ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Module 32 CAD-RP Transition ME 6104.

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. Systems Realization LaboratorySystems Realization Laboratory

Module 32 CAD-RP Transition

ME 6104

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Learning Objectives

• Learn about STL files.

• Learn the problems that can arise when creating STL files:

– holes

– inconsistencies

– degeneracies

• Learn about support structures and when they are needed.

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Part Preparation Steps• Verify/Correct CAD Model

- Evaluate sliceability by checking against STL file specification

• Orient CAD Model for Build

- trade-off time, accuracy, surface finish, supports required, post-processing time

• Generate Supports

- Significant clusters of downward facing triangles must be supported

• Assign Build & Recoat Styles

- SLA machine operating parameters such as layer thickness

• Produce Slice Cross-Sections

• Merge Files and Initiate Build

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STL FilesCollection of triangles that cover boundary of CAD model.

Two Important Properties

• Vertices oriented counterclockwise around each facet

• Vertex-to-Vertex Rule:

Each triangle must meet all adjacent triangles along common edges - every triangle must share exactly two common vertices with each adjacent triangle.

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STL FileSolid CUBE112 (only in binary format) Facet Normal -1.000000e+00 -0.000000e+00 0.000000e+00 Outer Loop Vertex 0.000000e+00 2.000000e+00 2.000000e+00 Vertex 0.000000e+00 -2.000000e+00 -2.000000e+00 Vertex 0.000000e+00 -2.000000e+00 2.000000e+00 Endloop Endfacet Facet Normal 0.000000e+00 -1.000000e+00 0.000000e+00 Outer Loop Vertex 4.000000e+00 -2.000000e+00 -2.000000e+00 Vertex 0.000000e+00 -2.000000e+00 2.000000e+00 Vertex 0.000000e+00 -2.000000e+00 -2.000000e+00 Endloop Endfacet

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Problems with .STL Files

• Shell Punctures

• Inconsistent Facet Orientations

• Incorrect Facet Normal Vectors

• Degenerate Facets:

• Topological - 2 or 3 coincident vertices in a facet

• Geometric - 3 distinct collinear vertices

• Internal Walls and Structures

From: J Bohn & M Wozny, “A Topology-Based Approach for Shell Closure,” Geometric Modelingfor Product Realization, Eds: P Wilson, M Wozny & M Pratt, Elsevier Science Publishers, 1993.

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Shell Punctures

• Holes in part boundary often caused by uneven tessellation of surfaces

• Can cause scan line problems

Stray Scan Vector

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Inconsistent Facet Orientations

• Indicate that material is on the wrong side of the facet

• Wrong vertex ordering

• Often caused by using a finite element mesher to create

part tessellations

• Confusion caused if specified normal is opposite to that implied by vertex ordering

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Incorrect Facet Normal Vectors

• Occurs if CAD package computes normal using the original curved surface, not the facet

Facet

Original Surface

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Degenerate Facets - Topological

• 2 or 3 coincident vertices

• Occurs typically due to round-off errors in converting from a CAD system’s double-precision numbers to the single-precision of STL files.

• Typically does not affect surface geometry or topology and degenerate facets can be deleted.

2 coincident verticesthird vertex

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Degenerate Facets - Geometric

• 3 distinct but collinear vertices

• Added by some stitching algorithms in order to avoid shell punctures.

• Undefined surface normal.

• Cannot just delete facet because it is important topologically.

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Internal Walls & Structures

• May arise due to designer modeling errors or shortcuts.

Poorly implemented Boolean operations, etc.

=> Non-manifold situations.

• Can cause structural weaknesses (SLS).

Or local shrinkage and part inaccuracies (SLA).

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Need for Supports

Base Supports Overhang

Float & Ceiling Unstable part orientation

From: J Hur & K Lee, “Efficient Algorithm for Automatic Support Structuer Generation in Layered Manufacturing,” submitted to ASMEComputers in Engineering Conference, 1996.

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Types of Supports

Web or Box Line

Point Gusset

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Generation of Supports

LineSupport

Project surfaces needing supportonto the X-Y plane, layout the supportpattern, then loft box, line, and/orpoint supports.

Pattern for box supports

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Challenges

• Multiple Materials, Graded Materials

• Many Thousands of Features

• Hierarchical Structures

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Many Features

• CAD systems bog down after hundreds of parts or features are added.

• What happens when thousands++ are required?

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Hierarchical Structures

Thousands of features at increasingly small size scales.

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Group Problem

This face needs to be supported. Develop a method for 1 Identifying where box and line supports are needed2 Constructing the box and line supportsAssume that the outline of the face on the X-Y plane is rectangularand aligned with the X-Y axes.

Normal

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Module 32 CAD-RP Transition ME 6104.

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