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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)


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