Assembling in Fusion 360:
There are several things happening that need to be teased apart to fully explain the joint assembly behavior. There is an important conceptual difference from other CAD tools that use constraints and Fusion 360 which uses Joints:
- Constraints use an approach where you add more and more constraints to remove degrees of freedom.
- Joints use an approach where you explicitly open degrees of freedom and these open degrees of freedom mimic mechanical behaviors.
The disk is a visual aid to try and help predict the position of the parts as they are assembled. We call the disk the Joint Origin.
to understand how the visual aids systems work, let's look at the visual elements that make up the Joint Origin disk.
First, there is the the plane that the Joint Origin disk lies in. As you assemble two components, the two Joint Origin will be made planar and the centers of the disks will align. This Joint Origin to Joint Origin planar alignment is the most important aspect to pay attention to. Almost all other positioning details can be changed while assembling. If you want to know as little as possible just know Joint Origin plane to Joint Origin plane align by default.
A good way to think about Joint Origins are as coordinate systems. The disk plane is X/Y. The white face points in a virtual Z+. The mustard color face points in a virtual Z- the open/filled disk distingushes X from Y. The open half is Y+. The line between open and filled is a virtual X axis.
As joints are created the two Joint Origins are aligned and the user can then refine the aligment options. There is a wrinkle coming a little later that affects this behavior.
Joint Origin to Joint Origin alignment has several user editable properties allowing positional refinement.
- Offset - You can edit the offset between the two Joint Origins. you can drag the offset or edit in the command dialog.
- Flip - Joint Origins have a front (White color) and back (Mustard color). Internaly Joint Origins match front to front back to back. In the command dialog the Flip button allows you to reverse this alignment if needed
- Angle - Joint Origins are dawn half filled to provide an angular reference. Internaly Joint Origins match fill to fill and open to open. The angle property allow you to edit the rotation alignment For example, editing a joint's angle to 180 degrees would align open to filled. You can drag the angle or edit it in the command dialog.
The orientation of the disk is controlled by the input geometry. Orientation is associative to the input so choosing input geometry is the second most important thing to pay attention to.
- Faces - Joint Origins can snap to a face's planar boundary segment vertices's, face boundary segment midpoints, planar face centroids, planar circular boundary centers. cylindrical faces allow snapping to cylinder top, bottom or midpoint.
- Edges/Sketch curve - Joint Origins will use the edge or sketch curve as the normal. For a linear edge this becomes the Joint Origin's virtual Z. For non-linear elements, the virtual Z is tangent to the edge/curve at the point along the edge. Joint origins can snap to either endpoint or midpoint of the edge.
- Construction points/Sketch points- Joint Origins will align to the point and use the active components Z axis as the normal.
While creating a joint, when your mouse is over an input, you can hold cntrl (Windows) or command (Mac) to lock to the highlighted input. This can make it easier to get to snap points that are obscured by other model geometry.
Now for the wrinkle. Parts come from all over and have all sorts of funny orientations. Most of the time user roughly position parts in the right relative position with respect to one another. For top down designs and imported designs relative part positions are alomst 100% right. If we followed the alignment rules above, as you created joints with input geometry parts might flip or move unexpectedly. So, to make this magically work joints determine what the minimum move is to align and will automatically adjust flip and angle so as to keep the component being joint as close as possible to its starting position. You can see this behavior when creating joints. Sometime the flip command will be on, other times off. Sometime an angle will be predefined other times not. Most of the time things just work. If you try and dig in and fully understand joints, it is important to know that the default joint preview is using minimum move to define the default values. Without this is can seem somewhat random what is happening.
Understanding Joint types:
Once two Joint Origins are aligned and their properties (offset, flip and angle) are known there is the last important property, the Joint type. Joint types mimic mechanical motion.
- Rigid joints - locks all degrees of freedom. As the model changes the joint origins are associative to their inputs. In most cases a single rigid joint can completely define the position of one component associatively to another. Rigid has 0 open degrees of freedom.
- Revolute aka Hinge - Allows a rotational degree of freedom that defaults to the virtual Z of the Joint Origins. You can override a revolute to Z, Y, X, or a custom axis. Revolute has 1 open degree of freedom.
- Slider - Allows a single translation degree of freedom. The sliding defaults to translation in the virtual Z of the Joint Origins. You can override translation to Z, Y, X, or a custom vector. Slider has 1 open degree of freedom.
- Cylindrical -Allows a rotational degree of freedom that defaults to the virtual Z of the Joint Origins AND a translation degree of freedom. The sliding also defaults to translation in the virtual Z of the Joint Origins. You can override the translation and rotation to Z, Y, X, or a custom axis/vector. Cylindrical has 2 open degrees of freedom.
- Pin slot - Allows a rotational degree of freedom that defaults to the virtual Z of the Joint Origins AND a translation degree of freedom. The sliding defaults to translation in the virtual X of the Joint Origins. You can override the translation and rotation to Z, Y, X, or a custom axis/vector. Pin-Slot has 2 open degrees of freedom.
- Planar - Allows a rotational degree of freedom that defaults to the virtual Z of the Joint Origins AND a translation degree of freedom. The translation is in both the virtual X and Y of the Joint Origins. You can override the translation and rotation to Z, Y, X, or a custom axis/vector. Planar has 3 open degrees of freedom.
- Ball - Allows a rotational degree of freedom (yaw) that defaults to the virtual Z of the Joint Origins AND a rotation degree of freedom (Pitch) in the virtual Z. The joint has an additional rotation degree of freedom in Y. You can override the Pitch or yaw to a custom axis/vector. Planar has 3 open degrees of freedom. Note: Ball joints, like real ball joints can exhibit gimbal lock.
It's best to pick the joint with the fewest open degrees of freedom needed. While it might be tempting to use a revolute to put that bolt in a hole, a rigid will be better. Fusion 360 will analyze the joint system and create rigid groups for all components that don't have any degrees of freedom.
Joint Options
Open degrees of freedom are shown under the joint in the browser ( as seen in the image on right). You can right mouse click on any degree and control several settings.
All joints have two parameters you can edit numerically; Angle and offset.
For each degree of freedom you can lock it. This sets the degree of freedom at its current value.
For each degree of freedom you can set limits. Limits allow you to define an optional minimum and maximum value. optionally, set a rest position or home where the joint will always return to after you play with a mechanism. Limits are the prefered way to simulate constrained movement. A pneumatic cylinder that has 100 mm stroke can be set by setting the sliding joints limits to a maximum of +100 and a minimum of 0. When you add limits or rest controls to a joint you will add these values to the parameters table as well allowing you to control these values with equations.
There are quite a more advaned topocis with joints but this seemd a good starting point.
Summary:
- Pick the geometry you use to define a joint origin internationally.
- Look for the planar joint origin aligment as a fast visual cue.
- Choose the joint with the fewest open degrees of freedom necessary.
Kevin Schneider
Fusion
