SolidWorks Featured Author Blog - October 2012 |
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The addition of a conic sketch entity in SolidWorks has been at the top of enhancement request for a number of years and now the wait is over with the introduction of them in SolidWorks 2013. Up to SW2013, users who needed conic sections (for instance a hyperbola) would have to create one using an equation driven curve.
Having come from a CAD background which included conics, this has been a passionate personal initiative of mine (and others) inside of SolidWorks for more than 6 years. There were times in past releases that we came close to introducing them, but because of quality issues and unity with the rest of the SW sketcher, we held back. For their 2013 introduction, Rob Jost, of Product Definition wrote the specification for their implementation and R&D Senior Manager, Ditmars Veinbachs oversaw its development.
One of the biggest issues preventing us from introducing them earlier is that we knew in order to do them correctly, they need to be a valid citizen with our solver and the capability to solve them lies with the solver. Now that the goal has been met, SolidWorks users have the freedom to use conics with any other 2D sketch geometry. The absence of them for so long, as compared to many other CAD packages, rest in great part on the solver issue. For instance, you can create conics in Rhino (and have been able to for a very long time) but you do not have the ability to parametrically constrain sketches in Rhino like you can in SolidWorks.
Why are sketch Conics important?
The answer to that question will ultimately come from our users, but we have recognized that conics are, among other things, a very efficient sketch entity for creating a bridge curve or an optical profile for lens designers. They are efficient in that, unlike a sketch spline, you can more simply constrain and drive them, especially in parametric/history-based CAD. The conic is strictly driven by the constraint type of its two endpoints and the Rho value which affects the shape (conic section) and can be driven with a single dimension. They also are robust in that they can never invert on themselves (will always be convex – or concaved depending on how you use them for a particular feature.)
Let’s take the example of how product designers use them to create a “bridge curve” when they need to drive the shape of a blend surface for instance.
Imagine that you require creating a surface that bridges between two existing surfaces, but in order to better define its shape, you will need to create some 2^{nd} direction curve/s. A Conic is a good candidate for this (see figure 1.)
Figure 1 – 2D sketch conic used as a bridge curve
You can then use a single conic to drive the second direction of the Boundary surface as shown in figure 2.
Figure 2 – conic sketch used in Boundary surface
The workflow for creating a sketch conic is very efficient in that it allows you to create tangent end conditions (with helpers) while you create it. Here’s the way that it works:
- Start a new 2D sketch that intersects the two surfaces that you’d like to create a bridge curve. Next create intersection curves (Tools>Sketch Tools>Intersection Curves) on the existing faces that intersect the current sketch like shown in figure 3.)
- Next, click the Conic sketch icon in your sketch toolbar of the Command Manager. If it does not appear, you might have to customize it into your sketch toolbar.
- Now while Conic sketch tool is active click the end point of the upper left endpoint of the intersection curve. Note the inference line for tangency at that endpoint appears.
- Next, click on the lower left endpoint of the other intersection curve. Note the inference line for tangency at that endpoint appears.
- Now, move your cursor to the intersection of the two inference lines and note a tangent constraint icon appears to the right of the cursor. Now left click to place. The result automatically infers tangency from the two intersection curves and insures that the conic is tangent with the two adjacent faces.
- The final step is to drag your cursor to change the shape of the conic – the Rho value.
- You are now free to move on or if you’d like to constrain it with a dimension you can right-mouse-button and invoke the dimension icon on your “mini-bar” and then click on the conic and dimension it accordingly.
- The final step before leaving the sketch is to make the two intersection curves toggled to construction since these are just for reference and will not be used in the surface feature.
- Now that you have a conic bridging the two surfaces, you can use this as the second direction curve to give more specific definition to your overall boundary surface. In this case, we are using it to create a conic fillet (see figure3.) You can download this SolidWorks example file here.
Figure 3 - example of conic sketch to create conic fillet
What are some of the current limitations for the SolidWorks 2013 Sketch Conic?
- Currently is only implemented as a 2D sketch entity (cannot be used in a 3D sketch)
- You can only apply a coincident or tangent constrain to its endpoints.
- You cannot make it equal-curvature with adjacent sketch geometry.
Upon its initial release, these are limitations that we are aware of and in future releases, we are hopeful that we can eliminate some of these restrictions.
There has been some discussion about why conic cannot be made Curve Continuous to adjacent sketch geometry. Keep in mind that by its very nature, conics cannot be curve continuous since there shape, per say, is driven strictly by its angle of intersection of a cone, whether it be a circle, ellipse, parabola or hyperbola. What could happen is if the adjacent sketches (that it was being constrained to) are splines, then they could be made Curve Continuous to the conic. But again, this is not the conic changing but the adjacent splines. Unfortunately, even a Curve Continuous relationship cannot be made between the conic and spline in its first release. This is something that we know designers will require and so we are seriously looking into for a future release. Until this happens, a workaround does exist; you can create construction lines before creating the Conic, making them Curve Continuous to the splines, then you can create your Conic sketch tangent to the construction lines (see figure4.)
Figure 4 - Conic sketch tangent to the construction lines
A degreed Industrial Designer, Mark has worked in the product design industry for over 25 years for such companies as Atari, Hewlett Packard and IDEO. Joining SW in 2004 as a Product Manager, he currently is Senior Product Manager for Definition.
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Mark, do you have an example where a 2 point spline would not create a stable feature (maybe by getting some inflections along the way in a sweep, loft or boundary feature) and a conic would successfully do the job?
BTW, very nice article!