Splines are often preferred to lines and arcs when modeling free form or organic shapes because they can create smooth and blended geometry. Since splines can have continuously changing curvature, they are difficult to define with dimensions and are typically left under-defined or a Fixed constraint is applied to prevent them from being modified.

The Fit Spline tool gives us an easy way to constrain a spline to specific dimensions.

Below is an example of a sketch created utilizing lines and arcs.

solidworks fit spline, SOLIDWORKS – FIT SPLINE

Tangency between lines and arcs creates a smooth transition but because of the abrupt change in curvature an edge is created that can be seen and felt. This can be undesirable in certain finished products.

solidworks fit spline, SOLIDWORKS – FIT SPLINE

Let’s use the Fit Spline command to combine the lines and arcs into one continuous spline and compare our results. We will edit our original sketch and select Fit Spline which is found under the Tools menu – Spline Tools – Fit Spline.

solidworks fit spline, SOLIDWORKS – FIT SPLINE

My original sketch was an open contour so I will leave Closed Spline unchecked.

I will leave the Delete Geometry option unchecked. This will keep my original sketch entities and they will be converted to construction geometry.

I will select the Constrained option which will parametrically link the spline to my original sketch geometry.

The Tolerance setting specifies the maximum deviation of the spline from the original entities.

With these options set, I will select all the entities I want converted to a spline.

solidworks fit spline, SOLIDWORKS – FIT SPLINE

Accepting these setting and exiting the sketch, we can see it is now one continuous smooth face with no abrupt changes in curvature. We could also use tools such as Curvature display or Zebra stripes to further investigate the quality of the surfaces and how well the surfaces blend together.

solidworks fit spline, SOLIDWORKS – FIT SPLINE

Because we utilized Fit Spline to convert the original lines and arcs to a spline, we can quickly and easily update the sketch dimensions which will automatically update the spline.

solidworks fit spline, SOLIDWORKS – FIT SPLINE

Next time you need to create a smooth blended contour whether it’s a solid or surface body and need to constrain it with specific dimensions, try using Fit Spline!

Brian Morris
Solidworks Elite Application Engineer
Computer Aided Technology

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