topic Re: spiral modeling in AutoCAD Forum

spiral modeling

riccardobaldino0 — Sun, 18 Aug 2024 07:51:55 GMT

Hi, let me start by saying that I have been using Autocad for a few weeks, 
unfortunately I am unable to model a spiral so that it rests on a sphere, 
I am attaching some images that portray my tests and the result that I would like to obtain, 
how can I do it?

Thank you so much

Re: spiral modeling

j.palmeL29YX — Sun, 18 Aug 2024 18:55:07 GMT

In short words:

- create a sphere as surface

- create a helix along the diameter of the sphere, any diameter.

- draw a line at the end of the helix to the helix's axis with a length equal or greater than the sphere's radius

- sweep the line along the helix as surface.

- start the intersect command and choose the both surfaces (sphere and sweeped line)

=> you will get a 3D spline along the sphere

- draw a circle perpendicular to the end of this spline

- sweep the circle along the spline

Attached an example file. (To examine the model turn off the layer Solids and turn on the layer Construction).

If you need more detailed assistance let me know.

(view in My Videos)

Re: spiral modeling

Washingtonn — Sun, 18 Aug 2024 16:49:33 GMT

The method below generates a distance between turns that varies with the sphere surface. It takes a little work but provides an accurate result.

Re: spiral modeling

j.palmeL29YX — Sun, 18 Aug 2024 18:28:15 GMT

If we want a curve with a constant distance between the turns in the top view (this is your intention?) we will get a result as shown in the attached image.

I know why you stopped your suggestion before the curve reaches its max diameter. To avoid the red marked defect you must cheat a little - and the curve will no be "precise". We will get a not exactly determined result with neither constant distance between the turns in height (as I suggested) nor constant distance in top view (as you tried to achieve).

Re: spiral modeling

Washingtonn — Sun, 18 Aug 2024 21:22:07 GMT

The OP specifically stated that the spiral was to rest on the sphere. The spiral should then be recognizable as a spiral when viewed from the top or bottom of the sphere (start or end of spiral).

Your second post makes an assumption and implies my response contained some slight of hand or trickery – it did not. I don’t know how you came to you conclusion that a cheat is necessary unless you failed to account for how splines work – the curve between the first 2 points will not match the ongoing radius needed for the spline to work or perhaps your spline doesn’t cross over both hemispheres at the same point. Add a new first point mirroring the original second point or draw the spline from pole to pole of the sphere.

I will say that slight modifications to points may needed, depending on the number of points used to develop the spline, but overall, the method I proposed should satisfy the OP’s request.

(view in My Videos)

Re: spiral modeling

j.palmeL29YX — Mon, 19 Aug 2024 05:47:33 GMT

@Washingtonn wrote:

The spiral should then be recognizable as a spiral when viewed from the top or bottom of the sphere (start or end of spiral).

Hmm, that I did not read in the OP's post. And his 2nd image (Immagine 2024-08-18 093614) let me interprete that the spiral should be placed on the surface of a sphere. Obviously I misunderstood the challenge ....

Re: spiral modeling

Washingtonn — Mon, 19 Aug 2024 11:08:42 GMT

It was meant to be on the surface of the sphere. In the video, I took the liberty of reducing the size of the sphere in order to display the spiral more clearly. Also, more points and slight adjustments to some points might be needed to make the spiral fit/appear better on the sphere - especially at the "cut" ends.

(view in My Videos)