CS 294-3: RAPID PROTOTYPING

Lecture #2 -- Friday 9/4, 1998.

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Lecture Topics

NC Milling -- The Physical / Engineering View:

  • The sweep of the tool removes material from the stock.
  • The sweep of the machine spindle and the tool assembly (i.e. cutting tool and tool holder) must not interfere with the current part geometry.
  • The sweep of the tool and its holder must not interfere with the fixturing devices.
  • The milling task should be accomplished with a minimum number of re-fixturing of the part and of tool changes.
  • The tool must be guided along a geometrical path so that it removes material only with its cutting edges.
  • The tool must be guided in time so that it removes material only at an acceptable rate per unit time.
  • All this takes a considerable amount of planning !

    "Macroplanning"

  • Decompose volume to be removed into simpler geometrical entities;
  • Determine a good sequencing of the operations that will minimize re-fixturing;
  • Determine a good grouping of setups and tools that will minimize tool changes.

    "Microplanning"

  • Find a strategy to remove these simpler volumes with complicated sweeps of one or more different cutting tools;
  • Avoid tool collisions between features (volumes to be removed) and fixtures;
  • Select appropriate parameters for table motion speed (feeds) and spindle rotation speed (cutting speed).

    "Tool-path Planning"

  • Motion planning for the individual paths;
  • Generates the actual code that will be fed to the machine tool.

    Abstracted, Conceptual Views of Milling:

    Just specify the individual volumes that need to be removed;
    - - - taking into consideration the geometrical obstructions.
  • Web Designer's Simplified View:
    - - Only one basic operation: removal of pockets of constant depth;
    - - (Drilling a cylindrical hole is a particularly simple pocket.)
  • Limiting Factors:
    - - Sharpness of inside corner radii;
    - - Depth of holes -- as afunction of diameter;
    - - Depth of pockets -- as a function of smallest dimensions;
    - - Proximities of holes and pockets;
    - - Proximity of holes and pockets to stock boundaries.

    Part Design for Homework #1

  • The Generic Part One Might Want to Do:
    - - Six or more access planes;
    - - No restrictions on connectivity (e.g., loosely interlocking rings);
    - - Stability and fixturing through RFPE.
  • Practical Limits in the Machine Shop:
    - - Only one (expensive) milling machine;
    - - Limited machinist time;
    - - Current RFPE is expensive in labor and time.
  • Restrictions to Cope with Above Limits:
    - - No use of RFPE;
    - - A common stock for all parts (2x2" bar);
    - - Only four access planes for all parts (sides of bar);
    - - Allowances for easy fixturing:
    - - -> Part must connect solidly to bar at both ends;
    - - Using a limited set of tools (end-mills):
    - - -> Remove only simple flat-bottom pockets.

    Web CAD

  • The Concept
    - - Removing "pockets" of material (from one access plane at a time)
  • What is a pocket ?
    - - A contour (composed of straight lines and circular arcs) plus a depth.
  • Example: How to Mill a Solid "A"
    - - a) Specify the complement of the "A" as individual pockets.
    - - b) Make one pocket as big as your work area,
    - - - then specify the outline of the "A" as an island,
    - - - then specify the hole in the "A" as another pocket.
  • Presentation and Demo by Jae Kim

    New Homework Assignment:
    DUE: FRIDAY 4/11/98, 9:45am.

    Using Web-CAD 1.0, design a 2x2inch prismatic segment, <= 4 inches long.
    Details will be discussed in class.

    Hand in:
    A printout {or a sketch -- if there are CAD tool problems} of your tentative design;
    put your name on your hand-ins.

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