Category: Fusion

When is a feature a sweep?

What do the following Fusion 360 3D modeling commands have in common?

• Extrude
• Revolve
• Coil
• Pipe
• Rib
• Web
• Hole
• Primitives (Box, Cylinder, Sphere, Torus)
• Ruled surface
• Sweep

The answer is that they all create features with a consistent cross-section. This is known as ‘translation.’ The cross-section is translated along a path. In Fusion 360, this cross-section is called a ‘profile.’

Under the hood, ASM (the Autodesk Shape Manager) uses the same algorithm to create all these features.

The difference between a swept form created with the Sweep command and a swept shape created with commands such as Extrude or Revolve is that a Sweep path is explicit. We create it ourselves. The other commands translate a cross-section along an implied path. For example, an extrude path is implied as perpendicular to the sketch plane, whereas a revolve path is implied as an arc around the axis.

The path is implied by Fusion 360. Fusion 360 passes the Profile (cross-section) and the path to ASM, which generates a 3D shape. It then passes it back to Fusion 360 to display on the screen.

Therefore, I know you created a sweep today – even if you didn’t use the Sweep tool to create it!

What can I do with a sweep?

The one rule of sweeps is that they must have a consistent cross-section. However, we do have some additional controls for sweeps.

You can scale the cross-section (think ‘Taper’ in the Extrude or Sweep command), and you can rotate the cross-section (think ‘Twist’ in the Sweep command).

What can’t I do with a sweep?

The sweep algorithm accurately maintains a cross-section along a path, so swept forms can only have one profile.

How can I sweep between multiple profiles?

Well, you can’t. At least, not with the sweep command! If you need to create a shape using multiple profiles, you’ll need to use the Loft command.

Loft with centerline

The ‘Loft with Centerline’ command uses a hybrid algorithm that can sweep multiple profiles along a path and ‘warp’ the shapes between the profiles simultaneously.

How does a guide rail control the shape of a sweep?

You can use guide rails and surfaces to control the rotation (twist) of a profile along a path. When sweeping a guide rail, imagine lines drawn between your path and your guide rail. As these imaginary lines rotate around the path, the profile will rotate with them.

How does a guide surface control the shape of a sweep?

A guide surface is similar, but this time the twist is in relation to the surface normal. ‘Normal’ can be thought of as perpendicular to a surface.

When should I use a sweep?

Since a sweep must maintain a consistent profile, we can’t create sweeps that are curvature continuous to other geometry. For this reason, it’s usually a good idea to start with swept features (for example, extrude or revolve) as your base features. Then, use Loft or patch to create features that are curvature continuous with the base features.

credit- Paul Munford via Autodesk

Part 1 of this series will cover the Autodesk Shape Manager and some key 3D modeling terminology to have in your vocabulary.

In this series, ‘Get Smart with Fusion 360 Modeling’, we will find out how Fusion 360 generates shapes for us. As a result, we’ll gain a better understanding of how we can achieve the geometry we want. For example, Did you create a sweep with Autodesk Fusion 360 today? That’s a trick question! I know you did, but you may not have used the sweep command to do it. Stay tuned to find out what I mean.

In the meantime, Part 1 will cover the Autodesk Shape Manager and some key 3D modeling terminology to have in your vocabulary.

Introducing the Autodesk Shape Manager (ASM)

To generate 3D shapes, Fusion 360 calls on the Autodesk Shape Manager (A.S.M). ASM is a modeling ‘Kernel’ – a software component that calculates 3D shapes. Fun Fact: ASM is also used by AutoCAD and Autodesk Inventor (among others).

ASM uses the Boundary Representation (Brep) method of calculating shapes. Breps are shapes based on Vertices. Two vertices can define an edge. A loop of edges can define a face. A collection of edges defines a volume (a 3D shape).

When we create 3D forms In Fusion 360 from commands such as Extrude, Sweep, or Loft – we are using the Fusion 360 user interface to pass coordinates to ASM, which calculates the Brep and passes it back to Fusion 360 to be shown on your screen.

Important terminology

Let’s make sure that we are all using the same language when we talk about shapes inside Fusion 360.

Geometry & Topology

Geometry can be thought of as the shape we want to create. Topology is how we get there.

The same geometry can be created using different topology. The same topology can be used to create very different geometry.

• Geometry – the mathematics that describes the shape.
• Topology – The vertices, edges and faces that form the shape.

Normal

‘Perpendicular to an edge or surface.’ For a solid model, the normal must always point ‘outwards’ from the solid volume. In Fusion 360, the back of a surface is indicated by a ‘Golden’ color.

Curvature Continuity

When we would like to create a smooth transition between two surfaces, we can describe the condition as ‘Curvature Continuous’.

Curvature continuity is the process of matching properties across two surfaces. We describe these properties using the letter ‘G’.

The properties are accumulative – G3 continuity includes G2, G1 & G0.

• G0 Continuous: The edges meet (also known as ‘Touching’ or ‘Position’).
• G1 Tangent: The surface normals match along the meeting edges (A fillet).
• G2 Curvature: The amount of curvature matches along the meeting Edges.
• G3 Acceleration: The rate that the curvature changes is equal, leading into the meeting edges.

Note: Mathematically, you could describe additional levels of ‘G’ (G+), but they won’t help us create awesome models.

credit- Paul Munford via Autodesk