I often see questions relating to the use of Fusion's Loft command, especially when using rail curves. This post is an introduction to this subject, and gives some hints about to to best use this feature in Fusion. This is not an exhaustive discussion of all the capabilities (though it is kinda long...)
This article assumes that you already know the basics of using Loft to create solid or surface bodies between multiple sketch profiles.
What is a “rail curve”, and why is it needed in Loft? Rail curves in Fusion Loft allow you control the shape of the body as it transitions between Loft sections.
Let’s look at a very simple case. Here we have 3 rectangular profiles on parallel planes in space. We’ll show a couple of views to make their relationship more clear:
If you use Loft to create a shape between these 3 profiles, this is what you get:
Pretty much as expected…
However, if you add a couple of rail curves, you can drastically alter the resulting shape. Here I show the same shape, but I’ve sketched a couple of 3D curves that I want to use to affect the resulting shape:
By using those curves as rail curves in the loft, this is the result you get:
The rail curves allow you to change the shape of the body in between the profile sections.
Loft rail curves are a very useful tool to get the exact shape you want. Next, we will discuss the two different types of rail curves, and then get into the details of how to create and use rail curves, and how to avoid some common problems with them
Centerline Rail Curves
Centerline rails are kind of like “rails for beginners”. They are easy to use. A loft can only have one centerline rail curve. This curve is just a general suggestion for how the loft behaves. It does not precisely specify the shape.
Here is our original Loft body, and next to it I have drawn a curve:
If I create this loft, and pick this curve as the centerline, this is what the shape will look like:
See how the body has been “pulled” in the direction of the centerline? This is even more apparent and useful in cases where the loft has just two sections. If we take another simple example of just two circular sections:
below is the resulting loft body:
But, if I add a centerline rail curve:
the result is much different:
Here is a screencast showing how to create a loft using a centerline rail curve:
“Precise” Rail Curves
I’m not sure that this term is the correct technical term, usually these are just called “rail curves”, but I think “precise rail curves” is a more accurate term. When you use this type of rail curve, you are specifying a precise path that the loft body must follow. These allow greater control, but are a bit tricky to work with, because of this very behavior.
Reviewing our earlier Loft with rails example, this time with the profiles and rails visible:
The rails are indicated with the red arrows. The thing to notice here is that the Loft body follows those rails exactly. So, the difference between centerline rail and “precise” rail is this: Centerline rails affect the shape in a general way, while precise rails affect the shape in a very specific way: The body is constrained to follow the rails exactly.
It is very important, however, to understand one point here: Because of this precision, there is one big requirement when you create the rails: The rail curves must intersect the profile curves exactly. A common beginner mistake is to not make this intersection a part of your design. And so, you will see the dreaded “Rail does not touch all of the profiles” error:
Next, let’s look at how to prevent this. The simple answer is pretty clear: You must guarantee that the rail curves exactly intersect the curves that define the profile. The main tools for this are those in the Sketch “Project” menu:
The Project tools allow you to reference geometry into a sketch. Often, these tools are used to project/intersect solid or surface geometry, but in this case we want to project/intersect geometry from one sketch into another, because both rail curves and profile curves are sketch objects.
There are lots of ways to construct loft geometry, but I like to think of it as using one of two major workflows: “profiles first”, or “rails first”.
Profiles first workflow:
In this workflow, we construct two or more profile sketches, then we create the rails such that they intersect the profile sketches.
In the example below, I start with 3 profiles, each constructed on plane offset from an origin plane:
Next, I create two rail sketches. Because I was not particularly careful when I created the profile sketches in this case, my rail curves will have to be 3D curves. So, I create a new sketch, selecting a plane (it doesn’t matter which, since this sketch will contain only 3D curves).
Next, remembering the requirement that the rail curves need to precisely intersect the profile, I need to make sure that I can reference points in the profile sketches that I want to use in my rail curve. So, I use the “Include 3D Geometry” command from the sketch Project menu. This command does pretty much what it’s name implies. It directly includes the selected geometry into the active sketch, at its current 3D location. In the picture below, I have included one corner point from each profile:
Next, making sure that “3d Sketch” is checked in the sketch panel, I can create a 3D spline through these 3 points:
Then, I can use Insert Fit Point, to insert more spline points along the curve, and the Move command to modify those new fit points however I choose. Because the 3 included points are locked down, I know that my spline will meet the rail requirement.
Similarly, I construct the second rail:
And then, I can construct the loft using those profiles and rails:
here is a screencast showing this workflow:
Next, we’ll look at starting with the rail curves
Rail curves first workflow
This method goes about the loft in the other order: Starting out with some rail curves, and then creating profiles so that they precisely intersect those rails.
To get started, I created a single sketch with two rails:
This particular sketch is 2D, but that is not a requirement of this method. Each rail could be a 3D curve.
Next, I defined some planes for the profiles:
These planes are just offset from one of the origin planes.
Next, I created a sketch on the first plane. The key step is then to use the Project Intersect command in that sketch, to create an intersection point between the rail curves and my profile sketch plane.
This is the result:
The red arrows indicate the intersection points. Now, I know that these points are accurate places where the rail curves intersect my sketch plane. All I have to do is to use those points when constructing my profile sketch, and I know that the rail will be OK. This is the resulting profile for this plane:
Then I repeat that for the other two profiles, resulting in:
And then I am able to create the loft:
Here is a video showing that workflow:
Summary
The above workflows are by no means the only techniques which will work for creating a loft. You can easily mix and match these techniques, and there are many other options and workflows that more advanced users will find to be beneficial. But, hopefully this article helps you to understand the basics of using rails within a loft, and how to avoid some common errors.
Jeff Strater
Engineering Director
