Anyone looking for information on quad bikes – you’ve come to the wrong place! This is slightly less dangerous, and probably far less fun! This is about Quadrilaterals – 4 sided polygons that are created by Pro/Engineer and other CAD programs to create lovely cosmetic surfaces.

Quad Surfaces use the information contained in their 4 constituent curves and any adjacent surfaces to define all the points on the surface. You can think about the surface as having a profile in 2 directions, traditionally called U and V. This profile changes smoothly from one side of the surface to the other. Different types of construction methods provide you with different means of control over the way in which the U and V parameters can be defined. The UV terminology for surface description seems to come up more often in rendering discorse than modeling. This is because the same UV parameterization is used to apply texture maps prior to rendering.

Let’s make things more interesting by making the following statement…
There is only one kind of surface feature and that is a “quad” – a four sided surface. There are many different ways to create these quads and there may be times when it doesn’t seem like you are really working with quads, but you are.

Let’s zip through each of the Pro/Engineer surface feature types and see how everything can be considered a quad.

1. Boundary Surf: 4 curves4 curve boundary These are the most obvious means of creating a quad. Here you directly create or select the 4 sides. Wildfire prompts you for curves in First Direction and Second direction for a regular boundary quad. While you can select more than 2 curves in one direction, note that this simply provides intermediate control (if you choose 2 curves in one direction and 3 in another, you can also think of this as creating two quads one next to the other) With ISDX, the terminology is slightly different. Under “Primary Direction” you need to choose the main 4 curves – select each one in turn around the surface you want to create, not just the 2 facing sides. The “Secondary Direction” option is only used for internal curve picks that provide additional control for the quad. This is similar to the “Influencing Curves” pick under the Option menu of regular boundaries.
2. Flat/Extruded Surfacesflat surface Flat (filled), rectangular Surfaces and Extruded Surfaces: no secret here – note that you can achieve identical geometry by using the 2 different techniques. In reality, a rectangular, flat surface has no advantage over an extrusion, since the extrusion can have a non-flat or non-continuous (stepped) form. Flat surfaces are much more useful when they create a non-rectangular shape. See (5) below.
3. Boundary Surf: 3 curves3 curve boundary A boundary surface created from 3 discrete curves, otherwise known as a Triangular surface, is a quad in which one side has basically been shrunk to zero. In some software packages, you can zoom in to see that there is actually a gap between the 2 “coincident”points. Triangular surfaces are often described as having one side “pinched”. Be aware that it is often difficult or impossible to create an offset triangular surfaces especially to the inside (concave direction). Because of this issue and the cosmetic problems that triangular surfaces can cause, there are some useful workarounds to avoid using them.
4. Boundary Surf: 2 curves2 curve boundary 2 Sided Boundary surface: a quad in which 2 opposing sides have been shrunk to zero. As with triangles, the surface quality may be poor, but on the other hand, these can be indispensible and very fast for doming small obround buttons. If you can create the same shape using a 180 deg. revolve, this would be a more robust option.
5. Flat Circular Surfacecircular surface A flat circular surface is simple rectangular surface which has then been trimmed to whatever shape. The same is true for any other non-rectangular shapes. For example, if you create a flat, sketched disk, it will appear as a single feature. In reality, it is a flat rectangle that has then been cut into a circular form – note that Pro is one of the few CAD programs where it is not possible to “un-trim” these kind of shapes and hence reveal the underlying geometry. As an exercise, create a flat disk in Pro and export an iges to Rhino. In Rhino, select the surface and do an Untrim (Surface / Surface Edit Tools / Untrim). You’ll end up with the initial rectangular quad that Pro/E created
6. Swept Surfacesvariable section sweep Sweeps: think of these as sweeping the U side along the V side and thus creating a 4 sided surface. Whether the surface has a constant section or is a complex variable section or swept blend, it can readily be described by U and V parameters.
7. Revolved Surfacerevolved-surface Revolves: a 360 degree revolve is one in which 2 edges in one direction are actually the same. If the edge of the surface touches the axis around which it revolves, then you will end up with a 3 sided surface that appears to have the same problems of pinching associated with the 3-sided boundary surface. However, it is a uniform surface that is straightforward to offset within the limitations of its smallest curvature.
   

Given this arsenal of tools, you might imagine that it would be quite easy to surface a product. You’d be wrong!
Most of the time, creating a professional model requires breaking down the product into quads that are not only robust and high quality, but also interact well with their neighbours – ie. are able to maintain tangency or curvature continuity.