So If I well underdand only a mathematical script can be obtain a true perfect helix :)
Or maybe it's will be more easy when the function "project" a curve on a surface will be made
For example a line to a cylinder :)

Hi Jonah,

I guess it is not necessary to resuscitate this thread, but...

jonah wrote:

>For a G1 fit between each spiral segment, you have to add 2 points to the ends of each spiral. So it goes like this:

>1. Make array of points as Schbeurd explains (someday you will have to tell me the correct way of pronouncing this nickname)... for this example, let's say there are 8 points in the array :

>2. Select points 2 & 3, then copy from position of point 1 to poistion of point 8.

>3. Select points 6 & 7, then copy from position of point 8 to poistion of point 1.

>4. Now draw your curve through all the points.

>5. Select curve, and trim excess curve length using original points # 1 and 8.

>If you make multiple copies of this curve, and align them end to end to end, they will have G1 continuity... I analyzed in Rhino, and it doesn't matter if you make 2, 3, or 4 extra points the result will still be G1. But if only using one extra point at each end will result in G0 continuity.

---

I couldn't understand why the sweep command creates a surfaces that can't be joined together, although the rail is "perfectly" smooth (G1). It appears that I have discovered the reason. The Rhino's _GCon seems to be quite confusing. While the command prompt displays G1 continuity between turns of helix, the actual tangency deviation can be as great as the tolerance that is hard-coded inside Rhino(?) I think it haven't much to do with Rhino's "Angle tolerance" setting. I haven't manage to find any relationship between that tolerance and indicated continuity.

Have a look at Rhino's command history window:
:Command: GCon
:First curve - select near end:
:Second curve - select near end:
:Curve end difference = 0
:Radius of curvature difference = 2.66454e-15
:Curvature direction difference in degrees = 0.85681
:Tangent difference in degrees = 0.661446
:Curves are G1.


Tangent difference in degrees = 0.661446 !!!

So now, I am not sure that method, you have described, is just ok. How do you feel about it, jonah?


Petr

Hi Peter. Well. Obviously it's not perfect. The sweep is still a polysurface. In Rhino in fact i have a method which is simpler than Moi and will generate it as a single curve segment, so the resulting pipe (thickening the curve) will result in a single Nurbs surface. But this is the most simple method i feel personally for Moi (pun intended!)... In either case, it's OK to use a polysurface for this purpose since coil springs are not manufactured from 3D models. It's only for illustrations of mechanical objects. I think if you really want the curve deviation to be minimized, then we just start with more points in the initial array, and also we create many more extra points on each end before creating the helix to be trimmed away... Not perfect, but makes it simple and quick :)

jonah

SORRY - I was thinking this was in response to my tutorial on making a coil spring! http://www.moi3d.com/forum/index.php?webtag=MOI&msg=317.1 How embarrassing for me :) ...If using this type of spiral to develop an object for manufacture (such as threading on a moulded bottle) then yes i agree it is better to make a single spiral curve by drawing the spiral in it's full length. For most illustrative purposes either method should be OK....

Don't worry about it. Btw, your "how to make a coil spring" tutorial (both here and on Rhino's newsgroup) is perfect. / Petr

thanks Petr !