From:  Michael Gibson
5233.14 In reply to 5233.13 
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