Morton,

One way to work around this is to construct the 2d helix from (trimmed/half) circles. There are errors due to that, but it will allow for easily adding different pitches(sizes) of other 2d helix.

A pic showing error(difference) between using flat (2d) helix and constructing from circles (helix in red, trimmed circles in black).





- Steve

ANother option would be to turn on control points for the helix and move the last point to be inline with the second to the last point. This will have the endpoints be tangent.

Great respons here! Thank you for the insightful answers.

I'm actually trying to join 3d helices - the trimmed circle solution might be a good bet.
Just wanted to make sure, that it was a correct behaviour I was seeing.

In the end I need to make a kind of tool to turn springs with changing diameters,
but with fewer turns than the attached screencap and more than one change in diameter.
I just need a good method for making a clean geometry. I'll post results when I have tested the arc method.

Thanks!

Hi Morton,

OK.

As mentioned, what you are seeing is correct, as the helix will change tangency, in this case, due to the angular slope.


Although changing the tangency will give out an incorrect result, you can make slight changes to help.


After creating the helix with the helix tool(in this case 3 helix, the middle helix being tapered)



Zoom in to where the helix endpoints join



Trim off a small part of the end of each helix (you can use trim:- Add trim points). Here I have trimmed one helix endpoint and adding a trim point to the end of the other helix.



Then select those 2 helix and use "Blend" G1 to blend.



Repeat for the other joining helix endpoints, then join all those curves (helix + blends).

You can then sweep that resulting curve with your shape for the cutout.

In this example, this was the result.




- Steve

Hi Morten, one thing to be aware of with arcs for a 3D curve is that each arc is going to be a planar curve, so if you string many of them together you will get a curve that has a kind of abrupt change in a certain sense where the arcs touch one another, rather than a continuous gradual change.

You might try using the Rebuild command on it to reconstruct a new smooth curve from points sampled on the original curve.

There isn't any icon for Rebuild, to run it you need to type it in (press Tab first, then type Rebuild and push Enter), or set it up on a keyboard shortcut.


You might have an easier job of this if you start with a straight cylindrical type helix and then use some of the deformation tools in Rhino to warp it into the final shape.

- Michael

Hi Morten, also if you can describe what you need with enough detail so an equation could be written for the position of each point on your curve, it would then be possible to write a script command that would make the curve for you by generating a bunch of points for a control point curve.

That's how this sine wave plugin was done:
http://moi3d.com/forum/messages.php?webtag=MOI&msg=1519.38

- Michael