Springs

[jmwright]

A question from a KiCAD community member sparked this issue. As the name implies, should springs be part of this collection?

[fragmuffin]

@jmwright yeah!, I can do springs.

There should also be an option for simplified rendering, perhaps 3 stages:

• full quality helix
• hollow cylinder
• solid cylinder

But this is something I need to cover generally... still not sure how to define an overall model quality. I was thinking of setting some sort of "polygons per mm^3" parameter.

The main challenge with springs is creating a tapered helix... that is, a helix with a changing pitch. Do you know how to generate this?... the only way I can think of is to manually create it as a 3d spline. More generally, there should be a way to generate a function

[jmwright]

I was thinking of setting some sort of "polygons per mm^3" parameter.

Are you talking about setting the quality of a tesselation?

The main challenge with springs is creating a tapered helix... that is, a helix with a changing pitch.

The 3D spline should be a good option. Another method for a decent approximation might be to stack helixes of multiple pitches on top of each other. It would take some decent math built into the function, but I think it should be doable. This method might be too slow at higher quality settings though.

More generally, there should be a way to generate a function

Do you mean generate a function to create geometry, or something else? How would that work?

[fragmuffin]

@jmwright

Are you talking about setting the quality of a tesselation?

I wanted to have a generalised concept of what to "make" when a Part or Assembly object is displayed or exported...

• if the desired quality is below a threshold: a cylinder is returned (a simplified model)
• if the desired quality is above (or equal) to a threshold: a circle is swept through a helix to create the full complexity model.

But this model quality factor would be dependent on the scale of the part.

For example: one giant spring could be holding up something with hundreds of tiny springs as part of it's assembly... when rendering the whole thing (simply for display purposes), one would want:

• the tiny springs to be rendered as cylinders, while
• the larger spring should be the highest quality.

Do you mean generate a function to create geometry, or something else? How would that work?

For example, a conical spiral is another curve that would be used to create fully collapsible springs. One way to represent this is:

a = 1; r =1
x(t) = r * t * cos(a * t)
y(t) = r * t * sin(a * t)
z(t) = t

I guess we've already discussed how to do this; to have x(t), y(t) and z(t) as functions, and give a range of t, and the number of points to be represented as a spline (linearly spaced).

def wire_points_generator(fx, fy, fz, t1, t2, steps):
    step_size = (t2 - t1) / steps
    for i in range(steps):
        t = i * step_size
        yield (fx(t), fy(t), fz(t))

from math import sin, cos
(a, r) = (1, 1)
conical_spiral_points = list(wire_points_generator(
    fx=lambda t: r * t * cos(a * t),
    fy=lambda t: r * t * sin(a * t),
    fz=lambda t: t,
    t1=0, t2=1, steps=100,
))

[jmwright]

I've never tried to make a 3D spline with FreeCAD, but it sounds like it's possible.

https://forum.freecadweb.org/viewtopic.php?t=12032

I wonder if it would work with your wire_points_generator setup as-is.

[fragmuffin]

Yeah, I'm sure it is possible, thanks for the link!

I think it would be great to have something that intelligently used more points for increased accuracy. For example, for a parabola:

x(t) = t
y(t) = t ** 2
z(t) = 0
{-5 <= t <= 5}

you'd want more points around to t=0 because the parabola's curvature is greatest there. Or perhaps it should be based of the rate of change of curvature).

I'll ponder this when I'm making springs :wink:

[fragmuffin]

Removed from the release project, but still relevant.