CAD/CAM /CIM (ME-704) - Unit 2 Short Notes

2.1 Fundamentals of CAM & CNC Technology

Computer-Aided Manufacturing (CAM) is the use of software to control machine tools and related machinery in the manufacturing process. Its primary objective is to automate and optimize production, reducing time and errors.

Evolution:

• NC (Numerical Control): Hardwired, punched tape input.

• CNC (Computer Numerical Control): Stored-program control, flexibility, on-the-fly edits.

• DNC (Direct Numerical Control): Networked control of multiple machines from a central computer.

Basic Components of a CNC System:

Types of CNC Machines: Machining Centers (multi-axis milling), Turning Centers (lathes with live tooling), EDM, Grinders, etc.

Axis Designation & Coordinate Systems:

• Machine Coordinate System (MCS): Fixed, unique to each machine (home position).

• Workpiece Coordinate System (WCS): User-defined origin (G54-G59).

• Absolute (G90) vs. Incremental (G91) Programming:

  [!TIP] Common Pitfall: Mixing G90/G91 modes mid-program causes crashes. Always declare mode at start.

2.2 CNC Part Programming

Manual Part Programming

Program Structure:

1. Header (Preliminary): O1000 (program number), G20/G21 (units), G90/G91, tool calls.

2. Data (Main): Motion commands, spindle/feed commands, tool changes.

3. End-of-Program: M30 (end & rewind) or M02 (end only).

Essential G-Codes (Preparatory):

Essential M-Codes (Miscellaneous):

Canned Cycles (Pre-defined sequences):

[!TIP] Canned cycles retract to R-plane; always set G98 (return to initial point) or G99 (return to R-plane).

Computer-Assisted Part Programming

Need: Complex geometries (3D surfaces) are impractical to code manually. APT (Automatically Programmed Tools) Language:

• Geometry Statements: Define points, lines, circles (P1 = POINT/0,0,0, L1 = LINE/P1,P2).

• Motion Commands: GOTO (position), GODLTA (incremental), CUT (machining along tool axis).

• Post-Processor: Translates generic APT CL (Cutter Location) file to machine-specific G-code.

CAM Software Workflow:

CAD Model → Setup (machine, stock, fixture) → Toolpath Selection (contour, pocket) → Simulation/Verification → Post-Processing → CNC Machine.

2.3 Tool Path Generation & Machining Strategies

Tool Path: The trajectory of the tool's centerpoint. Directly impacts surface finish, machining time, and tool life.

Milling Operations & Strategies

Turning Operations

• Rough Turning: Heavy depth of cut, high feed; tool path follows rough profile.

• Finish Turning: Light depth, high speed; follows final contour.

• Grooving: Dedicated tool, multiple passes if deep.

• Threading: Multiple passes with decreasing depth (G76 cycle).

High-Efficiency Machining (HSM):

• Trochoidal Milling: Cycloidal, arc-like motion. Advantages: Constant tool engagement, reduced cutting forces, higher feed rates, less heat.

• Key Parameter: Stepover is often < tool diameter (e.g., 25-50%).

Collision Detection & Simulation:

• Backplot/Verify: Toolpath simulation in CAM software (no machine motion).

• Machine Simulation: Full kinematic model of machine, tool, fixture, stock. Essential to detect collisions, over-travel, and gouges before shop floor run.

[!TIP] Always run full machine simulation for multi-axis jobs; backplot alone is insufficient.

2.4 Post-Processing & CNC Machine Communication

Post-Processor Role: Translator between neutral CL data (tool position, orientation) and machine-specific G-code (dialect, syntax, cycles).

Process:

1. Input: CL file (e.g., GOTO/10.5,20.3,5.0,0,1,0).

2. Translation: Post-processor maps CL to controller's format (Fanuc, Heidenhain, Siemens).

3. Output: Machine-specific .nc or .ptp file with correct G/M codes, arc formats, canned cycles, and syntax.

Customization: Post-processors are machine-controller specific. Must be tuned for:

• Unique M-codes (e.g., tool change M06 vs T1M6).

• Arc definitions (I,J,K vs R).

• Rotary axis output (A, B, C) and rotation limits.

[!TIP] A generic post-processor is a major source of errors. Always validate output with a "dry run" or simulation.

Program Transfer Methods:

• Punch Tape / Floppy: Obsolete.

• RS-232 Serial: Legacy, slow.

• Ethernet / TCP/IP: Modern standard.

• DNC (Direct Numerical Control): Real-time streaming from server to machine; manages multiple programs, version control.

2.5 Process Planning & Integration (CAPP)

Process Planning: Determining the optimal sequence of manufacturing operations to transform a raw material into a finished part.

Manual vs. CAPP:

• Manual: Experience-based, time-consuming, inconsistent.

• Computer-Aided (CAPP): Systematic, faster, repeatable.

Types of CAPP:

Inputs to CAPP:

• CAD geometry (features: hole, pocket, slot).

• Material, tolerances, surface finish.

• Production volume (batch size).

• Available machines, tools, fixtures.

Outputs of CAPP:

• Operation sequence (drill → mill → grind).

• Machine/tool selection.

• Cutting Parameters:

$$V_c = \frac{\pi \times D \times N}{1000} \quad \text{(Surface speed, m/min)}$$

$$f = \frac{N_f}{N} \quad \text{(Feed per rev, mm/rev)}$$

Where $D$ = tool dia (mm), $N$ = spindle speed (rpm), $$\displaystyle N_f $$ = table feed (mm/min).

• Time estimates (setup, machining, tool change).

CAD/CAM/CIM Integration Loop:

CAD (Design) → CAPP (Process Plan) → CAM (Toolpath & NC Code) → CNC (Manufacturing) → Feedback (Quality data to CAD/CAPP).

2.6 Tooling & Workholding for CNC

Cutting Tool Materials:

Tool Holders (Standard Tapers):

• CAT (V-flange, US), BT (Metric, common in Asia/Europe), ISO (Metric).

[!TIP] Never mix CAT and BT in same spindle; different flange dimensions.

Workholding Devices:

• Vises: For prismatic parts.

• Chucks: For round/cylindrical parts (3-jaw, 4-jaw).

• Clamps/Fixtures: Modular (flexible) vs. Dedicated (high-volume).

• Setup: The orientation of part in machine. Minimize setups (ideally 1) to reduce error and time.

Tool Management & Presetting:

• Tool Presetter: Measures tool length/diameter offline (in setting room). Data sent to CNC (G43 H# for length, G41 D# for diameter).

• Tool Life Monitoring: Based on time or part count.

2.7 Introduction to Rapid Prototyping & Additive Manufacturing (Link to CAM)

RP/AM: Additive processes building parts layer-by-layer from CAD data. Complementary to subtractive CAM (used for prototypes, complex geometries, tooling).

Key Processes:

STL File Format: De facto standard. Approximates CAD surfaces with triangular facets. No color, no tolerance info.

Process Planning for AM (CAM-like steps):

1. Build Orientation: Affects surface finish, support structures, build time.

2. Support Generation: Temporary structures for overhangs; must be removed post-build.

3. Slice & Toolpath: Software slices model and generates scan/laser path (analogous to CNC toolpath).

[!TIP] AM does not typically use G-code; uses proprietary machine instructions. However, 5-axis AM is emerging with complex toolpath strategies similar to CAM.

Integration: CAD → STL → AM Machine. No CAPP in traditional sense, but build orientation and support are critical process decisions.