CS 294-3: RAPID PROTOTYPING
Lecture #8 -- Oct. 20, 2000.

NC Machining -- 3-Axis Milling

The Physical / Engineering View -- (repeat)

• 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 View of Milling

• The Concept
  - - Removing "pockets" of material (from one access plane at a time)
  - - Only one basic operation: removal of pockets of constant depth;
  - - (Drilling a cylindrical hole is a particularly simple pocket.)
• 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.
• Limiting Factors:
  - - Sharpness of inside corner radii -- >= tool radius.
  - - Depth of holes -- is a function of drill diameter;
  - - Depth of pockets -- is a function of smallest slot/radius dimensions;
  - - Proximity of holes and pockets to stock boundaries -- avoid breakout;
  - - Proximities of holes and pockets -- avoid breakout;
  - - Joining of adjacent pockets -- must mill across "open edges."

WebCAD Demo by Mike Schmittdiel

First Part Designs for CS294-3

• The Generic Part You Might Want to Do:
  - - Six or more access planes;
  - - No restrictions on connectivity (e.g., loosely interlocking rings);
  - - Stability and fixturing through RFPE (reference-free part encapsulation).
  This is fine, if your primary goal is to test WebCAD and the process planning pipeline.

• Practical Limits in the Machine Shop:
  - - Only one (expensive) milling machine;
  - - Limited machinist time;
  - - Current RFPE is expensive in labor and time.
  This needs to be considered if you hope to have your part machined.

• Restrictions to Cope with Above Limits:
  - - No use of RFPE;
  - - Use prismatic plastic stock (2x2" bar); with only four access planes (sides of bar);
  - - Or use a flat plastic stock (12x12x0.5" plate); with only two access planes (top and bottom);
  - - Allowances for easy fixturing:
  - - -> part must connect solidly to 2x2" bar at both ends;
  - - -> or part must have some flat places along rim of 12x12" plate.
  - - Using a limited set of tools ( flat-end-mills):
  - - -> Remove only simple flat-bottom pockets.
  Observing these restrictions gives you a good chance that your part will be machined.

Homework Assignment:

A#8: Design a part with WebCAD that can be made on our 3-axis milling machine

• Design a feasible part with on-line WebCAD
  - - Note your experiences with this "light-weight" CAD tool.
• Submit part to CyberCut group
  - - Note your experiences with the submission process.
• Bring some hard-copy of your design to class (or make it accessible through a browser on the web),
  - - preferably with a VRML (COSMO-browser) rendering of it,
  - - so that we can discuss it in class.
• Keep your designs relatively simple and robust for this first round;
  - - you can go more extreme next week ...

• Be prepared to give a 5-minute oral presentation of your design effort to the class