5. Lucas-Hull’s DFA Method Notes

MEE3014 Module 05: Lucas-Hull’s DFA for Manual Assembly

Part Count Reduction

  • Real-life examples include:

    • Pedals

    • Peelers

    • Clips

Part Count Reduction Examples

  • Clothes Clips: Transition from a complex clip to a one-piece molding with an integrated spring made of DELRIN.

  • Electric Plug Totalizer Wheel: Acts as an indexing gear, integral bearing, integral spring, position stop, and print wheel.

Design for Assembly (DFA)

  • Focus on Lucas-Hull's DFA Method.

Lucas-Hull DFA Background

  • Developed in the early 1980s through collaboration between Prof. Ken Swift (University of Hull) and Lucas Corp in the UK.

  • A knowledge-based approach developed collaboratively with Prof. G. Boothroyd.

Comparison of Broothroyd-Dewhurst’s DFA and Lucas-Hull’s DFA

  • Broothroyd-Dewhurst’s DFA:

    • Based on "Tables of Data" compiled through time studies in factories/laboratories.

  • Lucas-Hull’s DFA:

    • Based on a "Points Scale" providing a relative measure of assembly difficulty.

    • Consists of three sequential analyses:

      • Functional analysis.

      • Handling (manual assembly) or Feeding (automated assembly) analysis.

      • Fitting analysis.

How to Perform Lucas-Hull DFA

  1. Product Specification.

  2. Product Design.

  3. Functional Analysis (loop back to Step 2 if needed).

  4. Handling / Feeding Analysis.

  5. Fitting Analysis.

  6. Design Assessment (return to Step 2 if needed).

Lucas-Hull’s DFA Method

  • Functional Analysis

  • Handling or Feeding Analysis

  • Fitting Analysis

Lucas-Hull DFA Functional Analysis

  • Parts are reviewed for their intended function(s).

  • Parts are divided into two groups:

    • Group A: "Essential" to the product's function.

    • Group B: "Not Essential" to the product's function (including fastening, locating, etc.).

  • Design efficiency is calculated using the equation: Ed=AA+B×100%Ed = \frac{A}{A + B} \times 100 \% where A is the number of essential parts, and B is the number of non-essential parts.

  • Targeted design efficiency: Ideally, 60% for the initial design.

Purpose of Design Efficiency in Lucas-Hull DFA

  • Reduces "part count" in the early design stage.

  • Used to "pre-screen" an initial design.

  • Ideally, a design efficiency of 60% is targeted for the initial design.

Determining Parts in Group A and Group B in Lucas’s functional analysis

  1. Does the part move relative to all the other parts already assembled? (Only gross movement should be considered)

  2. Must the part be of a material that is different from those of all the other parts already assembled or must it be isolated from these? (Only fundamental reasons relating to material properties are acceptable)

  3. Must the part be separate from all the other parts already assembled (because necessary assembly or disassembly of other separate parts would otherwise be impossible)?

Lucas-Hull DFA Functional Analysis Criteria

  • Flowchart to determine if a part is essential (A) or non-essential (B) based on its movement, material, and need for adjustment/replacement.

  • Relative Movement

    • Does the part move relative to parts already analyzed? If yes, is the movement essential for the product to function?

  • Different Material

    • Is the part made of a different material? If yes, is the material difference essential for product function?

  • Need for Adjustment / Replacement

    • Is the part separate to allow for maintenance, adjustment, or replacement? Is this essential?

    • Must the part be separate to enable adjustment or replacement?

Lucas-Hull’s DFA Method: Handling or Feeding Analysis

  • "Handling" is for manual assembly; "Feeding" is for automated assembly.

  • Parts are scored in four categories:

    • A. Size & weight (score between 0 and 3)

    • B. Handling difficulties (score between 0 and 0.8)

    • C. Orientation (score between 0 and 0.5)

    • D. Rotational orientation (score between 0 and 0.4)

Meaning of Scores in Handling/Feeding Analysis

  • Reflects the extent of difficulty to handle or feed parts into an assembly.

  • Size & Weight (A)

    • Convenient: 1

    • Very small (requires tools): 1.5

    • Large/heavy (requires more than 1 hand): 1.5

    • Large/heavy (requires hoist or 2 people): 3

  • Handling Difficulties (B)

    • No handling difficulties: 0

    • Gripping problem/slippery: 0.2

    • Sharp/abrasive: 0.3

    • Delicate: 0.4

    • Sticky: 0.5

    • Flexible: 0.6

    • Severely nest: 0.7

    • All the above apply: 0.8

  • Orientation (C)

    • Symmetrical: 0

    • End to end (easy to see): 0.1

    • End to end (not visible): 0.5

  • Rotational Orientation (D)

    • Rotational symmetry: 0

    • Easy to see: 0.2

    • Hard to see: 0.4

  • Handling or Feeding Score for a Part = Score of (A+B+C+D)(A + B + C + D)

Targets and Recommendations

  • FOR a “PART”

    • The target score (i.e. sum of scores from A to D) for a part is 1.5

    • If the score is > 1.5, the part should be considered for “redesign”

  • FOR an “ASSEMBLY”

    • The recommended “Handling or Feeding Ratio” for an assembly is 2.5

    • Handling or Feeding Ratio = Total Handling or Feeding Score of All PartsNumber of Essential Parts\frac{\text{Total Handling or Feeding Score of All Parts}}{\text{Number of Essential Parts}}

    • “Total Handling or Feeding Score” is the “total sum of all scores in category A, B, C and D of all parts”

    • “Number of Essential Parts” is the “number of A parts determined from the functional analysis”

Lucas-Hull’s DFA Method: Fitting Analysis

  • Scores parts based on performance in six categories:

    • A. Placing and fastening (score between 1 and 4)

    • B. Process direction (score between 0 and 1.6)

    • C. Insertion (score between 0 and 1.2)

    • D. Access and / or vision (score between 0 and 1.5)

    • E. Alignment (score between 0 and 0.7)

    • F. Insertion force (score between 0 and 0.6)

  • Related to "actions of placing, fastening, inserting and / or aligning" a part.

  • Calculated similarly to Handling / Feeding Analysis.

Meaning of Scores in Fitting Analysis

  • Reflects the extent of difficulty to fit the parts into an assembly.

  • Placing and Fastening (A)

    • Self-holding orientation: 1

    • Requires holding: plus one of the following

      • Self-securing (i.e. snaps): 2

      • Screwing: 4

      • Riveting: 4

      • Bending: 4

  • Process Direction (B)

    • Straight line from above: 0

    • Straight line not from above: 0.1

    • Not a straight line: 1.6

  • Insertion (C)

    • Single insertion: 0

    • Multi insertions: 0.7

    • Simultaneous multiple insertions: 1.2

  • Access and / or Vision (D)

    • Direct: 0

    • Restricted: 1.5

  • Alignment (E)

    • Easy to align: 0

    • Difficult to align: 0.7

  • Insertion Force (F)

    • No resistance to insertion: 0

    • Resistance to insertion: 0.6

  • Fitting Score for a Part = Score of (A+B+C+D+E+F)(A + B + C + D + E + F)

Targets and Recommendations for Fitting Analysis

  • FOR a “PART”

    • The target score (i.e. sum of scores from A to F) for a part is 1.5

    • If the score is > 1.5, the part should be considered for “redesign”

  • FOR an “ASSEMBLY”

    • The recommended “Fitting Ratio” for an assembly is 2.5

    • Fitting Ratio = Total Fitting Score of All PartsNumber of Essential Parts\frac{\text{Total Fitting Score of All Parts}}{\text{Number of Essential Parts}}

    • Where “Total Fitting Score” is the “total sum of all scores in category A, B, C, D, E and F of all parts”

    • “Number of Essential Parts” is the “number of A parts determined from the functional analysis”

Lucas-Hull DFA Example: Toy Aircraft Analysis

  • Manual Disassembly:

    • Upper body molding

    • Lower body sub-assembly

    • Motor / fan sub-assembly (bought-in, considered as a part)

    • Battery compartment cover sub-assembly

    • Captive screw

    • Ceiling mount kit (considered as a part)

    • Long and short screws for assembly

    • Some other parts (unseen here)

      • Packaging

      • Switch

      • Motor mount screws

      • Lower body moulding

      • Battery compartment cover

Part Lists of the Toy Aircraft

  • Packaging

  • Aircraft body assembly

  • Ceiling mount kit

  • Long and short screws for assembly

  • Upper body moulding

  • Lower body sub-assembly

  • Lower body moulding

  • Motor / fan bought-in part

  • Motor mount screws

  • Switch

  • Battery compartment cover sub-assembly

  • Battery compartment cover

  • Captive screw

Lucas-Hull’s Functional Analysis Results for Toy Aircraft

  • Identified "A" parts:

    • Upper and lower body mouldings

    • Packaging

    • Ceiling mount kit

    • Motor / fan bought-in part

    • Switch

  • Identified "B" parts:

    • Long, short, and motor mount screws

    • Battery compartment cover

  • Rationale:

    • Lower body moulding can theoretically serve as a battery compartment cover.

    • Screw fastening for securing batteries, though a required safety feature, could be achieved by other methods.

Lucas-Hull DFA Example: Calculations and Implications

  • Number of Essential A Parts: 6

  • Number of Non-essential B Parts: 4

  • Calculation of design efficiency:

    • Ed=AA+B×100%=66+4×100%=60%E_d = \frac{A}{A + B} \times 100 \% = \frac{6}{6 + 4} \times 100\% = 60\%

  • Implications of this design efficiency:

    • 60% is fairly good for an initial design, but there is still room for improvement by redesigning the toy aircraft (e.g., removing the battery compartment cover and its securing screw).

    • Design efficiency can be used as a decision gate and allows for a direct comparison of two or more alternative design concepts.

Remarks of Boothroyd-Dewhurst’s and Lucas-Hull’s DFA

  • Always try to simplify products.

  • How well-designed a product is can be reflected by its “part count”.

    • Lower the part count, the better the design (for similar functionality.)

  • Good designs or reliable products tend to have fewer parts.

  • Fasteners (e.g., nuts, bolts, screws, and rivets) can usually be designed out.

    • Leave them in where access or disassembly are required.

Next Lecture

You will learn about the details into Hitachi’s DFA method for manual assembly and also the concept of automated assembly.