COMPUTER CONTROL IN PROCESS PLANNING Unit 2 (ME CAD/CAM)

COMPUTER CONTROL PROCESS PLANING

UNIT 2

PART DESIGN PREPARATION

CCPP PART DESIGN PREPARATION 1

Design Drafting

Computer-Aided Drafting and Design (CADD)is

the process where a drafter/designer/engineer

creates drawings or models that define a given

product before it is ready to be manufactured.

The drafter is the key link in the design

engineering process and manufacturing steps,

and must possess a working knowledge of

design principles, material properties, and

manufacturing processes.


Basic Dimension

A theoretically exact size, profile, orientation, or location of a feature or datum target, therefore, a basic dimension is untoleranced.

Most often used with position, angularity, and profile)

Basic dimensions have a rectangle surrounding it.

1.000

Basic Dimension cont’d.

INDIVIDUAL (No

Datum

Reference)

INDIVIDUAL or

RELATED

FEATURES

RELATED

FEATURES

(Datum

Reference

Required)

GEOMETRIC CHARACTERISTIC CONTROLS

TYPE OFFEATURE

TYPE OFTOLERANCE CHARACTERISTIC SYMBOL

SYMMETRY

FLATNESS

STRAIGHTNESS

CIRCULARITY

CYLINDRICITY

LINE PROFILE

SURFACE PROFILE

PERPENDICULARITY

ANGULARITY

PARALLELISM

CIRCULAR RUNOUT

TOTAL RUNOUT

CONCENTRICITY

POSITION

FORM

PROFILE

ORIENTATION

RUNOUT

LOCATION

14 characteristics that may be controlled

Characteristics & Symbols cont’d.

Maximum Material Condition MMC

Regardless of Feature Size RFS

Least Material Condition LMC

Projected Tolerance Zone

Diametrical (Cylindrical) Tolerance Zone or Feature

Basic, or Exact, Dimension

Datum Feature Symbol

Feature Control Frame

CAD-input / output devices

II. Input Devices

a. Keyboard,mouse,joystick,scanners,digital camera, bar code

reader, touch Sreeen,Speech input device (microphone)

III. Output Devices

a. Monitor , Speaker, Printers ( different types)


TOPOLOGY Topologydescribes how elements are bounded and connected.

Geometry describes the shape of each individual element.


Topological entities

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Topological entities comprise all the entities that are used to

construct the structure or skeleton of a model

Geometric transformation

Graziadei/Leone

2O & 2A

October 29, 2012

Geometrical transformations

Transformations are a turn, flip, or slide of any figure.

Geometrical transformations

If a figure is represented by ABC, the image of this figure will be represented by A’B’C’ (ABC PRIME)

A figure and its image will be congruent to each other. They will have the same shape and the same size.

A

B

C

A

’

B

’

C

’

Pre-

image

Image

Transformations: Translations(slides)

A translation is a transformation whose points are all the same relative distance from the pre-image and which is pointing in the same direction.

Transformations: Reflections (flips)

A reflection is an isometry in which a figure and its image have opposite orientations

An isometry is when the distance between any two points in the pre-image must be the same as the distance between the images of the two points.

3

73

7

3

3

3

3

2

0

2

0

Doing reflections: easy tricks

Y-Axis

If the mirror line is the y-axis, just change each (x, y) into (-x, y)

Tricks

X-Axis

If the mirror line is the x-axis, just change each (x, y) into (x, -y)

Symmetry

If a line is drawn down the middle of an object, both sides would be identical.

Axial Symmetry

The points on the left side of the Y-axis will be

at the negative coordinates of the points on the

right side: a perfect mirror image.

Example:-

44

-

5

5

-

1

0

1

0

Transformations: Rotations (turns)

A rotation turns all of the points in a figure around a given point, called the center of rotation. The center of rotation is the only point that does not change during the rotation.

A’ =

r(A)

A’B’ =

r(AB)A

A’B

B’

Transformations: Rotation

180-degree turn = half turn

A positive angle rotation is when one figure is rotated counter-clockwise

A negative angle rotation is when the figure is rotated clockwise.

Transformations: Identity

If the point of the pre-image and of the image is exactly the same, this point is united.

If ALL of the points of the pre-image and image are the same, the entire figure is united.

Transformations: Expansions & Contractions

Produces an image that is the same shape as the original, but it is a different size.

Expansion if the scale factor is greater than 1

Contraction is the scale factor is between 0 and 1

Transformations: Projecting

Inverse translations

If a transformation maps the pre-image onto its image, then the inverse transformation maps the image back onto the pre-image.

Basically, it is the same process, but backwards.A

’

B

’ C

’

A

BC

t -

1

DATA STRUCTURESEach geometric model of an object in a CAD system is represented by aset of data. The data consist of numerical values, names, codes andsymbols. This data are stored in computer memory in an organizedmanner. This organization represents the relationship of each dataelement to other elements; included in this relationship is thetopological relationship of surface geometry. There are also otherrelationship, such as names and their associated geometries. A morecomplete survey on the different types of data structure for computergraphics can be found in two technical papers by Williams (1971) andGrays (1967). In this section only a general data structure of CAD will bedescribed.

The first item in the edge list, therefore, represents the number ofedges. In the fig, edge 1 is represented by the data “ 3,3,4,5,” where thefirst 3 indicates that there are three edges; 3,4 & 5. Each edge has twovertices (edge 3 has V1 & V3). Finally, a vertex may store all thecoordinates of vertices.

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For holes, circles, and other curves and curved surfaces, a code isnecessary to distinguish them for polyhedron surfaces.

For manufacturing purposes technological information’s, such asdimensions, tolerances, and geometric tolerances, are essential. Theymust also be included in constructing the CAD data structure. Mostcurrent systems explicitly (or more approximately externally) definesuch information in a manner similar to the way in which a draftsmanwould. Dimensioning and tolerancing information is not implicitdefined within the model. For a explicit dimensioning another list isadded to store the dimensioning information.


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Data Exchange Between CAD/CAM Systems (Data structures)

Why do we need Data Exchange?

Why do we need Data Exchange

Design projects require data to be shared between suppliers

Different companies often used different CAD systems

All CAD systems have their own database formats

They are mostly proprietary and often confidential

Data is stored in different ways e.g. 1.0,2.0,3.0 or X1.0,Y2.0,Z3.0, etc.

Data conversion between systems becomes necessary


Data Exchange Formats

IGES (Initial Graphics Exchange Specification) 1980 US NIST, National Institute of Standards and Technology

DXF

VDA – German Automotive Industry

SET – Airbus

PDES – American fore-runner of STEP

STEP

STL

VRML


GEOMENTRIC MODELING FOR PROCESS PLANNING

In the preceding section, the fundamentals of CAD were discussed. In this section a discussion 3-D representation schemes and their applications to process planning is presented.

There are seven different types of graphics representation schemes:

Wire frame

Primitive instancing

Spatial occupancy enumeration

Cell decomposition

Constructive solid geometry (CSG)

Boundary representation

Sweeping


GT codingIntroduction

It is true that classification helps to bring like things together, butwith out having a proper coding system which reflecting theclassification is introduced, handling, processing families of similardata or retrieving relevant information is more cumbersome andinefficient. Most of the time, coding is thought of in itscryptological sense, meaning to restrict and suppress thedissemination of intelligence and/or information duringcommunication. However, when we describe it in the businessand industrial context, quite the reverse is true. In industriesproperly designed codes are used as a shorthand, as a means tocompress information and improve its communicationeffectiveness through the business and the outside environment.


Types of Coding

Industrial codes can take several forms. Those, which use the alphabet and/or numerals, are called alpha or numeric codes or as combinations of the two as alphanumeric codes. But there are also special symbol codes which are not derived from either alphabetic or numeric sources. Here we can see the different types of codes as follows.

Attribute code structure

Hierarchical Type

Hybrid Type

Special Symbol codes

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Attribute code structure

In this type of structure, the interpretation of each symbol in the sequence is fixed and represents one feature. Thus, the value of any given digits (or position) within the code does not depend on the preceding digits. Another name of this type of symbol is poly-code. The problem associated with poly-code is that the code tends to be relatively long. On the other hand, the use of poly-code allows for convenient identification of specific part attribute. This can be helpful in recognizing parts with similar processing requirements. A typical attribute code is illustrated in the table below.

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Hierarchical Type

In this type of code structure, each code number is qualified by the precedingdigits (characters). Thus if the first digits define the type of material used,such as steel, the second digits will define a feature related to steel (likecarbon constraint), and the next digit will define a feature related to thefeature defined in the second digit and so on. A typical hierarchical codestructure is shown in Figure below. As it was shown in the diagram, each digit isdirectly related to the preceding digits. Thus the second digits “2” may definea power unit of the work part, the digit in the third position may then definethe type of power system (i.e. weather it is mechanical, hydraulic, or electric).In the fourth position if a digit “1” is preceded by “1”, then it may define thesub unit of the driving system that is a rotational part, or if it is proceeded by“3” it might have totally different meaning.

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Hybrid Type:

Most of the commercial parts coding system in industries are acombination of the two pure structure (i.e. poly codes and mono codes).The hybrid is an attempt to achieve the best feature of both poly-codesand mono-codes. Hybrid codes are typically constructed as a series ofshort poly codes. Within each of these shorter chains, the digits areindependent, but one or more of symbols in the complete code numberare used to classify the part population into groups, as in the hierarchicalstructure. This hybrid coding seems to best serve the need of both designand production.

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Special Symbol codes:

This is a type of code that used picture or symbols of an object other thannumbers or alphabets to represent an activity, event, words, etc. Thisspecial symbol codes can be hieroglyphical, which can best characterizedby the symbolic therbligs, created by Frank and Lillian Gilbreths or of morerecent vintage, the flow chart codes for tracking the handling of datawithin a system. During their study of work and time-and motion, they usedan epitomized hieroglyphic code symbols to represent a fundamentalmotions such as reach, grasp, think, etc. The hieroglyphic code symbolsdescribed by the therbligs codes pictorially represent the action theydescribe.

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Principles of Coding

As in the case of classification, an industrial coding has also its own principles.

These includes:

1. No code should exceed five characters without a break in the string. That means when the code number becomes shorter it becomes easy to handle and, fewer errors committed.

2. Identity codes should of fixed length and pattern. If we use varying-length codes within a given class of materials it will proliferate error rates and require justification in handling (right or left) to the longest code in use.

3. All-numeric codes produce fewest errors.

4. Alphanumeric combination codes are acceptable if the alpha field is fixed and used to break a string of numbers.


The optiz systemThe Opitz coding system uses the following digit sequence:

12345 6789 ABCD

The basic code consists of nine digits, which can be extended byadding four more digits. The first nine digits are intended toconvey both design and manufacturing data. The generalinterpretation of the nine digits is indicated in Fig in next slideThe first five digits, 12345, are called the “form code” anddescribe the primary design attributes of the part. The next fourdigits, 6789, constitute the “supplementary code”. It indicatessome of the attributes that would be of use to manufacturing(work material, raw work piece shape, and accuracy). The extrafour digits, “ABCD”, are referred to as the “secondary code” andare intended to identify the production operation type andsequence. The secondary code can be designed by the firm toserve its own particular needs.


EXAMPLE

Given the part design of Fig. 9.7, the form code for this part is discussed below.

The overall length/diameter ratio, L/D = 1.6, so the first code = 1. The part isstepped on both ends with a screw thread on one end, so the second digit codewould be 5 the third digit code is 1 because of the through hole. The fourth andfifth digits are both 0, since no surface machining is required and there are noauxiliary holes or gear teeth on the part. The complete form code in the Opitzsystem is “15100”. To add the supplementary code, we would have to properlycode the sixth through ninth digits with data on dimensions, material, startingwork piece shape, and accuracy.


The MICLASS systemThe MICLASS classification number can range from 12 to 30 digits The first 12 digits are universal code that can be applied to any part. Up to 18 additional digits can be used to code data that are specific to the particular company or industry. For example, lot size, piece time, cost data, and operation sequence might be included in the 18 supplementary digits.

The component attributes coded in the first 12 digits of the MICLASS number are as follows:

1st digit Main shape

2nd and 3rd digits Shape elements

4th digit Position of shape elements

5th and 6th digits Main dimensions

7th digit Dimension ratio

8th digit Auxiliary dimension

9th and 10th digits Tolerance codes

11th and 12th digits Material codes


THANK YOU

January 26, 2014 PART DESIGN PREPARATION 42

COMPUTER CONTROL IN PROCESS PLANNING Unit 2 (ME CAD/CAM)

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Transcript of COMPUTER CONTROL IN PROCESS PLANNING Unit 2 (ME CAD/CAM)