How to draw a helix with varying pitch

labern

Hi
I need to draw an item that has a helix that varies in pitch as it goes around a cylinder. as per attached picture.

I started to try and do it by adding multiple surfaces going up a cylinder and adding a different helix on each surface. but it didn't really work.
It would be good to be able to use spline tool around the cylinder. but you can't sketch on the surface.
So how would you do it?

konstantin_shiriazdanov

you can find parametric curve feature script and create a number of helices with varying pitch defined by equation like x=r*cos(t),y=r*sin(t),z=k*t^2, r - radius of helix, k ~0.1 - some free factor

philip_thomas

Labern - i am curious, how do you define this today?
If this was a feature, what would the input parameters be?
Philip . . .

mahir

I actually wrote the parametric curve featurescript, but it may not be necessary in this case. If the path of one "blade" curves 180deg or less, then you can just project a 2D spline onto a cylindrical surface. You might be able to create curves greater than 180deg by projecting multiple sketches, but tangent continuity might be difficult to maintain.

philip_thomas

@Mahir - the 'project' curve method that you describe would seem to be a bit challenging.
If i want a constant rate of change of gradient, the projection of any curve that exhibits that behaviour would become very skewed (rapidly decreasing gradient as you move away from normal to the surface). Is there a trick that i am missing?

mahir

@philip_thomas, you're right. I didn't think you needed a mathematically precise pitch. However, you can do a decent job of approximating via projection. The flat projection of a constant pitch helix is just a sine wave. The projection of a helix with constantly varying pitch is just a sin wave that's been slightly stretched. If you have a good side view of the part you're trying to model, it would be easy enough to trace that stretched sin wave.

Or, just do as konstantin suggested and use my parametric FS to generate the 3D curve  

labern

I imported 3 images of the part onto front, right and back planes.
Then extruded a polygon with 50sides. 
Plotted points along the polygon following the imported images.
Used the 3D spline feature script and selected all the created points to create a curve that matched the part. 
I then tried to sweep along the 3d spline but the sweep doesn't stay tangent to the polygon and ends up turning into it.
Using helix sweeps stay tangent to the part but don't suit in this case.

mahir

@labern, take a look at my example. You're right about the problem keeping the sweep direction tangent to the curve (normal to the surface). I created a Sweep Normal featurescript specifically to fix that problem. Take a look at my example below. It's not perfect, but I think it's a good first step.

https://cad.onshape.com/documents/57acdfaae4b005c413ed9b6f/w/3fd585a46d3af1b3ba413c53/e/052b9b1376888b5d8b16e4f1

philip_thomas

Another solution may be to generate the profile from a tool such as Swift Calcs

https://appstore.onshape.com/apps/Utilities/KQ5H3ZDPXICD2PJKVGR6RJZ3HFNAJ2IQQMBBZFQ=/description

mahir

Using the Parametric Curve FS in cylindrical mode wasn't too bad. Same link as my previous post.

philip_thomas

^^^^ I like it!

konstantin_shiriazdanov

philip_thomas said:

Another solution may be to generate the profile from a tool such as Swift Calcs

https://appstore.onshape.com/apps/Utilities/KQ5H3ZDPXICD2PJKVGR6RJZ3HFNAJ2IQQMBBZFQ=/description

@philip_thomas
about swift calc and related engeneering calculations, did you heard about wolfram cloud invironment/wolfram language? they developed a very powerful symbolic calculations environment, would be nice if OnShape had an integrated tab with wolfram notebook, and even some featurescript functions might be implemented in Wl