Leslie Langnau

To get to parametric modeling and to the newer direct modeling, CAD has undergone such radical changes as to make the early systems look unrecognizable to today’s engineers. Here’s a quick review.

Jean Thilmany, Senior Editor

Last year, parametric modeling turned thirty.

“Parametric modeling made solid modeling practical for the first time and was a huge time saver,” says Jon Hirschtick, chief executive officer at OnShape, a CAD software company. “The beauty of feature-based parametric modeling was that engineers could create solid models with an ordered list of understandable modeling features like sketch, extrude, fillet, and shell. By changing dimension values—or adding, editing, reordering or deleting features—a solid part’s geometry would automatically update,” he says.

Now, direct modeling has joined parametric design as a model-building maneuver, and other techniques join their repertoire. Yet, parametric modeling continues to roll along. Mostly because, as Hirschtick points out, it works well.

To get to parametric modeling and to the newer direct modeling, CAD has undergone such radical changes as to make the early systems look unrecognizable to today’s engineers.

With the kinds of advances Hirschtick and others have made to CAD software through the years, it sometimes can be difficult to remember the technology has only been around since the 1960s, and parametric modeling since 1988.

The Wayback Machine
Ivan Sutherland is often credited with creating the modern graphical user interface and kicking off CAD. He essentially came up with the idea of drawing on a screen.

In 1963 while still a Ph.D. student at the Massachusetts Institute of Technology, Sutherland created his Sketchpad system, the first program to use a graphical user interface to interact with users. The program comprised an x-y plotter display and the recently invented light pen, a computer-input device shaped like a wand.

“In the past, we have been writing letters to, rather than conferring with, our computers,” Sutherland wrote in his thesis. “For many types of communication, such as describing the shape of a mechanical part or the connections of an electrical circuit, typed statements can prove cumbersome. The Sketchpad system, by eliminating typed statements in favor of line drawings, opens up a new area of man-machine communication.”

Sutherland’s system displayed vector graphics rather than the raster graphics we’re used to today. Sketchpad users controlled the cathode ray tube’s electron beam via light pen to draw vectors on screen, creating shapes line by line. It was like operating a ray gun. You’d turn it on, draw a line, turn it off, move to the next point, and turn it on again in a process not entirely unlike the workings of an Etch-a-Sketch (which is, in fact, a simplified vector plotter), says Bernhard Bettig, a mechanical engineering professor at the West Virginia University who has taught a course on CAD history.

The phosphate that displayed the drawings tended to fade so users had to continually refresh the display. With very complicated displays they’d have to refresh often, he says. “And the system blinked a lot and when you got really complicated, you got a lot of blinking,” Bettig says. “But you still had lines on a screen, so it was a big deal.”

The systems allowed engineers to work out potential manufacturing errors on screen, to readily update their designs, and to render designs faster than they could by hand,” Bettig says. “These were wireframe drawings, which couldn’t depict volume and that could oftentimes lead to confusion.”

“Did a part open from the top, from the left, the right? It was ambiguous in terms of where the surfaces were. You didn’t know how to look at it,” he says.

The 1970s saw the advent of 3D modeling. Those early systems were based on solid modeling and stem from the work of two men on two continents who worked on separate approaches at about the same time. In 1976, mechanical engineering professor Herbert Voelcker’s group at the University of Rochester in New York used a process that came to be called constructive solid geometry, essentially a molding and joining of shapes.

Also in the middle 1970s, Ian Braid at Cambridge University in England released his solid modeler, Build, which delineated the boundary between solids and nonsolids to create models. As their methods varied, so did the eventual CAD systems based on those methods. Still, their underlying principle was much the same, writes David Weisberg in his 2008 self-published book “The Engineering Design Revolution: The People, Companies and Computer Systems that Changed Forever the Practice of Engineering.”

Yet, 3D CAD systems met with resistance from designers who said it was difficult to use.

“It was not until the introduction of parametric-based CAD that this resistance began to melt away,” writes Jami Shah in the book “Theoretical and Computational Basis Toward Advanced CAD Applications (Springer, 2001). Shah is an Ohio State University professor of engineering design.

In parametric design, the relationship between each element is used to inform the design of what will become complex geometries and structures. When using a CAD system driven by this method, engineers are called upon to build a geometry piece by piece, based on parameters like the depth of a hole, the diameter of a circle, or the thickness of a shape, Weisberg says.

One important and defining feature: the software tracks each step in the building process. When an engineer modifies the value of a dimension, the shape of the model changes accordingly. That single change can ripple through the model to automatically update each area affected. Engineers needn’t isolate and make those changes themselves.

In 1988, Parametric Technology Corporation, founded three years earlier by mathematician Samuel Geisberg, released the first commercially successful parametric modeling software, Pro/Engineer, Weisberg writes.
The goal was to create a system flexible enough to encourage engineers to consider a variety of designs, with the cost of making design changes as close to zero as possible, he says.

Pro/Engineer was implemented from the start as a solids-based system. Everything was done with double-precision solid geometry and NURBS surfaces.

To create a model, the user typically started by creating a profile of the object. This shape was then converted into a solid model by translating it through space or revolving it around a centerline. Additional geometry could be added or subtracted from the base model. Some of the geometry was in the form of features such as holes, bosses, and ribs, Weisberg writes.

“A key characteristic of Pro/Engineer was that as the model was created, the software recorded each step the operator took. This was referred to as a ‘history tree.’ The software also recoded geometric aspects of the model such as whether two surfaces were parallel or the fact that a hole was a specified distance from the edge of the part. Each dimension used to define the part was also recorded. If the user placed a through hole in a block and the thickness of the block was later increased,” he says.

The program was successful from the start in part because it didn’t have to support the legacy minicomputer and mainframe-based software that its competitors did at the time, Weisberg says.

“PTC developed Pro/Engineer from the start to be hosted on networked UNIX workstations. Its software was written in a higher-level language and the system used the latest software architecture techniques,” he writes.

Another modeling method
That’s the point at which things more or less rested until around the turn of the most recent century, which saw the introduction of a new method called direct modeling.

CAD systems driven by direct modeling don’t require engineers to use parameter-driven regenerations of a solid model. Instead, an engineer changes a solid model by pulling it, stretching it, and moving it as needed, rather like working directly with clay, says Holly Ault, mechanical engineering professor at Worcester Polytechnic Institute, Worcester, Mass.

Alternate terms for direct modeling include synchronous modeling and dynamic modeling, she adds.
“Direct modeling is an intuitive approach to creating geometry without the burden of history-based dependencies,” Ault says. “Construction methods are similar to those used in conventional solid modeling. The user can design a 2D profile and then develop the model using commands like extrude, revolve, mill, and bore. Without the presence of a parameterized history tree, manipulation of the geometry is greatly simplified.
The approach allows engineers to design directly on the model’s geometry, she adds.

Direct modeling creates geometry rather than features, so it’s prefect for conceptual modeling where the designer doesn’t want to be tied down with the interdependencies of features and the ramifications making a change might have,” Ault says.

CAD into the future
Of course, CADmakers don’t stand still. Many are looking at ways to include artificial intelligence to increase the ways engineers can use CAD tools for design.

Last year, for example, Autodesk released generative design to subscribers of its Fusion 360 Ultimate product development software. The design concept allows engineers to define design parameters such as material, size, weight, strength, manufacturing methods, and cost constraints–before they begin to design. Then, using artificial-intelligence-based algorithms, the software presents an array of design options that meet the predetermined criteria, says Ravi Akella, who headed the product management team for Autodesk’s generative manufacturing solutions before moving last year to become director of product development at Roblox. The feature focuses on helping designers define the problem they’re trying to solve, he says.

“The software asks the user preliminary questions. ‘What sorts of materials would you consider for your design? Where does it connect with other things as part of an assembly? What are the loads? What are the pieces of geometry?’” Akella says.

After a short period of time, the software then presents designers and engineers with an array of design options that best meet their requirements. Designers choose the best design. Or, if none of the options meet their needs, they can begin the generative process again, this time offering slightly different inputs.

CAD software will continue to evolve and who knows how AI will influence design. But it will be an interesting journey.