Shell Vs Thicken

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Part Design in CATIA V5: A Comparison of Shell and Thicken

Abstract  In this workshop you will see how to model a thinwalled 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 thinwalled 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 Angle Thicken has more material than Shell

Blue = Shell Yellow = Thicken

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Drafted Wall – Negative Angle

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Thicken has less material than Shell

Blue = Shell Yellow = Thicken

Transition Areas: Wall - Wall Shell

Thicken

Smooth Transition

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Transition Areas: Wall - Floor Shell

Thicken

Smooth Transition

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Hole Edges

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Hole intersects the bead

Shell Thicken

•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.

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?

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 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

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. Solid Model

1a

1c 1b. Select Boundary Surface

1c. Shell Result

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2. Solid Model, Extract, Thicken 2a. Solid Model

2a

2d 2c 2b 2b. Extract Surface

2c. Thicken Surface

2d. Remove Lump from Solid

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Why Remove Lump?

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 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.

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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Thicken

Shell

Transition Areas: Wall - Wall Solid Model

•Corrects difference due to drafted wall •Rough approximation of transition area

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Thicken

Shell

Transition Areas: Wall - Floor

•Corrects difference due to drafted wall •Rough approximation of transition area

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Recommendations When should I use a particular modeling method? 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.

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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 Link 1. Right-click and select Attribute Link

4. Text in the drawing is linked to the model

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2. Select the parameter or feature to link to

3. Confirm the parameter to link to

Summary What’s the difference between shell and thicken?

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Issues to Think About When Modeling

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Manufacturing method Accuracy of the solid model - How important is it?

Modeling strategies

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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.

4 methods

Recommendations 1. Solid Model, Shell 

This is the most straightforward modeling process.

1. 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 Cussen Cadpo – Westminster, CO [email protected] www.cadpo.com