Hi Ed, well you can actually use a scaling rail with 2-rail sweep as well, it's something that acts as an additional modifier on top of the basic sweep itself (either 1-rail or 2-rail).

If you find yourself trying to add a whole bunch of cross-section profiles to a sweep to try and control the side-view profile of it, that can be a good sign that you should be using a scaling rail for that instead.

Here's an example of that from a while ago:



You can kind of get the idea there that someone is trying to make the sweep fit to that guide curve that you see running along the upper part when viewed from the side.

But it can be frustrating to try to get the shape to match purely by adding more and more and more profiles, you may have to add quite an awful lot of them to control the profile and that tends to be a lot of work.

That's what a scaling rail will handle for you - it will pull those profiles up so that they match that additional "scaling" rail going down the center so you don't have to try and add many many profiles to try and get more direct control over the side profile.

Also, a one rail sweep with scaling rail is not quite the same process as a 2-rail sweep.


A 2-rail sweep works something like this - if you have 2 rails with a profile curve:



The profile will be moved by incrementally sliding it along each of those rail curves, so it will draw out a path something like this:



So one thing to note here is that the profiles are not necessarily always perpendicular to any one single rail curve, like here is a more exaggerated example:



You can kind of get an idea there how the profiles kind of slide around for 2-rail sweep. If you want something that is guaranteed to be a perpendicular construction off of a single backbone or spinal-column type curve, then one rail sweep will do that - it will move the profile curve along that spine curve and be perpendicular to it at every spot.



With 2 rails it just is not physically possible to be perpendicular to both rails at the same time at every spot unless your curves happen to be offset curves from one another.

Now when you use a scaling rail with a 1-rail sweep, it keeps the perpendicular-to-spine type arrangement and then will shrink or expand those profiles still in the perpendicular direction. So for example if this is the scaling rail that is added to that one rail sweep:



The profiles will get then expand in this kind of manner:



So one-rail sweep with scaling rail does not "slide" the profile along the scaling rail, it only slides along the spine rail and then grows to intersect the scaling rail, that's what is different about it from 2-rail sweep.

Hope this helps!

- Michael