CS 285: SOLID MODELING

Lecture #8 -- Mo, Feb. 14, 2000.

Preparation:

Homework #3 -- Escher Ball Design

• Intermediate checkpoint -- discussion of progress.
  (Since there are no discussion sections for this class, we have to integrate such discussions into the lecture).
• Are there conceptual difficulties ?
• What useful tricks did you dicover for this assignment ?
• Harlan's polyline editor.
  -- How could this be extended to our specific task of editing points in the fundamental zone of the Escher ball ?
• Put your final solution at the provided pointers in Assignment #3.

Review of the Symmetry Groups

• Hermann Weyl: "Symmetry", Birkhaeuser, 1955.
• Alan Holden: "Shapes, Space, and Symmetry", Dover, 1971.
• Alan Holden: "Orderly Tangles", Columbia, 1983.
• Robert Williams: "The Geometrical Foundation of Nature and Structure", Dover, 1972.
• Gruenbaum / Shephard: "Tilings and Patterns", Freeman, 1986.

in 2D Space:

• 7 frieze symmetries.
• 2 families of rotational groups Cn, Dn.
• 17 lattice groups.

in 3D Space:

• 7 families of rotational groups based on the 7 friezes wrapped around a cylinder: Cn, Dn, S2n, Dnd, Cnh, Cnv. Dnh.
• 7 groups of "really 3D" symmetries based on the Platonic and Archimedean solids.
• more than 200 3D lattice groups !

The symmetry of everyday objects (part 1 of Asmt#3).

• What objects did you analyze ? (hand in list).
• Brain teaser: -- Why does an ordinary wall mirror reverse left and right, but not up and down ??

Review of Various SFF Processes

• What are the salient differences ?
• Which process should you use for a particular part ?
• Fill in overview table.

More on Designing for FDM and QuickSlice

Key design concepts:

• In principle you need to form a single, closed, water-tight shell of your whole object.
• If your CAD environment cannot do that,
  (and even sophisticated modelers such as ACIS, Pro-E, Solidworks, ... sometimes have trouble),
  you should at least have every individual component described by a shell as specified above;
  these components should then describe the desired part as their union.
  (You may omit horizontal faces, if you are sure that you don't want to reorient the part for fabrication !)
• Some parts are still difficult to describe in this manner, we would really like the full set of CSG operations !
• QuickSlice does NOT deal with Boolean CSG operations !
• Jane Yen has put together a little summary of how QuickSlice typically deals with multiple contours in the same layer.
• This issue should be irrelevant in Assignment #3.

Fabricating your Parts -- Asmt #2B

Some things to note:

• After loading your STL file, adjust size and orientation -- then save the file again.
• After cleaning up your contours -- save the SSL file.
  At this point you may want to do a quick road-build to see whether the walls are thick enough to accommodate some internal fill structure, and also do a quick estimate of the build time to be sure that you are in the right ball-park.
• Then build the supports and save the SSL file again.
  This representation is convenient to composite many parts together for a simultaneous build.
• Now run the roads -- and save the SML file.

Later: -- Interfacing with Quickslice at the SSL-Level

• Design with SLIDE, using boolean CSG operations.
• Slice the design with Sara McMains' slicer program, which slices all shells individually and preserves the boolean relationships for the output contours.
• This output is represented in a machine-neutral representation called L_SIF.
• We then resolve the boolean operations in 2D -- one layer at a time. (This is much easier than in 3D).
• This new L_SIF representation with resolved boleans is then converted to SSL and fed to QuickSlice.
• In this way we maintain unambiguous control over the build geometry.
• QuickSlice still computes the support volumes and calculates the roads.

Current Homework Assignment: