Tricky 3D curve question

[line-weight]

I have met my 3D modelling match here. This staircase is direct modelled. I want the stringer that runs up the straight flight to carry on around the winders. In plan, it will follow a circular path, as per the extruded object I've created. However the top surface needs to continue along the pitch line, rising as it curves, as I've indicated with the red dots.

 

Any tips on the best way to do this?

 

 

[markdd]

can you post the drawing  or paste the object into a blank file?

 

 

[line-weight]

Sure, is attached below:

 

 

stairstring.vwx

Edited June 8, 2017 by line-weight

[RussU]

Ok... I took an outside arc length, and got to 1206mm... Then in a side view, I drew a rectangle, and then tapered it to height, extruded it to match your stringer thickness to 18mm...

Then in the 3d modelling pallet I chose the "deform" tool, and bent it 90° to match... see the attached file.

 

I hope that helps... Although I'm sure there's other ways to crack it too...

Stair2.vwx

Edited June 8, 2017 by RussU

[line-weight]

Interesting - thanks. I have not used the deform tool previously; you have prompted me to have a bit of a play with it.

[markdd]

Used the deform tool too but results didn't look convincing enough aesthetically so used your extruded curve, sliced the angle I liked from the rear with the slice tool, extracted the curved surface, extruded it to your correct thickness, and then deformed it around your curve again!!

 

Here is the result!

 

I guess its all about what looks right!

stairstring.vwx

[line-weight]

32 minutes ago, markdd said:

Used the deform tool too but results didn't look convincing enough aesthetically so used your extruded curve, sliced the angle I liked from the rear with the slice tool, extracted the curved surface, extruded it to your correct thickness, and then deformed it around your curve again!!

 

Thanks. Not sure I fully follow the steps but if I understand correctly you've effectively cut through it with a flat plane - the result of which is that it is not rising on an even gradient along with the pitch of the stairs at the points where they meet the stringer (neither does RussU's - however I can see how to use his method to achieve this).

Edited June 8, 2017 by line-weight

[Kevin McAllister]

Sweep a rectangle and a loci. The spacing between the loci and the centre of the rectangle is the radius between the centre of the stair and the centre of your extrude. Adjust the parameters of the sweep so its going in the direction and arc of your extrude. Set the pitch in the sweep dialog (likely negative in your example). Position the bottom of the sweep where you want your cut. Solid Subtract the sweep from your extrude.

 

KM

[markdd]

2 hours ago, markdd said:

Used the deform tool too but results didn't look convincing enough aesthetically so used your extruded curve, sliced the angle I liked from the rear with the slice tool, extracted the curved surface, extruded it to your correct thickness, and then deformed it around your curve again!!

Yeah sorry. A bit garbled! You've got me thinking though!

 

You could create 4 segments exactly along the path you want to follow including the heights and then use the Loft Surface tool.See pic 

[line-weight]

1 hour ago, Kevin McAllister said:

Sweep a rectangle and a loci. The spacing between the loci and the centre of the rectangle is the radius between the centre of the stair and the centre of your extrude. Adjust the parameters of the sweep so its going in the direction and arc of your extrude. Set the pitch in the sweep dialog (likely negative in your example). Position the bottom of the sweep where you want your cut. Solid Subtract the sweep from your extrude.

 

KM

Thanks. Sounds like a good solution although it relies on calculating the pitch manually. Ideally I like solutions that generate directly from existing geometry, as this seems less prone to small errors.

[Bas Vellekoop]

I don't trust the deform tool.

In this kind of cases I use the nurbs tools. You can reproduce this and make minor changes to the design without starting from scratch and ending up with a generic solid. 

 

Not near a pc so I hope you can follow this:

 

- draw several 3d loci that follow the pitch line and the circular path

- draw a nurbs  curve through the loci

- duplicate the nurbs curve and move into position in the z-axes

- loft 2 the nurbs curves

- shell the nurbs curves 18mm

Edited June 8, 2017 by Bas Vellekoop

[line-weight]

20 minutes ago, Bas Vellekoop said:

- draw a nurbs  curve through the loci

 

 

Does this give me a curve that is part of a true circle in plan though?

 

Trying it with 4 loci produces something that looks like it might be nearly right (viewing it in plan, against a "genuine" circle it doesn't follow exactly the same line but that might just be down to screen rendering).

 

Trying it with 3 loci gives something less convincing.

 

May seem pedantic but this could be significant in some scenarios, for example if I later added it to another solid generated from a "genuine" circle they would nearly-but-not-quite-match. And I'd like to know that the end points of that nurbs curve, viewed in plan, are at a tangent to the proper circle so that there is a smooth transition if placed adjacent to a straight section.

Edited June 8, 2017 by line-weight

[Phil hunt]

I think i had this one a few months ago and i think i used the curved wall tool with a deform command

 

curved wall.vwx

[Kevin McAllister]

1 hour ago, line-weight said:

Thanks. Sounds like a good solution although it relies on calculating the pitch manually. Ideally I like solutions that generate directly from existing geometry, as this seems less prone to small errors.

 

Here's a version using my Sweep approach. The Sweep pitch is is not a calculated angle, but rather the amount of travel upward in the Z direction in one full 360 degree revolution. In your case this is your stair rise 205 x 12. I type the math right into the pitch field and let VW do the work.

 

The Deform method and the NURBS sweep method rely on a fair amount of interpolation.

 

Kevin

 

stairstring_KM.vwx

[bcd]

Without digging into the geometry of this is strikes me that it's not possible given the starting geometry. You will not be able to maintain a constant pitch on the baseboard as it transitions into the curve and back out without while maintaining the same board height on the upper & lower stairs. Some form of up-easing and overhand easing is going to be required much like it would for a handrail (if the handrail were pitched in the turn).

[Kevin McAllister]

9 minutes ago, bcd said:

Without digging into the geometry of this is strikes me that it's not possible given the starting geometry. You will not be able to maintain a constant pitch on the baseboard as it transitions into the curve and back out without while maintaining the same board height on the upper & lower stairs. Some form of up-easing and overhand easing is going to be required much like it would for a handrail (if the handrail were pitched in the turn).

True and evident from the model. The stair rise is constant but the run varies along the outside edge. The winders are longer while the regular stairs are constant.

KM

[Phil hunt]

yep your all correct this would not be a good solution.....

[line-weight]

Yes it is true that there would need to be a transition between different pitches in elevation.

[RussU]

We also need to be aware of the top of the shape "banking" as it goes around. At the moment, at any given point, the "normal" points to a z-neutral value, without any pitch or twisting in or out. The slicing option will give you a bank or twist in the top face.

 

I found the deform tool to be very good as long as you present good construction lines.

 

We know that the pitch is a constant going around the edge, as the winders around the equal rad are the same and the rise from step to step would be constant.

 

I tried to lay out a 3d nurbs curve, but couldn't match the Rad in plan very easily or accuately.

 

Has anyone tried to extrude a helix object and then "shell" it? perhaps?

 

[bcd]

How is this going to be constructed? Are you using square or profiled boards?

[AlanW]

This is a method I often use for the stringer of a curved stair. See ozcad tutorial

 

 

[line-weight]

15 hours ago, bcd said:

How is this going to be constructed? Are you using square or profiled boards?

 

Possibly bendy ply or MDF onto a formwork, but it's just a design option at this stage. As it happens I will probably go with an alternative. It's still very useful to have the discussion about how to create something like this though.

[bcd]

I agree - it's a very interesting problem and worth exploring. If you develop out the side of the flights flat in 2d you will be able to draw lines & curves representing the top of the baseboard to explore heights & transitions.

[line-weight]

19 minutes ago, bcd said:

I agree - it's a very interesting problem and worth exploring. If you develop out the side of the flights flat in 2d you will be able to draw lines & curves representing the top of the baseboard to explore heights & transitions.

Yes, and in an ideal world it would be nice to have the whole of the stringer (straight and curved sections) drawn in such a way that a 2d elevation of its profile can be easily edited (including those transitions between pitches) and remapped onto the model.

[bcd]

This is what I get in 2d when I unfold a extract of the solid intersection between and extended baseboard and a solid addition of the stairs and offset the run line by 250mm

You could play with the transitions, extrude this to the thickness and use the deform tool to bend it on the curve.

stairstring_2d_baseboard.vwx

Edited June 9, 2017 by bcd