topic Re: Variable diameter Spring / Helix. in AutoCAD Forum

Variable diameter Spring / Helix.

Anonymous — Thu, 29 Mar 2018 16:26:56 GMT

Hello !

I am writing to ask if using AutoCad 2017 it is possible to draw an helix of variable diameter, in which the helix starts with a small diameter, then the diameter increases with each turn till half the total height, then the diameter starts decreasing with each turn, to the total height of the spiral.

The external contour of the so achieved spiral is a curve, please look at the attached picture.

What I am trying to do is to model a spiral spring whose external shape follows a curved shape, not a double cone ( like one conical spiral inverted and aligned over another equal one and then “ unioned “ ).

Thank you very much for any help that someone can provide.

One of the attempts that produced the nearest results to what I want to do, was projecting an helix positioned outside an curved closed surface ( created by lofting three different diameter circles ), the resultant external surface profile is a curve.

Unfortunately, after projecting the helix ( I tried the three different methods available in AutoCad ) the results were not as expected.

The helix projection to the surface creates cross section ellipses ( closed splines ) not a spiral path along the prolate spheroid shape inside it.

Only the “ Surface projection VIEW “ method worked, the “ Surface projection UCS/VECTOR “ methods did not projected anything.

I am arriving to the conclusion that this is not a task AutoCad can perform, but if you ( or anyone ) reading this post come with new ideas I am always willing to try them.

Below is a photograph of the method I tried to create the “ Oval “ spring.

Best regards
PR

Re: Variable diameter Spring / Helix.

peterm — Thu, 29 Mar 2018 16:35:09 GMT

Use the HELIX command.

You set the base diameter, then the end diameter, then the height.

Make two of these, one getting bigger and then one getting smaller. Then position them to continue one onto the other.

Finally sweep a circle of the correct thickness through the helixes and union the resulting solids

Re: Variable diameter Spring / Helix.

Patchy — Thu, 29 Mar 2018 16:36:32 GMT

Have you seen this?

https://www.youtube.com/watch?v=HtADR5MZt9w

Re: Variable diameter Spring / Helix.

Kent1Cooper — Thu, 29 Mar 2018 17:14:40 GMT

@Anonymous wrote:

....
What I am trying to do is to model a spiral spring whose external shape follows a curved shape, not a double cone ( like one conical spiral inverted and aligned over another equal one and then “ unioned “ ).
....

Both Posts 2 & 3 missed that part....

Here's a possible start, though the coil is not circular in cross section:

Half-ellipse, Offset for shell thickness, close the ends, Revolve it around the center axis for an egg-shaped shell.

Helix a little shorter than the shell, Rectangle extending from Helix end to center axis, Sweep the Rectangle along the Helix.

Shell and swept-rectangle thingie are together in the middle image -- Intersect them for the right image.

I haven't yet thought of a way to convert that coil into a path along which you could Sweep a Circle to get a circular cross-section, but there may be a way.

Re: Variable diameter Spring / Helix.

Anonymous — Thu, 29 Mar 2018 18:28:22 GMT

Hello Mr Kent Cooper

Thank you very much for your contribution !

You almost achieved what I am trying to do !

Your method was the best approach I have seen to solve this “ problem “.

I am going to follow your description / steps and try to do it myself.

Afterwards, as you say, it will be just a matter of extracting the helicoidal path.

I am starting my mini Easter holydays today, next Monday I will post news.

Have a nice Easter !

Yours faithfully

Re: Variable diameter Spring / Helix.

Anonymous — Thu, 29 Mar 2018 19:18:17 GMT

Hello again !

I couldn’t wait to try your method Mr Kent ! ( Super AutoCad Man ? )

I have followed your instructions, and guess…

Success !!!

This looks almost what I need, if the resultant intersection cross section was circular the problem was solved.

Let’s see if it’s possible to “ extract “ the helical path from that “ weird “ shape and then use it to sweep a circle along it and obtain the desired uncommon spring !

Again, Have a nice Easter !!!

Re: Variable diameter Spring / Helix.

Kent1Cooper — Thu, 29 Mar 2018 20:09:51 GMT

@Anonymous wrote:

....

Let’s see if it’s possible to “ extract “ the helical path from that “ weird “ shape and then use it to sweep a circle along it and obtain the desired uncommon spring !

....

That's the hard part, from a 3DSolid object. Thinking about it, I expect it would be easier to come up with a routine that would construct the spring shape as a Spline in 3D [along which a Circle could be Swept], by calculating a series of points along it to whatever degree of accuracy is required. Toward that end, I wonder:

Does it need to go all the way to the tips as yours appears to, rather than stopping a little short with an open center in the ends, as mine does?

Would it be acceptable for the outer shape of it all to be defined in arc-shaped geometry [yellow] rather than elliptical [red]?

[Tilted on its side to take up less space in the Post.] I think that would be much easier to work out the calculations for, at least as I'm imagining approaching it. I can't quite tell whether yours is arc- or ellipse-based.

I'm picturing asking the User for a center at the base, a height, a radius of the opening at the end [which could be 0], a radius at the middle/equator, and a number of times around. The routine would calculate the radius of the virtual [yellow] side-view arc, and start a Spline, calculating a point at the end radius and 0-degree direction from the center-base point, then another moving up some tiny calculated fraction of the overall height and calculating the radius there based on the arc geometry, and at a direction swung around a few degrees, and repeat all the way up.

Sort of a 3D expansion/elaboration of the idea of SPIRALS.lsp, available >here<, if you want to see something that steps by increments and calculates successive locations as it goes [but in 2D].

Re: Variable diameter Spring / Helix.

Anonymous — Mon, 02 Apr 2018 12:33:49 GMT

Hello Mr Kent Cooper

This weekend I had an idea on how to draw the spring based on your technique of solids intersection, I think it solves the problem.

I modified your process to become a surfaces intersection process, instead.

Your suggestion to create a routine to accomplish the task is great !

It can simplify the process and save lots of time.

You asked if the spring need to have the ends closed; for my application it is not a requirement, and as can been seen on my example pictures below, indeed they are very open.

The outer shape can be generated by an arc ( it was the method I used on the example pictures below ) or any type of curve as needed by the user.

The input options you choose for the routine seems perfect for me, everything needed is covered !

Now…

To the…

Variable diameter coil “ recipe “

Step One

A- Create an axis of revolution for your spring outer shape.

B- Create your spring outer shape using an arc or “ custom “ curved shape and place it at the correct “ spring radius “ distance ( distance away from the axis of revolution ).

C- Centred on your axis of revolution, create one helix with the desired pitch and height, use the axis end points to define the helix height, then go to “ Properties “ constrain the helix height and adjust diameter, helix direction CW/CCW and number of turns to your taste, the diameter should be such that the line that will be swept around it will intersect the spring outer shape profile.

D- Draw a line at one end of the spiral with sufficient length to intersect all the spring outer shape when swept around the helix.

E- Finally, revolve the coil outer shape curve around the axis of rotation, thus creating an oval surface, sweep the line at the end of the helix and create a spiral surface.

See picture below:

Step two

A- Do an intersection of the two previously generated surfaces.

B- A spiral with the required pitch, height and external profile will result, this spiral is made of several splines.

C- Delete all auxiliary geometry used to build the surfaces ( the original helix, axis of revolution, external profile, and helical surface generating line ) and retain only the desired external profile helix.

D- Select all splines, join them and create one 3D spline that will be the path to sweep the coil “ wire “ cross section, ( on my example it is circular ).

See picture below:

Step Three

A- Draw a shape that represents the cross section of your coil “ spring wire “ ( usually it is circular, but could be square cross section ).

B- Move the newly created wire cross section shape from its centre to one of the ends of the previously created path generating 3D Spline.

C- Sweep the cross section shape along the 3D Spline and obtain your variable diameter coil !

See picture below:

Other views:

Mr Kent Cooper, I want to Thank You Very much for your Invaluable contribution without which it would be impossible for me to devise this “ recipe “.

I hope this information will have any use to someone trying to model something similar.

Best regards