New ideas make complex solids simple

Feb. 12, 1999
Modeling power, simple operations, and integrated rendering features let one person turn complex ideas into operable and manufacturable products

Simon Floyd
Simon Floyd Design.
Melbourne, Australia
[email protected]

The turbine engine in the picture helps illustrate new features now becoming available in solid-modeling software. As remarkable as it may sound, they let just one person model the 1,165 parts in the prototype automobile engine, and in only 20 days. Of course the idea for the engine had been has been incubating for several years, but design, evaluation, documentation, and modeling took less than a month.

The CAD field has advanced in recent years with the introduction of clever new ways to speedily devise complicated parts. Three developments in particular are notable. One is a more sophisticated model-snap facility that works in 3D to simplify the selection of precise locations to which additional geometry might be attached. A second is a more intelligent means of manipulating and positioning modeled shapes. The idea is to have modeling software recognize a location in space or on a model without having to establish arbitrary datum planes that older programs require.

A third is the ability to drag basic shapes and complex 3D components from a catalog for use in constructing models. Some earlier solid modelers let the user construct designs by intersecting geometric primitives such as cones, cylinders, or boxes. These early systems were so limiting they were difficult to use. Simple mistakes often meant scrapping everything and starting over. Modern software, in contrast, has perfected the idea of a basic-shape library so that simple shapes and complex parts can be easily modified and reused to more quickly model new parts.

Think assemblies
Three-dimensional solid modeling is an excellent communication tool for concepts. But most engineers don’t use their CAD software in preliminary stages. It’s usually a back-end tool. The point of new software such as the IronCAD package is to provide tools that are useful for developing fast-changing concepts as well as producing drawings.

It’s fair to say that older versions of so-called high-end modeling software are not conducive to evaluating or creating designs on the fly. Older software requires that users have a well developed idea of how their mechanism works before they begin modeling because of the way the software handles parts and assemblies. But few engineers hold finished designs in their heads. They often develop the design as they work.

What’s more, engineering projects can quickly reach a point where a hand sketch isn’t enough to convey ideas. Sketches are fine for sparking ideas or flow charts, but a 3D assembly view is mandatory to show a whole mechanism and how each section works.

Modern software such as IronCAD provides tools to manage assemblies composed of multiple modeled parts. Moreover, the software frees designers from having to keep track of managing a complicated constraint hierarchy. Most designs change so much it is impossible to predict a correct constraint from part to part. Each iteration generally requires keeping several design aspects and scrapping others. As the assembly forms, it changes dynamically. In the case of the turbine engine assembly, I revised concepts about four times before arriving at an engineering solution. This illustrates why modeling programs must let their users make revisions easily.

Assembly complexity can grow so quickly it makes it difficult to design in 2D. The effort associated with designing the back end of the turbine engine illustrates the point. I worked on this rotor section early in the assembly. Combustion is critical to generating shaft motion so power and torque can be harnessed. Animation helped show how it works — how the rotor turns, and how each blade passes each combustion and exhaust port. The animation was invaluable because it produced useful feedback from turbine experts with whom I consulted. On the strength of the animations, they convinced me one large-bladed rotor would suffice over two rotors in the expansion area.

New modeling features such as those in IronCAD facilitate quick and easy concept development. For example, the most notable new features include a software tool called a TriBall. It dynamically positions faces, features, parts, and assemblies. And a Smart Snap feature quickly pinpoints construction locations on 3D models, such as midpoints of edges and faces, and centers of holes. A shape catalog is a reservoir of frequently used model shapes. What’s more, the shapes are dynamic so they can be resized for a range of applications.

On screen, the TriBall has several red handles to mark its three axes. A hotkey or icon calls up the tool and users can simply drag it to any appropriate reference location. The software device serves as an intuitive means of performing both common and complex transformations. The tool provides visual feedback so users can always ‘see’ what they are doing. For instance, when one axis of the ball is selected it changes color to indicate it’s locked in position so it can translate or rotate about that axis. “Grabbing” a red handle with the cursor allows dynamic snap feedback in that axis or lets users enter a dimension. This tool relieves users of any need to manage an x-y-z coordinate system that all other CAD systems force on users.

For example, consider the problem of evenly delivering compressed air to the combustion chamber. An air tube follows a complex path around the engine. It’s precise twists require a lot of 3D manipulations. The approach used to describe such paths is through lofting planes — many 2D cross sections of the pipe positioned in a required path. In the case of IronCAD, an operator manipulates each cross section of the loft into place using the Triball. A lofting function then stretches a skin or surface from one cross section to the next. Without the positioning tool, users would have to define dozens of datum planes and then sketch the cross sections on them, operations that take 10 to 20 steps for each plane, versus two or three for the TriBall.

New solid modelers also incorporate intelligence about important 3D entity locations such as line midpoints, or circle and face centers. They additionally provide alignment feedback and dimension information.

For example, one might have to modify a part by shortening a particular boss from a certain edge by 10 mm. IronCAD allows dragging a handle in the height direction on the boss to stretch it all the way to the edge where it snaps in place. Then it’s easy to measure back 10 mm from the face. For instance, should the software indicate the boss is 50 mm, a user can type the value -10 into the length field so it reads like an equation [50 – 10]. The software then shortens the boss to the right length. Once users get familiar to the operation, it’s surprising how dimensionally unaware one can be and still never misjudge a length or area.

The idea of building construction models from a list of predefined basic shapes has been around for a long time. Previous implementations, however, used static 2D shapes or 3D primitives. New generations of software have overcome the drawbacks of earlier systems. The IronCAD program, as an example, incorporates what’s called a shape catalog. Shapes in this catalog can include everyday entities such as bosses, ribs, and screws — anything that makes sense to reuse.

To use a shape, users need only drag and drop it onto the part model. One widely chronicled benefit of a shape catalog is to enforce the use of uniform components — only the bolt types and sizes furnished in the catalog, say, just as spelled out in some ISO standards. Once users create and test a particular boss, they can store it in the catalog for others to apply.

The construction of a shape from the catalog begins with operations on primitive solids such as spheres, blocks, and cylinders, all of which initially are in the catalog. Users might begin a design by dragging a block onto the design area and then resizing it by pulling on a “handle,” a small red ball that appears on a surface when the cursor approaches. A user might construct a long rod by dragging the top of a cylinder to an appropriate length. Similarly, bosses or ribs can be dragged onto a model and resized by pulling on their handles or by entering a dimension.

Blending and drafting functions also get smarter in modern modelers. The features allow cleanly extracting cast parts from molds. Even machined parts require defined blends because spinning tools can’t produce square corners easily.

The task of defining complex blends used to be possible only in expensive high-end modelers. This is because the operations involve computing intersections between two or more second or higher-order curves. Now even midrange modelers can handle such chores. For example, in cases where the IronCAD software cannot complete a complex blend, it gives users an explanation why the blend failed and how to correct it. This is useful when users ask in error for impossible blends — when entered radii values exceed the amount of material on the part. The software returns an explanation and allows modifications so errors are corrected rather than just stranding users as older CAD systems might.

Other advantages of modern modelers includes features that allow quickly generating cross sections through complex areas and integrated rendering and animation functions. They’ve previously been available as separate functions. But when combined with recent technology, concepts turn a lot faster into understandable models and saleable products.

How rendering improves engineering

Rendering modules are often separate software programs but worth the effort to learn because giving components lifelike colors provides another avenue of communication. As it’s shown in the lead image, for example, the cowl over the compressor section could be polycarbonate, a strong, translucent plastic. Selecting that material from the database and dragging it to the cowl produced the image. The tactic lets viewers see into the engine, and still recognize the shape and position of its cover. One might think glass provides the same appearance. But it does not. Its different refractive and reflective qualities produce a remarkably different look.

Most importantly, rendering encourages communication and understanding. One look and you recognize the engine and that it’s more than a CAD model.

Rendering’s function is to let users show parts in their intended materials, such as stainless steel, cast iron, machined metal, or rubber. The software also lets users tweak material properties such as the index of refraction to produce a different appearance.

Selecting the same material for nearby parts might obscure each other, but giving each a slightly different gloss value, a reflection term, lets users more easily recognize different parts of the same material. The trick is particularly useful for mating parts of the same material.

Industrial designers will find the rendering module useful because they’re often concerned with reflections off surfaces, highlighting, and so on. The features are also critical for making a lasting impression during a presentation.

As useful as rendered images can be, animated ones are sometimes better. Animating a gearing system, for example, helped time the inlet of compressed gas and fuel. The animated-gear section showed an engineering colleague how it was supposed to work before appropriate ratios were calculated. The concept design showed a valve system driven by a solenoid on an electric motor, as opposed to a purely mechanical design. Viewers can watch the valve open and close as other components moved by. This was important to getting the message across and improving the engine.

The animation has progressed further to now show turbines turning and valves operating so users can envision the entire process. A movie might show the complexity of the design by spinning the model and zooming in to show the alternator wheel turning. Viewers will see details such as windings in the alternator.

A turbine engine for tomorrow’s cars

It’s almost become a “given” that aircraft turbine engines won’t work in automobiles or private water craft because they currently cost a lot and perform poorly at low rpm. But the technology is worth a second look because existing reciprocating designs have reached a peak in development with about a paltry 25% efficiency on the high end. Efficiencies in turbine engines reach above the figure so the technology seems the next logical development.

After mentioning the idea of a turbine auto engine and generating a few hand sketches, I examined helicopter engines, turbocharging turbines, and jet-turbine techniques and came up with a concept that was different than most thrust-generating designs. The engine uses a centrifugal or radial compressors rather than axial designs based on the former’s lower cost.

The calculations conducted to verify successful operation have been relatively basic — mostly from text books. More work would be required on the precise rake and cross section of the turbine blades to increase the engine’s efficiency.

© 2010 Penton Media, Inc.

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