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

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory 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.

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory 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

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory STL Files Collection 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

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory STL File Solid CUBE1 12 (only in binary format) Facet Normal e e e+00 Outer Loop Vertex e e e+00 Vertex e e e+00 Vertex e e e+00 Endloop Endfacet Facet Normal e e e+00 Outer Loop Vertex e e e+00 Vertex e e e+00 Vertex e e e+00 Endloop Endfacet

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory 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 Modeling for Product Realization, Eds: P Wilson, M Wozny & M Pratt, Elsevier Science Publishers, 1993.

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Shell Punctures Holes in part boundary often caused by uneven tessellation of surfaces Can cause scan line problems Stray Scan Vector

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory 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 1 2 3

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Incorrect Facet Normal Vectors Occurs if CAD package computes normal using the original curved surface, not the facet Facet Original Surface

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory 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 vertices third vertex

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory 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.

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory 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).

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Need for Supports Base Supports Overhang Float & CeilingUnstable part orientation From: J Hur & K Lee, “Efficient Algorithm for Automatic Support Structuer Generation in Layered Manufacturing,” submitted to ASMEComputers in Engineering Conference, 1996.

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Types of Supports Web or BoxLine Point Gusset

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Generation of Supports Line Support Project surfaces needing support onto the X-Y plane, layout the support pattern, then loft box, line, and/or point supports. Pattern for box supports

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Challenges Multiple Materials, Graded Materials Many Thousands of Features Hierarchical Structures

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Many Features CAD systems bog down after hundreds of parts or features are added. What happens when thousands++ are required?

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Hierarchical Structures Thousands of features at increasingly small size scales.

ME6104: CAD. Module 32. ME6104: CAD. Module 32. Systems Realization Laboratory Group Problem This face needs to be supported. Develop a method for 1 Identifying where box and line supports are needed 2 Constructing the box and line supports Assume that the outline of the face on the X-Y plane is rectangular and aligned with the X-Y axes. Normal