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How to create a parametric helix with multiple loops

After trying with different methods to create a parametric helix with multiple loops, such as in the conceptual design environment with points, extrusions, repetitions, etc., I have chosen this method, in the standard family editor, as the most practical, easiest to edit, and easiest to use. In this article, I want to describe the method.

We need 3 families, in this way:

Family
#
Template Sample name Description Parameters created in this family Parameters tied to others
1 Profile.rft “Profile for helix.rfa” This will contain a 2D closed shape that represents the section of the helix at any point. This family will be nested into Family 2.
  • Elevation [length]
2 Generic model.rft “Helix_one loop.rfa” This will contain one complete loop of the helix, made with swept blend solids, using Family 1 as the profiles, with different elevations. This family will be nested into Family # 3.
  • Radius [length]
  • Elev half loop [length]
  • Tie 1.1 to 2.2, only for the “Profile for helix_high” type in the profile family.
3 Generic model.rft “Helix_n_loops.rfa” This will contain a linear array, from the Front view, to repeat Family 2, as needed.
  • Radius [length]
  • Elev at half loop [length]
  • Constraint for array [length]
  • # of loops
  • Tie 2.1 to 3.1
  • Tie 2.2 to 3.2

.


Family 1

This family starts with the profile.rft template. It requires one reference plane, and one length parameter, such as “Elevation”, to control the offset of the profile from a reference plane. Make sure that the shape moves up and down when the Elevation parameters is changed. Having the section of the helix as a separate, simple 2D shape, makes the helix very easy to reuse for multiple purposes, simply changing this section, and doing the nesting into the next families again.

No types are required in the profile family. Save, and load this family into Family 2.


Family 2

This family starts with the generic model.rft template. It requires one reference plane, created from the Front view, named such as “half loop”. The purpose of this family is to create one complete loop of the helix. Create parameters as shown in the table, above. From a Front view, create a reference plane. Name it, such as “half loop”, and control its height with the “Elev half loop” parameter.In this family, once Family 1 has been nested, select the profile from the Project Browser, and make 2 new types. Name one of them such as: “Profile for helix_low”. The Elevation parameter of this type should be zero. Make another type, and name it such as “Profile for helix_high”. Tie the Elevation parameter of this type to the local “Elev half loop” parameter, to take control of the vertical distance ran by the helix at half a loop. Set this elevation to starting default value such as 4′.

Now, from the plan view, start a solid by the Swept Blend method. For the path, create a semi-circle, with a radius such as 6′. The starting point determines the start of the helix, and Profile 1. Lock the center of the arc to the intersection of the origin reference planes. Control the radius with the Radius parameter of this family. From a Front elevation, set Profile 1 to be “Profile for helix_low”, and set Profile 2 to be “Profile for helix_high”.

For the profiles, use “Profile for helix_low” for Profile 1, and “Profile for helix_high” for Profile 2.

When the Swept Blend is finished, the result should be half a loop of the helix.

Use Set > Workplane, to set the “half loop” reference plane as the new workplane, and make another Swept Blend, beginning where you finished the first half, repeating the same steps.

Assign again the profiles. The “low” at the beginning, the “high” at the end of the arc.

When this second swept is finished, we will have a complete loop of the helix.

Save, and load this family into Family 3.


Family 3

This family starts with the generic model.rft template. Family 2 has been nested and placed at the intersection of the origin reference planes. The purpose of this family is to repeat Family 2, vertically. In this Family 3, from the Front view, create two reference planes. One of them is to create a base offset, to lift the helix, if necessary, from a reference level. The other one is to control the top of the linear array. Align and lock Family 2 to the reference plane that will control the base offset. Create and tie parameters as shown in the table, above. Control the two reference planes with the correspondent parameters. Make a formula to control the array by number of items, as shown in these images.

Then, make a linear array of Family 2. Tie the number of items to the “# of loops” parameter. Align and lock the last item on the top to the top reference plane. If everything went well, you will have now a parametric helix.

Remember, “nothing is ever easy“. If something goes wrong, try again…


This family can have multiple applications, just by changing the profile (Family 1) and adjusting the dimensions of Family 3. These are some of the applications I can name:

  • In architecture: a recreational slide, or a pedestrian ramp, or a vehicular ramp, or other architectural features with the spiral form.
  • In mechanical engineering: draining or agricultural devices using the Archimedes’ screw, or a spring.
  • In structural engineering: ramps, or a seismic spring device.

Depending of the field of your work, I am sure that you will find several other applications.


See you in our next blog…

© 2012 Planta1.com, inc. , Alfredo Medina | permission to reproduce this article is granted if the name of the author and the URL of this article receive proper credit.

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