Dfa guidelines


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Dfa guidelines

  1. 1. Product design for manual assembly DFA tool: To reduce manufacturing and assembly costs. Effectively analyses the ease of assembly, Quick results simple and easy to use. Free association of ideas, comparison of alternative designs, identification of assembly problem areas, evaluation of solutions logically, Ideas, reasoning and decisions made during the design process become useful for future reference. Database for assembly times and cost factors for various design situations and production conditions. The ease of assembly depends on manual or general purpose automation or special purpose automation.
  2. 2. Manual assembly: Handling, insertion and fastening Guide lines for part handling: Design parts with end to end symmetry and rotational symmetry about the axis of insertion Provide features that will prevent jamming of parts stacked in bulk Avoid features that allow tangling of parts Guide lines for part insertion and fastening: Provide chamfers to guide insertion of two mating parts Generous clearance should be provided Where ever possible avoid holding down of parts Use pyramid assembly, progressive
  3. 3. Manual insertion and fastening consist of a finite variety of basic assembly tasks. Peg in hole, screw, weld, rivet, press fit. Factors effecting handling times. Part symmetry, Part thickness and size, Part weight Parts requiring two hands: Heavy, Very precise, large, flexible, part does not posses holding features Combination of factors Parts that severely nestle or tangle: Small, vision obscured, high temp. Chamfer on insertion operations: peg into a hole, part with a hole onto peg. Chamfer on peg is better, curved chamfers are better. Insertion time depends on length, diameter ,chamfer and clearance. Avoid jams and disc assembly problems: length and clearance Holding down time: clearance, grip size and insertion length
  4. 4. Further Design guidelines Avoid connections Design so that access for assembly operations is not restricted. Avoid adjustments Use kinematic design principles Design parts to prevent nesting. Nesting is when parts that are tacked on top of one another clamp to one another, for example, cups and coffee lids. Design parts with orienting features to make alignment easier.
  5. 5. To determine whether it is possible to combine neighboring parts: •Must the parts move relative to each other? •Must the parts be electrically or thermally insulated? •Must the parts be made of different material? •Does combing the parts interfere with assembly of other parts? •Will servicing be adversely affected? If the answer to all questions is “NO”, you should find a way to combine the parts. During the assembly of the product, generally a part is required only when; 1.A kinematic motion of the part is required. 2.A different material is required. 3.Assembly of other parts would otherwise be prevented. If non of these statements are true, then the parts do not need to be separate entities and may be combined.
  6. 6. Design for Assembly Principles • Minimize part count • Design parts with self-locating features • Design parts with self-fastening features • Minimize reorientation of parts during assembly • Design parts for retrieval, handling, & insertion • Emphasize ‘Top-Down’ assemblies • Standardize parts…minimum use of fasteners. • Encourage modular design • Design for a base part to locate other components • Design for component symmetry for insertion
  7. 7. DFA Process Product Information: functional requirements Functional analysis Identify parts that can be standardized Determine part count efficiencies Step 2 Step 1 Analyze data for new design Step 3 Identify handling (grasp & orientation) opportunitiesStep 4 Identify insertion (locate & secure) opportunitiesStep 5 Step 6 Identify opportunities to reduce secondary operations Identify quality (mistake proofing) opportunities Benchmark when possible Determine your practical part count Step 7
  8. 8. DFA Analysis Worksheet
  9. 9. Product Information: functional requirements Functional analysis Identify parts that can be standardized Determine part count efficiencies Step One
  10. 10. Considerations/Assumptions • The first part is essential (base part) • Non-essential parts: – Fasteners – Spacers, washers, O-rings – Connectors, leads • Do not include liquids as parts (e.g.. glue, gasket sealant,)
  11. 11. Part Identification • List parts in the order of assembly • Assign/record part number
  12. 12. So take it apart!
  13. 13. Count Parts and Interfaces • List number of parts (Np) • List number of interfaces (Ni)
  14. 14. Determine if Parts Can be Standardized • Can the current parts be standardized?: • Should they be? • (Only put a “Y” if both answers are yes…)
  15. 15. Theoretical Part Count Efficiency Theoretical Part Count Efficiency Theoretical Min. No. Parts Total Number of Parts Theoretical Part 1 Count Efficiency 10 Theoretical Part Count Efficiency = = * 100 = 10% * 100 GoalRule of Thumb – Part Count Efficiency Goal > 60%
  16. 16. DFA Complexity Factor – Definition • Assessing complexity of a product design • Two Factors • Np – Number of parts • Ni – Number of part-to-part interfaces – Multiply the two and take the square root of the total – This is known as the DFA Complexity Factor S Np x S Ni
  17. 17. DFA Complexity Factor – Target • Smaller is better (Minimize Np and Ni) • Let Npt = Theoretical Minimum Number of parts – from the Functional Analysis – Npt = 5 • Let Nit = Theoretical minimum number of part to part interfaces – Nit = 2(Npt-1) – Nit = 2(5-1) = 8 Part 2 Part 3 Part 4 Part 5 Part 1 DCF = S Np x S Ni DCFt = S Npt x S Nit DCFt = 5 x 8 = 6.32
  18. 18. Determine Relative Part Cost Levels • Subjective estimate only • Low/Medium/High relative to other parts in the assembly and/or product line
  19. 19. Cost Breakdown • Media paper 21.4% • Centertube 3.6% • Endplates (2) 3.0% • Plastisol 2.6% • Inner Seal 4.0% • Spring 0.9% • Shell 31.4% • Nutplate 21.0% • Retainer 4.8% • Loctite 0.3% • End Seal 7.0%
  20. 20. Step Two Determine Practical Minimum Part Count
  21. 21. Determine Practical Minimum Part Count • Team assessment of practical changes • Tradeoffs between part cost and assembly cost
  22. 22. Implementation Risk HighMediumLow Short Term Medium Term Long Term Idea Classification
  23. 23. Fastener Cost • Select the most inexpensive fastening method required plastic bending riveting screwing snap fit
  24. 24. General Design Principles Self-fastening features
  25. 25. General Design Principles Asymmetric Part Symmetry of a part makes assembly easier Symmetry eliminates reorientation
  26. 26. Step Three Identify quality (mistake proofing) opportunities
  27. 27. Mistake Proofing Issues • Cannot assemble wrong part • Cannot omit part • Cannot assemble part wrong way around. symmetrical parts asymmetrical parts
  28. 28. Step Four Identify handling (grasp and orientation) opportunities
  29. 29. Quantitative criteria • Handling Time: based on assembly process and complexity of parts – How many hands are required? – Is any grasping assistance needed? – What is the effect of part symmetry on assembly? – Is the part easy to align/position?
  30. 30. Handling Difficulty • Size • Thickness • Weight • Fragility • Flexibility • Slipperiness • Stickiness • Necessity for using 1) two hands, 2) optical magnification or 3) mechanical assistance
  31. 31. Handling Difficulty size slipperiness sharpness flexibility
  32. 32. Eliminate Tangling/Nesting
  33. 33. Step Five Identify insertion (locate & secure) opportunities
  34. 34. Quantitative criteria • Insertion time: based on difficulty required for each component insertion – Is the part secured immediately upon insertion? – Is it necessary to hold down part to maintain location? – What type of fastening process is used? (mechanical, thermal, other?) – Is the part easy to align/position?
  35. 35. Insertion Issues • Provide self-aligning and self locating parts
  36. 36. Insertion Issues • Ensure parts do not need to be held in position
  37. 37. Insertion Issues • Parts are easy to insert. • Provide adequate access and visibility
  38. 38. Insertion Issues • Provide adequate access and visibility
  39. 39. Step Six Identify opportunities to reduce secondary operations
  40. 40. Eliminate Secondary Operations • Re-orientation (assemble in Z axis) • Screwing, drilling, twisting, riveting, bending, crimping. Rivet
  41. 41. Eliminate Secondary Operations • Welding, soldering, gluing. • Painting, lubricating, applying liquid or gas. • Testing, measuring, adjusting.
  42. 42. Error = Sum all Y’s in Error Columns Proofing Theoretical Min. No. Parts Handling = Sum all Y’s in Handling Columns Index Theoretical Min. No. Parts Insertion = Sum all Y’s in Insertion Columns Index Theoretical Min. No. Parts 2nd Op. = Sum all Y’s in 2nd Op. Columns Index Theoretical Min. No. Parts Assembly Metrics
  43. 43. Analyze All Metrics First consider: Reduce part count and type Part Count Efficiency and DFA Complexity Factor Then think about: Error Proofing Error Index Then think about: Ease of handling Handling Index Ease of insertion Insertion Index Eliminate secondary ops. 2nd Op. Index Set Target Values for These Measures
  44. 44. Step Seven Analyze data for new design
  45. 45. Minimize part count by incorporating multiple functions into single parts. Several parts could be fabricated by using different manufacturing processes (sheet metal forming, injection molding).
  46. 46. Modularize multiple parts into single sub-assemblies.
  47. 47. Design to allow assembly in open spaces, not confined spaces Do not bury important components
  48. 48. Parts should easily indicate orientation for insertion. Parts should have self-locking features so that the precise alignment during assembly is not required, or provide marks (indentation) to make orientation easier.
  49. 49. Standardize parts to reduce variety.
  50. 50. Design parts so they do not tangle or stick to each other.
  51. 51. Distinguish different parts that are shaped similarly, or hard to distinguish, by non-geometric means, such as color coding
  52. 52. Provide alignment features on the assembly so parts are easily oriented.
  53. 53. Design the mating parts for easy insertion. Provide allowance on each part to compensate for variation in part dimensions. Case 1 Case 2
  54. 54. Design the first part large and wide to be stable and then assemble the smaller parts on top of it sequentially. Case1 Case 2
  55. 55. If you cannot assemble parts from the top down exclusively, then minimize the number of insertion direction. Never require the assembly to be turned over. Case 1 Case 2
  56. 56. Joining parts can be done with fasteners (screws, nuts and bolts, rivets), snap fits, welds or adhesives.