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8/7/2019 Top Down Modeling
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ME 380Advanced Computer Aided Engineering
Chris Morgan
Precision Systems Laboratory
University of Kentucky
Introduction to Top Down Design
March 22, 2006
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Top-Down Design Overview
Top-down design is a design that first specifies top-level criteria and
model structure, then passes that information down to all pertinent
subsystems. The overall, or top-level, design intent is built into the model
from the start, ensuring that top-level modifications correctly propagate
through and update entire system
Advantages of top-down design:
Design workload distribution is facilitated because each component or
subassembly contains required information from higher levels of the design.Individual designers can be assigned tasks without fear of imcompatible
components.
References and parent/child relationships can be accurately controlled.
(External references which occur when a reference for a feature orcomponent does not belong to that part or assembly can also be controlled.)
Designers can retrieve only the skeleton structure of an assembly, rather than
the entire assembly. This facilitates parallel design work and maximizes
computer resources.
Mechanical Engineering Design with Pro/E, Archibald
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Steps for Top-down design
1) Define the design intent Define a Product Design Specification (PDS)
Draw a rough 2-D sketch in a Layout file (not precision scaled)
2) Define the model structure The model structure is defined by breaking a large assembly downinto sub-assemblies and parts, which builds a Bill Of Material(BOM) before the parts are designed. Parts and sub-assemblies canbe added after this step if necessary
3) Introduce skeleton models Skeleton models are the primary means of capturing design intent
and passing it to lower-level components
Each assembly or sub-assembly can have one, and only one, skeleton
model. Skeleton models are 3D parametric layouts capturing important
design criteria for the assembly
Typical skeleton models contain datum planes, point, curves,
coordinate systems, surfaces, and sometimes, volumes
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Steps for Top-down design (cont.)
4) Communicate design intent throughout the assemblystructure
Information such as mounting locations, space claim requirementsand motion requirements should be included in the top-level skeleton
model and propagated down to sub-assembly skeletons as required
5) Populate the assembly Once top-level and subsidiary assembly structure and skeleton
models are complete, the assemblies can be populated with
individual parts Existing parts can be assembled to the model, or parts can be created
within an assembly
6) Manage part interdependencies
Correctly establishing relationships between parts or part featurescan be difficult, particularly with bottom-up design
Managing parent/child relationships and references is crucial for anydesign, but particularly important when designing in assembly mode
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Layouts
2D sketches that represent the overall layout of assemblies
Layouts are used for: Defining global parameters and relations
Developing basic part geometry envelopes Defining how components are mounted with respect to each other
Defining fits between parts or determining part size
Documenting the overall assembly
Automatic component assembly
Geometry shown in layouts is not associative
Advantages
Document design information in one central location
Document design information before creating solid models
Investigate design options without involving the entire assembly
Easily make design changes because all of the design information iscontained in one location
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Steps to use layouts
Create the layout with desired parameters, globalrelations, datum planes and axes.
Declare the layout to an skeleton (In a file clickFile-
Declare-Declare Lay)Declare all parts (that need to reference the layout) to
the layout
Explicitly declare datum planes and axes for partsthat require it
Write part relations to access global parameters
Note- Pro/E recognizes datum planes and axes ondifferent parts that have been declared to the samelayout datums as being the same entities
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Layout assembly for Engine
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User Parameters
Parameters contain important information about a part or assembly. Thisinformation could be:
Dimensions
Tolerances
Surface Finish
Thread notes Cost
Etc
There are five types of user parameters1. Integer-
Number of teeth on a gear, number of holes
2. Real- Cost, Model Size, dimensions
3. String Vendor, designer name, part number
4. Yes No Family Table values, logical expressions
5. Note Contains the ID of a model note
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Relations
Relations are used to define relationships between model parameters andother model parameters or dimensions.
Relations are defined using mathematical and logical expressions and modelparameters.
Relations can be created on any level of the model structure assembly,part, skeleton, layout, or sketcher mode.
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Skeleton Models
Each assembly model may contain oneskeleton model.
Skeletons are part models, but they do notappear in the assembly BOM.
The geometry in skeleton models can bereferenced by part files.
Use the Shared Data menu to publish and copy
geometry from one model to another.To access parameters from a layout the layout
must be declared to that skeleton
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Exercise: Create the layout
The sketch used for the layout has been already drawn in Corel Draw and isavailable for download on the website
Download the Stirling_sketch.dxffile by clicking on the link next to theTop Down Modeling slides, and place the dxf in your ENGINE directory.
Set the working directory to the Engine directory and start Pro/E. Start a new layout namedENGINE_LAYOUT.lay
Set the orientation to portrait and the paper size to A
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Import DXF
Insert the downloaded dxf file by selectingInsert-Shared Data-From file
Accept the default values in theImport DXF dialogue box and clickOK.
When asked, Drawing is smaller than format. Scale to fit format? select
No. When asked, Move bottom left corner of drawing to screen origin? select
Yes.
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Move the sketch
Drag a box around all of the lines
to select the sketch.
Use your mouse to move the
entire sketch to the bottom centerof the page as shown.
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Change the line style
Hold down CTRL and select the lines shown in red below. Hold down the rightmouse button and selectLine Style.
Chang the style tocenterline and selectApply, then Close.
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Change the line style
Hold down CTRL and select the lines shown in red below. Hold down the rightmouse button and selectLine Style.
Chang the style tophantom and selectApply, then Close.
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Create Dimensions
Click thecreatedimension icon
Create the followingdimensions. Place the
dimensions by middleclicking.
Make sure you select theproper orientation of thedimension (horizontal or
vertical) When asked for the
symbol, type the name asshown (dont forget theunderscore) when askedfor the value just hit
Enter.
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Create Dimensions
SelectFile-Properties
Select thedrawing_text_height option and change the valueat the bottom to 0.1.
SelectAdd/Changeand clickOK.
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Create Dimensions
Rearrange yourdimensions sothey fit on the
page.
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Edit parameters
ClickTools-Parameters, noticethe parameters from the layouthave been inserted
Notice all of the dimensionshave been set to parameters thatyou can edit
Highlight thePTC_COMMON_NAMEparameter and click the deletebutton
ClickOKto exit.
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Insert the table
Click on theadd table buttonon the toolbar
Under the Table Create menuclickDescending-Leftward-
By Num Chars and pick apoint near the top right cornerof the paper outline
For the first column widthselect 6
For the second column widthselect 30
Middle click
Pick just below 1, repeat 7
times Middle click to finish
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Fill in the table
Double click the cells in the firstcolumn and enter the Names ofthe Parameters which representthe design intent (For example
CHAMBER_DIA) select OK. In the second column enter the
same names, but place & in frontof each name (For example
&CHAMBER_DIA)
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Fill in the table
Continue to fill out the table
as shown to the right.
When a new row is needed
select Table-Insert-Row and
select the last horizontal
line of the table.
Go back and double clickthe values in the second
column and change the
values as shown in the table. Save the
ENGINE_LAYOUT.lay
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Fill in the table
Continue to fill out the table
as shown to the right.
When a new row is needed
select Table-Insert-Row and
select the last horizontal
line of the table.
Go back and double clickthe values in the second
column and change the
values as shown in the table. Save the
ENGINE_LAYOUT.lay
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Create the ENGINE assembly
Create a new assembly
named
ENGINE_ASSY.asmAccept Use default
template
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Click on the Component Create icon at the right
Select Skeleton Modeland accept the default name of
ENGINE_ASSY_SKEL, select OK.
Select Copy From Existing and selectBrowse.
Find your template.prt file and select OK.
Add skeleton model to assembly
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Add CHAMBER_BODY Component
ClickComponent Create and create a part calledCHAMBER_BODY, clickOK.
Select Copy From Existing-Browse and find the template.prt
SelectLeave Component Unplaced, clickOK.
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Add all components
Repeat the steps on theprevious slide and add thefollowing components
BOTTOM_COVER
TOP_COVER
STAND
DISP_GLAND
POWER_CYLINDER
BEARING_MOUNT
The model tree should looklike the one shown at theright.
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Add a Bulk Item
You can add components to the assembly that will
not be modeled (such as glue, oil, small nails, etc.)
Add a component for the oilNotice that Skeleton modelis grayed out because an
assembly can only have one skeleton model
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Save all files
Save and close the assembly file
Save and close the layout file
Exit Pro/E
At the command prompt type:
purge
(type this command at the end of each day to purge the
previously saved versions of all files)