Surface design and editing tips

Surfaces are defined by a rectangular mesh of control points, which determines the shape and smoothness of the surface.

The rectangular nature of the control point mesh means that a surface has four boundary curves. In some cases, surfaces are constructed where one or two (opposite) boundary curves are degenerate or form a single point such as the poles of a sphere. In other cases where surfaces wrap around, such as a cylinder, two opposite boundaries can be the same curve and are called seams. With seams and degeneracies, a surface may appear to have only three, or two, or even no boundaries (a sphere), but the four boundaries are always defined. With these four boundaries, you can break a surface into rows and columns so surfaces have a table-like structure.

Surface design tips

• Design first, edit last. Put all the surfaces in place first, then trim and fillet the surfaces together to create the final shape and boundaries.
• Use quality source curves. Quality curves have as much detail as possible designed into the curves before the curves are used to make surfaces. Curves are much simpler to design and edit. With Parametric modeling active, changes to your source curves propagate automatically into the surface.
• Build as much detail into a single surface as possible. It's easier to split and reduce surfaces than to merge and combine them. Similarly, machining with isoline operations uses one surface at a time and filleting and trimming work with two surfaces at a time so the more detail the better.
• Learn which surface methods are approximate and which are exact. Approximate methods are not better or worse than exact methods, but have more degrees of freedom in filling in the space between input curves or surfaces. Other surface methods are conditional. They are exact to what they do, but when used with approximate surfaces, the resulting surface is still approximate.
  • Approximate: surface intersection, trim, untrim, fillet, lofting, merge, modify, offset. (Another key is any operation that includes a tolerance setting. That is a flag that the source material is being approximated.)
  • Exact: extrude, surface of revolution, sweep, ruled, sphere, cylinder, flat, from a feature.
  • Conditional: split, region, reverse, coons, cap. Coons is a mathematical definition of how four boundary curves describe a surface. Learn which dialogs give you the option to modify-in-place or create a new object.
• Learn when and how to use the option to Modify-in-place or Create a new object. Both methods have their uses. Modifying an object in place breaks the parametric link for which constructor was used for the original object and prohibit parametric modeling from updating an object in the future. This limits you to further modification operations only. Creating new objects can result in excessive clutter on the screen.
• Do not use self-intersecting curves or surfaces. Curves and surfaces with that characteristic are not viable for predictable editing, construction or machining.

Surface editing tips

• Do not include portions of the boundary when constructing curves for trimming. Trimming curves may cross or end on boundaries.
• Do not use surface/surface intersection to get a boundary of a surface. Use surface boundary extraction.
• Intersecting tangent surfaces, such as a surface and its fillet, produces inaccurate or broken intersection curves. Instead, extract curves and trim with a curve.
• Do not intersect coincident surfaces (surfaces that overlap). Extract curves and trim with a curve instead.
• Avoid cutting through the pole of a surface with a degeneracy such as the tip of a sphere or surface of revolution.
• Avoid cutting along a seam of a surface (across is fine). A seam exists, for example, at the left/right boundary of a cylinder. You can detect seams with the Surface Boundary dialog, and then canceling out after you know where the seam is. Cutting a seam can usually be avoided by rotating the object.
• Surfaces can be trimmed multiple times.
• Curves for surface/curve trimming can extend off the surface and/or cut the surface multiple times.