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![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](https://image.slidesharecdn.com/assemblymodelling-230309054901-06426287/75/assembly-modelling-pdf-22-2048.jpg)
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assembly modelling.pdf
- 1.
- 2. We would belooking 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 needsto 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 - Assemblyis 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 - Naturalway 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 - Largeassemblies 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 issatified 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 normalsand the two points specifying the mating conditionscan be expressed interms of MCS(xyz) system as follows
- 10. Coincident condition requiresfour eqns
- 11. Concentric condition holdsbetween two cylindrical faces, is achieved by forcing the shaft and the hole axes to be collinear
- 12. Concentric requires thecentrelines of shaft and hole to be collinear
- 13. Tangent conditionis applicablebetween 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 holdsbetween 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 anda 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 thenumber 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 Bottomup 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 assemblycont… - 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 assemblyapproach - 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 assemblybegins 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 ofcreating 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 topdown - 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. Generateassembly 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 orientingthe 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 - Groupof 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 anda 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 thenumber 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 thewidely 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 ofelectric clutch assembly
- 36. PrecedenceDiagram The precedence diagramis 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 methoddevelops 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 electricclutch assembly
- 40. Precedence Graph Unlike theprevious 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 ofelectricclutch assembly