Spline Schmline, part 4: Conics

This is a topic that gets hit from time to time on the SolidWorks forums. Recently, it resurfaced again. This time it’s not someone asking why SolidWorks doesn’t have conics or explaining what they are or someone trying to offer a way to create conic sections from a plane and a cone, or a way to use a 2 point spline as a conic. This time someone actually went one better. This fellow wrote an equation that you can use to create a conic curve.

SolidWorks now has the ability to create an equation driven curve. In the past you used to have to write the equation in Excel, export a set of points and then make a spline through the points. The new equation driven curve is much more efficient.

Still, this doesn’t address the real need for conics in SolidWorks.

In the geometrical sense of the word, “conics” can be taken to mean “conic section”, which is the classic, line, circle, ellipse, parabola, hyperbola progression that you can create by sectioning a cone with a plane. You learn that in high school, I think.

In the CAD sense of the word, a conic is limited to something in the range of a circular arc to a parabola, so it is a well defined shape. It has constant convexity. In other CAD programs it is often driven by a “rho” value, which changes the shape within the range of possible shapes. It is like a 2 point spline, but it cannot have a convexity inflection point (always U shaped, never S).

Users of other CAD packages report that these shapes are almost indispensible as a sort of non-arc based transition (fillet) between other shapes.

SolidWorks does have a sketch entity for Parabola, but it is notoriously difficult to wrangle, and may not be capable of the same range of shape as a general conic curve. Please join me and others in an enhancement request campaign to get SolidWorks to add the Conic as a sketch entity. I know there are people at SolidWorks who see and understand the value of this type of tool, but another useless AutoCAD-esque widget for the masses might tickle the marketing fancy more than something that may be difficult to understand how it works, or why you would use it.