|SolidWorks Featured Author Blog - November 2012|
SolidWorks 2D and 3D splines are an essential tool for anyone creating complex shapes and mastery of them can make the difference when creating aesthetically pleasing shapes. Unfortunately thou, they a daunting for many because they are so unlike analytical sketch geometry that is simple to comprehend, constrain and dimension. Let’s take a walk thru them, to help better familiarize and understand them.
2D verses 3D splines
Currently in SolidWorks, you can create 2D and 3D splines, but it is worth noting that there are major differences between them although we have and (are continuing) to make them more consistent with each other. What lies at the heart of their differences is the fact that they each use different constrain solvers. Although we would have preferred them to both use the same, it is not technically possible at this time. We have been working to resolve but it is not easy. So let me just help you make the best of them at this point in time. My first recommendation is that, whenever possible, create 2D sketch splines over 3D sketch splines; the solver is more robust in 2D than 3D since there is the additional vector of 3D space to solve for. If you need to use 3D sketch splines (and in many cases of complex shapes it is your only alternative) the issue that usually arises is over-constraining the spline when applying constraints or when parent geometry changes. Of course, SolidWorks unique power to create parametrically constrained (and therefore controlled) splines makes them adaptable to parent feature design changes. As mentioned before, there are differences between 2D and 3D sketch splines when it comes to constraining them. First rule of thumb is, per the over-all design intent, consider whether, curve continuous constrains are necessary. In all CAD systems (not just SW) C2, C3 and beyond connections require more careful consideration (overhead) on the part of the user.
Here are a few tips to keep in mind that can help you avoid these issues:
In summary, 3D sketch splines use a different, solver which must solve for 3 dimensions verses two, so try to keep your 3D sketch splines, simple, don’t use equal curvature, if it isn’t necessary, and remember the face curvature constraint.
2 different Spline types
There exists two different ways of working with SW splines: thru point splines (default) and polygonal control splines. In fact, most think that the polygon control is just an option but don’t realize that these are two very different mathematical representations for the spline – let me illustrate in the following video
So as you can see in the video, when you turn on the polygon control, you are actually turning off Parameterization. To get back to a thru point spline with automatic parameterization is not possible at this time, although we should have this fixed in a near future service pack. When that is fixed, I’ll try to remember to announce it. I believe the reason this has not been more well-known is that we’ve observed most customers using one or the other instead of switching back and forth. Please let us know it you use both and have seen this issue.
The following is video is an update to the Guitar video I did a few years ago. I think it is a great example because, first, because a guitar shape is so familiar to us all, and second, it works well to illustrate the principles of how to reshape splines in a logical and controllable manner.
In summary, familiarize yourself with the various widgets that are part of 2D and 3D sketch splines:
C3 Spline in SolidWorks
What about designs that require smoothness of curvature beyond just matching (equal curvature.) There is one instance of option for a 2D and 3D sketch spline in which the user can specify a higher degree of curvature matching, but there are certain limitations:
- Can only be a 2 point spline
- Must apply equal curvature constrain to both ends
- If the above two our met, then need to select the spline and make sure that “Degree Raising” option is check marked in the spline’s property manager.
Fit spline – a “poor man’s” spline
Many engineers shy away from splines for the simple fact that it is difficult to constrain and drive them with dimensions. Fortunately for them there is an alternative which allows them to use splines but control them with dimensionable non-spline sketch entities. Fit Spline is a sketch function which allows for a user to create a “fitted” spline over existing sketch geometry that is contiguously connected or even non-contiguous sketch geometry that has small gaps.
The idea is simple: create a serious of lines, arcs, conics, ellipses which “represent” the spline shape; dimension them fully as you would any sketch and then select them and create a Fit spline over them. By default, any changes you make in the underlying geometry (which is now construction) will “drive” the fit spline. When you create the Fit spline, you will be able to adjust how close the fit splines adhere to the underlying geometry. Remember that you have to always give up how accurately the Fit spline follows under lying geometry if you want it smooth. This is because the underlying geometry is usually only C1 at their connection points. I term it poor man’s spline because in many cases, you will not be able to achieve the smoothness of a single manually created spline.
- If you are going to create C2 3D sketch spline, only create one sketch spline per sketch feature.
- Use the “Relax Spline button often” when you are modifying splines. It has a nice way of re-parameterizing (smoothing) entire spline.
- When drawing a spline shape, use as few points as possible to start with and only add interim points when absolutely necessary to describe shape.
- When placing interim spline points, place them roughly at the inflection areas of the shape – i.e. the peaks and valleys.
- When modifying a spline’s shape, start first with repositioning the interim spline points, followed by changing the vector handle and then lastly adjusting the weighting of the handle.
- When adjusting the weighting of a handle, hold down your <alt> key to get symmetrical weighting; this will result in a smoother C2 match across the interim spline point.
- Only create an asymmetrical weighting as a last resort, since the C2 match is not as good (but still maintains C2) across interim spline point.
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.