3D CAD Models For Prototyping, Tooling, And Machining
Three dimensional (3D) CAD (computer-aided-design) models have enabled major productivity gains in product design and engineering. Prior to the advent of economical desktop computers skilled draftsmen spent hours laboring with graphite lead pencils on velum and Mylar to achieve engineering quality documentation for fabrication and production manufacturing. The process required tedious checking to eliminate errors wherein modification required erasures and redrawing over and over on the same sheet. Large projects had many drawing documents linked to one another that required procedural revisions and archiving.
Two dimensional (2D) CAD (computer-aided-design) was a great step forward in moving from the drafting table to the desktop computer. The graphical display capabilities of the desktop computer enabled a user to graphically draft using mouse, keyboard strokes, text line commands, and tablet with stylus. Each CAD system had its own (UI) User Interface. While manual drafting may have actually been faster in the early years, two dimensional (2D) CAD (computer-aided-design) technologies allowed endless changes to be made with sharp clean printed copies printed without limitation. It was easy to export the CAD drawings in PDF format for viewing and printing on any non-licensed computer.
Three dimensional wire framing was the next step in CAD wherein two dimensional (2D) sections were given depth to extrude. Sections were revolved around an axis and swept along trajectories to create 3D forms. Surface modeling was another advancement wherein surface skins were created using typical wire framing three dimensional commands. Solid modeling was the biggest advancement in three dimensional (3D) CAD (computer-aided-design). Solid models could be created using similar commands from surface modeling and wire framing but the result was that the model took on mass properties useful for analysis, sectioning, and Boolean operations of merging for adding, subtracting, and merging.
Solid modeling offered enormous advantages resulting from mass property data. The solids data allows the software to be aware of the respective position of all of the mass within an assembly. This allowed for clearance and interference checking. Drawings are no longer line, arc, and spline entities in a data file but were actually view of a solid model akin to a camera taking pictures of an actual object. In addition to view angles, sectional views can be defined as needed to define and dimension the part.
Solid modeling greatly reduces the incidence of design and data errors in drawings because the drawing views are associatively linked to the solid model. The modeler task is to evaluate the form fit and function of the model in the 3D solid to provide the proper clearance and tolerance allowances between parts. The modeler has the tools to assure that this process is completed. Part clearance and global interference checking as well as drawing views and sections provide tools needed to assure proper fit and function. If the solid model is correct, the drawings will be correct as well. All dimensions will be precise as well. Since the dimensions come from the model, as they were used to create the model in solids, the drawing, the parts, and assembly are totally consistent.
Parametric dimensioning was the next great advancement in solid modeling. Parametric dimensioning is akin to spreadsheets for financial tools. In a spreadsheet, if a number is one cell is changed, all cells that reference that cell automatically update to reflect the revised calculations. In CAD modeling, parametrics work in a similar way. Consider a block with dimensions of length, width, and height. If the dimension for the width is modified, the block will automatically change its size to reflect the new dimension. Accordingly the mass properties of the part will change as well reflecting the new and higher volume. Parametrics add a level of complexity to CAD modeling that allows the modeler to create "intelligent models" that respond to changes in a predictable and automated way. Parametrics take CAD modeling to a new level wherein a true professional modeler can skillfully capture design intent with a flexible model for efficient modification and part iterations. Parametrics in assemblies allow the modeler to observe how a mechanism will function. Parametrics are the foundation on which sophisticated mechanism modeling studies can be performed to observe parts in motion.
Surface modeling is a specialized advanced form of modeling that allows the modeler to create forms that cannot be created with basic geometry. These tools are essential to capture organic forms popular with contemporary Industrial Designers. Such tools are essential for developing CAD models into a form that can be used for tooling in processes such as plastic injection molding. The 3D data extracted from such a solid model can be directly used by software programs such as Mastercam and Surfcam to drive multi-axis CNC machines to create cores and cavities or to directly machine parts from solid stock. Surface modeling tools typically include multi-option sweeps, surfaces constructed from boundary curves, and freeform manipulation of surface using drag handles to push and pull the surfaces. Among CAD programs Parametric Technologies Corporation (PTC) offers the most flexible surface modeling tools for a desktop system. It ISDX (Interactive Surface Design Extension) allows the modeler to manipulate many boundary and surface tools and instantly observe the result.
Surface modeling is a world apart from geometric primitive modeling wherein the modeler creates a number of individual surfaces and knits them together at boundary intersections to enclose a volume. Once the volume is completely closed, the surface enclosure can be solidified meaning that the model takes on solids properties and defines a measurable mass. The volume can be shelled which hollows out the volume leaving a shell representing a molded part. Advanced surface modeling involved control curves which can be lines, arcs, or more commonly, an abundance of splines. Splines are curves that have variable curvature or radii (which is the mathematical reciprocal of curvature). Advance splines used in CAM software are called NURBS (non-uniform rational B-splines) and are a more advanced mathematical form of the B-spline (basis splines) which utilizes polynomial expressions to create smooth curvature transitions along the curve. B-splines are patched together algorithmically to create a continuous undulating curve. The B-spline is a generalization of the Bezier spline that avoids interpolation errors that can occur in higher order polynomials. Pierre Bezier defined the Bezier spline for design of automobiles. Control polygons are a graphically represented grid of mathematical drag points used by the modeler to manipulate splines and complex surfaces. Spline geometry is interpolated from these drag points based upon the respective algorithms of the CAD modeling engine. For more precise definition of a surface, the control polygon is made denser yielding more drag points. The file size to support a denser grid expands greatly. CNC machining requires the more advanced NURBS based curve and surface system.
CAD systems are defined as B-rep (boundary representation) geometry systems. When the CAD model is view in non-shaded mode, boundaries are observed between surfaces. A simple block with rounded edges will be defined as planar surfaces bounded by cylindrical rounds and spherical corner surfaces. In surface modeling, the boundaries are visually and mathematically more complex. Keeping complex surface models free of excessive boundary patches is an art for a surface designer and is highly advantageous for turning the data over for CNC machining because they are less prone to interpolation and boundary intersection errors.
Analysis tools that come with solid modeling software allow the CAD modeler to evaluate the quality of curves and surfaces for smoothness and surface boundary transition condition. These tools give the model the information required to improve the model to a high level of quality that results in better aesthetics and machining data that will not generate machine tool path errors. A surface that may have a tiny spike in curvature transition can be interpreted by the CAM software and cause a cutter tool to gouge the surface of a core or cavity causing costly repair. A professional surface modeler can avoid these problems with the use of analysis tools. Analysis tools provide data on part weight by factoring in the specific gravity of the selected material and calculating the volume of the material required to manufacture the part. This information allows a manufacture to quickly and accurately determine the cost to manufacture the part.
Rendering capabilities make solid modeling a virtual reality theatre of design. The ability to apply colors and finish to parts for realistic shaded images allows the CAD modeler to visualize parts and assemblies as they would appear as manufactured. This is useful for design and especially useful as a communications tool for others who are involved in the project. Shaded views combined with software conferencing tools such as gotomeeting.com and Skype conferencing enable effective remote meetings. The ability to hide parts of an assembly and displace section views enables the CAD modeler to easily evaluate parts and assemblies. Multiple display conditions can be defined and called up quickly to facilitate discussion. Screen captures are easily converted to JPEG file formats and imported into presentation programs such as Powerpoint for slide presentation with annotation.
Given that there are many publishers of CAD software, the industry long ago recognized the need to be able to import and export data between CAD, CAM (computer-aided-manufacturing), rapid prototyping, and graphical visualization systems. IGES (Initial Graphics Exchange Specification) is most commonly used. STEP (Standard for the Exchange of Product model data.) is a later development that includes more information than the IGES standard notably (PDM) Product Data Management information. STL (stereolithography) files are files created from surface data by creating a tessellation mesh of triangulated planes. These are used in the stereolithography process wherein the data is used to "grow" prototype parts in a tank wherein a liquid is solidified in thin successive layers according to the cross sections of the part building from bottom to top. It is literally a process of layering, or printing hence lithography, building up a part by layering cross sections. STL files are commonly used in rendering programs as they are less data intensive than IGES files. The tessellated mesh representation does not require the advanced math algorithms that splined forms require. Data export is used for FEA (finite element analysis) which is done with programs such as Ansys, Cosmos, Nastran, and Algor. These systems also use tessellated mesh applying partial differential and integral equations to approximate the structural, fluid, thermal, and electrical dynamics.
Rapid prototyping has advanced dramatically as the need to validate part design with simulations early in the design process has prompted manufactures to offer many new materials to represent various polymers, ceramics, and metal parts.
In summary, 3D CAD parametric solid modeling has streamlined the product development process dramatically in the past two decades. These tools allowing designers and engineers to bring products to market far more rapidly while achieving greater product quality and reduced costs. The accuracy and completeness of CAD data allows for efficient remote manufacturing sourcing to any place on the globe and for engineering data to be rapidly transferred between design, engineering, and manufacturing facilities. Product development today is truly operating in a network of service and manufacturing providers.