DES - Mechanical Design
Theory 8
Fall 2024
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Assembly Models
It is often convenient to model complex parts, such as a combustion engine, as a
gathering of subparts – e.g. cylinders, pistons, etc. In NX 2312, fitting and constraining
multiple .prt files forms an Assembly. A new Assembly is generated from the same
interface as a .prt file.
Note 1. The file extension for an assembly is also .prt. One can access
the Assembly tab in any model, regardless of having generated it as a .prt
file. This can be enabled on the Top Ribbon by Right-clicking and further
selection. All part files that form the assembly must, by default, be saved in
the same folder as the assembly file.
With the assembly file created, the environment is
set for the adding of components (the icon can be seen in
Figure 2. As shown in Figure 1, one or multiple parts can
be loaded into the Absolute origin – meaning their original coordinate system will match that of the assembly,
or at another specified position. It is good practice to
add a Fix constraint on the first loaded part. By default,
the part is loaded with a Reference set that includes the
solid only – other options can be found under Settings,
allowing more or all construction features of the part file
(Datum planes, sketches, etc.) to be loaded.
Once the first part is positioned and fixed, one can
continue adding components in groups or separately until the assembly is complete. Their original placement
is not important, since they will always require further
constraining. The concept of constraining is very similar
to that used for sketches - in the case of assemblies, the
position of components must be enforced. The dynamic
behavior of components constrained to each other, e.g.
mating gears, will not be simulated in this environment.
Fig. 1 – Add Component
Fig. 2 – Assembly tools
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Assemblies are useful when
• modeling complex parts consisting of subcomponents
• indicating assembly procedures through assembly and
exploded drawings
1.1
Constraints
Assigning constraints to a part or multiple parts
can be done by selecting the constraint type from the
Position section, tools shown in Figure 2. When hovering over the constraint icon, a tooltip will appear
describing the type of constraint, this can be used
as a guide when deciding the constraint type. While Fig. 3 – Assembly Constraints
most are self-explanatory, such as Parallel and Distance. When using the constraints it is useful to have the Prefer Center/Axis Over
Object enabled, for aligning objects with a center axis - such as a circular shaft and a
hole.
The Concentric constraint, marked green in the Figure 3, performs the same operation, to which it adds an edge alignment, i.e. it will set a distance of zero between the
selected edges on the two components.
Note 2. As opposed to defining sketches, there is no direct indication of an
assembly being fully constrained. The evaluation should be based on the
correct position and it being enforced in the Assembly Navigator.
The Assembly Navigator (Figure 4) includes all the
assigned constraints and the loaded part files, allowing
their selection, movement, deleting, etc. Components of
an assembly may be accessed as part files directly from the
assembly environment, by double-clicking, thus enabling
their part tree with all features. Returning to the assembly will be done by choosing it from the Assembly Navigator. Components may be added to other components,
creating subassemblies. These may also be imported into
a new assembly.
All changes performed on a child part while loaded Fig. 4 – Assembly Navigator
in the assembly will appear within the latter. Proper
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DES - Mechanical Design
constraining will ensure that no conflicts will arise between constraints. If this should,
however, be the case, constraints may be supressed or deleted where needed.
The aforementioned features that can be imported along with their respective components may also be used as constraining elements. When loading duplicates of the same
component, they will appear packed in the Assembly Navigator. By right-clicking they
can be separated into individual parts.
1.2
Assembly Drawings
Just like component parts, assemblies must be represented in a standardized way.
Drawings are created the same way as with singular components, without dimensions
unless critical. An example of an assembly drawing is given in Figure 5. Notice the
section cut, which is often necessary when components are fitted through others.
Fig. 5 – Assembly Drawing in template
1.3
Exploded View Drawings
Whenever parts interlock or merely hide each other in certain views, an extended
view of the assembly with all components visible becomes useful. The so-called Exploded
View enables the assembly to be clearly defined through all components.
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DES - Mechanical Design
The explosion itself is performed in the assembly environment, where Explosions
may be selected from the far right of the Top Ribbon. Editting the explosion allows
components to be moved apart in any direction which facilitates a holistic view of the
assembly. Once a satisfactory view is obtained, it may be saved from Menu→View
→Operation→Save As. This new view is then selectable in the Drafting environment,
among the standard Base Views. Figure 6 illustrates the explosion of the previous
assembly. Again, the drawing lacks dimensions.
Fig. 6 – Exploded View Drawing contents
1.3.1
Parts list
The above figure contains a Parts List and a series of balloons associated to it. The
contents are self-explanatory, including part name and quantity. The icon can be found
to the right of the Top Ribbon in Drafting. Balloons may be enabled from the subsequent
window, and the entire Parts List may be edited for growth direction, header position
and even sorted by right clicking on it.
Attributes may be added to the components in the parts list in a separate column.
By hovering near either end of any preexistent column and right-clicking, a new column
may be added. The cells are editable, but direct entries are not to be used. Instead, the
components may be opened individually and from File→Properties a new attribute
title can be created. In the figure, the title “Standard” was used for both the spring and
the plate, with different values (DIN2000 and ISO1000, respectively).
Once assigned to the individual parts, returning to the assembly drafting will enable
the attributes to be added to the Parts List. This is performed by selecting the entire
new column, right-clicking and going to Settings, followed by choosing the newly named
title from the icon highlighted orange in Figure 7.
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DES - Mechanical Design
Fig. 7 – Parts list settings
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