DESIGN AND DRAFTING

SUSTAINABLE CAD

CAD offers tools that significantly improve the ability to apply sustainable design practices. Software is available that assists all elements of sustainable design, from manufacturing material selection and usage to product life cycle assessment. A powerful example of sustainable design with CAD is developing a digital prototype of a product as a 3-D solid model. Digital prototyping was described in the Prototyping section earlier in this chapter. Digital prototyping can support sustainable design by leading to lower costs, reduced material consumption, and optimized use of energy. CAD allows the design process to occur in significantly less time, using fewer engineers and technicians and reducing physical prototypes, which are expensive and time-consuming to create and test. The following information describes how Utility Scale Solar, Inc. uses CAD technology to optimize the cost and material used in solar energy production.

Manufactures solar tracking equipment for large-scale solar power plants (see Figure). Solar tracking equipment, such as the USS Megahelion™ MH144 heliostat, accurately follows the sun as it moves across the sky to position solar reflecting surfaces, or solar panel arrays, for the best collection of solar energy. Solar collection units are very large, about three stories tall, and each solar power plant includes thousands of units. Therefore, reducing the weight and increasing the efficiency of solar tracking equipment can provide significant material and energy savings.

The patent-pending Megahelion drive and heliostat products are resistant to wind, dust, dirt, weight, and weather, which are common issues affecting the performance of solar tracking machinery. The Megahelion uses fewer moving parts, stronger components, and a system that distributes forces over a larger surface area than conventional drives, resulting in a fluid motion with fewer breakdowns and much lower ownership and operating costs. Unlike traditional drives that use gears or conventional hydraulics, the Megahelion™ drive uses flexible hydraulic cells to position the drive shaft.

USS relies heavily on modern CAD technology for digital prototyping. USS uses Autodesk Inventor and Algor® software for design, dynamic simulation, and finite element analysis (FEA). USS also uses Autodesk Vault Manufacturing software to manage CAD data and Autodesk Showcase® software to prepare images and 3-D visualizations for sales and marketing. According to Jonathan Blitz, USS’s chief technical officer, “The software has significantly streamlined what we are doing and made it much easier to visualize and communicate our designs. The ability to then subject these designs to realistic forces and loads has given us the confidence to remove the mass and streamline the components without sacrificing structural integrity.”

An example of CAD optimization at USS is the redesign of an endcap for the Megahelion solar tracker. The figure shows the original endcap design’s 3-D solid model and FEA analysis. The original component weighs 650 pounds, is overdesigned, and uses a cylindrical drum with a flat endcap. The objective was to redesign the part to distribute loads more effectively, enabling a reduction in material use and mass.

The focus of the endcap redesign was changing to a hemispherical shape that would bear weight, and wind loads more efficiently and naturally than a flat end plate. The figure shows a digital prototype of an early, nonoptimized redesign. USS used Autodesk Inventor 3-D solid modelling and stress analysis tools to simulate and test design options, including varying the hemisphere’s depth, the shell’s thickness, and the number of reinforcing ribs. Autodesk Inventor parametric optimization capabilities allowed USS engineers to optimize the design for reduced mass and automatically validate the design against project requirements.

After analysis, USS determined a more optimal design with a wall thickness of .5 in., an endcap depth of 6 in., and six ribs (see Figure). The simulation results show that stress and safety factors are within the specifications set by the design team. Compared to the original endcap design in Figure, the redesigned endcap uses less material in low-stress areas, shows less dramatic stress concentrations, and distributes the load more evenly and efficiently. The mass of the new design is 481 pounds, making it 26% lighter than the original part. USS now has an accurate concept of a product that should perform better, require less material and energy to produce and handle, and cost less to manufacture and transport.

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PRODUCTIVITY WITH CADD

CADD software continues to improve in various ways. CADD programs are easier to use than ever before and contain multiple tools and options, allowing you to produce better quality and more accurate drawings in less time. CADD multiplies productivity several times for many duties, especially for multiple and time-consuming tasks. A great advantage of CADD is that it increases the time available to designers and drafters for creativity by reducing the time they spend on the actual preparation of drawings.

As CADD applications improve, the traditional requirements of a drafter often become less important, while the ability to use new CADD software and application-specific tools increases. Productivity gains realized by the use of CADD tools are directly related to the proper use of those tools. In the constantly changing CADD world, you must be prepared to learn new drafting tools and techniques and be open to attending classes, seminars, and workshops on a regular basis.

Design Planning

Some of the most important and productive time you can spend working on any project or drawing is the time you use to plan. Always plan your work carefully before you begin to use the tools required to create the drawing. A design plan involves thinking about the entire process or project in which you are involved, and it determines how you approach a project.

A design plan focuses on the content you want to present, the objects and symbols you intend to create, and the appropriate use of standards. You may want processes to happen immediately or to be automatic, but if you hurry and do little or no planning, then you may become frustrated and waste time while designing and drafting. Take as much time as needed to develop design and project goals to proceed confidently.

Consider creating a planning sheet during your early stages of CADD training, especially for your first few assignments. A planning sheet should document all aspects of a design and the drawing session. A sketch of the design is also a valuable element of the planning process. A design plan and sketch help you establish the following:

• The drawing drafting layout: area, number of views, and required free space.
• Drafting settings: units, drawing aids, layers, and styles.
• How and when to perform specific tasks.
• What objects and symbols to create?
• The best use of CADD and equipment.
• An even workload.

Ergonomics

Ergonomics is the science of adapting the work environment to suit the needs of the worker. There is concern about the effects of the CADD working environment on the individual worker. Some studies have found that people should not work at a computer workstation for longer than about four hours without a break. Physical problems, ranging from injury to eyestrain, can develop when someone is working at a poorly designed CADD workstation. The most common injuries are repetitive motion disorders, also known as repetitive strain injury (RSI), repetitive movement injury (RMI), cumulative trauma disorder (CTD), and occupational overuse syndrome (OOS). Carpal tunnel syndrome is a common repetitive motion disorder. Most computer-related injuries result from the sedentary nature of working at a computer and the fast, repetitive hand and finger motions typical while using keyboards and pointing devices. Proper workstation ergonomics, good posture, and frequent exercise help to prevent most computer-related injuries.

Ergonomic Workstations

The figure shows an ergonomically designed workstation. In general, a workstation should be designed so you sit with your feet flat on the floor, your calves perpendicular to the floor, and your thighs parallel to the floor. Your back should be straight, your forearms should be parallel to the floor, and your wrists should be straight. For some people, the keyboard should be either adjustable or separate from the computer to provide more flexibility. The keyboard should be positioned, and arm or wrist supports can be used to reduce elbow and wrist tension. In addition, when the keys are depressed, a slight sound should be heard to ensure the key has made contact. Ergonomically designed keyboards are available.

The monitor should be 18″-28″, or approximately one arm’s length, away from your head. The screen should be adjusted to 158-308 below your horizontal line of sight. Eyestrain and headache can be a problem with extended use. If the position of the monitor is adjustable, you can tilt or turn the screen to reduce glare from overhead or adjacent lighting. Some users have found that a small amount of background light is helpful. Monitor manufacturers offer large, flat, nonglare screens that help reduce eyestrain. Some CADD users have suggested changing screen background and text colors weekly to give variety and reduce eyestrain. The chair should be designed for easy adjustments to give you optimum comfort. It should be comfortably padded. Your back should be straight or up to 108 back, your feet should be flat on the floor, and your elbow-to-hand movement should be horizontal when you are using the keyboard, mouse, or digitizer. The mouse or digitizer puck should be close to the monitor so movement is not strained and equipment use is flexible. You should not have to move a great deal to look directly over the cursor to activate commands.

Positive work habits

In addition to an ergonomically designed workstation, your own personal work habits can contribute to a healthy environment. Try to concentrate on good posture until it becomes second nature. Keeping your feet flat on the floor helps improve posture. Try to keep your stress level low because increased stress can contribute to tension, which can aggravate physical problems. Take breaks periodically to help reduce muscle fatigue and tension. You should consult with your doctor for further advice and recommendations.

Exercise

If you feel pain and discomfort associated with computer use, stretching exercises can help. The figure shows exercises that can help reduce workstation-related problems. Some people have also had success with yoga, biofeedback, and massage. Consult with your doctor for advice and recommendations before starting an exercise program.

Other Factors

A plotter makes some noise and is best located in a separate room next to the workstation. Some companies put plotters in a central room, with small office workstations around them. Others prefer to have plotters near the individual workstations, which acoustical partition walls or partial walls can surround. Air-conditioning and ventilation systems should be designed to accommodate the computers and equipment. Carpets should be antistatic. Noise should be kept to a minimum.

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What are the basics of solid modelling?

Each solid modeling software offers unique methods for creating and working with solid models. Simple solid modelling programs allow you to build models using solid primitives, which are objects such as boxes, cones, spheres, and cylinders that you combine, subtract, and edit to produce a final model. The process of adding and subtracting primitive shapes is known as a Boolean operation in geometry (see Figure). Boolean operations also apply to more complex solid models defined by features and surfaces.

In contrast to modelling with solid primitives, feature-based solid modelling programs allow you to construct solid models using more intuitive feature tools. A feature often begins with a 2-D sketch, followed by a sketched feature such as extrusion or revolution created from the sketch. Additional features include adding or subtracting solid material to generate a final model. Many feature-based solid modelling programs are highly sophisticated and include many advanced tools and functions that significantly automate the design and documentation process. Parametric solid models are the most common models created using feature-based solid modelling software. Parametric refers to the method of using parameters and constraints to drive object size and location to produce designs with features that adapt to changes made to other features. Some solid modelling programs generate nonparametric solids, known as basic solids or dumb solids.

Feature-based solid modelling programs often maintain a history of the modelling process, which typically appears in a feature tree or history tree (see Figure). History-based solid modelling is most often associated with parametric solid modelling. The software stores and manages all model data, including calculations, sketches, features, dimensions, geometric parameters, the sequence in which each piece of the model was created, and all other model history and properties.

Parametric Solid Modeling

One of the most common 3-D solid modelling techniques is feature-based, parametric solid modelling. Autodesk Inventor, Pro/Engineer, NX, and SolidWorks are examples of feature-based, parametric solid modeling. Many parametric solid modeling programs are surprisingly similar in the way they function. In fact, once you learn the basic process of creating a model using specific software, you can usually transition to different parametric solid modeling software.

Parametric design tools allow you to assign parameters or constraints to objects. Parameters are geometric characteristics and dimensions that control model geometry’s size, shape, and position. A database stores and allows you to manage all parameters. Parametric design is also possible with some 2-D CADD programs. The parametric concept, also known as intelligence, provides a way to associate objects and limit design changes. You cannot change a constraint so that it conflicts with other parametric geometry. Parameters aid the design and revision process, place limits on geometry to preserve design intent, maintain relationships between objects, and help form geometric constructions.

Parameters are added by using geometric constraints and dimensional constraints. Geometric constraints, also known as relations, are characteristics applied to restrict the size or location of geometry. Dimensional constraints are measurements that numerically control the size or location of geometry. Well-defined constraints allow you to incorporate and preserve specific design intentions and increase revision efficiency. For example, if the two holes through the bracket shown in the Figure must always be the same size, then geometric constraints must be used to make the holes equal, and dimensional constraints must be used to size one of the holes. The size of both holes changes when you modify the dimensional constraint values.

• Model Work Environments

  
  

Parametric solid modelling software often includes several work environments and unique file types for different applications. A part file allows you to create a part model, such as the engine block shown in Figure. A part is an item, product, or element of an assembly. Some systems include separate files or work environments for specialized part modelling and related applications, such as sheet metal part design, surface modelling, analysis and simulation, and rendering.

An assembly file allows you to reference component files to build an assembly model. Components are the parts and subassemblies used to create an assembly. A subassembly is an assembly that is added to another assembly. The figure shows an engine subassembly that references the engine block part shown in Figure.

• Part Model Elements

  
  

Part models allow you to design parts, build assembly models, and prepare part drawings. A part model begins as a sketch or group of sketches used to construct a feature. Add features as necessary to create the final part model. Primary part model features include sketched, placed, work, catalogue, and patterned features. Develop additional model elements, such as surfaces, as needed to build a part model.

Every part model usually contains at least one sketch and at least one sketched feature. A sketch is a 2-D or 3-D geometry that provides the profile or guide for developing sketched features (see Figure). A parametric sketch includes geometric constraints that define common geometric constructions such as two perpendicular lines, concentric circles, equal-sized objects, or a line tangent to a circle. Dimensional constraints specify the size and location of sketch objects. Examples of sketched features built from a sketch include extrusions, revolutions, sweeps, and lofts. Normally, the initial feature on which all other features are built, known as the base feature, is a sketched feature, such as the extrusion shown in Figure.

Adding placed features requires specifying size dimensions and characteristics and selecting a location, such as a point or an edge. No sketch is necessary. You typically use a dialogue box or other on-screen tool to describe size data. The figure shows two of the most commonly placed features: chamfers and fillets. Placed features are also known as built-in, added, or automated features. Shells, threads, and face drafts are other examples of placed features.

• Assembly Modeling

  
  

One option for developing an assembly is to insert existing components into an assembly file and then assemble the components with constraints or mates. This is an example of a process that some designers refer to as bottom-up design, and it is appropriate if all or most components already exist. Depending on your approach and the complexity of the assembly, you can insert all components before applying constraints, as shown in Figure. A common alternative is to insert and constrain one or two components at a time.

Another option is to create new components within an assembly file or in place. This is an example of a process that some designers refer to as top-down design. Both assembly techniques are effective, and a combination of methods is common. However, for some applications, developing components in place is faster, easier, and more productive. Developing components in an assembly file usually creates an assembly and a separate part or assembly file for each component.

Once you insert or create assembly components, the typical next step is to add assembly constraints, also known as mates. Assembly constraints establish geometric relationships and positions between components, define the desired movement between components, and identify relationships between the transitioning path of a fixed component and a component moving along the path. There are multiple types of assembly constraints, such as a mate or similar constraint that mates two or a combination of component faces, planes, axes, edges, or points. Component geometry and design requirements determine the required constraints.

• Editing Parametric Solid Models

  
  

The parametric nature of parametric solid modelling software allows you to edit model parameters anytime during the design process. You can manipulate parameters assigned to sketch and feature geometry, parts, and assemblies to explore alternative design options or to adjust a model according to new or different information. The model stores all of the data used to build the model. Often, modifying a single parameter is all that is required to revise a model. Other times, a completely different product design is built by editing several existing model parameters. The example in Figure shows how changing a few model parameters can significantly alter a product design. In most cases, the tools and options used to edit models are similar or identical to the tools used to create the model originally.

Parametric geometry allows you to make any necessary changes to the design of a model, allowing you to assess design alternatives almost immediately by changing, adding, or deleting sketches, features, dimensions, and geometric controls. Parameter-driven assemblies allow changes made to individual parts to reproduce automatically as changes in the assembly and assembly drawing. Adaptive parts in assemblies are effective when you may not know the exact dimensions of a part or you may not fully understand the relationship between assembly components. Adaptive parts modify automatically if another part changes. Paramedic geometry also allows you to develop equations that drive your models, allowing a few dimensions to define the entire model or even create a family of related parts.

• Extracting Drawing Content

  
  

Some parametric solid modelling CADD systems, such as Autodesk Inventor, Pro/Engineer, NX, and SolidWorks, combine 3-D solid modelling with 2-D drawing capabilities. You can create any type of part, assembly, or weldment drawing from existing models. The figure shows an example of a part drawing extracted from a part model. When you edit a model, the corresponding drawing adjusts to reflect the new design. You can also edit a model by modifying parametric model dimensions inside a drawing.

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Surface modeling technique

Curves and surfaces are the principal elements of surface models created using NURBS technology. Curves provide the basic form and geometry necessary for constructing surfaces. A NURB curve is a complex mathematical spline representation that includes control points. A spline is a curve that uses a series of control points and other mathematical principles to define the location and form of the curve. The term spline comes from the flexible spline devices used by shipbuilders and drafters to draw smooth shapes. Spline control points typically lie some distance from the curve and are used to define the curve shape and change the curve design (see Figure). Typically, adding control points to a spline increases the complexity of the curve. Surface modeling uses splines because of their ease and accuracy of construction and evaluation and their ability to approximate complex shapes through curve design.

A surface model usually includes multiple NURB surfaces known as patches. Patches fit together using different levels of mathematical formulas to create what is known as geometric continuity. In the simplest terms, geometric continuity is the combining of features into a smooth curve. In actual practice, geometric continuity is a complex mathematical analysis of the shapes used to create a smooth curve. The figure shows NURBS used to design a surface. NURB geometry offers the ability to represent any necessary shape, from lines to planes to complex free-form surfaces. Examples of applications for freeform shapes include automobile and ship design and ergonomic consumer products. The figure shows a surface model of a sailboat with standard and freeform geometric shapes.

Creating Surfaces

Common methods for creating surfaces include direct and procedural modeling and surface editing. Different surface modeling techniques can often be applied to achieve the same objective. Usually, a combination of methods is required to develop a complete surface model. Surface modeling, like other forms of CADD, requires detailed knowledge of software tools and processes and knowing when each tool and process is best suited for a specific task.

Surface modeling typically involves creating a series of curves that form the spans for defining a surface. Transition surfaces are added as needed to fill gaps within the model. Direct surface modelling is a basic method for developing existing surfaces created using multiple curves by adjusting the position of the surface control points or poles (see Figure). Another approach to surface construction is the use of procedural modeling tools to create surfaces from curves. Common options include extruding a curve, sweeping a profile curve along a path curve, lofting through multiple curves, and using curves to define a boundary (see Figure).

Most surface modeling software provides tools that allow you to construct additional surfaces from initial surface geometry. Examples of these techniques are offsetting, extending, and blending surfaces. Tools are also available for trimming intersecting surfaces. The figure shows basic examples of constructing additional surfaces from initial surface shapes. Many other advanced surface modeling tools are also available. Some surface modelling packages provide the ability to manage the structure of surface objects and maintain a modelling history. For high-quality surfaces, analytical tools such as comb curves and zebra analysis are used to check the continuity of curvature so that reflections and highlights can be managed for the best aesthetic quality.

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What are Cad Plotting and File Templates?

A drawing created in CADD initially exists as a soft copy. A soft copy is the electronic data file of a drawing or model. A hard copy is a physical drawing created on paper or some other media by a printer or plotter. Hard copies are often a required element of the engineering or construction process, and they are more useful on the shop floor or at a construction site than soft copies. A design team can check and redline a hard copy without a computer or CADD software. CADD is the standard throughout the world for generating drawings, and electronic data exchange is becoming increasingly popular. However, hard-copy drawings are still a vital tool for communicating a design.

Each CADD system uses a specific method to the plot, though the theory used in each is similar. In general, you must specify the following settings in order to plot:

• The plot device is the printer, plotter, or alternative plotting system to which the drawing is sent.
• Plot device properties.
• Sheet size and orientation.
• Area to the plot.
• Scale.
• The number of copies.
• Software and drawing-specific settings.

Drafting Equipment, Media, and Reproduction Methods is an important consideration when plotting. Some CADD applications such as Autodesk Inventor, Pro/Engineer, NX, and SolidWorks highly automate plotting, especially the process of scaling a drawing for plotting. Other systems, such as AutoCAD and Micro Station, require you to follow specific procedures to scale a plot correctly. AutoCAD, for example, uses a 3D Model space and paper space system. Model space is the environment in which you create full-scale drawings and designs. Paper space, or layout, is the environment in which you prepare the final drawing for plotting to scale. The layout often includes items such as viewports to display content from model space, a border, sheet blocks, general notes, associated lists, sheet annotation, and sheet setup information.

• Scale Factor

The scale factor is the reciprocal of the drawing scale, and it is used in the proper scaling of various objects such as text, dimensions, and graphic patterns. Most modern CADD software automates the process of scaling a drawing, allowing you to focus on choosing a scale rather than calculating the scale factor. However, the scale factor is a general concept with which you should be familiar, and it remains an important consideration when working with some CADD applications.

When drawing with CADD, always draw objects at their actual size or full scale, regardless of the size of the objects. For example, if you draw a small machine part and the length of a line in the drawing is .05 in. (1.27 mm), draw the line .05 in. (1.27 mm) long. If you draw an aircraft with a wingspan of 200′ (60,960 mm), draw the wingspan 200′. These examples describe drawing objects that are too small or too large for layout and printing purposes. Scale the objects to fit properly on a sheet according to a specific drawing scale.

When you scale a drawing, you increase or decrease the displayed size of objects. AutoCAD, for example, uses model space and paper space for this process. Scaling a drawing greatly affects the display of items added to full-scale objects, such as annotations, because these items should be the same size on a plotted sheet, regardless of the displayed size or scale of the rest of the drawing.

Traditional manual scaling of annotations, graphic patterns, and other objects requires determining the scale factor of the drawing scale and then multiplying the scale factor by the plotted size of the objects. Text is a convenient object for describing the application of scale factors. For example, to plot text that is .12 in. (3 mm) high on a mechanical drawing plotted at a scale of 1:4, or quarter-scale, multiply the scale factor by .12 in. (3 mm). The scale factor is the reciprocal of the drawing scale. A 1:4 scale drawing has a scale factor of 4 (4 4 1 5 4). Multiply the scale factor of 4 by the text height of .12 in. (3 mm) to find the .48 in. (12 mm) scaled text height. The proper height of .12 in. (3 mm) text in an environment such as model space at a 1:4 scale is .48 in.(12 mm).

Another example is plotting 1/8 in. high text on a structural drawing plotted at a scale of 1/4″ 5 1′ 0″. A 1/4″ 5 1′ 0″ scale drawing has a scale factor of 48 (1′ 0″ 4 1/4″ 5 48). Multiply the scale factor of 48 by the text height of 1/8 in. to find the 6 in. scaled text height. The proper height of 1/8 in. text in an environment such as model space at a 1/4″ 5 1′ 0″ scale is 6 in.

• Electronic Plots

Exporting is the process of transferring electronic data from a database, such as a drawing file, to a different format used by another program. For some applications, exporting a drawing is an effective way to display and share a drawing. One way to export a drawing is to plot a layout to a different file type. Electronic plotting uses the same general process as hard-copy plotting, but the plot exists electronically instead of on an actual sheet of paper.

A common electronic plotting method is to plot to a portable document format (PDF) file. For example, send a PDF file of a layout to a manufacturer, vendor, contractor, agency, or plotting service. The recipient uses common Adobe software to view the plot electronically and plot the drawing to scale without having CADD software, thus avoiding inconsistencies that sometimes occur when sharing CADD files. Another example is plotting an AutoCAD file to a design web format (DWF or DWFx) file. The recipient of a DWF file uses a viewer such as Autodesk Design Review software to view and mark up the plot.

File Templates

A file template, or template, is a file that stores standard file settings and objects for use in a new file.

All settings and content of a template file are included in a new file. File templates save time and help produce some consistency in the drawing and design process. Templates help you create new designs by referencing a base file, such as a drawing file, that contains many of the standard items required for multiple projects. A drawing file template is often known as a drawing template. File templates function and are used differently depending on the CADD program.

Most CADD software provides tools and options for working with and managing templates. For example, AutoCAD makes use of drawing template (DWT) files that allow you to use the NEW command to begin a new drawing (DWG) file by referencing a saved DWT file. A new drawing file appears with all the template file settings and contents. Other CADD programs have different but similar applications. This automates the traditional method of opening a file template and then saving a copy of the file template using the name of the new file. You can usually specify or reference any existing appropriate file to use as a template, or you can begin a new drawing using a default template.

Template options and specifications vary depending on the file type, project, and design and drafting standards. For example, you might use a template for designing parts and another for assemblies, or a template for metric drawings and another for customary drawings. A template includes settings for specific applications that are preset each time you begin a new design. Templates may include the following, depending on the CADD format and software, and set according to the design requirement:

• Units settings.
• Drawing and design settings and aids.
• Layers with specific line standards.
• Colour, material, and lighting standards.
• Text, dimension, table, and other specialized annotation standards.
• Common symbols.
• Display settings.
• Sheets and sheet items, such as borders, title blocks, and general notes.
• Plot settings.

Cad Plotting and File Templates

Create and maintain a variety of file templates with settings for different drawing and design disciplines and project requirements. Store your templates in a common location that is easily accessible, such as the local hard disk or the network server. Keep backup copies of templates in a secure location. You may discover additional items that should be included in your templates as you work. Update templates as needed and according to modified settings or new standard practices. For example, use the OPEN command to open a DWT file when using AutoCAD. Then, add content to the file as needed. Once you resave the file, the modified template is ready to use. Remember to replace all old template copies with updated versions.

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How do you reuse elements in a 2D CAD design?

One of the most productive features of CADD is the ability to reuse drawing content, that is, all of the objects, settings, and other elements that make up a drawing. Drawing contents such as objects and object properties, text and dimension settings, drafting symbols, sheets, and typical drawing details are often duplicated in many different drawings. The most basic method of reusing content is to apply commands such as MOVE, COPY, and ROTATE; these allow you to modify or reuse existing objects instead of re-creating or developing new objects.

File templates, described later in this chapter, are another way to reuse drawing content. Customized templates provide an effective way to start each new file using standard settings. Another method to reuse drawing content is to seek out data from existing files. This is a common requirement when developing related drawings for a specific project or working on similar projects. Sharing drawing content is also common when revising drawings and when duplicating standards used by a consultant, vendor, or client. Most CADD programs provide several other options that automate the process of sharing drawing content, including reusing pre-drawn symbols and entire drawings. Specific CADD applications throughout this textbook provide additional information about reusing content with CADD.

CADD Symbols

A major benefit of drawing with CADD is the ability to create and store symbols in a drawing for future use. Saved reusable symbols are known by names such as symbols, blocks, cells, and reference files, depending on the CADD software. You can insert symbols as often as needed and share symbols between drawings. You also often have the option to scale, rotate, and adjust symbols to meet specific drawing requirements. Using symbols saves time when creating drawings and increases productivity.

Draw the elements of a symbol as you would any other geometry. A symbol can usually consist of any object or group of objects, including annotation, or it can be an entire drawing. Review each drawing and project to identify items you can use more than once. Screws, punches, subassemblies, plumbing fixtures, and appliances are examples of items to consider converting to reusable symbols. The process of converting objects to symbols varies with each CADD software. The common requirements are to select the objects to define as the symbol, specify an insertion base point that determines where the symbol is positioned during insertion, and save the symbol using a unique descriptive name. The figure shows some common drafting symbols. Once you create a symbol, the symbol is ready to insert in the current file or be added to other files as needed. As you define symbols, prepare a symbols library in which each symbol is available for reference. A symbol library is a collection of symbols that can be used on any drawing.

File Referencing

CADD software usually provides additional ways to reuse content, such as a system for referencing existing files. For example, AutoCAD provides an external reference, or Xref, the system to automate and manage file referencing. File-referencing tools provide an effective way to use and relate existing base drawings, complex symbols, images, and details to other drawings. File referencing also helps multiple users share content. Whether you have the option to reference a variety of files depends on the CADD software. For example, AutoCAD allows you to reference a drawing (DWG), design web format (DWF and DWFx) raster image, digital negative (DNG), or portable document format (PDF) file. Referencing a file is similar to inserting an entire drawing or a portion of a drawing as a symbol. However, unlike a symbol, which is usually stored in the file in which you insert the symbol, reference file information does not add to the host drawing. File data appears on-screen for reference only. The result is useful information but with a much smaller host file size than if you inserted a symbol or copied and pasted objects. Another benefit of file referencing is the link between reference and host files. If you make any changes to reference files, update the host drawings to display the most recent reference content. This allows you or a design drafting team to work on a multi-file project with the assurance that any revisions to reference files display in host drawings.

CAD content and symbols

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What are basic cad drafting techniques?

The process of preparing a 2-D drawing varies, depending on the CADD software and preferred design techniques. Software centred on 2-D drafting typically requires you to construct geometry such as lines, circles, and arcs and add dimensions and text. In contrast, to prepare a 2-D drawing when using software or a design process that focuses on building a 3D model, you typically extract 2-D views from the 3-D model. Two-dimensional drawing concepts and theories are the same regardless of the technique used to produce the drawing.

Designing and drafting effectively with a computer requires a skilled CADD operator. To be a proficient CADD user, you must have detailed knowledge of software tools and processes and know when each tool and process is best suited for a specific task. Learn the format, appearance, and proper use of your software’s graphical user interface and customize the GUI according to common tasks and specific applications to increase proficiency. You must also understand and be able to apply design and drafting systems and conventions, plus develop effective methods for managing your work.

Drawing and Editing

CADD software includes commands for creating and modifying all elements of a drawing for any design requirement. Study the CADD drawing in as you explore drawing and editing with a computer. This drawing of a medical instrument part includes straight lines, circles, arcs, text, dimensions, and numerous symbols created accurately and efficiently using a variety of drawing and editing commands. The CADD applications throughout this textbook provide specific information about drawing and editing with CADD.

Line Standards and Layers

CADD programs often include a layer or similar system to organize and assign certain properties to objects. In CADD terminology, layers are elements of the drawing that allow you to separate objects into logical groups for formatting and display purposes. For example, a multi-view mechanical drawing can have layers for each unique line type, Including object lines, hidden lines, dimensions, and section lines (see Figure). You can display all layers to show the complete drawing or hide specific layers to focus on certain items. Lines and Lettering provide detailed information on types of lines. Some CADD systems automatically or semi-automatically set drawing elements on separate layers, and others require that you create your own layering system.

Layers allow you to conform to drawing standards and conventions and help create unique displays, views, and sheets. The following is a list of ways you can use layers to increase productivity and add value to a drawing:

• Assign each layer a unique color, line type, and line weight to correspond to line conventions and to help improve clarity.
• Make changes to layer properties that immediately update all objects drawn on the layer.
• Turn off or freeze selected layers to decrease the amount of information displayed on-screen or to speed screen regeneration.
• Plot each layer in a different color, line type, or line weight, or set a layer not to plot.
• Use separate layers to group-specific information. For example, draw a floor plan using floor plan layers, an electrical plan using electrical layers, and a plumbing plan using plumbing layers.
• Create several sheets from the same drawing file by controlling layer visibility to separate or combine drawing information. For example, use layers to display a floor plan and electrical plan together to send to an electrical contractor or display a floor plan and plumbing plan together to send to a plumbing contractor.

CADD programs often include a layer or similar system to organize and assign certain properties to objects. In CADD terminology, layers are elements of the drawing that allow you to separate objects into logical groups for formatting and display purposes.

For example, a multi-view mechanical drawing can have layers for each unique line type,

Layers Used in Industry

The drawing typically determines the function of each layer. You can create layers for any type of drawing. Draw each object on a layer specific to the object. In mechanical drafting, you usually assign a specific layer to each different type of line or object. The following is an example list of common layers and basic properties assigned to each layer for mechanical drafting applications.

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Layer Name	Line Type	Line Weight	Color
Object	Solid (continuous)	.02 in. (0.6 mm)	Black
Hidden	Hidden (dashed)	.01 in. (0.3 mm)	Black
Center	Center	.01 in. (0.3 mm)	Green
Dimension	Solid (continuous)	.01 in. (0.3 mm)	Red
Construction	Solid (continuous	.01 in. (0.3 mm)	Yellow
Border	Solid (continuous)	.02 in. (0.6 mm)	Black
Phantom	Phantom	.01 in. (0.3 mm)	Magenta
Section	Solid (continuous)	.01 in. (0.3 mm)	Brown

Architectural and civil drawings, for example, can require hundreds of layers, each used to draw a specific item. For example, create full-height floor plan walls on a black A-WALL FULL layer that uses a .02 in. (0.5 mm) solid (continuous) line type. Add plumbing fixtures to a floor plan on a blue P-FLORFIXT layer that uses a .014 in. (0.35 mm) solid (continuous) line type. Draw roadway centerlines on a site plan or map using a green C-ROAD-CNTR layer that uses a .014 in. (0.35 mm) centerline type.

Layer names are usually set according to specific industry or company standards. However, simple or generic drawings may use a more basic naming system. For example, the name Continuous-White indicates a layer assigned a continuous line type and white color. The name is Object-Red, and it identifies a layer for drawing object lines that are assigned the colour red. More complex layer names are appropriate for some applications, including drawing numbers, colour codes, and layer content. For example, the name Dwg101-3Dim refers to drawing DWG101, color 3, for use when adding dimensions. The CAD Layer Guidelines from the American Institute of Architects (AIA), associated with the NCS, specifies a layer naming system for architectural and related drawings. The system uses a highly detailed layer naming process.

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What is Virtual Reality?

A walk-through can be characterized as a camera in a computer program that creates a first-person view of walking through a building, around a product or building, or through a landscape. A fly-through is similar to a walk-through, but the first-person camera view is like a helicopter flying through the area.

Fly-through is generally not used to describe a tour through a building. Walk-through or fly-through is the effect of a computer-generated movie. The computer images represent the real architecture or the VR presentation in which the computer images turn or move as you turn your head in the desired direction. Realistic renderings, animations, and VR are excellent tools to show the client how the building will look inside and out or how a product will operate. Design ideas can be created and easily changed at this stage.

VR requires special interface devices that transmit the simulated world’s sights, sounds, and sensations. In return, these devices record speech and movement and transmit them back to the simulation software program. Virtual reality technology is a logical step in the design process.

A VR system provides the capability of interacting with a model of any size, from molecular to astronomical. Surgeons can learn from virtual patients and practice real operations on a virtual body constructed from scanned images of the human patient. Home designers can walk around inside a house, stretching, moving, and copying shapes to create a finished product.

Buildings can be designed and placed on virtual building sites. Clients can take walk-through tours of a building before it is built and make changes as they walk through. Scientists can conduct experiments on a molecular level by placing themselves inside a model of chemical compounds. Using telerobotics, a person can see through the eyes of a robot while in a safe virtual environment in order to guide a robot into a hazardous situation.

Passive VR

Through-the-window VR, also referred to as passive VR, is a common basic VR application. Passive VR is the manipulation of a 3-D model with input from a mouse, trackball, or 3-D motion control device. This allows more than one person to see and experience the 3-D world. A variation on this is a flat-panel display with handles for movement. The window VR unit in Figure is designed to allow natural interaction with the virtual environment. Museum and showroom visitors can walk up, grab the handles, and instantly begin interacting. Observers can follow the action by moving beside the primary user. A variety of handle-mounted buttons imitate keyboard keystrokes, joystick buttons, or 3-D motion control device buttons.

Another type of through-the-window VR consists of a special stereoscopic monitor and sensing devices. The viewer wears lightweight, passive, polarized eyewear. The monitor sends the images directly to the screen to generate 3-D images by users wearing the special glasses. This technology also allows several persons to view the same image on the screen.

Head Mounted Display (HMD)

To interact visually with the simulated world, you wear a head-mounted display (HMD), which directs computer images at each eye (see Figure). The HMD tracks your head movements, including the direction in which you are looking. Using this movement information, the HMD receives updated images from the computer system, which is continually recalculating the virtual world based on your head motions. The computer generates new views quickly, preventing the view from appearing halting and jerky and lagging behind your movements. The HMD can also deliver sounds to your earphones. The tracking feature of the HMD also can be used to update the audio signal to simulate surround sound effects. The three most important HMD attributes are

• field of view (FOV)
• resolution
• weight

• Field of View (FOV)

Field of View (FOV) The human visual field is approximately 2008 wide for both eyes, about 1508 for each eye, and 908 vertically. The portion of the visual field that is visible to both eyes is called the binocular overlap and is about 1008. The greater binocular overlap allows a stronger sense of depth. The necessary vertical field of view depends on the application. For example, driving simulators typically require only a narrow vertical field of view because the out-of-window view in most cars is limited in the vertical direction. In addition, scientific research into motion and balance often requires taller vertical fields of view so that test subjects can see below and above them.

• Resolution

Resolution Higher resolution throughout the visual field brings out fine detail in a scene (such as the ability to read text on a car’s dashboard), makes images look more realistic, and increases the amount of information that can be displayed. The characteristics of a computer monitor are often specified as a size measure (such as 21 in.) and as input resolution (such as 1920 3 1200 pixels). Input resolution is useful in determining compatibility with a particular image generator, and pixel density is at least as important in determining visual quality. A reasonable estimate of the visual sharpness for a person with 20/20 vision is 60  pixels/degree. This means that to match human visual quality, an HMD with a field of view of 408 3 308 (H 3 V) would need to present 2400 3 1800 pixels.

• Weight

A lightweight and balanced HMD helps users feel comfortable and allows for greater freedom of movement. Professional HMDs can be as light as 350 g (12 oz) or as heavy as 2 kg (4.5 lbs). A way to assist HMD weight is to install a boom mechanism that suspends the HMD from the top, although this typically restricts movement and makes the system more cumbersome. There is a dramatic range in the weights of offered HMDs.

Binocular Omni-Orientation Monitor (BOOM)

The Binocular Omni-Orientation Monitor (BOOM), developed by Fake Space, Inc., is a head-coupled stereoscopic display device (see Figure). The display is attached to a counterbalanced multilink arm system. The person can guide the counterbalanced display while looking into it like binoculars. The system is guided with tracking attached to the counterbalanced arms.

Cave Automatic Virtual Environment (CAVE)

The Cave Automatic Virtual Environment (CAVE) projects stereo images on the walls and floor of a room. CAVE was developed at the University of Illinois at Chicago to allow users to wear lightweight stereo glasses and to walk around freely inside the virtual environment. Several persons can participate within the CAVE environment as the tracking system follows the lead viewer’s position and movements, as shown in Figure.

Haptic Interface

The sense of touch is the most challenging physical sensation to simulate in a virtual world. A haptic interface is a device that relays the sense of touch and other physical sensations. In this environment, your hand and finger movements can be tracked, allowing you to reach into the virtual world and handle objects. Haptic interfaces of this type hold great potential for design engineers, allowing various team members to manipulate a product design in a virtual environment in a natural way. Although you can handle an object, it is difficult to generate sensations associated with the human touch, for example. These sensations are felt when a person touches a soft surface, picks up a heavy object or runs a finger across a bumpy surface. Very accurate and fast computer-controlled motors generate force by pushing against the user to simulate these sensations.

Haptic devices are synchronized with HMD sight and sound, and the motors must be small enough to be worn without interfering with natural movement. A simple haptic device is the desktop stylus shown in Figure. This device can apply a small force, through a mechanical linkage, to a stylus held in the user’s hand. When the stylus encounters a virtual object, the user is provided feedback that simulates the interaction. In addition, if the stylus is dragged across a textured surface, it responds with the proper vibration.

In the future, engineers may use VR to increase productivity in various areas, including virtual mock-up, assembly, and design reviews. These applications may include the realistic simulation of human factors, such as snap-fits, key component functions, and the experience of virtual forms. Virtual assemblies may include fit evaluation, maintenance path planning, manufacturability analysis, and assembly training.

Web-Enabled Virtual Reality Modeling Language (VRML)

An emerging area in the world of virtual reality is Web-enabled virtual reality modeling language (VRML). VRML is a formatting language that is used to publish virtual 3-D settings called worlds on the World Wide Web (www). Once the developer has placed the world on the Internet, the user can view it using a Web-browser plug-in. This plug-in contains controls that allow the user to move around in the virtual world as the user would like to experience it. Currently, VRML is a standard authoring language that provides authoring tools for the creation of 3-D worlds with integrated hyperlinks. The current version of VRML is viewed using a basic computer monitor and, therefore, is not fully immersive. However, the future of VRML should incorporate the use of HMDs and haptic devices, making for more truly immersive environments.

In the future, engineers may use VR to increase productivity in various areas, including virtual mock-up, assembly, and design reviews. These applications may include the realistic simulation of human factors, such as snap-fits, key component functions, and the experience of virtual forms. Virtual assemblies may include fit evaluation, maintenance path planning, manufacturability analysis, and assembly training.

VR Opportunities

A field of opportunity is available in the creation of virtual worlds. These worlds are detailed 3-D models of a wide variety of subjects. Virtual worlds need to be constructed for many different applications. Persons who can construct realistic 3-D models can be in great demand. The fields of VR geographic information systems (GIS) are combined to create intelligent worlds from which data can be obtained while occupying the virtual world. In the future, many cities will have virtual models on their Web sites.

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What is Computer-Aided Manufacturing (CAM)?

Computer-aided manufacturing (CAM) uses computers to assist in the creation or modification of manufacturing control data, plans, or operations and to operate machine tools. Computers are

integral to the manufacturing process. Computerized tools such as welding machines, machining centres, punch press machines, and laser-cutting machines are commonplace. Many firms are engaged in computer-aided design/computer-aided manufacturing (CAD/CAM). In a CAD/CAM system, a part is designed on the computer and transmitted directly to computer-driven machine tools that manufacture the part. Within the CAD/CAM process, there are other computerized steps along the way, including the following:

STEP 1 The CAD program is used to create the product geometry. The geometry can be in the form of 2-D Multiview drawings or 3-D models.

STEP 2 The drawing geometry is used in the CAM program to generate instructions for the CNC machine tools. This step is commonly referred to as CAD/CAM integration.

STEP 3 The CAM program uses a series of commands to instruct CNC machine tools by setting up tool paths. The tool path includes the selection of specific tools to accomplish the desired operation.

STEP 4 The CAM programmer establishes the desired tool and tool path. Running the postprocessor generates the final CNC program. A postprocessor is an integral piece of software that converts a generic CAM system tool path into usable CNC machine code (G-code). The CNC program is a sequential list of machining operations in the form of code that is used to machine the part as needed.

STEP 5 The CAM software simulator verifies the CNC program (see Figure).

STEP 6 The CNC code is created. Figure 3.26 illustrates the CADD 3-D model, the tool and tool holder, the tool path, and the G-code for machining a part.

STEP 7 The program is run on the CNC machine tool to manufacture the desired number of parts.

Computer Numerical Control (CNC)

Computer numerical control, also known as numerical control (NC), is the control of a process or machine by encoded commands that are commonly prepared by a computer. CNC is a critical aspect of CAM in which a computerized controller uses motors to drive each axis of a machine, such as a mill, to manufacture parts in a production environment. The machine’s motors rotate based on the direction, speed, and length of time specified in the CNC program file. A programmer creates this file, and it contains the programming language used to establish the operation performed on the machine tool. Examples of CNC programming language include G-codes, which are primary functions such as tool moves, and M-codes, which are miscellaneous functions such as tool changes and coolant settings. CNC is a major innovation in manufacturing. CNC has led to increased productivity because the consistency of the process has lowered manufacturing costs, increased product quality, and led to the development of new techniques. Persons possessing CADD and CNC skills can find various opportunities in manufacturing industries.

Computer-Integrated Manufacturing (CIM)

Computer-integrated manufacturing (CIM) brings together all the technologies in a management system, coordinating CADD, CAM, CNC, robotics, and material handling from the beginning of the design process through the packaging and shipment of the product. The computer system controls and monitors all the elements of the manufacturing system. The figure illustrates an example of CAD within a CIM process. The field of CIM incorporates the disciplines of CAD, CAM, robotics, electronics, hydraulics, pneumatics, computer programming, and process control. Computer-integrated manufacturing enables all persons within a company to access and use the same database that designers and engineers would normally use.

Within CIM, the computer and its software control most, if not all, portions of manufacturing. A basic CIM system can include transporting the stock material from a holding area to the machining centre, which performs several machining functions. From there, the part can be moved automatically to another station where additional pieces are attached, then on to an inspection station, and from there to shipping or packaging.

Additional Applications

In addition to design and manufacturing, CADD provides usable data and supports many other areas of the engineering design process. Most sales and marketing materials, technical publications, and training documents reference some form of CADD data. Often, existing drawings and models provide the majority of critical content required for items such as product brochures and installation and service manuals. Technical illustration involves the use of a variety of artistic and graphic arts skills and a wide range of media in addition to pictorial drawing techniques. The figure shows an example of a technical illustration partly created by directly reusing existing CADD data from the design process.

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