The design is what I posted
I just wanted to know if there is a more accurate and faster
but I think I already answered with what I posted

Hi Tony,

> I just wanted to know if there is a more accurate and faster
> but I think I already answered with what I posted

More accurate and faster than what though? Which method did you use to generate that Scale1.3dm file?

Something like get 2 helix curves for the top and 2 planar arc curves for the bottom and then loft between things? That should be a good method.

- Michael

I made a half turn helix and then a section of a planar curve and sweep

as in the file scale1.3dm

Post the new file

Sorry for my english :)

Hi Tony, your method there is probably the fastest. There is a slight accuracy loss with it because with 2 rail sweep the profile slides along the rails and since each rail is of different length the "in between" sections will wobbly just very slightly instead of staying absolutely vertically aligned. However it will be a very minor amount of wobble and it will be aligned exactly at the start and the end so it's probably fine.

The method with the absolute most accuracy would probably be something like - take a bottom planar outline like this:



And extrude that up to make the base form:



That base surface is now extremely accurate since it is an extrusion of 2D arcs, so those walls have the 100% precise shape of a cylinder with no variation at all.

Then to form the sloped top construct a sweep like I showed earlier, with this as the profile for the sweep:



And just this one helix for the sweep rail, making this 1-rail sweep ramp surface:



Boolean difference the "whole wall" with the ramp surface:



Delete the portion you don't want.



Generally this method of constructing a more simple larger primitive and then slicing it gives the highest accuracy because you know most of the shape comes from the exact primitive shape.


The error in your 2-rail sweep method is quite small though, so it's not like it's a bad method, it's just that 2 rail sweep is not going to guarantee that all the in-between sections are all co-planar with one another as the profiles are sliding along different length rails.

- Michael

Also another factor making a very slight inaccuracy in your 2-rail sweep method is that end tangents of a helix are not pointing in the same flat direction as the arc - a helix has a slight incline to its tangent direction and during the 2 rail sweep the profiles will follow a kind of average tangent between the 2 rails and so that will basically angle the profiles a little bit as the sweep traverses its path.

Basically if you want to have the most accuracy if you want a shape that is supposed to be a fragment of a cylinder it will be the most accurate to actually construct it literally that way by starting with a cylinder and then cutting it.

- Michael

Project an inclined line to the "cylindre" is not more simple than draw the first helix ?
Or maybe the projection of a line is not the same drawing than the helix!

Effectively there is not the same! :)

"scale" (climb up) definition: to climb up by means of a ladder.

Hi Pilou - no a projection of a line onto a cylinder is a different shape than a helix.

- Michael

Ok thanks for the help michael
:)

Another solution :)
Draw internal Helix
draw external Helix
Loft straight
Boolean diff
:)