Computer Aided Design - Lecture 02

8/2/2019 Computer Aided Design - Lecture 02

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Lettering

Need to communicate more technical information that is not pictorialin nature but is required in manufacturing/fabrication process e.g.dimension, structures, written instructions, notes and several otherelements

That enable designer, engineer or architect to provide completedescription of an object

Written information on a drawing is conveyed through lettering


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Lettering Techniques

Freehand Lettering

Typed Lettering

Computer-based Lettering


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Lettering Techniques

Typed & Computer-Based Lettering

Speedy More variety in available styles More options for placement of text in drawings

Attributes of Computer-Based Lettering

Font:

(Size, Style) Size = Height of Letter Measured in points (1 = 72 pts, 12 pts=1/6) Size selection is dependent on scale of the drawing and the text e.g.

Title > Notes and Call outs


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Applications of Lettering

Title Block

Part Name

Dimension Information

Bill of Materials

Notes


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Dimensioning

Def:The technique of specifying size, shape and location information on adrawing using lines, symbols, figures and notes.

An object is first broken down into its constituent shapes (cuboids,

pyramids, cylinders, cones) and then each shape is dimensioned.

Importance:Under-dimensioning and over-dimensioning both cause riskof incompatibility and fitting problem in the designed product

Types of Dimension

Size Dimension

Location Dimension


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Tolerancing

Allowable limits of deviations from the exact value

Control variation of different parts

Must be mentioned on the drawing through general notes orwhen writing dimensions


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Techniques of Tolerancing

Plus and Minus Tolerancing

+ve orve variation limits are specified

Bilateral Tolerancing Allowable limits lye on both the sides of standard value Variation is allowed in both directions

Unilateral Tolerancing Allowable Limit is to one side of the standard value Variation is allowed in only one direction

Conventions +ve tolerance is written above dimension line -ve tolerance is written below dimension line Standard value is written in-line


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Cumulative Tolerancing

If the dimension (size or location) of an object/feature is

affected by more than one tolerance

In this case the separate tolerances area added up to showthe cumulative tolerance of the particular object/feature

Example of holes in surface of sheet at specific distances


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Geometric Tolerances

Allowable deviation limits from the true geometry

of an object

e.g. angularity, parallelism, position etc.


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Projection

A method/protocol by which an image of a three-dimensional objectis projected onto a planar surface

A 2D drawing of any entity

Entity can be a point (dimensionless), line (1D), plane (2D) or solid(3D)

Going from Spaceland to Flatland for simplicity of representation of3D objects

An image/view of a 3D object in a 2D plane is projection

Demonstration Example: Projection of a Globe
http://en.wikipedia.org/wiki/Three-dimensional_spacehttp://en.wikipedia.org/wiki/Three-dimensional_spacehttp://en.wikipedia.org/wiki/Three-dimensional_spacehttp://en.wikipedia.org/wiki/Three-dimensional_space


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Orthographic Projection

Drawing an object from different directions (views) such that the viewingdirections are at right angle (orthogonal) to each other.

Usually a front, side and plan view are drawn and are enough for aperson looking at the drawing to see all the important details.

Consider an object placed inside a transparent box

Look at right angle (90o) to the six planes of the box one by one anddraw all the six views of the object on the six sides of the box

Projecting these essential views of the objects in to a single plane isOrthographic Projection


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Orthographic Projection


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Orthographic Projections


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Principal Planes


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1st Angle Orthographic Projection


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1st Angle Projection onto a Plane


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3rd Angle Orthographic Projection


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3rd Angle Projection on to a Plane


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Orthographic Projections of Entities

OP of Point

OP of Line

OP of Plane

OP of Solids

Computer Aided Design - Lecture 02

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