*4.4. Determine and Verify Root Causes (D4)*

This discipline aims to find the root causes of problems. According to Šk ˚urková (2017), cause–effect diagrams can be used to map causes with their corresponding effects or problems. The general problem in this case study is that the assembly does not work; hence, a fishbone diagram is developed—also known as Ishikawa diagram—as depicted in Figure 8 to identify the root cause. As can be observed, several causes were identified across five aspects: materials, methods, environment, workforce, and machinery.

**Figure 8.** Cause-effect diagram to find out the root cause of the problem.

Regarding the environment, the reason why the returned assemblies were defective is because the company lacked a functional test to confirm that they worked. However, to perform this evaluation, the cables first had to be correctly assembled—and even then, it would have been impossible to know if the assemblies worked properly. As for the materials, it was found that the cable end terminals were incorrect, since the warehouse employees mistakenly provided the wrong types to the operators. Additionally, two more problem causes were associated with the work method. Usually, the motors supplied to the company come with already-integrated cables, and the employees only need to cut these cables as specified by the customers, and then rivet the excess. However, sometimes the cables are not always cut at the right length or inverted.

In terms of machinery, it was found that the tools of the company were obsolete and needed required to be replaced. Finally, regarding the workforce aspect, the diagram indicates that the assembly cables were not always riveted properly, yet correct riveting makes it possible for the motor to be connected to the cables, which in turn enables the functional test to be successfully performed. Similarly, it was found that the employees may poorly handle the motors, and in the case of the rejected assemblies, this could have an impact on their performance. Another possible cause of having defective assemblies is that the motors might have been damaged during their delivery.

Moreover, since most of the assemblies were returned because of inverted cables, this issue is considered as the main root cause of the problem (see Figure 3). In most of the assemblies, the black, white, and blue cables had been inverted. At first, this can be a problem related to the company work method; however, a functional test could have also solved the problem. In addition, with a functional test, the company could have prevented non-working motors and abnormal noise problems. During functional tests, motors usually display a "not working" message, in which case the position of the cables must be thoroughly reviewed. Finally, to prevent the problem from re-occurring, a program on Visual Basic® was developed to conduct motor functional tests (see Appendix A). The test uses binary values (0 and 1) that allow employees to confirm an assembly's functionality before it is delivered to the customer. Figure 9 introduces the truth table for the motor, with values 1 = true (ON) and 0 = false (OFF).

**Figure 9.** Truth values for the motor.

The binary values are translated into decimal values to be used in the program; first a formula table is built, as depicted in Figure 10, wherein each row corresponds to one cable. Then, in each row, the first ten powers of 2 are displayed, i.e., 2<sup>0</sup> = 1, 21 = 2, ..., 2<sup>9</sup> = 512, from right to left, and it is assigned one binary value from Figure 9 corresponding to a power of two, starting at 2<sup>0</sup> = 1. Finally, each binary value in each row is multiplied by its corresponding power of 2, and the sum of the products is the resulting decimal value that is reported on the right side of each row. Once the four decimal values were obtained, they were used in the program commands to be executed, consequently beginning a new project according to customer specifications.

**Figure 10.** Translation of binary values to decimal values.

Once the Visual Basic® program was designed, the Parmon's parallel port monitor application was used to verify that the decimal values were correct when the program was executed, as shown in Figure 10. The Dec column contains the decimal values corresponding to the binary values from the binary column. In all the decimal values shown in Figure 11, the motor being tested was turned on. Once the motor finished its cycle, the program indicates that the motor is turning in the opposite direction regarding the position it had started in. The goal of this test is to confirm that the motor works properly without abnormal noise.

**Figure 11.** Parmon's parallel port monitor application and motor functioning: (**a**) decimal value 10 enabled, the motor starts turning; (**b**) decimal value 9 enabled, the motor continues its cycle; (**c**) decimal value 5 enabled, the motor continues its cycle; (**d**) decimal value 6 enabled, the motor finishes its cycle.

#### *4.5. Develop Permanent Corrective Actions (D5)*

In this discipline, the following corrective actions are implemented:


The three corrective actions significantly improved the production system, since they helped solve problems of inverted cables, noisy motors, dysfunctional motors, and wrong component features. Additionally, the brand-new insertion method was added in the datasheet of part number A, and it was stored in an electronic file to be updated when necessary. However, one important factor to consider is that, regardless of whether the motor was properly assembled or not, it was still likely to fail or generate abnormal noise.

#### *4.6. Implement and Validate Corrective Actions (D6)*

An operation method for the functional test was developed (see Appendix B). Specifically, each connector being tested only had to be connected to the box containing the driver. The process time established by the customer was 7.28 min, but it is managed to decrease in 4.61 min (i.e., 36.68% less time) after the process was documented and a functional test was conducted. In the end, the operation method helped employees avoid mistakes when assembling the cable. The corrective actions were validated by comparing the analysis results from the defective assemblies before and after implementing these actions. Actually, the defective products decreased by 76%, which validates the implemented corrective actions [24].

### *4.7. Prevent Recurrences (D7)*

The manufacturing process of part number A comprises eight tasks: manual cable cutting, semi-automatic cable riveting, cable end terminal insertion, cable labeling, performing electrical and functional tests, conducting final inspection, packaging, and shipping. Once these tasks were identified, a series of checklists was designed to monitor their successful completion and ensure continuity in the manufacturing process. At the shipping stage, all this documentation was assigned a customer revision number, which would allow the resulting datasheet to be immediately updated as customer specifications change, thereby informing the production, quality, and cutting departments of such updates.

Finally, in this discipline, an executable version of the Visual Basic® program was developed. The program forbid employees from changing any of its settings, since it only allows them to open it and perform the test in a pre-configured mode to prevent misconfiguration problems.

#### *4.8. Recognize Teamwork and Individual Contributions (D8)*

In this stage, all the teamwork members were acknowledged for their individual and group performances. Although each member had his/her own ideas, and different suggestions were proposed during the problem-solving process, the teamwork remained united and worked towards a common goal.

#### **5. Conclusions and Industrial Implications**

The principal goal of this work was successfully accomplished. The 8Ds method implemented in the manufacturing company managed to decrease the number of assembly defects in part number A from 67 to 16, which represents a decrease of 76.12%. Figure 12 shows a comparison about the frequency of each defect before and after implementing the 8Ds method. Note that the frequency of all defects decreased. For example, the frequency of inverted cables, the most common defect, decreased from 35 to 2. Similarly, the frequency of motor disfigured decreased from 10 to 3, and the noisy motor decreased from 9 to 3, to just mention the higher frequency defects.

**Figure 12.** Comparison of the frequency of defects before and after applying the 8Ds method.

Simultaneously, the 8Ds method implementation allowed increasing customer satisfaction. In the 16 case studies reported in Section 3, the 8Ds method was applied to help corporations to comply with delivery times, reduce scrap and defect costs, implement new processes or develop new products, improve quality assurance systems, minimize supply chain and customer complaints, and improve services. However, solving these types of problems involves having a solid and effective communication system among the affected departments, which should also share a common goal.

In addition, by implementing the 8Ds method, the company managed to decrease production time, machine downtimes, scrap costs, operational defects, the rate of late deliveries, and customer complaints. Regarding the manufacturing system, the 8Ds method increased efficiency and productivity in the application of statistical methods and techniques at low operational costs. Table 5 shows a comparison of the main indicators before and after implementing the 8Ds method. It is important to note that the total defects were reduced by 76.12%, while the customer complaints were reduced by 100%. Similarly, production, inspection, and packing times for the part number A were reduced by over 30%; and machines stoppages were reduced by over 77%. This reduction of time cycles allowed for increasing the production level by 34.22%.


**Table 5.** Comparison of the main indicators before and after applying the 8Ds method.

Moreover, the implementation of the 8Ds method had a positive impact on the company's competitiveness in terms of quality and safety. Furthermore, the 8Ds method had a significantly positive effect on employees and managerial responsibility, participation, and commitment, which streamlined and improved the company problem-solving process, especially by helping delegate equal responsibilities to the lowest organizational levels. Finally, the 8Ds method implementation allows collecting information concerning a problem in a quick manner, and reduces the communication time between the quality teamwork and operators.

When problems arise, a method, technique, or abstract tool ought to be implemented to find the best solution. On some occasions, the implementation process may require making small modifications in the organization, whereas in other cases, engineers must be more careful to spare the company losses. Additionally, in the implementation of any method, communication is a key element of success. A solid, rapid, and effective communication system encourages employees to be creative and be engaged in the problem-solving process and motivates employees to be prepared for any further change. In other words, the 8Ds method has a two-fold goal: to solve problems and to increase active employee participation in the problem-solving process. In order to achieve these goals, experts recommend the following strategies:


As future work and based on the findings obtained in the present case study, the authors of this research plan to implement the 8Ds method in some companies from the 914 manufacturing industries located in Baja California state to solve problems related to defective products and/or production process efficiency. Additionally, the authors plan to extend the 8Ds method implementation, as well as other industrial engineering tools (PDCA cycle, standardized work, poka-yoke, DMAIC, to mention few) not only to companies in the manufacturing sector, but also in another sectors, such as construction, education, agriculture, and food services.

Finally, the authors encourage researchers from the industrial engineering field to publish their case studies on the applications of different techniques, methods, or tools, supported by the CRA.

**Author Contributions:** Conceptualization, A.R.-V. and J.L.G.-A.; data curation, A.R.-V. and K.C.A.-S.; formal analysis, J.L.G.-A. and E.J.M.; funding acquisition, A.R.-V.; investigation, A.R.-V. and K.C.A.-S.; methodology, J.L.G.-A. and E.J.M.; project administration, A.R.-V. and J.L.G.-A.; validation, J.L.G.-A.; visualization, E.J.M.; writing—original draft, A.R.-V.; writing—review and editing, J.L.G.-A. and E.J.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research has received no external funding.

**Acknowledgments:** The authors would like to acknowledge the manufacturing company where the 8Ds method was implemented. Additionally, the authors would like to thank the Tijuana Institute of Technology, the Autonomous University of Baja California, the Autonomous University of Ciudad Juarez, and the University of La Rioja for allowing the use of their facilities for this research. Finally, the authors would like to thank CONACYT and PRODEP for their constant support toward develop projects and research.

**Conflicts of Interest:** The authors declare that there is no conflict of interest.
