CAD & CAE FAQ's

CAD & CAE FAQ's
[ Home ] [ Up ]

What do all these acronyms mean?

CAD – computer aided design, 2D & 3D design on a computer screen

CAM – computer aided manufacturing, using a part geometry database to control a numerically controlled (NC) manufacturing machine such as a milling machine or lathe

CAE – computer aided engineering, doing higher level work on a 2D or more typically 3D part model; for example 3D solid modeling of some type or finite element analysis to calculate stresses & strains, as well as dynamic, thermal, or fluidic response to input stimuli.

FEA – finite element analysis

How much computer hardware do I need?

Basically as much as you can justify spending for a Windows or UNIX based workstation. At least one high speed central processor (CPU). A 19" to 21" high-resolution color monitor with a fast graphics board is the norm. Lots of memory & fast, high capacity SCSI hard drives. A mouse or digitizing tablet plus a C, D, or E sized plotter rounds out the hardware. You may want a plotter than can handle photographic output as well as CAD line drawings to handle architectural renderings, 3D mechanical solid model visualizations, or graphics arts design from non-CAD software such as Adobe PhotoShop, Corel Draw, or Corel Ventura Publisher. A fast 100 MBPS ethernet connection is also nice for large file transfers.

How long does it take to become proficient with AutoCAD, or any other powerful CAD software?

With training it takes a relatively short time. Without training, it takes much longer. Without training, some people will never become proficient. I have had some people in my AutoCAD courses that have been using AutoCAD for over a year and they still had terribly inefficient work habits. The three-day Introduction to AutoCAD course solved these problems for most of them.

What is the difference between an integer based and a floating point based CAD package?

Integers are whole numbers; i.e., 5, 99, -66. Floating-point, or real, numbers include fractional parts; i.e., 3678.99, and –0.003. Integer based packages are very fast due to exclusively using integer arithmetic which is much faster than floating point arithmetic. Unfortunately this results in limited capability such as limited discrete zoom ratios which is not satisfactory for Mechanical Engineering, Civil Engineering, or Architectural work. As the drawing gets larger and larger, the allowed resolution gets smaller and smaller. For example if you are designing a 1000’ long bridge and now want to detail a 1" rivet hole your allowable resolution may be a minimum of several feet – which is useless.

Integer based packages are really intended for Electrical Engineering (EE) applications where they work very well. All parts for schematic capture fall on a grid. Not only are parts standardized but they are only shown in schematic form; i.e., a functional electronic circuit symbol and not the physical housing of the part itself. Printed circuit board work also is normally done with integer based CAD software although it is sometimes handled well with a floating point based package. EE based CAD tools also need to understand part connectivity so that a netlist can be generated. A net-list describes all of the interconnections in the circuit. If desired, circuit simulation is done with an external floating point based simulation program.

Mechanical Engineering, Civil Engineering, Ocean Engineering, etc. as well as Architecture all deal with designing large, complex, and irregularly shaped parts for which you need powerful graphical drawing tools. From a drawing perspective only, ME, CE, Architecture, etc. applications have much more stringent drawing requirements than are those for EE. EE applications, on the other hand, need to understand connectivity. With AutoCAD, which is floating point CAD software, the zoom ratio is on the order of 16 trillion to one. This allows you to, for example, make a scale drawing of the solar system, zoom down to the Earth and the Moon, then zoom into a lunar crater, zoom to the speck that you see in the crater which is the lunar lander, and then zoom inside the lunar lander and read the control panel!

What is an entity, object, or primitive?

Entities, objects, and primitives are just different names for the same thing used by different CAD vendors. These are the most basic or primitive parts that you will use to create your drawings. They include: points, lines, arcs, circles, text, dimensions, crosshatch, polylines, blocks, 3D surfaces, 3D solids, etc.

What is a CAD drawing database?

This is a software rendition of a paper drawing. Whatever you draw on the screen of the CRT has to be broken down into its primitive components and then stored in the drawing database of graphics entities. For example every 3D line would have the X,Y,Z coordinates of its end points stored, insertion layername, default color, default linetype, etc. All of the entities in the drawing make up the CAD drawing. database.

How do I translate a drawing from one CAD package to another?

Most CAD software uses a proprietary database to store the actual drawing that you create. This is not a problem until you want to port one of your drawings between two different CAD packages. What you need is a drawing translator of some sort. The translator opens the source drawing file and looks for specific entity types and then writes them to the output drawing file to the equivalent entity type for the new drawing file.

Or the translator may use a third, neutral database form such as: IGES, DXF, etc.. Any entity type that is not supported by both systems will always create a problem unless they can automatically be converted into a different and common entity type. You also could do the non-supported entity conversion manually within the CAD software before or after the transfer.

How can I get my old paper drawings into a CAD system?

You have several options to do this.

A. Recreate the drawing from scratch

Tape the paper drawing up by your CAD system and manually enter all of the data into a CAD drawing database. This isn’t quite as bad as it sounds, a fast CAD operator can work fairly efficiently. They may find old mistakes, which they can correct and they may create new ones. You may be able to correct old drawing errors.

B. Digitize the drawing with a digitizing tablet

Tape the paper drawing onto a large (C, D, or E sized) digitizer tablet and manually trace over the drawing with the digitizer puck while entering CAD drawing commands. Requires a large, properly calibrated digitizer and a sure hand. Can be time consuming and can create new errors as well as fix old ones. It will replicate any drawing errors from the original drawing. Smaller digitizers require you to shift, retape, and recalibrate the drawing. Should be faster than option A, but will input all entities as drawn, not necessarily as they should be.

C. Video scan the drawing

This can be challenging. Doing a video scan of the drawing is trivial and works very well, but all you have is a digitized video image of the piece of paper. It doesn’t have any intelligence in the form of a drawing database of graphics entities. Converting the video digitized image into a CAD drawing database of entities is the hard part. This is done with powerful artificial intelligence software that looks at items in the video image (raster file of pixels) and tries to turn them into equivalent CAD entities that it can then store into a CAD drawing database. Considering the enormous complexity of the task, this scanning software is remarkably successful. It will normally require some manual clean up of each drawing. This is potentially the fastest method for large quantities of drawings.

What is 3D wireframe design?

Drawing with 3D line entities. The model can be complex but it can only be displayed as a wireframe model. Not only are surfaces not visible, but it is impossible to do any type of "solid" computations. It is possible to draw objects that physically cannot exist. For example look at some of the German artist, Escher's prints.

What is 3D surface modeling?

Surface modeling is one step up from wireframe modeling. It allows the creation of complex 3D curved surfaces similar to what you would require for an automobile fender, a Clorox bottle, or a jet engine inlet nacelle. Some solid modelers support this to varying degrees.

What is 3D solid modeling?

Designing with 3D solid primitives such as: rectangular boxes, cylinders, cones, spheres, etc. Creating a complex 3D model by performing Boolean operations on two or more primitives via: join, cut, intersect, and union. The final model will be mathematically correct. By definition, it cannot be an optical illusion.

What is a 3D parametric, feature based solid modeler?

This is a newer and more powerful version of solid modeling that eliminates some of the problems associated with older modelers. Older solid modelers created models that were very difficult or impossible to change. Parametric feature based solid modelers have the ability to scale the model up or down and change dimensions of any of its features. Features could include the length, width, height, and wall thickness of a model as well as hole, groove, and fillet locations, and their dimensions. Any geometric feature of a model can be changed. Model features can also be driven from a spreadsheet containing all of the model dimensions.

What are the advantages of solid modeling over 2D CAD work?

Working from a single, common geometry database. You don't have to create geometry multiple times for: 3D visualization, 2D detail design & drafting, 2D or 3D Finite element analysis, 3D NC manufacturing, and creating visualization diagrams for marketing or a manual. It's also much easier and faster to make changes to the model. Change is a fact of life for any design project so it's better to have tools that are powerful enough to cope with the inevitable changes.

It's also very easy to port the 3D solid geometry to an FEA software tool. This is an enormous time saver, easily paying for the entire cost of converting from 2D/3D CAD to a parametric, feature based solid modeler all by itself. The end result is a virtually seamless "end to end" set of software tools for mechanical engineering. Everything from 3D solid design and visualization to 3D FEA, to detailed 2D drawings, to 3D NC CAM is handled by a single geometry database.

What are the disadvantages of solid modeling over 2D CAD work?

The disadvantages of solid modeling over conventional 2D CAD design are: the requirement for faster and more powerful hardware to run the solid modeling software; increased cost of the hardware, and software; getting up the learning curve of the solid modeling software; and learning to think differently about the design process. Just as you had to when converting from manual design and drafting on a drawing board to 2D CAD, the thought processes change when going from 3D wireframe design to 3D solid modeling.

What is Finite Element Analysis (FEA) ?

FEA involves building an accurate 2D or 3D geometry model of the component to be analyzed. The model is broken up into discrete elements with nodes at their corners. Material properties are assigned for all materials used in the part. Boundary conditions are used to model physical connections to the part by setting the appropriate degrees of freedom (DOF) for all boundary nodes. Each node has up to six DOF comprised of three for translation, and three for rotation. Boundary conditions can also be used to model: dynamic, thermal, fluidic, and electrostatic connections. This part of the modeling process is done with a CAD like preprocessor to the FEA software, or geometry data can be imported from your existing CAD or solid modeling software which is usually an easier way for most people to work.

Once the geometry, materials, and boundary conditions are set, the next step is for the engineer to decide on the analysis type and then run the FEA software to solve thousands of simultaneous differential equations to obtain a physical displacement at each node. This strain data is then used to compute stress data at each node. A graphical postprocessor is used to digest all of this data and display it superimposed over the geometry model of the part with color coded stress (or other parameters) contour lines. Models can also be sliced to display stress contours inside the part.

What are efficient CAD work habits?

Take our Introduction to AutoCAD course and find out. If you don’t learn to work efficiently, you probably won’t. I've had numerous students in the Introduction to AutoCAD course who have already been using AutoCAD for one to two years and they still were working very inefficiently.