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From CAD User Mechanical Magazine  Vol 17 No 02 - FEBRUARY/MARCH 2004

VX introduces the concept of Designer Fillets in the quest to find the ultimate in Class 'A' surfaces, as David Chadwick discovers.

There can be as much complexity in the shape of a plastic bottle for skin lotion or toilet cleaner, perhaps, as in the surface of a piece of high-tech equipment, such as an automobile, or the wing of an aircraft. Apart from, that is, the aerodynamic and stress analysis that the latter parts require - and assuming that the use of plastic containers as projectiles in domestic rows do not require their subjection to similar analysis.

To achieve the same Class 'A' surfaces that automotive manufacturers demand, consumer product manufacturers have availed themselves of the same advanced surface modelling tools. What is a Class 'A' surface? The simple answer is that it is a perfectly smooth surface with no anomalies, in which all adjoining surfaces have curvature continuity. This means that where two surfaces meet, the graduation of one into the other is achieved without discernible abrupt transitions. The techniques used to create Class 'A' surfaces typically reside in top level surface modelling software developed for the motor industry, rather than mid-range mechanical CAD packages that have evolved from 3D solid modelling for mechanical assemblies.

VX, however, falls between both of these stools, having been developed primarily as a designers tool, using either surface and solid modelling technology (hybrid modelling), whichever is the most suitable for the task and as the model dictates. The company has just launched VX Version 9, which, apart from enhancing the performance of the numerous suites within the package, introduces the concept of Designer Fillets to improve the ability of the designer to create advanced smooth contiguous surfaces.

Designer Fillets
If you take two adjoining 2D lines, or a couple of tangential surfaces, the intersection between them can be turned into an arc (2D) or a fillet (3D), each of which is inserted with a constant radius. However the transition from each line or surface can often be too abrupt for the design.

To smooth out the approach to the curve, VX applies mathematical algorithms, modifying the arc or the fillet, producing more of a natural, visually appealing parabolic curve. According to Mike Lang, Technical Director of VX, fillets should look simple - you shouldn't see a fillet line in a model. They should also be simple to create. "Achieving tangent and curvature continuity in complex shapes on other systems is hard work. Designers using VX, though, don't have to be geniuses to achieve the ultimate surface".

VX software is designed to be easy to use to create free-form shapes. It's hybrid modelling technology allows users to create 3D surface profiles from 2D sketches that can be modified directly, or through altering, say, the dimensions in the parts history. (One of its most useful tools is the context sensitive menu of drawing aids that appear when you right click the mouse over a part of the design).

When two surfaces are selected for filleting, a window appears that enables the designer to select the appropriate weight or fairing for the curve to suit the desired style. Weighting and fairing affects the way that curves go through points. In effect, the shape of the part has more influence on the curve, rather than the curve itself.

A reduction in the weight of a curve will allow it to retain its tangency, but sharpen the change in curvature. This can be seen most effectively by reducing the weight almost to zero.

Fairings - the shape of the curve - can be influenced by energy, variation, jerk, bend or tension - each of which will produce a subtle difference in the mathematical fit through the curve.

Echo Attributes
Part of the process of obtaining Class 'A' surfaces is being able to see what's happening to the curve or the surface as it is being developed. VX uses a tool called Echo Attributes that allows users to apply iso lines, gaussian shading and wireframes to the curves that can pick up the contours of the shape, highlighting its imperfections. Strange only in name, Echo Attributes controls many attributes of the model, and allows users to make modifications throughout the design process.

Increasing the scale of the iso lines allows designers to pick up smaller imperfections in surfaces. Where blue iso lines lose their curve they change to white. The shifting colours of gaussian shading are also particularly adept at detecting subtle blemishes. Echo Attributes also has numerous other modifiable elements, including the ability to apply colours to lines and surfaces, and to alter the transparency of the surface. Curvature plots on non-designer fillets show regular arcs, unlike designer fillets that show the weighting of the curve at each point.

Commenting on the creation of good surfaces again, Mike Lang said "good design work relies on good wireframe technology. If you don't have basic curve geometry, you won't be able to produce a good surface." He also pointed out that VX software is very forgiving, providing designers with numerous tools to sort out problems in the surface as they occur.

Mike also recommends that designers always go through the routine of checking curves, especially if the design has come in from an outside source - perhaps containing older style Bezier curves with lots of points. VX software is quite capable of adjusting such designs, reducing the complexity of the surfaces considerably whilst retaining the ability to create Class 'A' surfaces from them.

VX Version 9
Besides adding Designer Fillets to the VX suite of design and manufacturing tools, some of the other enhancements are particularly interesting. Design Optimisation, for instance, allows designers to modify the dimensions of an object by selecting those dimensions that can be changed, and inputting a new attribute - capacity or weight. A typical example would be a plastic bottle which, when designed, is found to be capable of holding somewhat less than a litre. Designers can specify a litre as the new capacity and apply Design Optimisation whilst constraining the diameter of the bottle - and the software would automatically modify the other bottle dimensions to meet the specified capacity.

Staying with the bottle, designers can use Constant Panel Radius to 'hold' that portion of a bottle that will form the background to the label, whilst modifications are made to the overall shape of the bottle. A seemingly minor feature - unless you have ever attempted to attach a sticky label to a ball, or any other surface that curves in more than one direction!
Two other useful functions are split surfaces and surface offsets, that allow surfaces to be easily subdivided to facilitate the creation of complex curved surfaces - and surface offsets that can be used to build up surfaces, perhaps on bosses and other features.

Version 9 also has improved shape modification, enabling users to pull parts on the surface instead of just being able to control curve points, with the shape modification propagating through the shape. If you wish to use this feature on imported geometry it would be wise to reduce the number of points to optimise shape control, by applying NURBS control to filter the points, reducing the volume, whilst still retaining a useable surface within the necessary tolerances.

Model Control
Supporting the model is, of course, the model history. This depicts each command used to create the model. If the designer clicks on a command, it will expand to show the attributes underlying the command, each of which can be modified (new dimension etc.) which are then re-applied to the model. Groups of commands can also be combined and named to form Custom Features that can be exported for use in other designs, retaining full parametric capabilities. CU
www.vx.com

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