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Skeleton Models to Achieve Top-Down Assembly Design

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Suggested Tec hnique for Using Skele t on Models to Ac h ieve Top-Dow n Assem bly Design

ro/ENGINEER incorporates top-down design tools that allow for the creation of a well -structured, logical design which provides a more concurreorking environment and minimizes the creation of unwanted external references. These tools include advanced component creation tools, assekeleton models, copied geometric and datum references, and reference control and investigation utilities.

Advanced component creation tools provide the ability to create components in the context of an assembly. With this approach, the assemb

structure can be created using empty components. Part components, subassemblies, and subcomponents (components of subassemblies)

be created to any degree required (any level of assembly) before any geometry is actually created. Once this structure is defined, the

component geometry can be defined by selecting the part or assembly from the Model Tree and clicking Edi t > Ac t iva te .

Skeleton models are specialized components of an assembly that define skeletal, space claim, and other physical properties that may be u

to define geometry of components. Users can make use of skeleton models for managing external references by making all other compone

(at that level of assembly; not necessarily subcomponents) reference onlyskeleton geometry, though this is not mandatory. Typically quiltfeatures and datum features are created (including curves and planes) in the skeleton part and are then used as references to act as the b

the scenes backbone of the assembly. [Note: to help further differentiate specifically skeleton geometry from normal part geometry, the

config.pro option, "skeleton_model_default_color", can be used to configure the color of quilt features and solid geometry (if any) in the ske

part.]

Copy Geometry features provide the ability to copy geometric and datum references from any other component (including skeletons) into a

selected skeleton or a regular part being modified, while preserving not only the names, colors, line styles, and other properties assigned to

original parent entities, but also the relative positions of these entities based on the assembled positions of the components. Each Copy Ge

feature may only copy references from a single skeleton or regular part, but multiple occurrences of these features may be created in a sing

model. Note: Although not discussed in the context of this document, external copy geometry features can also be used for copying geome

information.

Reference control and investigation tools, including the Global Reference Viewer provide the ability to trace and easily understand the

references that are made among features in a design. Specifically, these tools clarify the external reference relationships that exist among

models in an assembly.

this example, a rolling desk chair will be started: the chair assembly structure will be defined in the model tree, but only portions of the baseubassembly will actually be created in this article. Since it is early in the design stage, certain important pieces of information about the model ill undecided. For example, the chair may have five wheels or six, or perhaps the diameter of the central shaft may change. A skeleton modele constructed that simulates the overall shape of the model, and allows for modifications to the overall design to propagate downwards to thedividual components of the assembly.

rocedure1. Create an empty part called start.prt to act as the start model for the components (do not use the default template). The part will consist

three default datum planes and a datum axis between DTM1 and DTM2. Store the file to disk by clicking File > Save . Create an emptyassembly (again, not using any templates) called chair.asm, and then create two empty subassemblies, base.asm, and seat.asm. These

subassemblies can be created by clicking . This will open the Component Create dialog box. Enter the name of the object, click thSubassembly and Standard radio buttons in the Type and Sub-Type sections as shown in Figure 1. Next, click Empty > OK from thCreation Method section of the "Creation Options" dialog box as shown in Figure 2.

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Figure 1

Figure 2

2. Create the skeleton model for the top-level assembly by clicking , and clicking Skeleton Model as the Type. The default name for askeleton model is assemblyname_skel0001.prt. Since only one skeleton model will be used in this assembly, remove the "0001" and accept tdefault name, in this case, chair_skel.prt. Click Copy From Exist ing from the Creation Method section of the Creat ion Opt ions dialbox, and Browse to the model, start.prt, in the working directory. This will allow the skeleton model to consist of a copy of another part,which in this example is the start.prt that was created in Step 1. The advantage of using the Copy From Exist ing option is that the nepart will not inherit any external references to other components that the original part may have had. The Copy From Exist ing pick willassemble the skeleton part (which now consists of three planes and an axis) by placing the model at the default origin of the parent assemCreate two quilts in the skeleton part, shown below in Figure 3, by selecting the skeleton part (from the screen or from the Model Tree), righ

clicking and selecting Act iva te . Click to create the seat and back such that the axis, A_1, is normal to the seat surface. Next, click

to create the central shaft and base revolved about the axis, A_1. Be sure to click from the dashboard to create the features as surfaNote that the geometry of the skeleton model appears in light blue to differentiate it from other components.


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Figure 3

Figure 4 shows the skeleton model in the Model Tree. Note that the icon for skeleton models is different from the icons for regular parts.

Figure 4

3. The skeleton model created in step 2 is a very crude representation of what the finished chair will look like, but it occupies roughly the samspace, and will serve as a reference for how the final geometry should look. The base.asm subassembly will consist of the central shaft, legspurs, and wheels. In the top-level skeleton model, it is represented entirely by one feature, a revolved protrusion. Information about thegeometry representing these areas can be copied from the top-level skeleton model, and be used to define the components of the

subassembly. Create a skeleton model for the base.asm subassembly by activating the base.asm and clicking . Create a new, empty ske

part, named base_skel.prt. Next, activate base_skel.prt and click Insert > Shared Data > Copy Geometry . Click to de-activatePublish Geometry collector. Select all the surfaces produced by the revolved surface in the chair_skel.prt.

Next, click References from the dashboard and click in the References collector to activate the selection of datum references. Select

A_1, from chair_skel.prt as shown in Figure 5. Finally, click to complete the feature. This will create a copy geometry feature in the

skeleton model of base.asm which contains surface copies of the central shaft, axis, and the disk that represents the legs, spurs, and wheels

These surfaces can now be used as a reference to build the components within the base.asm subassembly. If any geometry in those

components directly reference these surfaces, that geometry will update if the surfaces are modified in the top-level skeleton model. For m


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information on the copy geometry feature, refer toSuggested Technique for Copying Geometric and Datum References.

Figure 5

4. With base_skel.prt still active, insert a sketched datum curve -- -- on top of the disk to represent one leg, spur, and wheel. Constrain oendpoint to axis, A_1 and the other endpoint to the outer edge of the disk surface; make sure that there is an angular dimension for later rapatterning (HINT: to sketch the curve, select either datum plane DTM1 or DTM2 as the Reference plane for sketching). Next, insert a datumaxis passing through the outer end/vertex of the curve and normal to the flat circular surface (these should be the only two references neceto create the feature). Lastly, insert a datum plane through axis, A_1, and through the axis just created. Select the datum curve, axis, and p

just created, group them together by right-clicking and selecting Group , and rename the group "leg." Finally, pattern this group: with the grstill selected, right-click and select Pat tern, select the angular dimension for the curve and enter a suitable dimension increment and numof instances (in the example, 60 degrees was the increment with 6 instances). The assembly is shown in Figure 6 below (the datum pointdisplay is turned off in the image).

Figure 6

5. Activate base.asm in the Model Tree, and insert a new part called leg.prt; click Copy From Exist ing from the Creation Method section oCreat ion Opt ions dialog box. Again, Browse to start.prt in the working directory. This will bring up the start part (consisting of the thrdefault datum planes and a datum axis) in the assembly window, and will allow it to be assembled into the base.asm assembly. Mate DTMleg.prt against the flat circular surface in base_skel.prt, Al ign axis A_1 in leg.prt with the axis in the pattern leader Group LEG inbase_skel.prt, and Al ign DTM1 in leg.prt to the plane in the pattern leader Group LEG in base_skel.prt. This will assemble leg.prt as sh
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in Figure 7 (the datum planes in base_skel.prt are blanked in the image), and later, leg.prt will be able to be reference patterned to everyGroup LEG in the skeleton model. For more information about reference patterning components, refer toSuggested Technique for UsingReference Patterns to Assemble Components.

Figure 7

6. Activate leg.prt, select and sketch the extruded protrusion shown below in Figure 8. Sketch the feature on DTM3 of leg.prt, and useDTM2 as the Reference plane for sketching. While sketching the feature, the round at the tip of the part should be concentric to axis A_1 inleg.prt, and the arc on the inner portion of the part (which curves to match the profile of the center shaft) should be concentric to the cente

axis of the skeleton model, body_skel.prt, and should be aligned to the circular surface of the central shaft (HINT: specifically select the ce

shaft surfaces from the Copy Geometry feature in body_skel.prt, not the revolved surface in chair_skel.prt). Lastly, the depth should be

and the bottom surface of the Copy Geometry feature in base_skel.prt (not the bottom of the revolved surface in chair_skel.prt) should beselected. Note that this (and aligning the inner arc of the sketch to the central shaft Copy Geometry surfaces) creates a dependency, or exreference, in the protrusion to the skeleton model, which in this case is desired because now, if the diameter of the central shaft changes, tsize of the leg will update to match that size. Next, create the coaxial hole shown at the tip of the leg using axis A_1 in leg.prt. Finally,Reference patternthe component around the base of the chair. This will place a leg at each location of the Group LEG in the skeleton mod

Figure 8

7. Create a new part within base.asm called "spur", and click the Locate Defaul t Datums and Axis Normal To Plane radio buttons fro

the Creat ion Opt ions dialog box, as shown below in Figure 9.
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Figure 9

8. The Locate Defaul t Datums option provides the ability to create a component with default datums, define its placement constraints tolocate it relative to the rest of the assembly, and create initial features without forcing external dependencies. The Creation Options dialog bdisplays three options for the Locate Datums Method: Three Planes , Ax is Norma l To P lane, and Al ign Csys to Csys:

Three Planes - select three orthogonal planes in the assembly. The system then creates a new part with datum planes, which it us

place the new component with respect to the rest of the assembly.Axis Normal To Plane - select a single datum plane or planar surface in the assembly and an axis that is normal to it. The system

then creates a new part with datum planes and an axis, which it uses to place the new component with respect to the rest of the asse

Al ign Csys to Csys - lines up the x, y, and z axes of the selected coordinate systems.

After selecting the references, the component is automatically activated to allow features to be created in the new part. The features will

automatically use the part default datum planes for their references, thereby avoiding the creation of external dependencies on the assemb

Once a feature is created, the system places the new part in the assembly the way that its default planes are mated (by Mate Offset with ze

offsets) to the selected references in the assembly. In the case of Ax is Norma l To P lane, the system also aligns the part's axis with the

selected assembly axis. The offset dimensions can then be modified, or the component placement redefined, if so desired.

Select Axis Normal To Plane , and choose the flat surface at the bottom of the coaxial hole and the axis A_1 in leg.prt, as shown belo

Figure 10. Note that this does notcreate external references because the system is mating and aligning the default datums and axis of thispart to the selected references, and notusing them as sketching and orientation planes for the base feature in the new part.

Figure 10


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9. Before creating the base protrusion in spur.prt, click Tools > Assembly Set t ings > Reference Contro l . Select None on the Objetab of the External Reference Control dialog box and Al l Forb idden References from the Select ion tab. See Figures 11 and 12 beloThis will prevent anyexternal references from being created. For more information on reference control, refer toSuggested Technique forControlling the Scope of External References.

Figure 11

Figure 12

10. Create a cylindrical protrusion in spur.prt. Sketch on DTM1 in spur.prt and make the feature coaxial to A_1 (in spur.prt). Since the referecontrol is set to None , selecting any other axis (for example, in the skeleton model or leg.prt) will not be allowed; therefore, simply make diameter of the protrusion equal to the value input for the diameter of the coaxial hole in leg.prt (since the sketched circle cannot be align

the profile of the hole). Reference pattern the component. This will place a spur at each location of the leg.

11. Figure 13 shows a view of the Global Reference Viewer opened from the top-level assembly. Note that only two parts have externalreferences. The Copy Geometry feature in the base_skel.prt skeleton model is dependent on the geometry of the top-level chair_skel.prt

skeleton model because it was created by copying surfaces from one model to the other, and the protrusion created in leg.prt is dependenthe Copy Geometry feature in the base_skel.prt skeleton model because of the extruded protrusion's sketch and depth references. For mo

information on using the Global Reference Viewer, refer toSuggested Technique for Using the Global Reference Viewer to Manage ExternReferences.
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Figure 13

12. Since skeleton models were used to create this assembly, it is highly configurable, and easily modifiable. The chair could have five legs inof six, simply by changing the number of patterned groups in the base subassembly's skeleton model. If a larger diameter is required for thcentral shaft, the surface in the top-level skeleton model can be modified, and the location and size of the legs will update accordingly (seeFigures 14 and 15 below).


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Figure 14

Figure 15

his example is designed to introduce the user to the philosophy of top-down design through the preceding tasks. This example can be further

xpanded by adding another skeleton model to drive the geometry of the seat subassembly, and additional subassemblies and skeleton models

ould be added to the base to create the wheels. It is extremely important to stress, however, that in the use of top-down design, as much inform

s possible should be put in the top-level assembly skeleton model (from which subassemblies' skeletons should ideally copy data). Thus, if this

ere a real-world project, one might consider creating the feature groups containing the datum curves, etc, that define the legs in the top-level

keleton model.

his centralizes the overall assembly's information and makes the design more organized for the benefit of multiple users. Further, it is considere

ood modeling practice to organize the information in the top-level skeleton model into Publish Geometry features (not discussed in this article).

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