Assembly modeling

1. Assembly Modelling

2. We would be looking at
•Differences between part and assembly modeling
• Mating conditions
• Bottom up assembly modeling approach
• Top-downassembly modeling approach
• WCS and mate methods to assemble
• Managing assemblies
• Assembly analysis

3. Assembly modeling needs to take care of two issues
Hierarchy
* Individual parts and subassemblies must be assembled
in the right sequence,which is stored in a tree for each
assembly.
* The tree may not be unique, as there may be more than
one sequence to create the same assembly
Mating
* Mating conditions are used to determine the spatial
relationships/orientations between parts
eg. Shaft and a hole – concentric condition
two planar faces – coplanar condition
These two differentiates from part modeling

4. Assembly modelling
- Assembly is a collection of independent parts
- Need to understand the nature and the structure
of dependencies between parts in an assembly
(part to be moved/whichmoves it)
- the model should have spatial positions and
hierarchical relationshipsamong the parts and
mating conditions
- Assembly analysis (but part analysis separate)
- Relationshipsbetween parts and assembly

5. Assembly tree
- Natural way to represent the hierarchical relationships between
the various parts of an assembly is an assembly tree
- Root would be the assembly (depth 0) and the nodes would be
parts, subassemblies(n-1level) .Eg. Engine assembly…
Assembly
Part Part Part Part Part Part
Sub
Assembly
Sub
Assembly
Depth 0,
Hierarchy n
Depth 1,
Hierarchy n-1
Sub
Assem
bly
Sub
Assem
bly
Sub
Assem
bly
Part
Depth n-1,
Hierarchy 1
Depth n,
Hierarchy 0

6. Assembly Planning
- Large assemblies require quite a bit of planning
- For future design updation in parts and assembly
- Update to be automatic in all levels
- An assembly model should be fully parametric and flexible
Three important things to be considered
1. Identify the dependencies between the components of an assembly
Decide whether bottom up or top down approach eg….
2. Identify the dependencies between the features of each part
symmetry, geometry arrays(patterns), planning them helps to optimize
the assembly eg…
3. Analyse the order of assembling the parts
Ease of assembly process on the shop floor..
determines the ease of creating the assembly
Affects the manufacturing processes of the parts

7. Mating conditions..
• Coincident
• Concentric
• Tangent
• Coplanar
• Parallel faces
• Perpendicular faces
6 DOF in e3 space, constraintsand their number,
functionality of the part

8. Coincident condition is satified by forcing n1 and
n2 to be opposite to each other and the two
faces are in same spacial location such that
Part 1 and Part2 fulfill the condition

9. The unit normals and the two points
specifying the mating conditionscan be
expressed interms of MCS(xyz) system as
follows

10. Coincident condition requires four eqns

11. Concentric condition holds between two
cylindrical faces, is achieved by forcing the
shaft and the hole axes to be collinear

12. Concentric requires the centrelines of shaft and
hole to be collinear

13. Tangent conditionis applicable between two
planar/cylindrical or cylindrical/cylindrical
faces,
This is achieved by forcing a cylindrical face to
be tangent to a planar face.
The difference between tangent and coincident
is that the former uses atleast one cylindrical
face while later uses two planar faces

14. Co-planar condition holds between two planar faces
when they lie in the same plane
The coplanar condition is opposite of the coincident
condition and is satisfied by forcing two normals
n1 and n2 to be in the same direction

15. Concentric condition
Shaft and a hole
---------------------------
Coplanar same as coincident..
--------------------------
So total number of equations
Coincident and coplanar : 16 equations
Concentric :18 equations
For an assembly of N parts the total number of equations
can be written as
M = 6(N-1) + 16NA + 16 NC + 18 NF +2 NR
Total number of variables
V=12(N-1) + 12(NA+NC+NF)
Where NA coincident condition, NC coplanar, NF
concentric, NR free rotation

16. To reduce the number of variables per matrix
- Rotation matrix is negated..thus from 12 the
variables reduces to 6 (α, ᵦ, ᵧ, x,y,z)
Thus
M =4NA + 4NC + 6 NF + NR
V=6(N-1)

18. Testing mating conditions
- To check under or over constraints. Under
constraints will be a floating component in the
assembly modeling space
- Move/drag/rotate commands helps
- Its important in the context of creating animation
sequence, exploded views and provides an
oppurtinity to ensure that an assembly functions
as it is supposed to according to its design

19. Three assembly approaches
Bottom up
Top down
Combination of both

20. Bottom up approach
- Common, tradition and logical approach
- Individual parts are created, inserted into
assembly, use mating constraints for location
and orientation
- A blank assembly model in assembly module is
created – parts are imported one at a time-first
part is the base or the host part- on top of
which other parts are assembled- mating
conditionsapplied between them

21. Bottom up assembly cont…
- When we insert parts, it’s the copies of the parts
- Copies are known as instances, multiple instances of same part
can be used
- A link is maintained between the each instance and the its
original ( as a pointer in DB and programming)
- Any change made in original gets updated in all instances
- These assembly links are bi-directional, updation can be from part
to assembly or change in the instance in assembly and update the
part
Advantages:
- Its preferred if the parts have already been constructed,as in the
case of off the shelf parts, allows designers to focus on individual
parts
- Simple to maintain the relationship and regeneration behaviour
of parts than top down approach

22. Top down assembly approach
- Bottom up approach appeals to small assemblies consisting for
eg., hundred or maybe a thousand components
- Top down is suitable for large assemblies consisting of tens of
thousands of parts and sub assemblies
- Its an effective tool and a organised approach to manage the
design of large assemblies
- It allows a project leader to break up product specifications, assign work teams and
enforce downstream design changes at a high level
- It helps foster a systemsengineering approach to product design
- The assembly layout communicates design criteria to subsystem developers including
suppliers. The design control allows distributed design teams to work concurrently with a
common product framework
- It allows detail design to begin while the assembly layout is finalised
- This approach lends itself to conceptual design phase. Captures the design intent of a
product in the early design phases [doesn’t need to worry about detailed design]
- Allows designers to validate design concepts before implementing them, what if analysis
could be done

23. -Top down assembly begins with assembly layout sketch
called assembly sketch or skeleton model
- layout defines components in the context of an assembly
- these parts are empty as they do not have any external
references to actual parts and subassembly files yet
-The layout defines the skeletal, space claim and other
physical properties that may be used to define the
geometry and the relationships between the componets
-Space claim shows where each part in the assembly belongs
- when a layout is done any interferences,clearences, or
overlapping can be forseen
-Locations relative to each can be changed

24. The process of creating an assembly using top down
approach is as follows :
-CAD/CAM system is started
-Assembly mode enabled and a new assemble file created
-Create the sketch of parts in the tree indicating tentative
location thus capturing the design intent
- More sketches if necessary
-Sketches could define the skeletal size,shape and location
within assembly
-Edit the assembly sketches till finalisatin
- now start creating the individual parts
-Evaluate the assembly once the parts are created, modify
if necessary
-Save and exit

25. Advantages of top down
- If layout sketches are changed, assembly, parts
gets updated – changes made at one place..
- Certain subassemblies location can be changed
and gets updatedin the final assembly, thus the
assembly layout sketch does not have to be
master plan for the design

26. Assembly load options
- When a component is inserted, a reference
pointer between the assembly and component is
created
- Two options fully or partially loaded
- In the former entire info of the part is loaded in
assembly, latter subset of info is loaded
- Large assembly, partial loading is recommended

27. Managing assemblies
- Hide, freeze, copy, delete and smart mates , all using
property managers or design tree
Working with sub assemblies
- Sub assemblies can be nested in multiple levels in
the assembly tree
- Creation of separate sub assembly file, then insert
parts
- Create empty sub assembly at any hierarchical
level and add components
- Create sub assembly by selecting group of parts
- Sub assemblies can be edited as like assemblies

28. Assembly analysis
1. Generate assembly drawing
2. Generate parts list (BOM)
3. Generate exploded view
4. Generate sectional view
5. Perform interference checking
6. Peform collision detection
1. Parts cannot be assembled correctly into the assembly or it
cannot move or rotate freely in its final position
7. Perform mass property calculations2

29. Positioning and orienting the parts in a Assembly
- Position (location) and orientation of a part in the assembly
requires computing a 4 x4 homogenous transformation
matrix
- This matrix relates the part’s local coordinate system(part
MCS) to the assembly global coordinate system(assembly
MCS) ie connecting the assembly MCS origin to the part MCS
origin
- Matrix is given by
- This method is called as WCS method
- A WCS is completely defined by specifying its X and Y or its XY
plane, the proper WCS used to merge a part into its asselbly
such that XY plane coincides with the XY plane of the part
MCS

31. Mate Method
- Group of parts must be solved simultaneously to find the
transformation matrix
- Mating conditions along with the properties of the
Transformation matrix provide the necessary equations
to solve for the 12(N-1) variables
- Development of constraint equations from matrix
conditions
Coincident condition requires that the directions of the
two unit normals must be equal and two points must lie
in the same plane at which the two faces mate
Coincident condition requires 4 equations in one direction
thus 12 equations are necessary

32. Concentric condition
Shaft and a hole
---------------------------
Coplanar same as coincident..
--------------------------
So total number of equations
Coincident and coplanar – 12+rotations – 16 equations
Concentric – 6 equations *3 – 18 equations
For an assembly of N parts the total number of equations
can be written as
M = 6(N-1) + 16NA + 16 NC + 18 NF +2 NR
Total number of variables
V=12(N-1) + 12(NA+NC+NF)
Where NA coincident condition, NC coplanar, NF
concentric, NR free rotation

33. To reduce the number of variables per matrix
- Rotation matrix is negated..thus from 12 the
variables reduces to 6 (α, ᵦ, ᵧ, x,y,z)
Thus
M =4NA + 4NC + 6 NF + NR
V=6(N-1)

34. One of the widely used methods for representationof assemblies is based on graph
structures.
In this scheme, an assembly model is represented by a graph structure in which each node
representsan individual part or a sub assembly.
The branches of the graph represent relationshipamong parts. Four kindsof relationships
exist: part-of (P), attachment(A), constraint (C) and sub assembly (SA).
• The "part-of" relation represents the logical containmentof one object in another. For
example, the head and shaft of a screw are "part-of" the screw itself.
• There are three types of attachmentrelationships:rigid, non rigid, and conditional.
– Rigid attachment occurs when no relativemotion is possible between two parts.
– Non rigid attachmentoccurs when parts cannot be separated by an arbitrarilylarge
distance but relativemotion between the two parts is possible.
– Conditional attachmentis related to parts supported by gravity, but not strictly
attached.
• Constraintrelationshipsrepresent physicalconstraintof one part on another.
• Subassembly relationshipindicatesthat an assembly is merged into a higher assembly.

35. Graph structure of electric clutch assembly

36. PrecedenceDiagram
The precedence diagram is designed to show all the possible
assembly sequences of a given product.
To develop the precedence diagram for a product, each individual
assembly operation is assigned a unique number and it is
represented by an appropriate circle with the number inscribed.
The circles are connectedby arrows showing the precedence
relations.
The precedence diagram is usually organized into columns.
All the operations that can be carried out first are placed in the first
column, and so on. Usually, one operation appears in the first
column: the placing the base part on the work carrier where
whole assembly process occurs.

38. Liaison-SequenceAnalysis
The liaison method develops all possible assembly
sequences in two steps.
First it characterizes the assembly by a network
wherein nodes represent parts and lines between
nodes represent any mating conditions between
parts.
These mating conditionsreferred to in this method
as liaisons. The network itself is known as liaison
diagram.

39. Liaison-Sequence of electric clutch assembly

40. Precedence Graph
Unlike the previous two methods, this method is fully automatic.
It is based on the virtual link data structure and requires the mating
conditions as input to automatically generate assembly sequences for
various assemblies.
Once the mating conditions are provided, they are organized in the form of
a mating graph.
The parts in an assembly are then structured in a hierarchal assembly tree.
Then assembly sequence is generated with the aid of interference
checking. In this method, the assembly sequence is referred to as a
precedence graph.

41. Precedence graph of electricclutch assembly