Manufacturing System Design for High Product Quality

1. Manufacturing System Design for High Product Quality Jingshan Li, Dennis E. Blumenfeld, Ningjian Huang Robert R. Inman and Samuel P. Marin Manufacturing Systems Research Lab General Motors Research & Development Center Warren, Michigan, USA 5th International Conference on Analysis of Manufacturing Systems May 21 2005

2. OUTLINE 1. Motivation 2. Manufacturing system design impacts quality 3. Research opportunities 4. Andon System 5. Repair and Rework System 6. Conclusions

3. 1. MOTIVATION • System design and quality management are important elements in manufacturing industry. • Substantial research efforts have been devoted to both of them, but independently. • Little research attention has been paid to investigate the interactions between manufacturing system design and product quality.

4. Manufacturing Operation Manufacturing System Validation Manufacturing System Design Product Design TQM SPC JIT Lot sizing TQM QFD DFQ TQM Process capability Tolerancing ? • Scarcity of research on attempting to improve quality in manufacturing system design phase • Does manufacturing system design impact quality?

5. 2. MANUFACTURING SYSTEM DESIGN IMPACTS QUALITY • Evidences from automotive industry: • Harbour report – quality and productivity are positively correlated → improving production system can improve quality. • American Axles & Manufacturing – quality improvement due to production system changes, e.g., conveyor, inspection, buffers, etc. • Ford/Jaquar – improve quality by adopting Toyota production systems. • GM – strip out buffers to improve paint quality in paint shops. • Toyota – pay attention to production system’s impact on quality, e.g., Andon, additional inspection stations, selected stationary assembly stations, etc.

6. Experiments and analysis: • Ergonomics – poor workstation layout and high line speeds may hurt quality performance of manual operations. • Andon system – stopping the line to fix every problem can improve throughput of good jobs when average repair times are short. • Repair and rework system – appropriate design of repair subsystem can improve quality buy rate. • Verdict:Manufacturing system design does impact product quality. • Largely unexplored area with promising opportunities

7. Strategic issues: Flexibility Agility Level of automation Modularity Outsourcing Scalability Emerging Technology Tactic issues: Andon Assembly Line Movement and Balancing Batch Size Buffer Location and Size Centralized/Decentralized Equipment Feedback Loops Inspection Line or Machine Speed Parallel versus Serial lines Plant Layout Repair and rework loops 3. RESEARCH OPPORTUNITIES

8. 4. ANDON SYSTEM • Andon – a visual control device to monitor quality on assembly line. • The worker can pull the Andon cord to trigger a light as a call for help, and stop the line if needed to correct the problem.

9. Current literature contains many popular articles that are descriptive or provide qualitative studies of Andon use. • It is claimed and taken for granted that, in spite of line stoppages and productivity loss, overall system performance is improved. • Why?Under what conditions?

10. Two different Andon strategies: • Empower workers to stop the line for every problem, so that all jobs are corrected first time, or • Encourage workers to reduce the number of Andon calls, so that line only stops for severe problems. • Which one is the right way?

11. WHY? WHEN? HOW? • Need for quantitative model to • analyze performance of a transfer production line with Andon. • discover the conditions for successful Andon use. • investigate trade-offs between productivity and quality.

12. mk m1 m2 Andon cord Model Formulation • Transfer line with k machines (m1, m2,..., mk) linked to one Andon cord • Performance index: Throughput of good quality jobs

13. Type of systems • No Andon:Job moves to next machine at end of cycle, no matter whether job has problem or not. • Full Andon: If job has problem or is not complete at end of cycle, Andon cord is pulled and line stops to allow extra time for repair (up to a maximum time tm). • Partial Andon: If job has severe problem at end of cycle, Andon cord is pulled and line stops to allow extra time for repair (up to a maximum time tm).If job has minor problem at end of cycle, job moves to next machine.

14. Assumptions • Machines are synchronized (all jobs start work at the same time) with identical cycle time c. • At end of cycle, each machine has fixed probability ithat job has defect or is not complete. A defective job has probability aito have a severe defect. • Repair (correspondingly, severe repair) times are independent and exponentially distributed with parameters i(correspondingly, i, and i < i),with truncation at maximum time tm. • At most one Andon pull per cycle. • Independence of operations and quality failures.

15. No Andon: Full Andon: Partial Andon: • One-machine case • Throughput of good quality job, G

16. Illustration c = 1, l = 0.25, a = 0.5 m = 0.9, n = 0.8 Full Andon Throughput of good jobs, G (jobs per unit time) Partial Andon No Andon Maximum extra time for repair, tm

17. 0.8 0.79 m = 0.7, n = 0.5 m = 0.8, n = 0.7 0.78 0.77 0.76 0.75 Full Andon 0.74 No Andon 0.73 No Andon 0.72 0.71 Full Andon 0.7 Partial Andon Partial Andon 0 1 2 3 4 5 Illustration c = 1, l = 0.25, a = 0.5 Throughput of Good Jobs, G Maximum extra time for repair, tm

18. Theorem: Under assumptions, • If  + cµ > 1, then GNo Andon < GPartial Andon < GFull Andon • If  + c < 1<  + cµ, then GPartial Andon < GNo Andon < GFull Andon • If  + c <  + cµ < 1, then GFull Andon < GNo Andon GPartial Andon < GNo Andon

19. Line Condition Short repair times Long repair times Best Strategy Full Andon No Andon • Insights • Implementing Andon can improve throughput of good quality jobs when average repair times are short (i.e., when repair rate is high). • Partial Andon is never the best strategy. Even when repair times for severe defects are short, Full Andon is better than Partial Andon.

20. Rules of thumb • If average repair time is less than the cycle time, then Full Andon will improve throughput of good quality jobs. • If average time to repair severe defects is less than the cycle time, then any type of Andon will improve throughput of good quality jobs. • It is worth repairing all defects rather than severe ones only. • Right way: Stop line for all problems. • Toyota: hundreds of Andons per shift with total line stoppage time of 10-15 minutes.

21. Multiple machine case • Throughput of good quality job, G No Andon: Full Andon: Partial Andon:

22. Throughput of good quality job, G No Andon: Full Andon: Partial Andon:

23. k = 5, c = 1, l = 0.1, a = 0.5 m = 0.9, n = 0.8 Full Andon Throughput of good jobs, G (jobs per unit time) Partial Andon No Andon Maximum extra time for repair tm Illustration • Right way:Stop line for all problems.

24. 5. REPAIR AND REWORK SYSTEM • Repair and rework systems are often used in many manufacturing industries: automotive, electronics, packaging, process, etc. • In automotive assembly plants, product quality is typically characterized by • First Time Quality (FTQ): good job ratio of all first time processed jobs • Quality Buy Rate (QBR): good job ratio of all jobs, including first time jobs and reworked jobs.

25. Inspection New Jobs Confirmation (OK Jobs) Main Line Component Replacement Rework Minor Repair • Layout

26. Quality buy rate(Q): where n and nr are the numbers of first time jobs and reworked jobs, respectively, q1 and qr are first time quality and rework quality, respectively.

27. Observations: Minor repair capacity is limited. • Whenqr < q1,we obtainQ < q1. • Jobs that only need minor repair will be routed to rework when the minor repair capacity is insufficient. • Oftenqr < q1. • When minor repair capacity is insufficient, rerouting the jobs needing minor repair to rework reduces the quality buy rate of the main line. • In addition, it will waste more materials and resources and lead to loss of throughput.

28. Need for a quantitative model to analyze quality buy rate as a function of minor repair capacity Analysis results show that quality buy rate can be improved by appropriate design of minor repair capacity The study has been applied in an automotive paint shop

30. FTQ QBR • Illustration:

31. 6. CONCLUSIONS • Quality is critical. Manufacturing system design does have a significant impact on product quality. • Need to fully understand how it impacts quality and how to incorporate quality with productivity and flexibility in making manufacturing system design choices. • Lack of research makes it be a largely unexplored area with promising research opportunities, valued and important to industry. • Need to motivate research in the interactions between manufacturing system design and product quality. It will open a new area of manufacturing systems engineering.

32. Thanks • Prof. Chris Papadopoulos • Prof. Semyon Meerkov • Prof. Stanley Gershwin

33. Backups

34. Assembly line movement – how assembly line progress likely affects quality as well as throughput. Synchronous or asynchronous line? Stationary station or continuous moving line? • Assembly line balancing – not only from the point of view of worker utilization, but also to identify quality bottlenecks. • Plant layout – how layout affects quality? e.g., U - shaped lines produce better quality products. • Number and location of inspection stations – integrated quality and productivity model, information feedback, etc. • Number and location of rework loops – more rework loops or less? What should capacity of each be? • Feedback loops – feedback from inspection, production data analysis, etc.

35. Buffer location and size – buffer accommodate variation, lean inventory contributes to quality, what are tradeoffs? • Parallel versus serial lines – Parallel line improves productivity, but increases variations. It is difficult to trace root cause, but it may help quality due to slower speed. • Centralize versus decentralized equipment – centralized operations benefit from economic scale, better utilization and is easier for quality control, decentralized operations are responsible for dedicated assembly plants, have less logistic cost, less inventory and quicker feedback from assembly. • Batch size – large batch may improve quality by avoiding disruptive changeovers, small batch sizes allow quick defect detection but have frequent changeovers.

36. Flexibility – e.g.: fixtures on machines (loading/unloading) or on conveyors (improve throughput but more variability and degraded repeatability and reproducibility), need to delineate tradeoffs between cost, flexibility, throughput and quality for different strategies. • Agility – producing multiple products add variability which may damage quality, machine maintenance may require highly trained labors to obtain high quality, need to achieve both agility and quality without huge investment. • Level of automation – automatic operation provides better quality, manual has more flexibility, need to understand impact of automation on productivity, quality and flexibility.

37. Scalability – capacity expansion by speeding up or adding new machines or plants? Single large machines/plants or many small ones? • Modularity – easier for final assembler, but difficult to control, what is the impact on quality? • Outsourcing – American automakers spin off parts divisions, Toyota rarely hands complex modules to outside suppliers due to quality concerns. • Emerging technology – how to take advantage of data collection, communication, analysis capabilities and intelligent agents to design production system for improved quality?

38. No Andon Full Andon Partial Andon

39. No Andon Full Andon Partial Andon

40. m=0.9 n=1.2

41. m=0.85 n=0.9

42. m=0.7 n=0.9

43. m=0.7 n=0.8

44. Theorem: Under assumptions, • If (k-1)cµ+ + cµ > 1, then GNo Andon < GPartial Andon < GFull Andon • If (k-1)c+ + c < 1 < (k-1)cµ+ + cµ, then GPartial Andon < GNo Andon < GFull Andon • If (k-1)c+ + c < (k-1)cµ+ + cµ < 1, then GFull Andon < GNo Andon GPartial Andon < GNo Andon

45. Rules of thumb • If average repair time is less than cycle time plus average time within a cycle working on defective jobs, Full Andon improves throughput of good quality jobs. • If average time to repair severe defects is less than cycle time plus average time within a cycle working on defective jobs, then any type of Andon will improve throughput of good quality jobs. • It is worth repairing all defects rather than severe ones only. • Right way:Stop line for all problems.

46. Andon cord Andon cord Buffer Extensions • Non-identical machines • System with multiple Andon cords

47. Assumptions: A job can be reworked/repaired multiple times. No scrap. Constant percentages of good quality jobs. All reprocessed jobs have identical good job ratio. All routing probabilities are constants. • Notation: • αx,αr,αs: routing probabilities after main line inspection. • βsx, βss,βsr, βxs, βxr: routing probabilities after minor repair and component exchange. • N: minor repair capacity..

48. Need to develop a quantitative model to analyze quality buy rate as a function of minor repair capacity design appropriate repair capacity to achieve desired quality buy rate investigate the trade-offs between investment costs and saving from productivity and quality improvement

49. Theorem: Under assumption, the quality buy rate can be calculated as: where

50. Corollary: Under assumptions, the quality buy rate is monotonically increasing with respect to q, qr and N (when minor repair capacity is insufficient). FTQ QBR