.e. it builds the parts up in layers ofmaterial from the bottom. 9ach layer is automatically bonded to the layer below and the process

is repeated until the part is built. his process of bonding is underta#en in different ways for thevarious materials that are being used but includes the use of 6ltraviolet (6D) lasers! Carbon

8io,ide lasers! heat sensitive glues and melting the material itself etc.

(2.4) Post processing

he parts are removed from the machine and post processing operations are performedsometimes to add e,tra strength to the part by filling process voids or finish the curing of a part

or to hand finish the parts to the desired level. he level of post processing will depend greatly

on the final re$uirements of the parts produced! for e,ample! metal tooling for in0ectionmolding will re$uire e,tensive finishing to e0ect the parts but a prototype part manufactured to

see if it will physically fit in a space will re$uire little or no post processing.

(%) 5arious RP Processes

"everal P techni$ues are being developed and commercially available. he first commercial

process! "tereo5ithography ("5)! came to the mar#et in 1GF@. 2owadays! more than & different

processes (not all commerciali*ed) with high accuracy and a large choice of materials e,ist.hese processes can be classified in different ways but the most popular way is according to the

form of material used as an input. his can be given as follows+

• 5i$uid based processes

(i) "olidification of a li$uid polymer

(ii)"olidification of an electrostat fluid

(iii)"olidification of molten material

•

• 8iscrete Particle based processes

(i) %using of particles by laser

(ii)Hoining particles by binder

•

• "olid "heets based process

(i)


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Some of t$e commercially popular RP processes are described belo&/

(%.1) Stereolit$ograpy

"tereo5ithography ("5) is the best #nown rapid prototyping system. he techni$ue builds three

dimensional models from li$uid photosensitive polymers that solidify when e,posed to laser beam. he model is built upon a platform in a vat of photo sensitive li$uid. A focused 6D laser

traces out the first layer! solidifying the model cross section while leaving e,cess areas li$uid. In

the ne,t step! an elevator lowers the platform into the li$uid polymer by an amount e$ual to layerthic#ness. A sweeper recoats the solidified layer with li$uid! and the laser traces the second layer

on the first. his process is repeated until the prototype is complete. Afterwards! the solid part is

removed from the vat and rinsed clean of e,cess li$uid. "upports are bro#en off and the model isthen placed in an ultraviolet oven for complete curing.

%igure &F.' "tereolithography


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Application Range

• Processing large variety of photosensitive polymers including clear! water resistant and

fle,ible resins

• %unctional parts for tests

• ools for pre series production tests.

• =anufacturing of medical models

• =anufacturing of electroforms for 9lectro 8ischarge =achining (98=)

• %ormfit functions for assembly tests.

Ad,antages

• Possibility of manufacturing parts which are impossible to produce conventionally using a

single process.

• Continuous unattended operation for 7 hours.

• igh resolution.

• Any geometrical shape can be made with virtually no limitation.

!isad,antages

• 2ecessity to have support structures

• Accuracy not in the range of mechanical part manufacturing.

• estricted areas of application due to given material properties.

•

5abour re$uirements for post processing! especially cleaning.


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(%.2) Selecti,e "aser Sintering

"elective 5aser "intering ("5") is a &dimensional printing process based on sintering! using a

laser beam directed by a computer onto the surface of metallic or nonmetallic powders

selectively to produce copies of solid or surface models. he process operates on the layerby

layer principle. At the beginning a very thin layer of heat fusible powder is deposited in thewor#ing space container. he laser sinters the powders. he sintering process uses the laserto raise the temperature of the powder to a point of fusing without actually melting it. As the

process is repeated! layers of powder are deposited and sintered until the ob0ect is complete. he

powder is transferred from the powder cartridge feeding system to the part cylinder (the wor#ing

space container) via a counter rolling cylinder! a scraper blade or a slot feeder. In the unsinteredareas! powder remains loose and serves as natural support for the ne,t layer of powder and ob0ect

under fabrication. 2o additional support structure is re$uired.

%igure &F.> "elective 5aser "intering ("5")

Application Range• Disual representation models.

• %unctional and tough prototypes.

• Cast metal parts (by use of wa,).

• "hort run and soft tooling.


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Ad,antages

• Dirtually any materials that have decreased viscosity upon heating can potentially be

used.

• 8o not re$uire any postcuring e,cept when ceramics are used.

• 2o need to create a support structure! which saves time

• Advanced softwares allowing concurrent slicing of the part geometry files while

processing the ob0ect

!isad,antages

• aw appearance on the part surface due to hardening of additional powder on the

borderline of the ob0ect

• 2ecessity to provide the process chamber continuously with nitrogen to assure safe

material sintering

• Careful handling of to,ic gases emitted from the fusing process

(%.%) "aminated 6b7ect manufacturing ("6-)

In this techni$ue! layers of adhesivecoated sheet material are bonded together to form a prototype. he original material consists of paper laminated with heatactivated glue and rolled

up on spools. A feeder?collector mechanism advances the sheet over the build platform! where a

base has been constructed from paper and doublesided foam tape. In the ne,t stage! a heatedroller applies pressure to bond the paper to the base. A focused laser cuts the outline of the first

layer into the paper and then crosshatches the e,cess area (the negative space in the prototype).


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Crosshatching brea#s up the e,tra material! ma#ing it easier to remove during postprocessing.

8uring the build! the e,cess material provides e,cellent support for overhangs and thinwalledsections. After the first layer is cut! the platform lowers out of the way and fresh material is

advanced. he platform rises slightly below the previous height! the roller bonds the second layer

to the first! and the laser cuts the second layer. his process is repeated as needed to build the

part! which will have a woodli#e te,ture.


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• obust capacity of dealing with imperfect "5 files! created with discontinuities!

•


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• 9asy and convenient date building.

• 2o worry of possible e,posure to to,ic chemicals! lasers! or a li$uid polymer bath.

• 2o wastage of material during or after producing the model

• 2o re$uirement of cleanup.

• Juic# change of materials

!isad,antages

• estricted accuracy due to the shape of the material used: wire of 1.@ mm diameter.

(%.) Solid Ground Curing

"olid ground curing ("3C) is almost similar to stereolithography ("5A). In both one uses

ultraviolet light to selectively harden photosensitive polymers. 6nli#e "5A! "3C cures an entire

layer at a time. %irst! photosensitive resin is sprayed on the build platform. "econdly! themachine develops a photomas# (li#e a stencil) of the layer to be built. his photomas# is printed

on a glass plate above the build platform using an electrostatic process similar to that found in photocopiers. he mas# is then e,posed to 6D light! which only passes through the transparent

portions of the mas# to selectively harden the shape of the current layer. After the layer is cured!

the machine vacuums up the e,cess li$uid resin and sprays wa, in its place to support the model

during the build. he top surface is milled flat! and then the process repeats to build the ne,tlayer. -hen the part is complete! it must be dewa,ed by immersing it in a solvent bath.

%igure &F.G "olid 3round Curing ("3C)

Ad,antages


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• 5arge parts! ' K ' K &' mm! can be fabricated $uic#ly.

• igh speed allows productionli#e fabrication of many parts or large parts.

• =as#s are created with laser printingli#e process! then full layer e,posed at once.

• 2o postcure re$uired.

• =illing step ensures flatness for subse$uent layer.

• -a, supports model: no e,tra supports needed.

!isad,antages

• Creates a lot of waste.

• 2ot as prevalent as "5A and "5"! but gaining ground because of the high throughput

and large parts

(%.4) %! Printing

&8 printing is very reminiscent of "5"! e,cept that the laser is replaced by an in#0et head. he

multichannel 0etting head (A) deposits a li$uid adhesive compound onto the top layer of a bed

of powder ob0ect material (


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%igure &F.1 &8 Printing

"urfaces of the parts produced by layer manufacturing processes suffer from poor surface finish

and this is due to the inherent characteristics of the process itself li#e stair stepping effect!

shrin#age etc. 5east build time in P is generally preferred but stair stepping effect and poor

surface finish restricts it. his has been induced on to the surface of the parts during the variousstages that a particular part has to come across during a P cycle! typically data preparation

stage! part orientation! part geometry! deciding layer thic#ness etc. %igure &F.7 shows these

effects on P product.(') Some issues in RP

ecause of layer by layer deposition of t$e material and due to t$e finite t$icness of eac$

layer8 situation similar to stair case &ill be resulting on t$e surface and t$is effect is no&n

as stair stepping effect. *rom t$e figures it can be seen t$at layer t$icness &ill directly

affect t$e maimum cusp $eig$t attained and t$e stair case effect on t$e surface.


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%igure &F.11 "taircase effect in P Parts

%igure &F.1 9ffect of layer thic#ness on stair stepping effect

3o reduce t$e surface roug$ness8 one may go &it$ ,ery fine layers8 but t$is &ill beincreasing t$e o,erall build time and build cost considerably. If &e c$oose maimum

allo&able layer t$icness t$en8 t$is &ill be generating a part &it$ $ig$ surface roug$ness.

So an optimum layer t$icness must be decided. 3$e solution for t$is is to go for adapti,e

slicing or local adapti,e slicing.


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Adapti,e slicing is slicing t$e entire part &it$ different t$icnesses according to t$e local

surface geometry and maimum cusp $eig$t t$at can be reac$ed as s$o&n in t$e figure

abo,e. In local adapti,e slicing8 maimum layer t$icness &ill be considered and t$en

c$eced for maimum slice t$icness and eac$ layer is furt$er di,ided accordingly if

reuired.

(') : Computer Aided Process Planning

(1) Process Planning

Products and their components are designed to perform certain specific functions. 9very product

has some design specifications which ensure its functionality aspects. he tas# of manufacturing

is to produce components such that they meet design specifications. Process planning acts as a bridge between design and manufacturing by translating design specifications into manufacturing

process details. It refers to a set of instructions that are used to ma#e a component or a part so

that the design specifications are met! therefore it is ma0or determinant of manufacturing cost and

profitability of products. Process planning answers the $uestions regarding re$uired informationand activities involved in transforming raw materials into a finished product. he process starts

with the selection of raw material and ends with the completion of part. he development of process plans involves mainly a set of following activities+

• Analysis of part re$uirements

• "election of raw wor#piece

• "election of manufacturing operations and their se$uences

• "election of machine tools

• "election of tools! tool holding devices! wor# holding devices and inspection e$uipments

• "election of manufacturing conditions i.e. cutting speed! feed and depth of cut.

• 8etermination of manufacturing times

(2) 3$e manual eperience#based planning met$od


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he manual e,periencebased process planning is most widely used. It is mainly based on a

manufacturing engineer4s e,perience and #nowledge of production facilities! e$uipment! their

capabilities! processes! and tooling. he ma0or problem with this approach is that it is timeconsuming and developed plans may not be consistent and optimum. he feasibility of

developed process plan is dependant on many factors such as availability of machine tools!

scheduling and machine allocation etc. Computer aided process planning is developed toovercome this problems to some e,tent. (%) Computer Aided Process Planning

As mentioned in article &G.1! the primary purpose of process planning is to translate the designre$uirements into manufacturing process details. his suggests a system in which design

information is processed by the process planning system to generate manufacturing process

details. CAPP integrates and optimi*es system performance into the interorgani*ational flow.

%or e,ample! when one changes the design! it must be able to fall bac# on CAPP module togenerate manufacturing process and cost estimates for these design changes. "imilarly! in case of

machine brea#down on the shop floor! CAPP must generate the alternative actions so that most

economical solution can be adopted in the given situation. A typical CAPP framewor# is shown

in figure &G.1.

%igure &G.1 A Computer Aided Process Planning (CAPP) framewor#

-hen comapred with manual e,periencebased process planning! CAPP offers following

advantages+

• "ystematic developemnt of accurate and consistent process plans


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• eduction of cost and lead time of process planning

• educed s#ill re$uirements of process planners

• Increased productivity of process planners

• igher level application progams such as cost and manufacturing lead time estimation andwor# standards can be interfaced

wo ma0or methods are used in computer aided process planning+ the variant CAPP method and

the generative CAPP method

(%.1) 3$e ,ariant CAPP met$od

In variant CAPP approach! a process plan for a new part is created by recalling! identifying andretrieving an e,isting plan for a similar part and ma#ing necessary modifications for the new

part. "ometimes! the process plans are developed for parts representing a fmily of parts called

4master parts4. he similiarities in design attributes and manufacturing methods are e,ploited forthe purpose of formation of part families. A number of methods have been developed for part

family formation using coding and classification schemes of group technology (3)! similiarity

coefficient based algorithms and mathematical programming models.

3$e ,ariant process planning approac$ can be realied as a four step process/

1. 8efinition of coding scheme. 3rouping parts into part families

&. 8evelopment of a standard process plan

7. etrieval and modification of standard process plan

A number of variant process planning schemes have been developed and are in use. ne of the

most widely used CAPP system is CA=I developed by =c8onnell8ouglas AutomationCompany. his system can be used to generate process plan for rotational! prismatic and sheet

metal parts.

%.2 3$e generati,e CAPP met$od

he ne,t stage of evolution is towards generative CAPP. In the generative CAPP! process plansare generated by means of decision logic! formulas! technology algorithms and geometry based

data to perform uni$uely many processing decisions for converting part from raw material to

finished state. here are two ma0or components of generative CAPP+ a geometry based codingscheme and process #nowledge in form of decision logic data. he geometry based coding

scheme defines all geometric features for process related surfaces together with feature

dimensions! locations! tolerances and the surface finish desired on the features. he level ofdetail is much greater in a generative system than a variant system. %or e,ample! details such as

rough and finished states of the parts and process capability of machine tools to transform these

parts to the desired states are provided. Process #nowledge in form of in the form of decision

logic and data matches the part geometry re$uirements with the manufacturing capabilities using


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#nowledge base. It includes selection of processes! machine tools! 0igs or fi,tures! tools!

inspection e$uipments and se$uencing operations. 8evelopment of manufacturing #nowledge

base is bac#bone of generative CAPP. he tools that are widely used in development of thisdatabase are flowcharts! decision tables! decision trees! iterative algorithms! concept of unit

machined surfaces! pattern recognition techni$ues and artificial intelligence techni$ues such as

e,pert system shells.(') Ad,antages of CAPP and future trends

CAPP has some important advantages over manual process planning which includes+

• educed process planning and production leadtimes

• %aster response to engineering changes in the product

• 3reater process plan accuracy and consistency

• Inclusion of uptodate information in a central database

• Improved cost estimating procedures and fewer calculation errors

• =ore complete and detailed process plans

• Improved production scheduling and capacity utili*ation

• Improved ability to introduce new manufacturing technology and rapidly update process

plans to utili*e the improved technology

here are number of difficulties in achieving the goal of complete integration between various

functional areas such as design! manufacturing! process planning and inspection. %or e,ample!

each functional area has its own standalone relational database and associated database

management system. he software and hardware capabilities among these systems posedifficulties in full integration. here is a need to develop single database technology to address

these difficulties. ther challenges include automated translation of design dimensions andtolerances into manufacturing dimensions and tolerances considering process capabilities and

dimensional chains! automatic recognition of features and ma#ing CAPP systems affordable to

the small and medium scale manufacturing companies.

Documents

Application Stuff of Cnc Machine