CAD/CAM Notes Unit 1 PDF

# Chapter-I: Fundamentals of CAD ## 1.0. Introduction Engineering design and manufacture form the core of the engineering profession. The work of engineers is the basis for prosperity. Engineering is a continuous education. Engineering education has changed for using logarithmic tables to slide rul...

# Chapter-I: Fundamentals of CAD ## 1.0. Introduction Engineering design and manufacture form the core of the engineering profession. The work of engineers is the basis for prosperity. Engineering is a continuous education. Engineering education has changed for using logarithmic tables to slide rules to pocket calculators to personal computers. In the present day scenario, computers have become indispensable tools throughout the life cycle of engineering products. Computers have a prominent, often a central role. Today, ICG and CAD/CAM are impacting the drafting, design and manufacturing. ## 1.1. CAD System-Definition Computer aided design (CAD) may be defined as the use of computer systems to assist in the creation, modification, analysis or optimization of a design. CAD is used for creating solid models of the components to be designed (Output is a DESIGN). Computer Aided Manufacturing includes every step that is involved in creating the designed component; converting it from raw material into final form (Output is a manufactured product). The computer 'systems' consist of the hardware and software to perform the specialized design functions required by the particular user firm. Modern CAD systems are based on interactive computer graphics (ICG) Interactive computer graphics denotes a user-oriented system in which the computer is employed to create, transform and display data in the form of pictures or symbols. The user in the computer graphics design system is the designer who communicates data and commands to the computer through any of several input devices. A typical ICG system is a combination of hardware and software. The hardware includes a central processing unit, one or more workstations (including the graphics display terminals) and peripheral devices such as printers, plotters and drafting equipment. The software consists of the computer programs needed to implement graphics processing on the system and application programs to facilitate the engineering functions. The ICG system is one component of a computer aided design system. The other major component is the human designer. Interactive computer graphics is a tool used by the designer to solve a design problem. In effect, the ICG system magnifies the powers of the designer. The designer performs the portion of the design process that is most suitable to human intellectual skills, the computer performs the task best suited to its capabilities and the resulting system exceeds the sum of its components. ## 1.2. CAD/CAM may be utilized for - Producing drawings & documentation of the designs. - As a visual tool for generating shaded images and animated displays. - Performing engineering analysis on geometric models. - For process planning and generation of NC part programming - Calculation of mass properties of the components - Coding of parts - Piping and instrumentation layout - Kinematic and dynamic Analysis - Architecture, GIS, report generation, parametric design. ## 1.3. DESIGN PROCESS Design is the act of devising an original solution to a problem by a combination of principles, resources and products in design. Design process is the pattern of activities that is followed by the designer in arriving at the solution. of a technological problem and the process is an iterative procedure. A preliminary design is made based on the available information and is improved upon as more and more information is generated. The design progresses in a step-by-step manner from some statement of need through identification of the problem, a search for solutions and development of the chosen solution to trial production and use. These descriptions of design are known as models of the design process. There are various models of the design process, which are iterative in nature. One such model defined by Shigley is presented here. ### 1.3.1. Shigley Model The Shigley model is shown in Fig. 1.1. It involves six basic steps, which are explained as follows: - **Recognition of need:** Recognition of need involves the realization by someone that a problem exists for which some feasible solution is to be found. This might be the identification of some defect in a current machine design activity by an engineer or the perception of a new product marketing opportunity by a salesman. - **Definition of problem:** Definition of a problem involves a thorough specification of the item to be designed. This specification will generally include functional and physical characteristics, cost, quality, performance, etc. - **Synthesis:** During the synthesis phase of the design process various preliminary ideas are developed through research of similar products or designs in use. - **Analysis and optimization:** The resulting preliminary designs are then subjected to the appropriate analysis to determine their suitability for the specified design constraints. If these designs fail to satisfy the constraints, they are then redesigned or modified on the basis of the feedback from the analysis. This iterative process is repeated until the proposed design meets the specifications or until the designer is convinced that the design is not feasible. The components, sub-assemblies or sub-systems are then synthesized into the final overall system in a similar iterative manner. - **Evaluation:** The assessment or evaluation of the design against the specifications established during the problem definition phase is then carried out. This often requires the fabrication and testing of prototype model to evaluate operating performance quality, reliability, etc. - **Presentation:** The final phase in the design process is the presentation of the design. This includes documentation of the design through drawings, material specifications, assembly lists and so on. ## 1.4. Computer Aided Design Process Computers as mentioned earlier, are very much useful in developing the geometric models, producing drawings & documentation of the designs, as a visual tool for generating shaded images and animated displays and for performing engineering analysis on geometric models. They assist the designers in almost every aspect of the design stages and reduce the burden of laborious calculations and drafting and the design engineers can utilize their time better creativity and better ideas. A comparison between the conventional design process and the CAD process is presented below through fig.1.2. ## 1.4.1. APPLICATION OF COMPUTERS FOR DESIGN Engineering, design has traditionally been accomplished on drawing boards with the design being documented in the form of a detailed engineering drawing. This process is iterative in nature and is time consuming. The computer can beneficially be used in the design process in CAD. The design task is performed by a CAD system, instead of a single designer working over a drawing board. The various design related tasks, which are performed by the CAD system, can be grouped into four functional areas: - Geometric modelling - Engineering analysis - Design review and evaluation - Automated drafting These functional areas are discussed in the following sections. ### 1.4.1.1. Geometric Modelling Geometric modelling is concerned with the computer compatible mathematical description of the geometry of an object. The mathematical description allows the image of the object to be displayed and manipulated on a graphics terminal through signals from the CPU of the CAD system. The software that provides geometric modeling capabilities must be designed for efficient use both by the computer and the human designer. Several commercial software packages are available, for creating the Geometric models, namely CATIA, PRO-E, SOLID-WORKS, UNIGRAPHICS and IDEAS. ### 1.4.1. 2. Engineering Analysis In the formulation of nearly any engineering design project, some type of analysis is required. The analysis may involve stress-strain calculations, heat transfer calculations or computation of natural frequencies, acceleration, velocity and displacement response or other parameters. Analysis can be carried out using standard CAD software like finite element modeling and analysis or by software developed internally in the design offices. Most CAD software suits incorporate engineering analysis software to carry out finite element analysis and to determine mass properties of the component. The analysis of mass properties is the analysis feature of a CAD system that has probably the widest application. It provides properties of a solid object being analyzed such as the surface area, weight, volume, centre of gravity and moment of inertia for a plane surface (or a cross-sectional of a solid object). The corresponding computations include the parameter area and inertia properties. Checking the accuracy of the design can be accomplished conveniently on the graphics terminal semiautomatic dimensioning and tolerance routines, which assign size specifications to surfaces indicated "by the user help to reduce the possibility of dimensioning errors. The designer can zoom in on part design details and magnify the image on the graphics screen for close scrutiny. Several commercial software packages are available for analyses are ANSYS, NASTRAN, NISA and HYPERMESH. ### 1.4.1.3. Design Review and Evaluation: A procedure called layering is often helpful in design review. For example, a good application of layering involves over-layering the geometric image of the final shape of the machined part on top of the image of the rough casting. This ensures that sufficient material is available on the casting to accomplish the final machined dimensions. This procedure can be performed in stages to check each successive step in the processing of the part. Another related procedure for design review IS interference checking. This involves the analysis of an assembled structure in which the risk that the components of the-assembly may occupy the same space. This risk occurs in the design of large chemical, plants; air-separation cold boxes and other complicated piping structure. One of the most interesting evaluation features available on some computer aided design system is kinematics; the available kinematics packages provide the capability to animate the motion of simple designed mechanisms such as hinged components and linkages. This capability enhances the designer's visualization of the operation of the mechanism and helps to ensure against interference with other components without graphical kinematics on a CAD system. A designer must often restart to the use of pin and cardboard models to represent the mechanism. Several commercial software packages are available for ADAMS and SYMBOLS. ### 1.4.1.4. Automated Drafting Automated drafting involves the creation of hardcopy engineering drawings directly from the CAD database. In some early computer aided design departments, automation of the drafting represented the principal justification for investing in the CAD system. Indeed, CAD systems can increase productivity in the drafting function by roughly five times over manual drafting. Some of the graphics features of computer aided design systems lend themselves especially well to the drafting process these features include automatic dimensioning generation of crosshatched areas, scaling of the drawing and capability to develop sectional views and enlarged views of particular part details, the ability to rotate the part or to perform other transformations of the image (e.g., oblique isometric or perspective views). AutoCAD is a widely used popular drafting software package. There are other software packages, namely Pro-E, Unigraphics, CATIA, I-deas, Solid works etc. ## 1.5. Computer aided drafting The CAD systems can generate drawing easily and efficiently, a large drawing that requires several days to produce manually may be completed in less than a day with automated drafting. Indeed, it is possible in some instances to achieve a productivity increase by as high as twenty times. To be able to create engineering drawings, a graphic system will need to include some additional functions such as dimensioning, text and cross-hatching etc. In order that drafting is done with the aid of a computer, the software must include a drafting set up that includes (i) units (ii) angle measurement, angle direction (iii) area (iv) co ordinate system (v) limits (vi) grid (vii) snap and (viii) line type and line weight. The package should comprise drawing structure, where in various views, title block, bill of materials, labels and notes are created. The basic geometric commands for creating the drawings on VDUs are (i) point (ii) poly line (iii) arc (iv)circle (v) spline (vi) ellipse (vii) rectangle (viii) polygon (ix) block (x) table (xi) text etc. Another important feature in computer aided drafting is layering. Layer is a logical grouping of data that are like transparent acetate overlays on a drawing. One can view layers individually or in combination. Layers are the equivalent of the overlays used in paper based drafting. Layers are the primary organizational tools in Auto Cad.. The display is to be controlled with reference to size, position and orientation. This is achieved through the display control commands in Auto Cad. The display control commands are: Zoom, Pan, Window, Aerial view and View port. The magnification of a view can be changed by zooming in or zooming out. Panning is shifting the drawing without changing the magnification. Window plots any portion of the drawing that is being specified. With Auto Cad, one can modify the size, shape and location of objects Using the editing commands like copy, mirror, offset, move, rotate, scale, sketch, trim, chamfer etc., the drawings can be modified. Further, the standard types of dimensions, such as, linear, radial, angular and ordinate dimensions can be created on the drawings. Geometric tolerances can also be added on the drawings with the Auto Cad. Some features of Auto Cad along with the description are discussed below: - **Annotation:** Drawings and diagrams are an extremely' efficient method' of recording and conveying complex information for human comprehension. In a typical engineering drawing, it is very common for it to contain not. only graphical data but also textual information such as labels, notes, specifications and dimensions of parts. - **Arrows and Pointers:** Arrows heads and pointers figure prominently on most engineering drawings and a quick and simple method of mitering them is essential. There are many different styles and sizes of arrow heads and pointers as shown in figure. It is important that the draughting system is flexible enough to allow a large variety of arrows and pointers to be generated for different applications. - **Dimensioning:**Most systems provide the user with a complete range of dimensioning conventions which cover British, American and German standards. Usually, a user specifies the positions of each of the witness lines, and that of the text or dimension using the cursor. The system then produces the witness lines to the correct length for the required dimension position, draws the dimension lines and inserts the appropriate dimension. Also the system automatically checks whether or not there is space to place thedimensions between witness lines and, if not, it is put externally with a split dimension line. - **Text:** Text is probably the most common type of annotation and appears in some form on every drawing produced. In addition to outline and dimensional information, most engineering drawings contain textual information such as notes, specifications, comments and titles. Most CAD/CAM systems allow the user flexible control over the format of text, namely the character height and aspect ratio, character set or font, and the direction in which it is written. - **Cross-Hatching:** Cross-sections are commonly indicated in a drawing by cross-hatching which is time consuming and tedious process if performed by hand. Most CAD/CAM systems can do this task automatically through menu commands. The appearance of cross-hatching varies depending on the hatch angle and the spacing between successive hatch lines as shown in figure. To create cross-hatching for sections, the user usually needs to specify points that make up the outline for the boundary of the area to be cross-hatched. ## 1.6. DIGITAL COMPUTER The three basic hardware components of a digital computer are as follows: - Central processing unit (CPU) - Memory - Operating Input/Output devices - Memory ### 1.6.1. Central processing unit (CPU) The central processing unit coordinates and controls the activities of all the other units. The CPU consists of three separate subsections: Control unit, Arithmetic Logic Unit (ALU), Memory ### 1.6.2. Control unit Control unit basically coordinates the operations of all the other components. It controls the input and output of information between the computer and the outside world through I/O devices, synchronizes the transfer of signals between the various sections of the computer and regulates the other sections to perform their individual functions. ### 1.6.3. Arithmetic Logic Unit (ALU) Arithmetic logic unit as its name implies, carries out the arithmetic and logic manipulations of data. It performs the calculations such as addition, subtraction, multiplication, division and comparison according to the programmed instructions. ### 1.6.4. Memory The memory section stores all the instructions and data of a program. Therefore the CPU must transfer these instructions and data to and from the memory throughout the execution of the program. The type of memory is a very important consideration in the design of the entire computer system. There are two basic categories of computer memory: - **Main memory (primary storage)** - **Auxiliary memory (Secondary storage)** #### 1.6.4.1. Main memory (Primary storage): The purpose of the main memory is essentially to provide a working area for the current program and it only retains information on a temporary basis until the termination of the program. The main memory is used to contain instructions and data of a program during its execution. Main memory consists available in two parts such as Random-access memory (RAM) and Read-only memory (ROM) RAM is used to store an ever-changing parade of programs and data. When you load, a program into the computer or create a word processing document, the data you enter using the keyboard is stored in random-access memory (RAM) also called main, primary or user memory. Usually, if you turn off the computer, any programs or data stored in this memory are lost; thus RAM is said to be volatile memory. ROM is static and unchanging; as a result' it is called non-volatile memory. The data in ROM is permanently recorded on memory chips by the computer's manufacturer, prior to their being sold or shipped. ROM is generally used to store programs and instructions that the computer frequently needs, sometimes called firmware. For example, it contains the instructions the computer follows to start up when you first turn it on and the system's set-up program that you use to change settings such as the time and date. #### 1.6.4.2. Auxiliary memory (Secondary Storage) Programs and data files are not generally kept in main memory but are stored on auxiliary memory devices and loaded into main memory when required. Primary storage is very expensive and has a limited capacity. Backing storage is used to supplement the main memory and to save the data on a permanent basis. Information saved in backing storage can be retrieved and quickly transferred to the CPU when it is needed. There are several types of devices commonly referred to as auxiliary devices, which constitute the secondary storage that can provide this kind of backing storage. There are two forms of secondary storage: - **Sequential access storage.** It is also known as serial access storage. A sequential access storage unit can be distinguished by the fact that to read one particular record in the file, all the records preceding it must also be read. If additional data is to be added into the file, it is only possible to do at the end of the file, otherwise the rest of the file will have to be rearranged. - **Direct access storage.** With direct access storage, individual records can be located and read immediately without reading any other records. As the information is readily available at random, direct access storage is also known as random access storage. <br> Sequential access storage expensive and is used mainly to store files as permanent records in an archive where they are very rarely accessed, whereas direct access storage is most suitable for storing files that require frequent access. For a computer to work, it must contain memory where it can store data and programs until they are needed. Memory, basically, falls into two categories: read-only memory (ROM) and random-access memory (RAM). Both are very fast because data is read to and from the chips electronically; they have no moving parts. ### Mass Storage Devices The most common hardware devices used for computer storage technology are: - **Magnetic tape storage** - **Magnetic disk storage** - **Magnetic drum storage** #### **Magnetic tape storage** Magnetic tape storage is a good example of sequential access storage technology. Data are stored on magnetic tape, similar to that used in audio systems. The major advantages of magnetic tape are that it is relatively cheap when compared to other types of storage medium and that it can easily hold a large amount of data for its size. Magnetic tape, unlike punched paper tapes or cards, can be used again by simply overwriting previously stored data. Since data are stored sequentially access time is relatively slow. However, the low cost per bit and high capacity of magnetic tape make it ideal for system backup. It is most suitable for applications, which may be required in payroll, personnel management, inventory control and customer invoicing where a large amount of data is to be processed sequentially. #### Magnetic disk storage Magnetic disk storage is also a random access storage device. The storage medium is a magnetically coated disk. There are several types and sizes of disks each best suited to a particular set of applications. e.g.: Floppy disk, hard disk, Magnetic drum storage, Compact disk (CD) ## 1.7. DESIGN WORK-STATION A work-station is a computer with a terminal with its own computing power to support major software packages, multi tasking capabilities for complex tasks and net working potential with other computing environment. Generally, a work-station will have a high-end graphics terminal. The processors are usually 32 or 64 bit processors and the processors are mostly RISC - (Reduced instruction set computing) based processors. Eg.: SUN, DEC, HP work-stations The features of a design work-station: - It is the interface between soft ware, user and the system. It should perform the following functions. - It must interface with C.P.U. - It must generate Graphics image for the user. - It must translate I/O commands into operating functions. - It must provide the digital description of graphical-image. - It must facilitate the communication between user and the system. - To perform all these functions following hard ware components are required. - Graphics Terminal (Display devices) - Operator input devices. ### 1.7.1. Graphics Terminal It is the most prominent component in a CAD system. Its purpose is to display the image of a design on the screen, so that it can be examined graphically manipulated according to operator's commands to make modifications. Typically, the primary output device in a graphics system is a video monitor. The operation of most video monitor is based on the standard Cathode Ray Tube (CRT) for generating the image on the screen. In CRT heated cathode emits continuous and high - speed steam of electrons. These are formed into a beam of electrons by a control - grid surrounding the cathode high-speed electron beam is focused by a focusing system on to a phosphor coated glass screen. These electrons energize the phosphorous-coating and cause it to glow at the points where beam makes contact. By controlling the point of contact of beam against phosphor coating desired image can be generated by two methods such as Vector display and Raster scan display. #### 1.7.1.1. Vector - Display It uses stroke-writing technique for generating the image on the screen. In stroke-writing technique image is constructed by sequence of straight line segments. Each line segment is drawn on the screen to next point where each point is defined by its x & y co-ordinate curves, coning-sections and free from curves (splines) are constructed by series of infite short lines. Phosphor elements on the screen glow for a very short-time. Major -"" problem in this type of device is maintaining the image for required amount-of time. Problem of retaining the image on the screen can be over-come by using either direct-beam refresh tube (or) Direct-.view storage tube. - Vector display - Direct beam refresh tube - Direct view storage tube #### 1.7.1.2. RASTER-SCAN Display In a raster- scan system, the electron beam is swept across the screen, one row at a time from top to bottom. As the electron beam moves across each row, the beam intensity is turned on and off to create a pattern of illuminated spots. Picture definition is stored in memory area called the refresh buffer or frame buffer. This memory area holds the set of intensity values for all the screen points. Stored intensity values are then retrieved from the refresh buffer and "painted" on the screen one row (scan line) at a time (shown in fig.). #### 1.7.1.3. Image maintenance methods A beam of electrons (cathode rays), emitted by an electron gun, passes through focusing and deflection systems that direct the beam towards specified position on the phosphor-coated screen. The phosphor then emits a small spot of light at each position contacted by the electron beam. Because the light emitted by the phosphor fades very rapidly, some method is needed for maintaining the screen picture. One way to keep the phosphor glowing is to redraw the picture repeatedly by quickly directing the electron beam back over the same points. This type of display is called a refresh CRT. Another method is direct view storage. - Direct-beam Refresh tube :- In this method image is retained on the screen by causing the direct-beam to replace the image repeatedly. - Direct-beam replaces the image many times per second to avoid noticeable flicker of the image - and densely filled screen. It is difficult to avoid the flickering of the image by this process. - Besides, this disadvantage there. are several advantages associated with directed beam refresh system. - Since, the image is regenerated continuously selective eraser and modification of the image can be readily accomplished. - It is also possible to provide animations of the image with refresh -tube - Direct - view storage tube: In Direct view storage tube image is retained on the screen, thus avoiding the need to re-write gun, directed at phosphor screen, which keeps the phosphor elements illuminated, once they have been energized by stroke writing electron beam. Resulting image is flicker-free. Since, it is flicker-free any amount of text (or) graphics can be added to image. Disadvantages are selective erasure, modifications and animations are not possible. - Raster - Scan Display Device: In this approach viewing screen is divided into large number of distinct-phosphor picture elements. Each phosphor - picture element is called as a pixel. Matrix of these pixels is called as Raster. In this approach image is created by sweeping the electron-beam along the horizontal line on the screen from left to right and energizing the pixels in that line during the sweep. When the sweep of one line is completed, electron beam moves to next-line below it and proceeds in a fixed pattern. - It sweeps the entire screen, at the rate of 30 to 60 times per second. - Number of separate pixels in Raster-Display might range from 256 x 256 to 1024 x 1024. - The picture quality can be increased by increasing the pixel density (or) Resolution. Selective erasure, modifications and animations are possible in this method. - L.C.D.: L.C.D.s has become common and popular in watches and toys. (L.C.D: Liquid crystal Display). The liquid crystal material is amorphous until subjected to an electric field, at which point it behaves like a crystal, reflecting light in one direction depending on polarization. It is a passive display, requiring an external lighting source. They have extremely low power consumption and are inexpensive. - Plasma - Display: In a Plasma display thin (25 mm) glasses envelop is filled with insert gas such as neon. There are two sets of Electrodes at right - angle to each other that don't touch at the point where they appear to cross, being separated by a critical distance. Plasma displays are flat, can be. Transparent, possess high light out-put, low power consumption and are relatively expensive. ## 1.8. I/O DEVICES The digital computer is a major and essential ingredient of CAD/CAM system. The digital computer is an electronic computing machine that can perform mathematical and logical computations and data processing functions according to a predetermined series of instructions known as a program. The essential components making up a computer system are referred to as hardware, whereas the various programs are commonly called the software. - **Input devices:-** - Cursor control devices - Digitizers - Key-Board Terminals. ### Cursor control devices The movement of the cursor can be controlled by many ways some of them are: - Mouse - Mechanical, Optical - Thumb wheels: Potentiometric devices for X and Y locations. - Joy - stick - Tracker-ball - Direction keys on Key-Board - Light - Pen (Detects light on the screen) - Electronic - Tablet and pen. #### 1.8.1. Mouse A mouse is small hand-held box used to position the screen cursor. Wheels or rollers on the bottom of the mouse can be used to record the amount and direction of movement. Another method for detecting mouse motion is with an optical sensor, for these systems, the mouse is moved over a special mouse pad that has a grid of horizontal and vertical lines. The optical sensor detects movement across the lines in the grid. Since a mouse can be picked up and put down at another position without change in cursor movement, it is used for making relative changes in the position of the screen cursor. One, two, or three buttons are usually included on the top of the mouse for signaling the execution of some operation, such as recording cursor position or invoking a function. #### 1.8.2. Joysticks A joystick consists of a small, vertical lever (called the stick) mounted on a base that is used to steer the screen cursor around. Most joysticks select screen positions with actual stick movement; others respond to pressure on the stick. Figure below shows a movable joystick. Some joysticks are mounted on a keyboard; others function as stand-alone units. ### 1.8.3. Digitizers A common device for drawing, painting, or interactively selecting coordinate positions on an object is a digitizer. These devices can be used to input coordinate values in either a two-dimensional or a three-dimensional space. Typically, a digitizer is used to scan over a drawing or object and to input a set of discrete coordinate positions, which can be joined with straight-line segments to approximate the curve or surface shapes. One type of digitizer is the graphics tablet (also referred to as a data tablet), which is used to input two-dimensional coordinates by activating a hand cursor or stylus at selected positions on a flat surface. A hand cursor contains cross hairs for sighting positions, while a stylus is a pencil-shaped device that is pointed at positions on the tablet. Tablet size varies from 12 by 12 inches for desktop models to 44 by 60 inches or larger for floor models. Graphics tablets provide a highly accurate method for selecting coordinate positions, with an accuracy that varies from about 0.2 mm on desktop models to about 0.05 mm or less on larger models. ### 1.8.4. Keyboard input devices These input devices employ a typewriter like keyboard and input the data and programs directly to the computer. ### 1.9. Output Devices: - Printers, Plotters, Computer out-put micro-films ## Printer A printer is an output device that converts data from computer and prints it in a readable form on paper. There are a number of different categories of printers used to produce printed output, such as line printers, serial printers and laser printers. - **Dot-matrix printer:** Dot-matrix printers use a print head containing pins or wires, arranged in a column to print characters. As the print head passes across the paper, the computer tells it which pins in the print head are to be fired to form a particular character. As the pins are fired they strike an inked ribbon against the paper. The printed dots ate arranged in an invisible matrix. The number of pins or wires and dots determine the characters resolution. The latest printers have 18 or 24 wires in their print heads. Dot-matrix printers have been killed off by newer technologies. - **Inkjet printer:** Colour inkjet printers use three separate inkjets, one for each of the primary colours (Red, Green and Blue). Inkjet printers are very quiet and provide laser-like quality at a much lower cost although supplies are relatively expensive. Although you can print on regular paper, better results are obtained when using special paper that doesn't allow the ink to soak in. Liquid inkjet printers use ink cartridges from which they spray ink into the paper. A cartridge of ink attached to a print head with 50 or so nozzles, each thinner than a human hair, moves across the paper. The number of nozzles determines the printer's resolution. A digital signal from the computer tells each nozzle when to propel a drop of ink onto the paper. On some printers, this is done with mechanical vibrations, - **Line printer:** A line printer prints a whole line at a time and operates on the same basic principle as a typewriter. The speed of line printers varies from 60 to 5000 lines per minute and there are normally 80 to 132 characters per line; so it is very suitable for high volume output, to ensure sufficient and uninterrupted supply of paper for these high printing speeds, continuous stationery is used. - **Laser printer:** A laser printer is capable of producing very high quality print very quickly and a wide selection of character fonts is also available. Most laser printers operate at speeds between 30 and 250 pages per minute. There are many other types of input/output devices which are used specifically for some applications. ## Plotters Plotters are also output devices and produce final drawings and documentation on paper. They may be classified as (i) Flat Bed Plotters (ii) Drum Plotters and (iii) Pen Plotters. These plotters are available in different sizes ranging in AO, AI, A2, A3 and A4. - **Plotters** - Flat Bed (paper is stationery and the pen holding mechanism moves in two axes. - Drum type (Paper attached to drum rotates back and forth while the pen mechanism moves in the. transverse direction. Pen plotters are vector plotters. They employ multiple pens. - **Electrostatic Plotters:** Dot matrix/raster plotters usually monochrome. Vector famed image is converted to raster form. - **Ink Jet Plotters:** Utilizes the dot matrix method of plotting and produce raster images. These plotters impel tiny droplets of jet of ink on the surface of paper, to produce the required image Typical applications: Colour plots of solid models, shaded images and contoured plots. - **Magnetic ink character recognition (MICR):** In MICR readers, the recording of data is based on specially designed fonts in which the characters are printed with ink containirig ferromagnetic materials. Magnetic ink characters are used primarily on bank cheques to indicate the code number of the bank, the customer account number and the cheque sequence number. These devices are capable of reading up to 1600 documents per minute. - **Optical character recognition (OCR):** In the operation of the OCR reader, a mechanical drum is used to rotate documents past an optical scanning station. A light source and lens system can distinguish the patterns of the character. These patterns are converted into electrical pulses, which are interpreted as each individual character. An OCR can be programmed to read a variety of character sets, and scan a document for marks made by hand or printed by line printers. ## 1.10. CAD SYSTEM CONFIGURATION Apart from the central processing unit, a computer system consists of other devices such as backing storage devices and input/output devices which are collectively called peripheral devices. A peripheral unit is defined as any hardware device that is connected to the CPU and performs a specific task such as input/output for the system. There are normally many peripherals in a computer system and the way in which they are arranged and what they are determine the configuration of the system. There are in general a number of established types of system configuration, which are as follows: - Simple configuration - Network configuration ### 1.10.1. Simple configuration A simple configuration of computer system is shown in Fig. 2.2. The system comprises essentially one CPU, various backing storage devices and several input/output devices. Such a configuration is rather small and its processing power is undoubtedly limited. When the number of users increases and their demands for system resources grow, this kind of system configuration cannot really cope and there is very little room to expand by way of introducing additional hardware, Because of the lack of flexibility of this type of configuration, a new concept, known as network configuration, has been developed and has now become widely used in many computer applications. ### 1.10.2. Network configuration A computer network system is formed by linking distinct computer installations together, through a communication system. The purpose of setting up a network is to share computing power and to improve the flow and to allow the exchange of information within organizations and beyond. Network systems may be considered as a step towards establishing a means by which people can communicate with each other through a computer. A local area network (LAN) allows organizations operating on one site to link their computers, user terminals and workstations and often peripheral devices in an efficient and cost effective way. A wide area network (WAN) links various sites, computer installations and user terminals and may even permit LANs to communicate with each other. There are several types of network configurations. The main classes are: - Centralized network - Distributed network - Ring network A Centralized network is shown below, which is useful for organizations that require a centralized database or centralized processing facility. ## 1.11. Mechanical Engineering Applications of CAD Following is a brief description of the applications of CAD in mechanical engineering - **Two Dimensional Drafting:** This is the most common use of a CAD package. 2-D drawings are used for manufacturing a product. - **Report Generating:** To generate reports and bill of materials. Spreadsheets and word-processors can be linked to provide a report writing facility. - **3-D Modelling:** To create the wireframe, surface and solid models. The 3-D models are for concept verification, manufacturing, FEA, etc. - **Finite Element Analysis:** FEA package is used for pre-processing, analysis, and post-analysis of structures. For this application, a CAD package contains both the modeling and analysis modules. - **Manufacturing:** manufacturing software is usually called CAM, and contains CAD software as one of the components. CAM software provides capabilities of carrying out 2 and 3-axes machining. ## 1.12. OPERATING SYSTEMS: Operating system is software that automatically manages and controls the computers, resources and makes the computer to function in required way. It controls and coordinates the peripheral devices. The O.S. schedules the operations of a computer. It also monitors the use of machine resources. An O.S. deals with the faults and errors and informs the user. An operating system may generally be defined as a program, which organizes a collection of hardware into a consistent and coherent entity so that it can be used for developing and running programs. It can assist users and their programs to