UNIT 7
Computer Aided Drafting
Manufacturing of a product is the main activity in engineering profession. The design of a product may start with trial designs in the form of sketches on paper. As the design improves and undergoes changes, the final form of design must be the scaled manufacturing drawings with finer details included. These drawings are two-dimensional representations of three dimensional objects designed.
During the process of design, the designer may have to carry out a large amount of computations so that an optimum design is obtained. A computer with good graphic capabilities helps the designer to,
(i) realize his ideas.
(ii) carry out complex computations.
(iii) present the results of computations in a useful form for decision making and possible improvement.
(iv) present the improved model for evaluation. Interactive Computer Graphics (ICG) is the tool of the designer.
Computer aided drafting
A part to be manufactured is defined first in terms of its geometry which also includes dimensions, tolerances, surface finish, and in some cases the type of fit between two mating parts. The two-dimensional representation of a part, called an engineering drawing or a blueprint, shows three orthogonal views of the part. Sometimes, when three views are not enough to define the part, additional sectional views, auxiliary views may have to be added for conveying the right information.
Any design is finally represented in the form of orthographic views and auxiliary views so that production can be carried out. Hence, the computer aided drafting is an important tool for Computer Aided Design. The Computer Aided Drafting (CAD) system is the computerization of technical, production, electronics and architectural drawings.
CAD is the product of computer era. Its development originated from early computer graphics systems. CAD can find its roots to the development of Interactive Computer Graphics (ICG). A system called Sketchpad was developed at Massachusetts Institute of Technology, U.S.A., in 1963 by Ivan Sutherland.
In the beginning, CAD systems were no more than graphics editors with some built-in design symbols. The geometry available to the user was limited to lines, circular arcs, and a combination of the two. The development of free-form curves and surfaces such as Coon's patch, Ferguson's patch, Bezier's curve, and 8-splines enabled a CAD system to be used for more sophisticated work. A 30 CAD system allows a user to do very sophisticated design and analysis work.
Computer aided drawing and drafting system uses the computer to assist in generation of blueprint data. CAD systems are essential in design and many computers based systems are commercially available. 20 drawing systems correspond directly to traditional engineering drawings, and they are developed to substitute manual drafting.
The advantages offered by computerized drafting systems can be summarized as:
(a) It increases the accuracy and productivity of designer.
(b) It allows design alterations to be made easily.
(c) It offers better drawing visualization through colours.
(d) It improves the quality of drawings produced.
(e) Drawings are easier to store and retrieve.
(f) Storage space required is less.
(g) Transfer of drawings is faster and cheaper.
(h) It permits the use of library of standard symbols for more productive CAD work.
Although the capital investment required in setting up a computer aided drafting system is high, the greater capabilities offered by computers and software are making the systems more affordable.
Computer (fig.1):
A computer system consists of
(a) Central Processing Unit (CPU), also known as processor
(b) Main memory
(c) Input devices
(d) Output devices
(e) Secondary storage devices.
The CPU controls and supervises the entire computer system. The actual arithmetic and logical operations are performed by CPU with the help of main memory. The main memory stores program instructions and processing data. Secondary storage devices provide slow but high capacity memory for storing program and large amount of data that is not currently being processed. Typical secondary storage devices are hard disk, floppy diskettes, magnetic tape, C.D. (compact disk), D.V.D., pen drive, etc.
During operation, data flows between the CPU, the main memory, and the various input/output devices. Moreover, the control information flows between these units to tell what to do with the data. Control information is used to manage the overall operation of the system.
Figure 1 Organisation of a computer
It must be remembered that the main memory is directly connected to the CPU. The main memory stores the program instructions and all the data being used for processing until the data are released as output under the instructions from the CPU. A control unit inside the CPU directs and coordinates all the operations of the system that are called for by the program. It involves control of the input and output devices, the storage and retrieval of information from memory, and the transfer of data between memory and ALU.
A CAD system consists of Hardware and Software. The system usually has the following major hardware elements:
Processor (CPU)
Every CAD system must have a CPU to process and store a very large amount of graphics data. The speed and processing power of a computer is mainly decided by the CPU speed and the amount of main memory (RAM) installed.
Display
The most common type of display device for CAD systems is a Cathode Ray Tube (CRT). The display device must be capable of displaying both graphical and alphanumeric data. Although the display looks like television monitors, they have special electronics for creating graphics images on the screen from computer processed data as per the instructions. But, they lack the television program receiving capability. Since both graphics and text are to be displayed simultaneously on the screen, the graphics monitor is usually divided into graphics area and a small text display area (see fig. 2).
The capabilities of display devices are evaluated in terms of resolution, colours and picture refresh rate. Picture refresh rate is important to get a steady and flicker free display. For a general-purpose graphics system, a display with 1024 (horizontal) by 768 (vertical) pixel resolution, 0.28 mm dot pitch and 256 shades of colours is the desirable configuration. The small dot pitch gives a high resolution and therefore good quality picture.
Input Devices
(1) [fig. 2(i)]: This is the most common input device. It is preferred for entering commands, text or value such as coordinates of a point or radius of a circle. The 101-key keyboards have special function keys to support special graphic functions. Whenever a key is pressed, the key character is identified by the computer and a character is displayed on the screen. If function keys are pressed or a combination of keys is pressed, the software takes the appropriate action.
(2) Mouse [fig. 2(iii)]: The mouse is a pointing device which is moved across a flat surface (usually on a mouse pad) by hand to indicate X-Y movement. The rotation of rubber ball underneath the device is translated and the corresponding cursor movement on the screen provides a visual feedback. The mouse is used as a pointing and pick-up device.
Either two-button or three-button mice are available in the market. The mouse has become very popular with microcomputers in recent years, and it is an indispensable pointing device. It is inexpensive, small, and convenient to use. However, fine and very precise sketching is very difficult to obtain with the mouse.
(3) Tablet or Digitizer [fig. 2 (ii)]: This is also a very common and popular input device for CAD. It has a flat surface on which a stylus is placed and moved. The movement of stylus on the surface in terms of X and Y coordinates of surface is detected by sensors embedded in the tablet. The stylus incorporates many buttons for input. The parameters of a digitizer are resolution, accuracy and repeatability.
The digitizers are available in very large sizes to be used with full size engineering drawings. They have a very high resolution. The resolution is measured in terms of lines per inch (LPI). Digitizers with 2000 LPI resolution are commercially available.
The most common method of using digitizer is for tracing a drawing or a portion of the drawing. The drawing to be copied is placed on the digitizer and the stylus is used to trace the portions required. Thus, old drawings, maps, etc., can be entered into a CAD system.
Graphic output devices:
A hard copy i.e. copy on paper of what is displayed on a graphics terminal can be obtained by a variety of graphics printers and plotters. The computer graphic output device may be thought of as paper and a pen or a pencil.
(1) Dot Matrix Printers (DMP) and laser Printers: Dot Matrix Printer is the most commonly used printer for text printing. The characters are formed by printing dots in a specific manner. The dot matrix printer [fig. 2(vi)] can also be used for printing of drawings, but the quality of output is poor.
(2) Pen Plotters [fig. 2(v)]: Pen plotters are the simplest output devices for CAD. A pen plotter consists of a device to hold the paper. Usually two orthogonal motorized carriages hold a pen and move it under computer control. There are three inputs to the pen plotter: (i) an X coordinate, (ii) a Y coordinate and (iii) a pen variable. The pen variable can specify the pen colour by pen number, the pen to be up (non-drawing position) or down (in contact with paper in drawing position).
The two varieties of plotters are flat-bed and drum plotters. The flat-bed plotter is limited by the paper size it can handle. The drum plotter utilizes a continuous roll of paper which rolls over the top of the drum. The capacity and capabilities of a plotter are evaluated by the size of paper it can handle, resolution, speed of plotting, number of pens it can handle, etc.
(3) Ink-Jet printers/plotters: These are dot matrix printers. The drawing which is made up of lines, arcs, characters and symbols is converted into dot form. Then the rows of dots are printed across the width of the paper by impelling a tiny jet of ink on the surface of paper. The jets are switched on and off at high frequency to create multicolor plots.
Typically, the resolution is 600 to 900 dots per inch, and each dot is arranged to overlap the adjacent ones. This provides a high-quality photo-realistic picture. These plotters are quiet during operation. These plotters are used for colour plots of drawings, shaded images, contour plots and artistic work.
(4) laser printers: A laser printer rapidly produces high quality text and graphics on plain paper. The fastest colour laser printers can print over 100 pages per minute. It can write with much greater speed than an ink jet and can draw more precisely, without spilling any excess ink. The toner powder of laser printer is cheap and lasts a long time compared to expensive ink cartridges.
Figure 2 CAD system hardware
CAD software
The CAD system creates an environment to prepare drawings interactively. Most CAD systems available commercially are menu driven. Commands can either be typed directly with the help of a keyboard or can be picked-up from the screen menu or from toolbar with the help of a mouse or can be selected from the digitizer menu. Some screen menus offer pull-down menus (also referred to as pop-up menus) and dialogue boxes.
For example, a variety of hatching patterns are displayed on the screen for better visualization and selection if a hatching command is chosen. The appropriate hatching pattern can be selected with the help of input device. The effect of every command immediately displayed on the screen so that selection and corrections can be done interactively and immediately.
The major functions to be performed by a computer aided drafting system are:
(a) Basic set-up of a drawing
(b) Drawing the objects
(c) Changing the object properties
(d) Translating the objects
(e) Scaling the objects
(f) Clipping the objects to fit the image to the screen
(g) Creating symbol libraries for frequently used objects
(h) Text insertion
(i) Dimensioning
(j) Creates various layers (Transparent sheets)
(k) Allows zoom-in and zoom-out of any components drawing
(I) Creates different numbers of print/plot layouts.
AutoCad
AutoCAD (a product and registered trademark of Autodesk Inc., USA) is a low cost yet very effective computer aided design and drafting software. AutoCAD is accepted as the industry standard and it is preferred by a large community of CAD users in the world. Although AutoCAD is available for a variety of computer systems, majority of AutoCAD implementations are available on IBM or compatible personal computers with various operating systems. AutoCAD comes with a very large number of user-selectable options to support a great variety of commercially available display devices, digitizers, mouse, printers and plotters. AutoCAD supports 2D drafting and 3D modelling.
The basic drawing entities are lines, polylines [refer to art. 26-5-4 of any width, circles, arcs, ellipses and solids. There are many ways of defining a drawing entity, and the software always prompts the user for all options. Each drawing entity has an associated line-type, colour, layer [refer to art. 26-5-5(2)] and thickness. The thickness is a property associated with 3D entities.
Before any drawing is started, the AutoCAD environment must be prepared for proper units of measurement, line-type, drawing size, layer, etc. In AutoCAD the drawings are always prepared at full scale, and the drawing size can be changed at any instant of time by using LIMITS command.
Classic screen layout of AutoCad:
The default AutoCAD 2010 drawing screen can be divided into four areas such as drawing area, command area, menu bar area and tool bar area. The various component of Graphical User Interface (GUI) of AutoCAD 2010 is shown in fig. 26-3.
Figure 3 Classic AutoCad layout
Function Keys:
F1 Online Help
F2 Toggles between Drawing screen to text screen
F3 Toggles between OSNAP - On and Off
F4 Toggles between Tablet - On and Off
F5 Switches among lso planes Top, Right and Left
F6 Toggles between Coordinates - On and Off
F7 Toggles between Grid - On and Off
F8 Toggles between Ortho Mode - On and Off
F9 Toggles between Snap Mode - On and Off
F10 Toggles between Polar Tracking - On and Off
F11 Toggles between Objects Snap Tracking - On and Off
Drawing entities (Draw commands)
Figure 4
(1) Line: A line is specified by giving its two endpoints. The LINE command can be used to draw a single line or a series of lines with the end-point of one being the start point of the next. When a series of such lines is created, each line is treated as a separate entity. To create a closed polygon, the user has to type in C (close option) for the To point: prompt. This causes the last and the first points to be joined by a line and thus creating a closed boundary.
(2) Pline: Polylines are interesting drawing entities. Polylines can include both lines and arcs connected at end-points. Thus, a polyline is a single entity with multiple segments. The polylines can be straight or curved, can be wide (like a TRACE) or tapered. Fillets and chamfers can be added where needed on a polyline. Curve fitting and hatching can easily be performed on a polyline.
(3) Polygon (fig. 5): A polygon also a polyline with equal length of sides. The regular polygon can either be inscribed in a circle or circumscribed about the circle. The polygon may also be constructed by specifying the length of one side and the number of sides of polygon (called edges). In this method a polygon is constructed in anti-clockwise direction from the two-edge end-points that have been specified.
Figure 5 Polygon
(5) Arc (fig. 6): This command is used to draw an arc accurately. Usually there are three parameters required for drawing an arc.
Different ways of drawing circular arcs are:
(i) 3-point arc: The arc is drawn by specifying three points on the chord of arc. The first and third points define the start and end-points of an arc respectively.
(ii) Start, Center: This option needs start points and center point of an arc. The third parameter may either be an end-point, included angle, or length of chord.
(iii) Start, End: This option asks the user to enter the start and end-points of an arc. The arc is completed by either specifying radius or included angle or center point.
Figure 6 Arcs
Drafting Aids:
(1) limits: In AutoCAD, drawings are drawn in full scale therefore limits are needed to size up a drawing area. The limits are set by specifying X and Y co-ordinates of Lower Left Corner (L.L.C.) and Upper Right Corner (U.R.C.) of drawing area. By default, L.L.C. Is set to (0, 0) and U.R.C. Is set to (12, 9) for imperial file and (420, 210) for metric file.
Note: By changing limits, the drawing display area does not change. It is required to use all option of the zoom command to display the limits inside the drawing area.
(2) layer: A layer can be thought of as a transparent sheet on which drawings can be prepared. Drawings can be logically divided into different layers, and layers can be selectively displayed either individually or in any combination. Each layer is identified by a name. If the drawing becomes too dense or complicated, some layers can be turned off so that they do not interfere with the work. The drawing can be edited on any one layer at a time, called current layer. The colour, line type and line weight are the properties associated with a layer. This controls an entity's colour, line type and line weight drawn on that layer.
(3) Dimensioning: The manufacturing drawing must be dimensioned for size and tolerances so that the right information can be conveyed. The appearance and size of dimension arrows, size and style of dimension text with or without tolerances, and the layer on which dimensions are placed can be controlled by setting dimension variables.
(4) Object snap (fig.7): A very useful drawing aid, the OSNAP identifies the points on drawing entities that are visible on the screen. For example, the start point of an arc can be the endpoint of an existing line. This option allows the user to pick-up the points very accurately with respect to drawing displayed. Some of the OSNAP modes are:
Figure 7
(5) Zoom: This is the most common method of magnifying a portion of current drawing on the screen. The portion of drawing to be zoomed is usually selected by a window. A window is identified by picking up the diagonal corners of a rectangle around the area of interest.
Editing of a drawing:
Figure 8
The fundamental commands to edit a drawing are:
(1) Move [fig. 9 (i)]: Moves selected objects to another location about a base point.
(2) Rotate [fig. 9 (ii)]: Rotates selected objects through a specified angle about a base point.
(3) Copy [fig. 9 (iii)]: Creates one or more copies of selected objects at another location. The function of COPY command is like the MOVE command except that it preserves a copy of the objects selected at the original location.
(4) Mirror [fig. 9 (iv)]: Creates a mirror image of the selected objects about a specified line.
Figure 9
(5) Array: This command creates multiple copies of selected objects in rectangular or polar form. This is a form of COPY command.
(6) Erase: This command deletes the selected entities. A record of entities erased is always maintained. The most recent entity can be unerased by OOPS command.
(7) Oops: This command retrieves all objects erased by the last Erase and after executing Block or Wblock command.
(8) Break: This command erases a portion of line, arc, circle or a 20 polyline between two selected points.
(9) fillet (Refermodule26-5, 26-13): This command is used to create around corner between two lines. The lines are shortened or extended to fit a tangent arc of specified radius. FILLET works on any combination of two lines, arcs, circles, non-parallel lines, or a single polyline.
(10) Chamfer (Refer module 26-8): This command works on two lines or a single polyline to create a bevelled edge.
(11) Extend: This command extends the lines, polylines and arcs to a boundary edge which can be a line, polyline, arc or circle. A closed polygon cannot be extended. When you invoke this command, you will be prompted to select the boundary edges. These edges can be lines, polylines, circles, arcs, ellipse, rays, splines etc. After the boundary edges are selected, you must select each object to be extended. An object can be both a boundary edge and an object to be extended.
(12) Offset: This command creates a parallel single copy of line, arc, circle, rectangle, polygon, or 20 polylines at a given offset distance. Each offset creates a new entity with the same linetype, color and layer settings.
(13) Stretch: The STRETCH command can either lengthen entities or shorten them, and thus alter their shapes. The centre points of arcs or polyline arcs are adjusted accordingly.
(14) Trim: This command trims the objects that extend beyond a required point of intersection. When you invoke this command, you will be prompted to select the cutting edges. These edges can be lines, polylines, circles, arcs, ellipse, rays, splines, text, blocks or even viewports. After the cutting edges are selected, you must select each object to be trimmed. An object can be both a cutting edge and an object to be trimmed.
(15) Scale: The SCALE command allows to shrink or enlarge the already existing drawing objects about a base point by specifying a scale factor.
(16) Pedit: A polyline is a single entity which is made up of a continuous series of line and arc segments. The PEDIT command is exclusively used for editing of polyline properties. The selected line, arc and polyline can be added to an existing polyline by a JOIN option. A smooth curve passing through all vertices of a polyline can be created by using FIT option. Similarly, a spline can also be constructed by using SPLINE option.
(17) Explode: This command breaks a polyline into its individual segments. These segments can then individually be edited, and rejoined again to form an edited polyline.
The advanced features of AutoCAD are:
(1) Built-in programming language AutoLISP provides programming environment so that AutoCAD commands can be called along with programs written for computations. This is very useful for parametric design and drawing.
(2) Drawing Exchange Files (DXF) and script files can be used to interface AutoCAD with programs written in any other higher-level language such as C. DXF and IGES file formats allow the exchange of drawing files among various drafting software’s.
(3) AutoCAD provides techniques to define and extract attributes of entities. This feature is used in extracting information from a drawing for processing by other programs or to transfer it to a database.
Symbol library (Block, Wblock and Insert)
A complete drawing or a part of a drawing can be given a specific name and then stored as a block. A collection of such blocks form a library which is very useful for drawing purpose. The blocks, also referred to as symbols, can be scaled, rotated or mirrored if necessary and inserted into the drawing at the appropriate location e.g. Bolt heads to be used at different locations in a drawing. Many suppliers of CAD add-on products offer symbol libraries of standard mechanical components, electronic components, architectural symbols, piping symbols, etc. The block and wblock command groups many selected entities together and treats them as a single object i.e. single block. Block commands saves the group of selected entities with block name and is inserted in the file in which it is created, but wblock command saves the group of selected entities as a drawing file and can be inserted in any drawing file. The blocks and wblocks can be scaled, rotated, stretched or mirrored. The explode command separates the entities of block and wblock.
INSERT: INSERT command is used to insert blocks and wblocks in current drawing. Blocks will be inserted only in the drawing file in which they created and wblocks will be inserted in any drawing files.
