How to Handle Large Scale Assembly
This tutorial shows some of the features offered by Assembly3 to work with
complex assemblies. The sample assembly used in this tutorial is not that
complex by itself, but by following the steps illustrated here, smaller
assembly can be easily scaled up to build much larger and complex ones.
The model was taken from FreeCAD forum member jpg87 in
this
post. It is the rotor head of a wind turbine as shown below,
The finished rotor assembly looks like this,
The assembly consists of the following parts.
The original parts file can be downloaded here
Now, let's begin.
Although optional, it is highly recommended to wrap each part object using an
Assembly container. In Assembly3, the assembly container serves a double
purpose of both constructing a Product, and representing a Part. For the
later, the container often does not contain any constraint. It is used to
manage the geometry elements in this Part for use by other Products. No
matter how many hierarchies the Product has, any constraining elements that
belong to this Part will be referencing the Element object in the
ElementGroup of the assembly container. As shown in the following screen
cast, it is better to plan ahead which elements of the part will be used for
constraining, and create them explicitly, with proper names. You don't have to
do these at this stage. You can create new elements of any assembly at any
hierarchy with simple drag and drop. But it is a good habit to do it manually
and explicitly.
Also, notice that we are creating element from Sketch element as much as possible. And when that is not possible, use feature early in the modeling history, or datum feature, or even the origin plane if possible, in order to avoid lost of element reference due to model changes.
Next, we are going to assemble the axis part, which consists of two pins and
a dual holes socket type of thing. Create a new file, and remember to SAVE
the file first in order for external linking to work. Create a new assembly,
and then simply drag the parts into it. This automatically creates a new Link
object to externally link to the dropped object. Because of some technical
reason, right now FreeCAD does not allow you to drag more than one objects from
different documents (you can if all dragging objects are from the same
document).
The axis needs two pins. You can either drag and drop the pin object twice to
create two separate links, or drop it once, and change the ElementCount
property of the link to make it an array. Before creating any constraints,
remember to choose a fixed base part first, by creating a Lock constraint
using one of its element. This simple assembly can be formed by two
PlaneCoincient constraints,
.
As shown in the above screen cast, after the assembly is done, we create two
new Element, so that this assembly can be used as a Part by other higher
level Product. These new Element link back to the lower level Element
inside the pin Part, and defines the interface of this axis Part.
Now, we start to create the main assembly of this tutorial. First, we are going
to assemble the axis parts to the dome. We need three axis parts. The screen
cast below demonstrate the use of Constraint Multiplication
feature. In order to use it, we must use a link array. Simply drag and drop the
axis part into the main assembly once, and then change the ElementCount to one,
which is one of the requirement of activating Constraint Multiplication button
.
Normally, we you click any geometry element in the 3D view, the tree view will
auto expand to the deepest object that actually owns the selected geometry.
When you are working on complex assemblies with multiple levels of hierarchy,
you may find it annoying to try find the top level assembly you are working on.
You may find the new navigation feature helpful. The screen cast
above shows another solution, that is, to freeze a sub-assembly. Once frozen,
the sub-assembly's PartGroup object will make a copy of all geometry shapes
of its parts. Any subsequent changes to the parts will not be reflected. A side
effect is that when selected in 3D view, the tree view will only expand until
the PartGroup, because PartGroup now owns all the geometry.
Let's continue and select the corresponding elements. A PlaneCoincent
constraint requires two elements. In order to activate the Multiply button,
you must select the element from the multiplying component FIRST, i.e. the
first array element of the axis part array. The newly created constraint will
be automatically selected, which will activate the Multiple toolbar button.
Proceed to click it. As shown in the screen cast, an error message shows up,
saying "Cannot create element in frozen assembly". This is because the
multplication command will try to auto expand the second element by creating
a new element containing all coplanar circular edges of the same radius, but
a frozen assembly does not allow creation of new elements. Simply unfreeze it,
select the constraint, and click the Multiply button again. The axis part
array will be multiplied to fit into all three holes.
Next, we'll assemble the ring to fix all three axis parts above. Note in the screen cast above that we forgot to include all three corner elements, and we can add them easily by drag and drop.
The first constraint is a simple PlaneConstraint. The following two
constraints, though, is not every intuitive. You need to know a little bit of
the notion of DOF.
Each free part has six DOF, three positional and three rotational.
A PlaneCoincident constraint takes away five of them, and leaving only one
free rotational DOF. In other word, after constrained, the second part can only
rotate along the normal axis of the constraining plane. The axis part is
constrained to the fixed dome with a PlaneCoicedent, leaving one DOF for this
two-part system, which is constrained again to the ring, which leaves two DOF
for this three-part system. And now we are going to connect another axis part,
which in fact is another two-part system. In other word, we are trying to
connect,
- a three-part two-DOF system, dome-axis-ring, to another
- two-part one-DOF system, dome-axis.
There are totally three DOF left, not enough for another PlaneCoincedent. You
can try with the AxialAlignment constraint
.
that takes away four DOF, meaning that the second part can still rotate and
move along the normal axis of the constraining plane. If you use
a AxialConstraint, it will work, but you'll get some complaining message at
the status bar when recomputing saying "redundant constraints". The default
constraint solver can handle some level of redundancy, but if left uncheck and
you keeps adding more redundancy, the solver sometimes may fail with no obvious
reason. The best solution is to use a two DOF constraint here, PointOnLine,
,
which restricts only two positional parameter. We select the first circular edge
in the axis part, which is used as the point at its center. And the second
element, the corner arc of the ring, will be used as the line, which is the
normal vector of the arc plane. See
here for all possible
interpretations of various geometry elements.
When combining the above three-part system and two-part system, we have
a four-part system, dome-axis-axis-ring, with three DOF. After the
PointOnLine constraint, there is one DOF left. Finally, we combine the last
one DOF two-part dome-axis system with another PointOnLine, and there is no
DOF left. The assembly is completely fixed.
We are coming to the last part. The first constraint is another simple
PlaneCoincident. But the second one is again not very intuitive.
As mentioned in the previous section, the dome-axis-ring part is completely
fixed with no DOF left, so after constraining the gear with the
PlaneCoincident, the whole system has only one DOF left, which means we need
to find an one-DOF-only constraint. With the slot shape in the gear, it is
natural to first think of, again, the PointOnLine constraint, but, it takes
away two DOF. Fortunately, this constraint works in 2D, which you can
tell by the
look of its icon. A 2D point has only two DOF, and PointOnLine takes away one
of them, and thus completely fixes the whole system. Notice how the constraint is
created in the screen cast. You'll need to first select the element in the axis
part to used as the point, and then the corner element in the ring to be used
as the line, and finally the third element in the gear as the projection plane.
Once an assembly is done, it is a good habit to freeze it. Besides the benefits mentioned above, it also allows to be partially loaded when linked externally. In other words, when you open a document containing an externally linked frozen assembly, the external document containing the frozen assembly will only be partially loaded. None of the assembly's children part will be fully loaded, and if those children part are externally linked, none of their documents need to be loaded, either.
In addition, there is another feature to help scale up even further. It's called a derived assembly, which lets you create a new assembly inheriting the parts from another one. The parts are added as locked parts. You can add new parts to be constrained against them. The way it helps scaling up is that it allows you to customize visibility of derived parts, as shown in the following screen cast
Imaging an assembly with complex internal mechanisms, you normally don't need to show its internals when used as a Part in higher level assembly. And this is where the derived assembly can be of great help here. By hiding the internal parts before freezing, we only save shapes of visible parts. And this makes it load much quicker then the full version.
Here is a summary of the tips mentioned in this tutorial to help dealing with complex assemblies,
-
Put each discrete part in its own file, and wrap it inside an assembly container;
-
Explicitly create and manage the constraining element of each assembly, and give them proper names, while preferring sketch element, datum feature, or features in early model history;
-
Explicitly use those elements in each assembly's
ElementGroupto create the constraints; -
Freeze sub-assembly to reduce assembly hierarchy for easy navigation and quick loading;
-
Use derived assembly to customize part visibility and simplify assembly geometry.