Utilizing Splines - Bad Spline Day

A short how to on utilizing spline controls; get control on your splines!

Everyone has bad hair days. For me, that’s every day. When it comes to using splines, it can often feel as if SOLIDWORKS is having a bad hair day. No matter what you do, you just can’t get them to go where you want them to go. A spline certainly is the free spirit of SOLIDWORKS sketch entities. It can also be one of the most powerful tools you have at your disposal for transitions and consumer-type surfaces.


So here are my TOP 5 TIPS for creating and managing splines in SOLIDWORKS:

1. Use Fewer Points

It is the single most important thing that you can do to create great looking splines! If you take nothing away from this post other than this, you will be 80% there! The temptation is to start tracing out the shape you want with the greatest resolution of points you can. If you’ve spent any time with the Polygon Lasso tool in Adobe, this temptation is heightened. We know that if we trace a path with a large number of small straight lines, it begins to look like a curve.  We’ve all created that illusion connecting the dots on the paper covers of our math text books (I can’t be the only one). However, this is NOT how the Spline tool works. The spline tool is a mathematic curve generated by the weight of the points or by the convex hull depending on your spline choice (more on that later).

In this image, the spline with more points has a poor curvature while the 2 point spline has excellent curvature.


Bottom line: Start with a 2 point spline. If you can’t get the curve you need, right click the Spline and add a spline point! Complex curves can still be modeled with 2 point splines. When the curve stops, your spline stops as well. Here we see 7 individual splines strung together and mirrored to produce the Fleur de Lis. 6 of the 7 are 2 point splines while the curve in red required a 3rd point.


2. Create Constraints

One way to tame a spline is to create bounds for the spline to conform to. This methodology is simple and effective. Here we can see a “frame” built from construction geometry to hold the spline. Don’t forget that you can add relations to the handles! In this case, the top and bottom handles are set to “vertical.”


3. Change Control Methods

Did you know that standard splines have 3 control methods? This allows you to get comfortable with the method that gives you the most control, or is easiest to use.

The options can be found here: Tools>Options>System Options>Sketch.


We see that we can control our splines with handles, the control polygon, or neither.

Neither gives us the least amount of control, but greatest ease of use. We see here that we control the spline by the location of its points.


The control polygon allows us to control the spline through an imaginary box of tangency lines around which the spline is interpolated.


Spline handles allows us to control the weight of each point. These handles will give us a great amount of control. We are able to set the direction as well as the magnitude at each point. This is my preferred method, but arguably the most difficult to master. Handles are made up of 3 controls. The diamond will allow you to make directional changes only. The arrow will allow you to make magnitude changes only. The circle at the end will allow you to change both at once.


4. Check Curvature

Monitor you curvature frequently so that you can be sure to create smooth geometry. SOLIDWORKS makes this easy for us, as all we have to do is select a spline, then select “Show Curvature” in the properties. Look for steep sections of combs and places where the combs switch sides of the curves. Avoid sharp changes in comb height.


5. Know Your Splines

By now you may have heard that SOLIDWORKS surfaces are based on a mathematical formulation called NURBS. It’s a whole lot of fun to say. Go ahead and call your coworker a NURB and see if it doesn’t put a smile on your face! I’ll wait…

A NURB is a Non-Uniform Rational B-Spline. This B-Spline (short for Basis-Spline), is the way we manipulate a standard spline in SOLIDWORKS. The B-Spline method uses weighted points to define the curve.

In contrast, the Style Spline (introduced in 2014) uses a Bezier definition. In an effort to avoid boring you to tears with equations, I’ll just say that the curve is derived from the direction and magnitude of the hull that it fits in. This manifests itself in the software as a backwards approach to the spline from what we are used to.  It allows us to draw what we called the “control polygon” instead of drawing the spline explicitly. The Style Spline can be found here:


The style spline has a number of benefits. For one, it is more similar to the way artistic programs create curves. If you are familiar with the Pen tool in Adobe, the Style Spline will feel more comfortable to you. Here is a side by side of a style spline and a curve created with the pen tool in Adobe Illustrator. We can see the similarity clearly. The style spline is on the left while the pen path is on the right.


Secondly, we are able to add dimensions and relations to the polygon. This allows us to dimension a spline directly for the first time! On that topic, I will leave you with a quick tip. SOLIDWORKS 2014 now allows us to add dimensions directly to any spline as a path length dimension! Just grab your trusty Smart Dimension, and select the spline!

Hopefully this post will help some of you overcome your fear of the Spline tool! Get out there and start making sleek products!