3A. DFA Methodologies Lecture Notes

Recap of Last Lecture

• Progression from Product Design to Manufacture:

  • Not a one-step process.

• Implications:

  • Bring manufacturability into the design arena.

  • Inform production planning earlier.

• Costs:

  • Incurred Cost: Cost when a resource is used or given up.

  • Committed Cost: Costs that have not been incurred but will be in the future based on decisions already made.

• "Walls" in Product Design:

  • Represent ineffective communication and poor decision-making.

  • Arise from people thinking from their own perspectives.

  • Need to overcome or break the walls.

Design for Assembly (DFA)

• Assembly is a key process in manufacturing.

• Assembly cost is determined in the product design stage.

• Key factors:

  • Number of parts: Reducing part count is important.

  • Assembly process: Simplify assembly sequence to reduce time.

• Once design decisions are made, changes are difficult later on.

• Keys to Reduce Assembly Cost:

  • Design with as few parts as possible.

  • Design parts with careful consideration for ease of assembly.

General Design Principles for DFA

• Reduce Part Counts

• Use Modular Design

• Use Standard Parts

• Design Multi-functional Parts

• Design Parts for Multi-use

• Design for Ease of Fabrication

• Avoid Separate Fasteners

• Minimize Assembly Directions

• Maximize Compliance

• Minimize Handling

Why Reduce Part Counts?

• Best opportunity for reducing manufacturing costs.

• Fewer parts mean reduced:

  • Purchasing, inventory, and handling costs.

  • Time for processing, development, and engineering.

  • Difficulty in assembly, service inspection, and testing.

How to Reduce Part Counts

• Identify targets for part count reduction early in the design stage.

• Criteria for combining parts:

  • A part that does not move relative to other parts.

  • A part that does not need to be made of a different material.

  • A part that would make assembly or service difficult.

• Use one-piece/unified structures where possible.

Today’s Lecture Learning Outcomes

• After Part A, students will understand:

  • DFA methods, their background, goals, and benefits.

  • Basic procedure of using the Boothroyd & Dewhurst method.

• After Part B, students will understand:

  • More detailed procedure of using the Boothroyd & Dewhurst method.

Background of DFA

• Traditional product development:

  • Design team designs and prototypes.

  • Manufacturing team creates manufacturing plans.

• Issue: Design changes were rarely made due to uncertainty about functional requirements being affected.

Cost of Changes

• Decisions early in design have a significant impact on committed costs.

• Difficulty of change increases dramatically as development progresses.

Factors Influencing Total Costs

• Quantity to be manufactured

• Manufacturing costs (plant, energy)

• Material costs

• Labor costs (direct & indirect)

• Assembly costs

• Packaging, storage & transportation

Development of DFA Methods

• General design advice is useful for initiating the design process.

• Research in the 70s and 80s showed a structured approach could have a greater impact.

• Led to the development of several DFA methodologies.

• DFA methodologies have flourished, and development continues today.

Technical Benefits of Using DFA Methods

• Formalizing the procedure:

  • Makes the design process repeatable.

  • Steers the designer towards good solutions using proven techniques.

• Documenting the procedure:

  • Makes DFA skills accessible to non-experts.

• Quantifying the design performance:

  • Provides measurable standards.

  • Allows comparison of competing concepts.

• Highlighting design deficiency:

  • Identifies targets for improvement.

Economic Benefits of Using DFA Methods

• Significant benefits reported by literature.

• Major companies like Ford and General Motors reported billions of dollars in savings.

• Ford experienced easier parts fitting.

  • Example: Front Bumper of Taurus reduced to 10 parts after DFA.

• Boothroyd (1994) surveyed 43 cases of DFMA application:

  • 12 cases involved reduction of separate fasteners - average reduction $72 \%$.

  • 31 cases achieved reductions in assembly time - average reduction $61 \%$.

  • 12 cases achieved reductions in product cost - average reduction $37 \%$.

Concerns of Using DFA Methods

• Slowing down the concept-generation phase in design.

• Giving the designer a "blinkered view" by focusing only on assembly issues.

• Requiring significant training (or management effort).

• Requiring "additional design effort" before long-term benefits are seen.

Three Different DFA Philosophies

• Based on "General Design Principles and Rules":

  • General Electric (US) (Emphasis on part manufacture).

• Based on "Quantitative Evaluation" Methods:

  • Boothroyd & Dewhurst DFMA method (US):

    • Developed by Prof. Geoffrey Boothroyd (University of Massachusetts) in the early 1970s.

    • 1980: Peter Dewhurst joined to create software for Apple II Plus.

    • 1983: IBM and Digital Equipment funded a PC version, leading to Boothroyd Dewhurst Inc.

    • 1985: Research added a "Design for Manufacture" (DFM) module.

  • Hitachi (or Assemblability Evaluation Method) (AEM) (Japan):

    • Developed in the late 1970s by Hitachi.

    • Coincided with Boothroyd's work.

• Based on "Knowledge-based Approach" Methods:

  • Lucas / Hull (or Swift) DFA method (UK):

    • Collaborative work in the late 1980s.

    • Based on collaborative work with Boothroyd in 1980.

    • Initially a paper manual, later a computer-based system with IF-THEN rules.

Motive to Develop DFA Methods

• Product Development Battle (National level, US & Japan) ➔ Need for DFA Philosophy (General) ➔ DFA Methodology (Specific).

Common Goals of DFA Methods (1)

• Part count reduction:

  • Reduces direct assembly costs.

  • Reduces overhead and direct savings in the upstream supply chain.

  • Achieves cost savings in design, tooling, purchasing, vendor appraisal, etc.

• Faster new product introduction:

  • Less overall development effort.

  • Fewer parts needed.

  • Achieves the same project outcomes in a shorter time.

• Assembly process simplification:

  • Allows the use of simpler assembly processes to join the parts.

Common Goals of DFA Methods (2)

• Easier automation:

  • Optimizes the feeding and handling of parts.

  • Allows the use of simpler and cheaper equipment.

• Improved quality:

  • Reduces complexity, reducing operator error, improving process capability, and raising quality.

  • "Higher assembly efficiency ➔ Lower rate of defects"

Boothroyd & Dewhurst DFMA

• Peter Dewhurst and Geoffrey Boothroyd pioneered a design method and database: DFMA.

• Predicts assembly time and labor costs, challenging designers to simplify product structure.

• Companies can determine costs in advance and choose cost-effective materials and processes.

Ways to Use Boothroyd & Dewhurst DFMA

• Concurrent Engineering

• Benchmarking Competitors

• Analyzing Supplier Costs

• Why DFMA? Reducing number of components & processes, eliminating excess components.

What is Boothroyd & Dewhurst DFMA?

• Design for Manufacturing (DFM):

  • Concerned with reducing overall part production cost.

  • Minimizes complexity of manufacturing and uses common axes and processes.

• Design for Assembly (DFA):

  • Concerned with reducing product assembly cost.

  • Minimizes the number and complexity of assembly operations.

  • Individual parts may be more complex in design.

Boothroyd & Dewhurst DFA Guidelines (1)

• Basic Guidelines:

  • Minimize part count.

  • Design parts with self-locating features.

  • Design parts with self-fastening features.

  • Minimize reorientation of parts during assembly.

  • Emphasize 'Top-Down' assemblies.

  • Standardize parts (minimum use of fasteners).

• More Guidelines:

  • Design parts for easy retrieval, handling & insertion.

  • Encourage modular design.

  • Design for a base part to locate other components.

  • Design for component symmetry for insertion.

Boothroyd & Dewhurst DFA Guidelines (2) - Example: CD Rom Drive Assembly

• Standard parts (one screw type).

• Parts are self-guiding.

• Die cast with minimal amount of holes.

• Standard cutters.

• Guide features.

• Common datum for all fixtures.

• One common plane for assembly.

• Tabs for robotic lift.

• One assembly direction “tops down”.

• No adjustments required.

• No hidden features.

• Bottom rails for conveyor.

• Test direction access from top.

• Easy to fabricate parts.

• Sub-assemblies reduce handling of small, hard-to-grip parts.

• Symmetry in two axis.

• Holes large enough (straightness issues if too deep).

• Avoid tangle with use of fixtures.

Boothroyd & Dewhurst DFA Guidelines (3) - Example: Riveted Staple Remover

• Comparison of riveted vs. adhesively bonded staple remover designs.

• Illustrates part reduction and simplification.

Boothroyd & Dewhurst DFA (1)

• Introduction to the method:

  • DFA is part of the Boothroyd & Dewhurst DFMA.

  • Four DFA methods: manual, robotic, automatic, and printed circuit board.

  • Manual DFA is based on a database of estimated assembly times from a motion-time-measurement (MTM) study.

  • The database contains estimated assembly times for different assembly operations.

  • Every assembly operation depends on the design of a part and how it is assembled.

  • Analyzing how a part is handled and inserted allows calculation of estimated assembly time.

Boothroyd & Dewhurst DFA (2)

• Elimination / Integration of parts:

  • Perform a functional analysis of the product.

  • Analyze each part in two ways: Possibility to eliminate (or integrate) the part, Possibility to redesign the part to be easier to assemble.

  • Three questions to decide whether a part is a candidate for integration or reduction:

    • Does the part move relative to other assembled parts during normal operation?

    • Does the part have to be of other materials (or isolated) from other assembled parts?

    • Does the part have to be separate because assembly/disassembly would otherwise be impossible?

  • If the answer to all questions is “no”, the part is a candidate for elimination or integration.

Boothroyd & Dewhurst DFA (3)

• Geometrical properties of parts:

  • Analyze geometrical properties of each part.

  • Two steps: Analyze how difficult the part is to handle, Analyze how the part is inserted during assembly.

  • Compare assembly movements with estimated assembly times in the database.

  • Every assembly operation is quantified with an assembly time.

  • Ideal handling time: $1.5$ s (for a $1x1x1$ inches part).

  • Ideal insertion time: $1.5$ s.

  • Ideal total assembly time: $3$ s for each part.

  • Any geometric feature that does not follow the ideal design is penalized with a longer assembly time.

  • Extra operations (e.g., screwing) are considered outside the ideal assembly process.

Boothroyd & Dewhurst DFA (4)

• Evaluation by assembly efficiency:

  • Assembly efficiency is an indication for redesigning a product.

  • Assembly Efficiency is obtained by dividing theoretical minimum assembly time by actual assembly time:

    • $E<em>{ma} = (N</em>{min} * t<em>a) / t</em>{ma}$

    • Where:

      • $E_{ma}$ is the assembly efficiency.

      • $N_{min}$ is the theoretical minimum number of parts.

      • $t_a$ is the basic assembly time for one part (usually $3$ s).

      • $t_{ma}$ is the actual time to complete the assembly of the product.

  • Improving assembly efficiency is done by eliminating unnecessary parts and redesigning parts to be easier to handle and insert.