Getting a curve to follow a shape that tapers?

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 From:  Lang (LANGLEY)
5208.1 
Hi guys

I'm trying to get a helix shape to follow a cap design that flows around the cap and tapers off towards the bottom part. I'm looking at using a helix and then sweeping with the right shape and boolean to finish. I'm not having much luck with this approach so I'm hoping one of you more advanced users out there can point me in the right direction or show me a better way.

Thanks

Lang
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 From:  Michael Gibson
5208.2 In reply to 5208.1 
Hi Lang - one way to get a curve that actually hugs the surface is to sweep a line along your helix so that you have a kind of fin type protrusion and then use Construct > Curve > Isect to get a curve where the fin piece intersects the outer surface.

See here for a few examples where Danny used this technique:

http://moi3d.com/forum/index.php?webtag=MOI&msg=4084.3
http://moi3d.com/forum/index.php?webtag=MOI&msg=3530.2

Anyway, that's one possibility for getting some curves that spiral around but actually hug your cap's outer surface. Then you use those curves for further constructions like doing a sweep along them.

You also might form the curve on the surface just by drawing a freeform curve and placing a lot of points snapped on to the surface too.

To get the tapering effect it may be easiest to construct 2 such curves for either side of the piece and then those can be used as the rails for a 2-rail sweep. Sweep using 2 rails where the rails come together can be an easy way to construct that kind of surface.

- Michael
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 From:  Michael Gibson
5208.3 In reply to 5208.1 
Also you may want to run the Rebuild command on your initial curves before revolving them to make your cap, that will remove the segmentation from your generator curves where the segments were smooth to one another and that will make the cap made up of bigger surface pieces instead of split at every segment of your original profile curves.

http://moi3d.com/2.0/docs/moi_command_reference10.htm#rebuild

- Michael
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 From:  Michael Gibson
5208.4 In reply to 5208.1 
For drawing directly on a surface, turn off "Straight Snap" in the bottom toolbar so that it does not kick in and also go to the View section on the side pane and uncheck "Display hidden lines", so that you won't snap to the back edges of the shape either. You may also want to go to the "Object Snap" button in the bottom toolbar and push the little arrow that pops up above it and uncheck "Axis" so that you won't snap to the x and y grid axis lines either. You do want Object Snap turned on so that you will snap onto the surface though. With things set up in that way you can then use Draw curve > Freeform > "Through points" and place points on the surface to form a curve like this:



That's another way to get some curves to work with - note that the curve drawn in this way will only be exactly on the surface at each of those picked points, so when you generate a sweep or something from it you will probably want to have the sweep be generated as some piece that sticks out a little bit so that you can be sure it will intersect the outer shape fully.

- Michael

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 From:  Michael Gibson
5208.5 In reply to 5208.1 
Hi Lang, I've attached an example 3DM model here.

This was constructed with these steps - 2 curves drawn with "Draw curve > Freeform > Through points" using the on srf object snap as described previously:



Drew in 2 profile curves and positioned them like so:



That can actually be the most tricky part to position those kinds of profile curves well - I used Transform > Align > Line to line and also Transform > Rotate > Rotate axis to adjust the position and rotation of those curves.

Then a sweep to build this tapered thing here:



Boolean difference with the cap as the base and the tapered thing as the cutting object:



Note there that I placed the "seam" of the profile curves in such a way that the seam of the closed sweep surface was all on the outside of the generated sweep, that helps to simplify things later on by reducing the number of edges that will be in the final result.

Then filleting:




Hope it gives you some ideas on at least one method to do it! Constructing things that swoop around in 3D instead of being flat is definitely a lot more challenging. You have to exercise a wider variety of tools and transforms.

- Michael

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 From:  archetype (FABIENF)
5208.6 In reply to 5208.5 
My approach involves Deform > Twist - you can pick the original curve used to revolve the shape, and apply 2 different twist angles using the same centerline as the main object, then rotate one slightly to open one end. Then using a combination of project and trim you can extract two new profile curves to sweep along the two twisted/helix curves created earlier. This swept surface closely (but not exactly…) follows the surface of the dome shape. Next, you shell it (centerline, both-sided) and work from there. I cut off some parts using boolean diff with a straight line for example.

The technique of using Twist on a revolve profile works in most situations where you need a helix to follow such shapes exactly. I think it supersedes DannyT's technique in many cases now that Twist is available to us.

Turns out you can even use the twisted curve to directly Trim the surface, yielding an exact segment of the shape you can shell and work with as well. This is what I've done in the attached image.

I've attached an example below - if you watch the styles closely you should be able to figure it out. Let me know if you need some more detailed instructions!

/ Fabien

EDITED: 22 Jun 2012 by FABIENF

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 From:  Michael Gibson
5208.7 In reply to 5208.6 
Cool approach Fabien!

And yeah if you project the curve onto the dome first, then when you twist it the twisted result will still hug directly along the surface if the surface is a surface of revolution and it has the same axis of rotation as the twist axis.

Because with an exact surface of revolution if you rotate any point that starts on the surface around the same axis, the ending point will also be on the surface too since it has that radial symmetry...

I'm still getting used to all the possibilities opened up by Twist and Flow myself! :) Definitely helix-ish stuff and Twist are friends.

- Michael
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 From:  Lang (LANGLEY)
5208.8 
Wow thanks guys for your input I'll be trying these later on.

Lang
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 From:  Lang (LANGLEY)
5208.9 In reply to 5208.8 
Hi Fabien

Would you mind giving me a more detailed description of what your doing with that twist method as I can't work it out.

I've tried the drawing the curve directly onto the dome that works well, thanks Michael, but I'm having all sorts of problems getting the sweep right. I see what you mean about how hard that may be to get right.

Thanks guys

Lang
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 From:  Lang (LANGLEY)
5208.10 
Hi all

I've been playing around with the twist and sweep function and am getting there I think. Still not perfect but may be good enough for what I need. I sweeped the curve with 3 different circles as these work quite well around the tapering shape of the dome. No matter how I do it otherwise I can't seem to get say an oval to work how I want it so maybe one of you more experience guys out there can help me out on that one?

I have another issue going on here with the file attached. Basically I can't fillet the edges around the sweeped boolean shape to soften the edges and have come across this problem a few times. The object is unified and joined so don't know why it won't fillet. I've also tried using a very small fillet amount to begin with but that won't work either. I reckon its going to be something really simple no doubt that I'm missing here.

Thanks again for any advice here folks

Lang

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