Scaling sketch splines, what is happening here?

brucePBRKZ — Mon, 09 Jun 2025 13:08:16 GMT

I have a sketch containing a simple profile made up of lines and splines. The model contains user parameters which define the overall size in terms of the width from which everything else is derived. Height is 1.1 x width, everything else is defined as a function of either width or height.

The initial value of width is 240 (which divides exactly by 2, 3, 4, 5, 6, 8, 10, 12, 15, 16 etc. and the problem domain (heraldry) defines things in terms of fractions of the width and height so this is convenient. The initial sketch is the outline of a shield and looks like this:

I am guessing that to scale a spline we need to maintain the angles and length as a proportion of the overall width, of the control tangents, so I have dimensions on the horizontal and vertical length of each control tangent which amounts to the same thing. This sketch is fully constrained.

If I use the scale command in the solid workspace, select this sketch and a scale factor of 0.1, I get a sketch of width 24mm which looks right. If I disable the timeline and edit the sketch I see exactly what I expect, every dimension is 1/10 the value it was before (there are no formulae or parameters which seem to be lost as a result of direct editing):

So at this point I am confident that I understand how to scale splines and if I were to simply change the user parameters so that width is 24 instead of 240 I will get the same result right? The result is, sadly, this:

As far as I can see, the control points are all in the same position in the two last sketches, yet the shape of the splines is different, it looks to me as if the curves bend inwards on the last sketch but outwards on the other two.

Can anyone explain what is happening here, or how I can get the same effect as the scale command with user parameters? I have attached the model as an .f3d file

Context

In case you are wondering why I am doing this, I am making wooden inlays on a CNC machine of coats of arms, I would like to have a single master model containing all the shapes which may be used, and then import that master model into a project for each example so that I can compose the design from the library of elements. I would like the master library to be size agnostic (although I may need multiple versions of some things with different levels of detail), hence the use of parameters to derive all dimensions from a single master value (width).

In the manufacture workspace I need to have extruded bodies which I can use to cut into the base layer and in negative to cut the inlay layer, but the library doesn't really care about the extrusion depth and all the other physical details so I am really trying to create a sketch library, which isn't really the way Fusion seems to want to work.

I have tried various things and the best workflow I have come up with so far is to have the master library just contain sketches and then either "save as" or copy and paste the sketch geometry needed for each project. Of course I get no link back to the master library this way so changes to any of the master shapes will cause a lot of manual updates if I ever get to the point where I have a lot of projects and want to change something fundamental.

If I extrude the shapes into bodies in the master library then I can use "insert derive" but that still doesn't allow me to easily change the size of the imported instance in each individual project.

Re: Scaling sketch splines, what is happening here?

davebYYPCU — Tue, 10 Jun 2025 06:20:36 GMT

Even with the detailed explanation, I can't work out if you are changing size of these shields as a Uniform, or non uniform scale. To avoid the spline shape change, can I suggest you scale bodies as either Uniform or non Uniform?

Might help...

Re: Scaling sketch splines, what is happening here?

brucePBRKZ — Tue, 10 Jun 2025 07:58:42 GMT

The scale command works perfectly, the shape change happens when I change the user parameter width from which all dimensions are derived, so it is a uniform scale in both cases.

Thanks for the suggestion.

Re: Scaling sketch splines, what is happening here?

brucePBRKZ — Wed, 11 Jun 2025 13:16:40 GMT

This answer https://forums.autodesk.com/t5/fusion-design-validate-document/spline-tangent-handles-how-do-they-work/td-p/7521488 from Jeff Strater leads me to suspect that Fit Point Splines cannot be relied upon, scaling them reveals some of the odd behaviour associated with them, especially when the dimensions become small.

I have changed my project to use Control Point Splines and now everything works as expected, so the solution seems to be "Don't use fit point splines:.

Re: Scaling sketch splines, what is happening here?

fabricio_labegaliniX5YMG — Tue, 04 Nov 2025 01:34:31 GMT

Same here. Now, just using control point splines.

topic Re: Scaling sketch splines, what is happening here? in Fusion Support Forum

Scaling sketch splines, what is happening here?

Re: Scaling sketch splines, what is happening here?

Re: Scaling sketch splines, what is happening here?

Re: Scaling sketch splines, what is happening here?

Re: Scaling sketch splines, what is happening here?