Fit Splines: Are There Any Limits?

Hello Everyone!

I would like to a moment to discuss an interesting but often overlooked command inside of SOLIDWORKS. That command is called the Fit Spline command. Now most of you have heard about splines but you may not be aware of the “Fit Spline”. “Fit Spline” is interesting in that you can take just about any sketch geometry and turn it into a spline by using this command. This can be a great way to get the functionality of a spline but using our traditional sketch tools and constraints. It also turns all of the selected sketch entities into a single sketch entity. When this single sketch entity is turned into a Feature (3D Geometry) it also creates a single face/entity. This can be an extremely useful way to get certain visual aesthetics.

When using the traditional spline commands it is very common to hear users make comments about how uncomfortable they are with dimensioning or restraining the sketch entities. “Fit Spline” makes this much easier and more approachable by allowing users to create sketch entities using the standard sketch commands such as Line, Arc, etc. and then applying a “Fit Spline” to those features. A couple of items to note about “Fit Splines” before we look at some examples. First, they are an approximation. This means that they are not exact and the software is making some assumptions when it creates the fit spline. Second, because of this approximation, you will not be able to use the measure command on the “Fit Spline” sketch geometry or the face that is created other than for length. Third, if there are any sharp corners on your selected sketch entities they will be converted to a tiny fillet after the “Fit Spline” command has been applied. With those details out of the way, let us look at some examples. We are going to look at a simple square shape and how using different techniques will affect the visual appearance and how we edit the geometry.

I will start will a basic square with sharp corners.

Notice that we can select individual faces on the geometry that we created (selected face highlighted in blue below).

Now if we take the same geometry but apply the “Fit Spline” command and rebuild, we can see that they are now treated as a single entity because of how the “Fit Spline” command functions. So if we select any of the faces associated with the underlying sketch geometry it highlights and selects all of the faces. The two faces that are not highlighted are created by the Boss Extrude command so they are considered separate. See image below.

Now let us take a closer look at one of the corners and the underlying sketch geometry. Notice that there is a small spline fillet that has been applied at the corner. This fillet does not have a dimension associated to it and we cannot change it.

Below we can see what that corner looks like close-up from an angled view.

Here is the same corner shown from further away.

Now notice what happens if we try to measure the fillet at the corner (image below)

The nature of this command and how it functions removes our ability to use the measure command except to give us the total length of the feature.

The same holds true if we try and measure the sketch as well. (image below)

Now what happens if we apply our own fillets to the sketch?

This gives us slightly more control over the size of the fillets in the corners but they are still converted to splines and approximated.

We still cannot take measurements other than length. We do have control over the size of the fillet in the corners though. We can see in the image below that I have changed the fillet from 10mm to 25mm.

Now depending upon the type of project that you are working on this may be perfectly acceptable. I just want you to be aware of the limits of this command. With that said, it is still a very powerful way to create geometry with certain aesthetic appearances.

Below is an image that shows a comparison with the Fit Spline part on the left and a standard sketched part with fillets on the right.

It gives you a nice clean look even while using the Shaded With Edges display style.

I hope that this has been informative and I also recommend checking out the blog “SOLIDWORKS: What Is A Fit Spline?” by Blake Cokinis to learn more about the “Fit Spline” command and other use cases.  https://www.cati.com/blog/2017/07/what-is-a-solidworks-fit-spline/

James Reeher
Application Engineer
Computer Aided Technology