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Shell vs Thicken CATIA V5
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Part Design in CATIA V5:A Comparison ofShell and Thicken
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Abstract In this workshop you will see how to model a thin-
walled part, such as a metal stamping or a plastic cover, using the Shell or Thicken command. This teleconference will show you the difference between developing thin-walled parts using Shell and Thicken, and the implications of these differences when modeling a metal stamping or a plastic cover. We will cover several modeling strategies, and discuss the pros and cons of each method. This workshop is intended to be interactive and you will be invited to ask questions you may have.
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Presenter Terry Cussen
8 years of mechanical design experience automotive seating, robotics, and technology
research and development. designing metal stampings, machined parts,
and injection molded plastic components.
CATIA V5 Engineering Courseware Developer at Cadpo in Westminster, CO.
Masters and Bachelors degree in Mechanical Engineering from Stanford University.
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Intended Audience Engineers and Designers who make thin-
walled parts Injection molded Stamped Machined Cast
CAD department teams investigating best practice modeling methods
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Agenda What’s the difference? Shell vs. Thicken
Overview Issues to Think About When Modeling Modeling Methods Recommendations Questions & Answers
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Vocabulary Wall
The faces of the parts – usually the part’s sides.
Edge The end of a wall where thickness can
be measured – usually the part’s top or bottom.
Boundary Surface The surface will be made into the edges
of the walls, before the shell operation.
Thin-Walled Part
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What’s the difference?
Shell uses the existing boundaries of the solid as the limiting faces.
Thicken adds material to a surface in the direction normal to the face.
There is no difference between the two functions when the boundaries are perpendicular to the walls.
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What’s the difference? Drafted Wall – Positive Angle Drafted Wall – Negative Angle Transition Areas
Wall – Wall Fillet Wall – Floor Fillet
Hole Edges
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Drafted Wall – Positive AngleThicken has more material than Shell
Blue = ShellYellow = Thicken
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Drafted Wall – Negative AngleThicken has less material than Shell
Blue =
Shell
Yellow
= T
hicken
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Transition Areas: Wall - Wall
Smooth Transition
Shell Thicken
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Transition Areas: Wall - Floor
Smooth Transition
Shell Thicken
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Hole EdgesHole intersects the bead
•Red arrow: no difference because wall is perpendicular•White arrow: thicken area is very different because the thickness is applied normal to the surface.
If this is not what is intended, you should make the hole after the thicken operation.
Shell
Thicken
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Issues to Think About When Modeling How will the part be made?
Injection molded, stamped, machined, cast
Why model the edges correctly? How can the part be modeled to
accommodate design changes? What is the impact on linked files (drawings,
assemblies, other models) already in existence?
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How Will the Part be Made? Injection Molded
Parts are formed between two halves of a mold. The edges of the walls are the parting plane.
Stamped Parts begin as a flat sheet and are progressively formed to
the final shape. The edges are are perpendicular to the walls.
Machined Parts begin as a block and material is removed to leave
the final shape. The edges of the walls are determined by the cutting tool
Cast
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Why Model the Edges Correctly? Communicate design intent to manufacturing
For a stamping, it shows if secondary finishing or cam trimming is required.
Can this be handled by a note on the drawing? For an injection molded part, it shows where the parting line is. Will the edge be checked on the drawing?
Provide best information to design reviews Performing fit or clearance measurements in the assembly Evaluate part for safety, e.g. sharp edges
Even modeling “correctly” isn’t exact - the solid model doesn’t represent exactly what is made. Stamped edges already have die roll and tearout. Wrinkling at corners, thinning due to deep draws, etc. Plastic parts have flash at the parting line.
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Planning for Design Changes How can the part be modeled to
accommodate design changes? Solid modeling, shell is the most straightforward. Ordered geometrical sets make surface models easier to
modify. Maintain a strict hierarchy to feature creation, so children are
always listed below parents. Ordered set may use Scan to play the sequence of the surface.
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What is the Impact To Linked Files? If you change modeling strategy, linked files
will be impacted. Drawing
Loss of associativity of dimensions
Assembly Elements will need to be published again Constraints reconnected if not made with published
elements
Other part models Rebuild? Opposite hand parts e.g.
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Modeling Methods
1. Solid Model, Shell
2. Solid Model, Extract Surface, Thicken, Remove Lump
3. Surface Model (GSD), Thicken
4. Solid, Model Boundary Surface, Shell
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1. Solid Model, Shell
1a
1a. Solid Model
1b. Select Boundary Surface 1c. Shell Result
1c
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2. Solid Model, Extract, Thicken
2a
2a. Solid Model
2b. Extract Surface 2c. Thicken Surface 2b
2c2d
2d. Remove Lump from Solid
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Why Remove Lump? If Body is hidden
The body’s mass is included in Measure Inertia Dimensions in drawing are still maintained, but they are
still associated to the shelled body, not the thickened. Both solids are shown in sections
If Body has Remove Lump operation The body’s mass is not included in Measure Inertia. Dimensions in drawing are not maintained and need to be
reconnected.
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3. Surface Model, Thicken Model the surfaces directly using Generative
Shape Design Thicken
Model the surface Final Surface Thicken
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4. Solid Model the Boundary, Shell
Model the solid body
Model the resulting boundary surface using solid features (draft, rib,
loft, etc) using surface features (offset,
sweep, split, sew)
Shell
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Transition Areas: Wall - Wall
•Corrects difference due to drafted wall•Rough approximation of transition area
Solid Model
Thi
cken
She
ll
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Transition Areas: Wall - Floor
•Corrects difference due to drafted wall•Rough approximation of transition areaT
hick
enS
hell
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Recommendations
1. Solid Model, Shell
2. Solid Model, Extract Surface, Thicken, Remove Lump
3. Surface Model (GSD), Thicken
4. Solid, Model Boundary Surface, Shell
Try method 1 first. This is the most straightforward modeling process. It will work for injection molded parts and metal stampings where the boundary surface is normal to the walls.
Use method 2 for stamped sheetmetal parts where the walls are not normal to the boundary surface.
Use method 3 only for complex surfaces requiring the GSD functions.
Use method 4 as a last resort to correct a model which already has many files linked to it, such as drawings and assemblies with constraints.
When should I use a particular modeling method?
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Injection Molded Part Build the solid model, shell If you have a surface, close the surface with a
planar surface which represents the parting plane, then shell.
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Stamped Sheetmetal Part First try method 1 – solid model, and shell. If the edges are not being modeled correctly,
use method 2 – solid model, extract, thicken, remove lump.
Make this decision before creating drawings and assemblies. If you started with method 2 and the design changes such
that method 1 would work, don’t bother remodeling the part – you may need to change back in the future.
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Other Items to Consider Feature order: before / after shell?
Positive features Negative features Draft, fillets, holes, boss, ribs
Do holes have axis after thicken? Different wall thickness at different parts of
the model Shell has this function build in, Thicken does not Not relevant to stamped parts, since the stock size is
uniform.
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Tech Tips Reduce the number of surfaces you need to
select. Cut holes or pockets after the shell/thicken command. Fillet sharp edges prior to shelling.
Create a parameter for thickness Use to set shell/thicken command. Calculate inside / outside radii based on thickness Link to drawing using Attribute Text Link
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Attribute Text Link2. Select the parameter or feature to link to
3. Confirm the parameter to link to4. Text in the drawing is linked to the model
1. Right-click and select Attribute Link
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Summary What’s the difference between shell and thicken?
Shell uses the existing boundaries of the solid as the limiting faces. Thicken adds material in the direction normal to the face. This can lead to differences.
Issues to Think About When Modeling Manufacturing method Accuracy of the solid model - How important is it?
Modeling strategies 4 methods
Recommendations1. Solid Model, Shell
This is the most straightforward modeling process.
2. Solid Model, Extract Surface, Thicken, Remove Lump For use on stamped sheetmetal parts where the walls are not normal to
the boundary surface.
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Questions and Comments
Terry CussenCadpo – Westminster, CO