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Chapter: Mechanical : Computer Integrated Manufacturing : Computer Aided Design

Features of CAD Packages

Computer-aided design (CAD) is the use of computer systems to assist in the creation, modification, analysis, or optimization of a design.

Features of CAD Packages;


Computer-aided design (CAD) is the use of computer systems to assist in the creation, modification, analysis, or optimization of a design. CAD software is used to increase the productivity of the designer, improve the quality of design, improve communications through documentation, and to create a database for manufacturing. CAD output is often in the form of electronic files for print, machining, or other manufacturing operations.


Computer-aided design is used in many fields. Its use in designing electronic systems is known as Electronic Design Automation, or EDA. In mechanical design it is known as Mechanical Design Automation (MDA) or computer-aided drafting (CAD), which includes the process of creating a technical drawing with the use of computer software.


CAD software for mechanical design uses either vector-based graphics to depict the objects of traditional drafting, or may also produce raster graphics showing the overall appearance of designed objects. However, it involves more than just shapes. As in the manual drafting of technical and engineering drawings, the output of CAD must convey information, such as materials, processes, dimensions, and tolerances, according to application-specific conventions. CAD may be used to design curves and figures in two-dimensional (2D) space; or curves, surfaces, and solids in three-dimensional (3D) space.


·        CAD may be used to design curves and figures in two-dimensional (2D) space; or curves, surfaces, and solids in three-dimensional (3D) space.


·        CAD is an important industrial art extensively used in many applications, including automotive, shipbuilding, and aerospace industries, industrial and architectural design, prosthetics, and many more. CAD is also widely used to produce computer animation for special effects in movies, advertising and technical manuals, often called DCC digital content creation. The modern ubiquity and power of computers means that even perfume bottles and shampoo dispensers are designed using techniques unheard of by engineers of the 1960s. Because of its enormous economic importance, CAD has been a major driving force for research in computational geometry, computer graphics (both hardware and software), and discrete differential geometry.


·        The design of geometric models for object shapes, in particular, is occasionally called computer-aided geometric design (CAGD).


·        Computer-assisted surgery (CAS)


·        Computer-aided surgical simulation (CASS)


·        Computational fluid dynamics (CFD)


·        Component information system (CIS)


·        Computer-integrated manufacturing (CIM)


·        Computer Numerical Controlled (CNC)


1. Drawing entities;


A drawing is created using a no. of entities. A large no. of options are provided to draw the entities depending upon the requirements. Common entities are

•          Point, * Line,            * Arc, * Ellipse,

•          Circle,            *Polygon,     *Spline,         *Rectangle, etc.,

2. Drawing Utilities;


Drawing utilities include several functions to have the creation and storage of drawings. Common utilities are;

•          *Screen size,           * Line type,  * Scaling       * Layers,

•          *Grid,            *Snap,,          *Units, ,        *file utilities, etc.,


3.. Editing commands in CAD;


It is necessary to make the corrections and alterations to the entities of a drawing. Editing commands are used for this process.


A few editing commands are listed below,

*Erase,          *Move,         *Array,          *Fillet,           * Chamfer

*Mirror         *Rotate,        *Trim,            *Copy            *Scale, etc.,

Various Edit and Inquiry commands in CAD;


Editing an entity or group of entities in Autocad requires the entity or the group to be selected. There are three ways of doing this :


1) Autocad can be set to allow the user to select the objects first, and then accept commands to process them. This is called noun/verb selection. This mode of operation can be enabled/disabled using the DDSELECT command which opens up a dialogue box.


2)  The commands can be given first, and the objects can be specified when the user is prompted for them.


3)  The SELECT command can be used to select a specific selection set, which can be referred to in subsequent editing operations.


Editing with grips :


Selected objects can be edited by manipulating grips that appear on the selected entity. The Grips mode can be enabled with the DDGRIPS command which opens up a dialogue box. The editing operations possible using grips are :

Stretch, Move, Rotate, Scale and Mirror


·           Erasing unwanted objects and retrieving accidentally removed ones :


The ERASE command permanently removes specified objects. To erase only the drawn object, enter "L" at the 'select oblects' prompt.


2.           The OOPS command restores only the most recently erased objects.


3.           Copying and Moving :


The commands available are :


The MOVE and COPY commands are for recreating the object at another place.

The COPY command retains a copy in the original place while the MOVE command does not.


4.     The SCALE command allows the size of objects to be changed. It scales the object about a reference point, by expanding/shrinking it equally in all directions. SCALE can be used to rescale an entire drawing in one go.


5.     The ARRAY command creates multiple copies of entities in a rectangular or polar pattern. To change the orientation of the array, use SNAP Rotate command or SNAPANG system variable.


6.           Changes,  Cuts  and  Constructions  :  These  commands  allow  you  to  change


properties  of  objects


color,   layer,etc.)  and  modify  objects  by  trimming

/extending their ends,


cutting sections out of them.  They can also be used to


draw fillet arcs, chamfer lines, parallel lines, offset curves, and construction markers.


The available commands are :




The CHANGE command is used to change the following : color, elevation, layer, linetype, thickness


Characteristics other than the above can also be changed by specifying a point instead of choosing one of the above properties. Then this "change point" is used to modify the object depending on whether the object is a line or a circle, etc. The "change point" method works for multiple entities also.


Variations of the CHANGE command are : DDCHPROP and CHPROP


·        The DDEDIT command allows editing of both text and attribute definitions. The command can be used either in paper space or in model space, whichever is active when the command is issued. It cannot be used on text attributes that are part of a block.


·        The BREAK command erases part of a line, trace, circle, arc or 2D polyline The end points of the part are specified by the user.


·        The TRIM command is used to trim objects such that they end exactly at cutting edges defined by other intersecting objects.


·        The EXTEND command is the complement of the TRIM command because it lets you extend an object till it meets another object.


·        The FILLET command connects two lines, arcs or circles by means of a smoothly fitted arc of specified radius.

·        The CHAMFER command is similar : it trims two intersecting lines a specified distance from the intersection and connects the trimmed ends with a new line segment.


·        The OFFSET command constructs an entity parallel to the specified one, either through a given point or at a given distance.


·        The DIVIDE command lets you divide an entity into several equal-length parts, placing markers along the object at the dividing points.


·        The MEASURE command is similar to the DIVIDE command : it measures an entity and places markers at specified intervals.


Polyline, Mesh, and Block Editing :


There are two basic commands for this : PEDIT and EXPLODE. PEDIT is used to edit 2D and 3D polylines, and 3D polygon meshes.


The EXPLODE command breaks up a complex entity as follows :


A Block or associative Dimension is replaced with copies of simple entities comprising the Block or Dimension. Polylines are replaced with simple and arcs; 3D polygon meshes with 3D faces and polyface meshes with 3D faces, lines and points.


Undoing commands :

·              U command : causes the most recent command to be undone.


·              REDO command : reverses the effect of the most recent U command.


·              UNDO command : can undo several commands simultaneously.


·        Inquiry commands are commands which allow the user to inquire into locations and relationships into entities.


·              Inquiry commands available in Autocad are :


·              LIST : lists data for an entity.


·              DBLIST : lists data for every entity in the drawing.


·              ID : gives co-ordinates of a point specified in the graphics window.


DIST : measures angle and distance between


4. 2D transformations in CAD;


In computer graphics drawing are created by serious primitives which are represented by the coordinates of their end points.


Certain changes in these drawings can be made by performing some mathematical operations on these coordinates. The basic transformations are Scaling, Rotation, translation, etc..,


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