Design Workflow for AM: From CAD to Part

Transcription

1 Design Workflow for AM: From CAD to Part Sanjay Joshi Professor of Industrial and Manufacturing Engineering Penn State University Offered by: Center for Innovative Materials Processing through Direct Digital Deposition (CIMP-3D) The Pennsylvania State University & Applied Research Laboratory July 25-28, 2017 University Park, PA 1

2 Digital Workflow: CAD to Part 3 D Geometry Data CAD Generate.STL Files Post Process Determine Orientation Process Planning Machine Fix/Repair STL Support Structures Execute Build Slice to Layer Thickness Tool Path Planning and Process Parameters 2

3 Input to AM Systems 3D Part Design as Solid Model - CAD 3-D Scan Data (Point Cloud, Polygonal Mesh) 2-D Slice Data (MRI, CAT Scan) Convert (Export) STL File 3

4 STL File A faceted representation of the boundary, where each facet is a triangle The facets are created using a process called tessellation, which generates triangles that approximate the object boundary 4

5 STL Example Units (request a 2D drawing) Binary vs ASCII Resolution & File Size Deviation Tolerance Angle Tolerance 5

6 STL File ASCII Format Example One triangle Model Name (1,0,0) (0,0,0) (0,0, 1) (1,1,0) 6

7 Approximation Errors Process of tessellation into triangles creates an approximation Error Trade off Size and Number of facets 7

8 Generating STL files from SolidWorks 2 variables control the generation of triangles Deviation Controls whole-part tessellation Small numbers generate files with greater wholepart accuracy Angle Controls smaller detail tessellation (0~30 deg) Small numbers generate files with greater smalldetail accuracy, but they take longer to generate 8

9 Exporting STL from Pro/E Deviation Control Chord Height Also known as chordal tolerance, specifies the maximum distance between the surface of the original design and the tessellated surface of the STL triangle (the chord). Angle Control Regulates how much additional tessellation occurs along surfaces with small radii The smaller the radii, the more triangles Setting varies between 0 and 1; unless a higher setting is necessary, to achieve smoother surfaces, 0 is recommended. 9

10 Impact of Chordal Tolerance 10

11 Problems with STL files Errors of approximation Much larger than original CAD file for a given accuracy parameter 364kb 2.19 MB 70 MB CAD STL conversion provided by vendor as part of CAD system CAD model may not be mathematically correct Tessellation algorithms are not robust Challenges controlling numerical errors Difficulty tessellating curved surfaces 11

12 Overlapping & Intersecting Facets Overlapping facets may result from numerical round off errors during tessellation, since floating point numbers are used Facets may sometimes intersect at locations other than their edges resulting in overlapping facets 12

13 Gaps & Missing Faces Tessellation of surfaces with large curvature can result in errors at the intersections of such surfaces leaving gaps or holes along edges of the part model 13

14 Digital Workflow Process Planning 3 D Geometry Data CAD Generate.STL Files Post Process Determine Orientation Process Planning Machine Fix/Repair STL Support Structures Execute Build Slice to Layer Thickness Tool Path Planning and Process Parameters 14

15 Dealing with Defective STL Files STL files must be checked for validity and repaired before the slicing process Automatic repair of STL files is often performed by software supplied along with AM machine (or special repair programs) Examples include: - MAGICS, NETFABB, POLYGONICA (3 rd party software) - INSIGHT, 3-D Lightyear (supplied by machine vendors) - MeshLab (open source) Promotional video for Polygonica shows examples of defective STL files:

16 Process Planning Build Orientation Build orientation impacts: Build time Requirements for support structures Thermal behavior Internal stress buildup Part properties 16

17 Process Planning Support Structures Support overhangs during build Base supports facilitate removal from build platform Anchor part to prevent distortion Different designs of supports 17

18 Process Planning Support Structures Support options Support failures Source: 18

19 Digital Workflow Slicing 3 D Geometry Data CAD Generate.STL Files Post Process Determine Orientation Process Planning Machine Fix/Repair STL Support Structures Execute Build Slice to Layer Thickness Tool Path Planning and Process Parameters 19

20 Slicing Generate the layer information Input Triangles Sort Triangles in Z Direction Intersect with Z plane Increment Z Output Boundary Data Smooth Boundary Create Boundary Polylines Sliced Files 20

21 Layers Material between slicing planes is called Layer Part is built by sequentially building and stacking layers, resulting in a quantized part along the build axis in steps equal to the layer thickness Layer thickness depends on machine, material, and process parameters 21

22 Errors related to Slicing Staircase effect Surface roughness of surfaces not orthogonal to build direction 22

23 Errors related to Slicing Dimensions in plane of the layer may be created oversized or undersized Features along building axis may be moved up or down one layer, and features smaller than the layer thickness may disappear 23

24 Digital Workflow Tool Path Planning 3 D Geometry Data CAD Generate.STL Files Post Process Determine Orientation Process Planning Machine Fix/Repair STL Support Structures Execute Build Slice to Layer Thickness Tool Path Planning and Process Parameters 24

25 Process Planning Tool Path Tool path planning and process parameters determine how each layer will be built: Laser power/energy density Scan speed Hatch spacing Hatch orientation Boundaries/contours Internal filling (sparse/dense) Nearly all of these are determined once a material is selected Each material has unique recipe for making parts 25

26 Digital Workflow Post Processing 3 D Geometry Data CAD Generate.STL Files Post Process Determine Orientation Process Planning Machine Fix/Repair STL Support Structures Execute Build Slice to Layer Thickness Tool Path Planning and Process Parameters 26

27 Post Processing Considerations Stress relief and heat treatment Part and support removal Cleaning Secondary operations: Hot isostatic pressing (HIP) Improving surface finish (e.g., shot peening) Machining (e.g., threading, tolerances) Assembly (e.g., mating surfaces/interfaces) Inspection and verification 27

28 Digital Workflow: CAD to Part 3 D Geometry Data CAD Generate.STL Files Key decisions at each step impact the build time, cost, and quality Know where the tradeoffs exist for your specific process/machine Post Process Determine Orientation Process Planning Machine Fix/Repair STL Support Structures Execute Build Slice to Layer Thickness Tool Path Planning and Process Parameters 28