# Thread: Wrapping a helix around a non uniform cylindical solid

1. ## Wrapping a helix around a non uniform cylindical solid

I am trying to creat a fluted dagger handle using Rhino CAD and have hit a wall. While I can create a helix around a perfect cylinder I cannot seem to create a helix (and then pipe and perform a difference) on the surface of a solide that has an irregular cylindical shape (like an hour glass or dagger handle in this case).

Does anyone know how this is done? Am I making sense?

Thank you.

Shane Harvey

2. I would start by drawing a helix along and around the "hourglass" shape you are trying to wrap with the curve. Once that is done, select the curve, then select Curve_From Object_Pullback. Select the hourglass shape as the "surface that pulls". This will get you close. When trying this, I noticed that the curve is not projected exactly right, it pulls the curve normal to the surface.

Hope this helps a little.

Cheers!

3. ## Closer

Spoiledbrat,

That was big step in teh right direction. Thank you. So long as the hourglass shaped solid's curves are gentle it seems to be close enough for what I'm doing.

However, it still doesn't exactly do what I'd like it to. That command seems to pull teh helix to the surface not uniformly from the axis of the hour glass so you end up with some odditities once the curve of the solid gets too severe.

Still, thank you. That helped. If anyone else can kick this can further down the road please let me know!

Shane

4. Well, if you want the detailed long drawn out method...

Place a point somewhere for a snap reference. Create a helix, starting at the point. Then, scale the helix 2D, using the point as the reference point (also, hit the copy function, so you end up with 2 helix(es?)). Once this is done, draw a line connecting the two helix(es?) at the end. Then, select both helix(es?), and use the surface_sweep2rails function. Select the connecting line when it asks for the cross section curve. Then, place your hourglass shaped handle within the helix, so that we can split our new helical shaving with your handle. once this is done, delete the outer piece of the helix we just split. Next, use the curve_from object function, and select the outer edge of the helical shaving. The split function may have segmented our curves, so select them all. Then, join them. You should now have the helix you desire.

Cheers!

Rob

5. Here is a slightly different way to do it:
1) Make a line that is the centerline of your hourglass shape.
2) Make a helix using the centerline, the number of turns you want and a diameter bigger than your hourglass shape.
3) Use Loft to loft between the helix and the centerline.
4) use Intersect and intersect the surface resulting from 3 above with your hourglass shape. The result will be the helix curve on the hourglass surface.
5) Pipe the intersection curve.
6) Boolean difference the pipe from the hourglass surface (you will likely need to cap it).

Zip file with steps attached. --ch

6. Originally Posted by spoiledbrat
<snip> Curve_From Object_Pullback.... When trying this, I noticed that the curve is not projected exactly right, it pulls the curve normal to the surface.
This is exactly right for the function - the command Pull "pulls" a curve to a surface along the normals of the surface, that's what it's supposed to do... --ch

7. Ch,

That is a great approach. Thanks for the well put together series of instructions as well.

Thank you gentlemen for the help.

Shane

8. ## Boolean Intersection failed

CH,

When I hit step four I repeatedly get the error Boolean Intersection failed. I've recreated the steps many times with small changes trying to identify the cause. Any ideas why this may be happening?

Shane

9. Hi,

Sounds like you're trying to use BooleanIntersection instead of Intersect. Boolean Intersection is for producing a volume that is the common area between two solids, whereas Intersect (Curve>Curve from Objects>Intersection) will produce the intersection curves and points between various types of objects. In this case the intersection of the lofted surface and your hourglass shape will be the spiral curve desired. --ch

10. ## Bingo

Ch,

You're correct. Thanks for your time on this. That worked.

Shane