# 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.

• 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;