CAD/CAM Digital Study Notes

Product Cycle and Definition of CAD Tools

• CAD/CAM Overview: Computer-Aided Design (CAD) and Computer-Aided Manufacturing (CAM) are utilized in engineering to produce drawings, generate shaded images, perform engineering analysis (Finite Element Analysis), and generate NC part programs.

• CAD Tools Definition: CAD tools are defined as the intersection of three specific sets:

  1. Geometric Modeling.

  2. Computer Graphics.

  3. Design Tools.

• Heuristic Procedures: Implementation includes analysis codes, heuristic procedures, and design practices improved by computer hardware and software to satisfy design goals efficiently.

• Technical Extremes: CAD tools range from geometric tools (graphics entities manipulation and interference checking) to customized application programs (analysis and optimization routines).

• Intermediary Tools: Common tools currently available include:

  • Tolerance analysis.

  • Mass property calculations.

  • Finite element modeling and analysis.

• Primary Purposes of CAD Definition:

  1. To extend utilization beyond drafting and visualization.

  2. To customize systems for special design and analysis needs.

  3. To influence the development of next-generation CAD/CAM systems.

Reasons for Implementing CAD Systems

• Increased Productivity: Reduces time for synthesizing, analyzing, and documenting the design.

• Improved Quality of Design: Permits detailed engineering analysis and investigation of a large number of design alternatives, reducing errors through greater accuracy.

• Improved Communication: Results in better engineering drawings, increased standardization, and better documentation with fewer errors.

• Manufacturing Database: Creates much of the database required to manufacture the product during the design documentation process.

• Design Efficiency: Reduces waste at the design stage and improves the overall efficiency of the process.

Conventional Design Process Phases

1. Recognition of Need: Identifying the necessity of a design, environmental operating conditions, and historical difficulties.

2. Definition of Problem: Determining shape, space requirements, material restrictions, and operating conditions.

3. Synthesis of Design: Preparing rough drawings, identifying loading conditions, defining shapes for cross-sections, and establishing mathematical models.

4. Analysis and Optimization: Checking for safe stresses, recalculating dimensions if unsatisfactory, and optimizing for parameters like minimum weight, volume, efficiency, or cost.

5. Evaluation: Measuring the design against specifications; often requires the fabrication and testing of physical models to assess reliability and quality.

6. Presentation: Presenting the component design with drawings in an attractive format.

Comparison: Conventional Design vs. CAD

• Geometric Modeling: Concerned with computer-compatible mathematical descriptions. Wireframes represent the basic form, while solid modeling (3D) is the most advanced.

• Engineering Analysis: Includes stress-strain calculations and heat transfer computation.

  • Mass Properties: Provides surface area, weight, volume, center of gravity, and moment of inertia.

  • FEA: The most powerful analysis feature for testing complex structures.

• Design Review and Analysis:

  • Interference Checking: Analyzes assemblies to ensure components do not occupy the same space.

  • Kinematics: Animates the motion of mechanisms like linkages and hinged components.

• Automated Drafting: Features automatic dimensioning, cross-hatching, scaling, and the development of sectional or enlarged views.

CAD Hardware and Hardware Configurations

• System Components:

  • CPU: The brain of the system; receives workstation information and read/stores data.

  • Secondary Memory: Magnetic tapes and discs used to store engineering drawings, instruction programs for plotters, and CAD software.

  • Workstation: A visible part providing interaction; must support multitasking and networking. Includes the VDU (Visual Display Unit).

• Hardware Classifications:

  • Mainframe-based systems: Used in large organizations for massive data and concurrent remote activities. Supports hundreds of workstations over vast networks.

  • Minicomputer-based systems: Facilitated by 32-bit word lengths and virtual memory; allows decentralization from mainframes.

  • Microcomputer-based systems: Popularized by the IBM PC; utilizes 32-bit word lengths with sufficient disk storage for CAD/CAM.

  • Workstation-based systems: Defined as stations with their own computing power, multitasking capabilities, and networking potential.

Graphics Display Technologies

• Scan Technology:

  • Random Scan: Graphics are generated by drawing vectors/line segments in an order controlled by software.

  • Raster Scan: The screen is scanned left to right and top to bottom. The display is divided into picture elements called pixels ($N \times M$ resolution). A typical resolution is $1280 \times 1024$.

• CRT (Cathode Ray Tube):

  • DVST (Direct View Storage Tube): Stores the picture as a charge on a phosphor mesh. Pictures remain until erased. Cannot edit individual areas. No flicker.

  • Vector Refresh: Uses a refresh buffer to store the display file. Requires refreshing $30$ to $60$ times per second to prevent flicker. Suited for animation.

  • Raster Refresh: Works like a television. Geometric information is converted to pixel values. Each refresh takes approximately $0.02\,\text{seconds}$.

• Liquid Crystal Display (LCD):

  • Uses polarizing characteristics of organic compounds to block or transmit light.

  • Guest-host Technology: Combines dichromic-dye guest molecules with host liquid crystal molecules for color.

• Plasma Panel Display:

  • Gas at low pressure between horizontal and vertical wire grids.

  • AC-activated: Uses capacitive coupling and has bistable memory, eliminating the need for continuous refreshing.

  • DC-activated: Requires continuous refreshing.

Hardcopy Output Devices: Printers and Plotters

• Impact Dot Matrix Printer: Electromechanical device using thin wires and ink ribbons. Average resolution ranges from $60$ to $240\,\text{dots per inch}$.

• Ink Jet Printer: Raster scan device.

  • Continuous Flow: Sprays a stream of droplets broken by ultrasonic waves; selective droplets are electrostatically charged and deflected.

  • Drop-on-demand: Uses a piezoelectric crystal to squirt ink only when a dot is required.

• Laser Printer: Electrostatic process using a photosensitive drum, a laser beam modulated via a rotating octagonal mirror, and toner (electrostatistically sensitive black powder) fixed by heated rollers.

• Plotters:

  • Flatbed: Medium is fixed; the arm or head moves in $2\text{D}$. Some use the Sawyer motor principle.

  • Drum Plotter: Paper is on a rotating drum (one axis) while the pen moves transversely (second axis).

Geometric Modeling Requirements and Facilities

• Essential Guidelines:

  • Completeness of part representation.

  • Unambiguous representation (one-to-one mapping between representation and solid).

  • Unique solid representations for comparison of identical models.

  • Accuracy without approximation.

  • Support for transformations (move, rotate, scale) and Boolean manipulations (union, intersect, subtract).

• Categorized Facilities:

  1. Geometric Modeling: analytic features (lines, arcs), 3D wireframe, solid primitives, skinning, and sculptured surfaces (Bezier, Coons).

  2. Editing/Manipulation: Resizing, relocating, filleting, chamfering, and windowing.

  3. Display Control: Zoom, Pan, Hidden line removal, Shading, and Animation.

  4. Drafting: Generation of orthographic, isolated, and axonometric views with automatic crosshatching.

  5. Analysis: Calculations for perimeter, area, volume, mass, center of gravity, and moment of inertia. Supports Finite Element Analysis (FEA).

Comparison of Modeling Techniques

• Wireframe Modeling: Collection of points and edges ($x,y,z$ coordinates). Requires little memory but is ambiguous and cannot calculate physical properties like volume.

• Surface Modeling: Represents the skin or envelope of an object with no thickness. Used for complex "sculptured surfaces" such as car bodies or ship hulls.

• Solid Modeling: Complete, unambiguous representation of enclosed volume. Includes vertices, edges, and surfaces.

  • CSG (Constructive Solid Geometry): Uses Boolean operations on primitives (blocks, cones, spheres).

  • B-Rep (Boundary Representation): Defines solids through their bounding surfaces.

Mathematics of Curves and Continuity

• Continuity Levels:

  • $C^0$ Continuity: Position continuity (end points meet).

  • $C^1$ Continuity: Tangent continuity (first derivatives are equal).

  • $C^2$ Continuity: Curvature continuity (second derivatives are equal).

• Bezier Curves: Defined by $n + 1$ control points with a polynomial of degree $n$:     $P(u) = \sum_{i=0}^{n} P_i B_{i,n}(u)$

  • Bernstein Polynomials: Serve as blending functions:     $B_{i,n}(u) = C(n,i) u^i (1-u)^{n-i}$

  • Properties: Convex hull property (curve stays within the polygon), variation diminishing property (complexity does not exceed the polygon), and affine invariance.

Numerical Control (NC) and Automation

• Definition: Programmable automation where the process is controlled by numbers, letters, and symbols.

• Basic Components:

  1. Program of Instructions: Step-by-step directions (often on $1\,\text{in.}$ punched tape).

  2. Machine Control Unit (MCU): Electronics that read and interpret the program.

  3. Machine Tool: The part performing the work (spindle, worktable).

• NC Coordinate Systems:

  • Standard Axes: $x$ and $y$ are in the plane of the table; $z$ is the vertical motion of the spindle.

  • Fixed Zero: Origin is at a fixed position on the machine table (usually southwest corner).

  • Floating Zero: Operator can set the zero point at any position.

  • Absolute Positioning: Locations defined relative to the origin.

  • Incremental Positioning: Locations defined relative to the previous point.

NC Motion Control Systems

• Point-to-Point (PTP): Moves the tool to a location; the path and speed of movement between points are not critical.

• Straight Cut: Moves the tool parallel to a major axis at a controlled rate for machining (e.g., milling rectangular shapes).

• Contouring (Continuous Path): Simultaneous control of multiple axes to generate curved geometries. Approximates curves using short straight-line segments.

NC Programming and APT Language

• NC Block Words:

  • $n$-word: Sequence number.

  • $g$-word: Preparatory functions.

  • $f$-word: Feed rate.

  • $s$-word: Cutting speed (spindle rotation).

  • $m$-word: Miscellaneous functions (e.g., coolant on/off).

• APT (Automatically Programmed Tools): Supports up to 5-axis control and $400$ word vocabulary.

  • Geometry Statements: symbol = geometry type / descriptive data (e.g., P1 = POINT / 5.0, 4.0, 0.0).

  • Motion Statements: motion command / descriptive data (e.g., GOTO / P1).

  • Reference Surfaces: Drive Surface (guides the side), Part Surface (guides the bottom), and Check Surface (stops movement).

Computer Controls in NC: CNC vs. DNC

• CNC (Computer Numerical Control): Uses a dedicated microcomputer for one machine tool. Offers "soft-wired" flexibility, allowing on-site tape editing and metric conversion.

• DNC (Direct Numerical Control): A central computer controls multiple machines in real-time. Eliminates tape readers. Stores programs centrally and allows data collection/reporting of shop performance.

• Adaptive Control (AC): Measures process variables (force, torque, temperature) and adjusts feed/speed to optimize performance.

  • ACO (Optimization): Maximizes indices like Material Removal Rate (MRR) relative to Tool Wear Rate (TWR).

  • ACC (Constraint): Maintains variables at or below set limits.

Group Technology (GT) and Process Planning

• Philosophy: Grouping similar parts into "part families" based on geometric shape/size (design) or sequence of processing (manufacturing).

• Part Family Identification Methods:

  1. Visual Inspection: Subjective and least accurate.

  2. Production Flow Analysis (PFA): Analyzes route sheets to group parts based on similar machine routings.

  3. Classification and Coding: Examination of individual attributes.

• GT Coding Systems:

  • Opitz System: Uses a 9-digit code (5 for form, 4 for supplementary) plus a 4-digit secondary code.

  • MICLASS: Comprehensive system with $12$ to $30\,\text{digits}$; uses interactive computer questioning.

  • CODE: 8-digit system developed by MDSI using hexadecimal values.

• CAPP (Computer-Aided Process Planning):

  • Retrieval (Variant): Edits standard process plans saved for part families.

  • Generative: Creates plans from scratch based on part geometry and machine capabilities.

Computer-Aided Quality Control (CAQC)

• Terminology:

  • Quality Assurance (QA): Activities maximizing design specification compliance.

  • Quality Control (QC): Activities involving inspection, detection, and correction.

  • Statistical QC: Includes Acceptance Sampling and Control Charts (UCL and LCL).

• Contact Inspection: Uses Coordinate Measuring Machines (CMM) with movable heads and sensing probes to measure $x$, $y$, and $z$ coordinates.

• Non-Contact Inspection:

  • Optical: Machine vision (pattern recognition), Scanning laser (time-lapse measurement), Photogrammetry (extracting 3D data from angle photographs).

  • Non-Optical: Electrical field (Reluctance, Capacitance, Inductance/Eddy currents), Radiation (X-ray for thickness/voids), and Ultrasonics (high-frequency sound waves).

Flexible Manufacturing Systems (FMS)

• Components: Processing workstations (CNC), Automated material handling (conveyors/AGVs), and Distributed computer systems.

• Tests of Flexibility: Variety test, Schedule change test, Error recovery test, New part test.

• FMS Categories:

  • Single Machine Cell (SMC): One CNC machine with local storage.

  • Flexible Manufacturing Cell (FMC): Two or three workstations with a handling system.

  • Flexible Manufacturing System (FMS): Four or more workstations connected mechanically and electronically.

• Layout Types: In-line, Loop, Ladder, and Open Field.

Questions & Discussion

• Q: What are the main types of 2D transformations?

  • A: Translation, Scaling, Reflection, Rotation, and Shearing.

• Q: Distinguish between analytical and approximated curves.

  • A: Analytical curves (circles, ellipses) follow fixed mathematical equations. Approximated curves (Bezier, B-spline) provide flexibility for complex shapes like automobile fenders.

• Q: What is meant by "Acid Test" in NC?

  • A: It involves testing the NC tape on the machine tool using a foam or plastic material to verify tool movements before actual production.

• Q: Why is retrieval CAPP called variant CAPP?

  • A: Because it has the capacity to retrieve an existing plan for a part family and alter it (create a variant) to suit a specific part.

• Q: What are the objectives of FMS?

  • A: To provide flexibility in small/medium part production, maximize utilization, and improve management control over part families.