Designing Capable and Reliable Products Episode 1 Part 6 pdf

table (see Appendix VI) for later mechanical deformation, the risk index, a p , is: A (automated process) A 1.0 B (medium skill level) B 1.3 C (poor access) C 1.6 D (pressure used for deformation) D 1.2 E (no heat applied) E 1.0 F (one stage operation) F 1.0 For the bending the tab washer operation, the Additional Assembly Variability Risk is given by: a p  A ÂB ÂC ÂD ÂE ÂF a p  2:5 Therefore, q a 5  2:5 A completed variability risks result table for the ®xing bolt assembly is shown in Figure 2.30, highlighting the assembly variability risks only. The risk indices can then be entered on the assembly sequence diagram as shown in Figure 2.31. It is evident that insertion of the ®xing bolt and bending of the tab washer operations will potentially be problematic on assembly, scoring assembly risk values greater than 2. The previous sections have illustrated the use of the various process risk charts and tables to obtain variability indices associated with the manufacture and assembly of products. It is important to be systematic with the application of the methodology and the recording of any product analyses, especially as products often contain many parts. It is also important to ®rst declare a sequence of assembly for the indivi- dual components before proceeding with an analysis. 2.6.6 Completing the Conformability Matrix The ®nal part of the analysis is based around the completion of a Conformability Matrix relating variability risk indices for component manufacturing/assembly Figure 2.31 Revised sequence of assembly for the ®xing bolt Objectives, application and guidance for an analysis 85 processes to potential failure modes, their severity and the costs of failure. A blank Conformability Matrix is provided in Appendix VII. The ®nal results of an analysis are best displayed in the Conformability Matrix to provide a traceable record of the costs of failure and how these costs are related to the conformance problems through the design decisions made. The symbols, shown in Figure 2.32, are placed in the nodes of the Conformability Matrix, and represent the levels of design acceptability obtained with reference to the Conformability Map given in Figure 2.22. The link with FMEA brings into play the additional dimension of potential vari- ability into the assessment of the failure modes and the eects on the customer. The Conformability Matrix also highlights those `bought-in' components and/or assemblies that have been analysed and found to have conformance problems and require further communication with the supplier. This will ultimately improve the supplier development process by highlighting problems up front. Once the variability risks, q m and q a , have been calculated, the link with the particular failure mode(s) from an FMEA for each critical characteristic is made. However, determining this link, if not already evident, can be the most subjective part of the analysis and should ideally be a team-based activity. There may be many component characteristics and failure modes in a product and the matrix must be used to methodically work through this part of the analysis. Past failure data on similar products may be useful in this respect, highlighting those areas of the product that are most aected by variation. Variation in ®t, performance or service life is of particular interest since controlling these kinds of variation is most closely allied with quality and reliability (Nelson, 1996). For each q m and q a risk value and the Severity Rating (S), a level of design accept- ability is determined from where these values intersect on the Conformability Map. The symbols, relating to the levels of design acceptability, are then placed in the nodes of the Conformability Matrix for each variability risk which the failure mode is directly dependent on for the failure to occur. Once the level of design accept- ability has been determined, it can then be written on the Conformability Matrix in the `Comments' section. C pk values predicted or comments for suppliers can be added too, although predicted C pk values can also be written in the variability risks results table. Figure 2.32 Conformability matrix symbols and their quanti®cation 86 Designing capable components and assemblies Figure 2.33 Conformability matrix example Objectives, application and guidance for an analysis 87 Example ± determining the failure costs for product design We will now consider calculating the potential costs of failure in more detail for the cover support leg shown earlier. The process for calculating the failure costs for a component is as follows: . Determine the value of q m or q a . Obtain an FMEA Severity Rating (S) . Estimate the number of components to be produced (N) . Estimate the component cost (Pc). For example, the characteristic dimension `A' on the cover support leg was critical to the success of the automated assembly process, the potential failure mode being a major disruption to the production line. An FMEA Severity Rating S8is allocated. See a Process FMEA Severity Ratings table as provided in Chrysler Corporation et al. (1995) for guidance on process orientated failures. The component cost, Pc £5.93 and the number planned to be produced per annum, N  50 000. The characteristic was analysed using CA and q m was found to be 9. The values of q m  9 and S  8 are found to intersect on the Conformability Map above the 10% isocost line. (If they had intersected between two isocost lines, the ®nal isocost value is found by interpolation.) If there is more than one critical characteristic on the com- ponent, then the isocosts are added to give a total isocost to be used in equation 2.15. The total failure cost is determined from: Total failure cost  isocost7ÂN  Pc 100 (2.15) Total failure cost  10  50 000  5:93 100  £29 650 This ®gure is of course an estimate of lost pro®t and may even be conservative, but it clearly shows that the designer has a signi®cant role in reducing the high costs of failure reported by many manufacturing companies. The results are repeated in the Conformability Matrix in Figure 2.33. 2.7 Case studies 2.7.1 Electronic power assisted steering hub design Under this heading, a ¯exible hub design for an automotive steering unit is analysed. The application of CA resulted from the requirement to explain to a customer how dimensional characteristics on the product, identi®ed as safety critical, could be produced capably. A key component in this respect is the hub. The component is made by injection moulding, the material being un®lled polybutylene terephthalate (PBT) plastic. The moulding process was selected for its ability to integrate a number of functional elements into a single piece and reduce assembly costs. The design outline of the hub is shown in Figure 2.34(a) together with an optical plate that is mounted on it. 88 Designing capable components and assemblies Figure 2.34(b) shows a line from the design FMEA related to the plastic moulded hub. It gives the component function, the potential failure mode, the potential eects and potential causes of failure. In addition, the columns of current controls, Occurrence (O), Severity (S), Detectability (D) ratings and associated Risk Priority Number (RPN) have been completed. A high severity rating was given (S  8) since faulty positional readings due to out of tolerance variation could cause loss of car control and driver injury. The Detectability was rated at D  8 because complex inspection processes would be required. The possibility of Occurrence was provisionally estimated at O  4, giving an RPN equal to 256. The design FMEA speci®es compatible dimensioning as the current control to avoid failure, and it was this aspect of the design that needed to be explored further. A quotation from a supplier had been received for volume production of the component. Some detail on the analysis of the hub design is given below. The hub performed several functions in the controller and therefore carried several critical characteristics. The positional tolerance of the recesses to accommodate a system of location pegs needed to be close. Faces on the hub for mounting the optical plate required precise positioning to provide the necessary spacing between two optical grids (one mounted on the hub and the other carried on a torsion shaft). The depth of the moulded recesses needed to be controlled as they were part of a tolerance chain. It is important to note that the recesses and faces were in dierent planes and the depth was across a mould parting line. The positional tolerance on a 10 mm dimension was Æ0:1 mm (tol 1 ), providing an angular position of 0.68. Additionally, the widths of the recesses needed to be held to 4 Æ 0:08 mm (tol 2 ), the dimensional tolerance on the faces to 1 Æ 0:07 mm (tol 3 )and the depth to 10 Æ0:12 mm (tol 4 ). Also, the thin sections of the hub gave two geome- trical concerns as these vanes were on the limits of plastic ¯ow and distortion was likely on cooling. The results of the analyses carried out by the business on the hub are given in the variability risks table shown in Figure 2.34(c). The four critical characteristics described above were examined. The positional tolerance (tol 1 ) set across the thin vanes resulted in high geometry to process risk (g p ) and gave q m  9. This equates to an out of control C pk , as does the q m value for the recess depth across the mould parting line (tol 4 ) which came to 8. The q m scores for the characteristics tol 2 and tol 3 suggest initially that the process will be in control giving estimated C pk values of 1.33 and 1.75 respectively. Following the completion of the variability risks table, a Conformability Matrix was produced. This was used to relate the failure modes and their severity coming out of the design FMEA to the results of the Component Manufacturing Variability Risk Analysis. The portion of the matrix concerned with the moulded hub can be found in Figure 2.34(d) and was completed using the Conformability Map. It is evident that the two characteristics described earlier as being out of control, tol 1 and tol 4 , give costs of failure greater than 10%. Also, the characteristics tol 2 and tol 3 which may have been regarded as having acceptable C pk values are shown to have costs of failure of greater than 10% and 0.2% respectively, due to the high Severity Rating S8 for the potential failure modes in question. Note that two additional failure modes are also illustrated. Case studies 89 90 Designing capable components and assemblies Figure 2.34 Hub analysis results Case studies 91 The analysis indicated that the conformance problems associated with the hub design had a cost of failure of more than 30%. This would represent at the annual production quantity required and target selling price, a loss to the business of several million pounds. As a result of the study the business had further detailed discussions with their suppliers and not surprisingly it turned out that the supplier would only be prepared to stand by its original quotation provided the tolerances on the hub, discussed above, were opened up considerably (more than 50%). Subsequently, this result supported the adoption of another more capable design scheme. 2.7.2 Solenoid security cover This case study concerns the initial design and redesign of a security cover assembly for a solenoid. The analysis only focuses on those critical aspects of the assembly of the product that must be addressed to meet the requirement that the electronics inside the unit are sealed from the outside environment. An FMEA Severity Rating (S) for the assembly was determined as S  5, a warranty return if failure is experienced. Cover assembly initial design The initial design of the cover assembly as shown in Figure 2.35 uses an O-ring to seal the electronics against any contamination. Concerns were raised about three mains aspects of the assembly, these being: . The compression of the O-ring may work against the needs of an adhesive cure on ®nal assembly with an end unit. . There is a risk that the O-ring will not maintain its proper orientation in the cover recess during subsequent assembly processes, and therefore may not be correctly positioned on ®nal assembly. Restricted vision of the inside of the cover is the key problem here. . The wire cable may present problems using either manual or automated assembly. The analysis in Figure 2.35 shows that there is a high risk of non-conformance for the insertion of the frame into the cover, the process relying on the position of the O-ring being maintained (operation a8). The situation is complicated by the restriction of vision during O-ring placement, and this is re¯ected in the analysis. Using the Conformability Map, it is possible to calculate the potential failure costs for this design scheme in meeting the sealing integrity requirement, as documented in the Conformability Matrix. The ®nal failure cost is calculated to be £805 000. This potential failure cost for this single failure mode is far too high, representing over 10% of the total product cost. A more reasonable target value would be less than 1%. An alternative design scheme should be developed, focusing on reducing the risks of the ®nal assembly operation to reduce the potential for non-conformance as highlighted by the analysis. Cover assembly redesign Unfortunately, the design of the wire could not be changed to a more simple arrange- ment, for example using a spade connector integrated with a recess for the O-ring. The wire is part of the customer's requirements and will inevitably present problems using 92 Designing capable components and assemblies Figure 2.35 Cover assembly design analysis Case studies 93 either manual or automatic assembly operations. Looking to the O-ring, a better design would be to eliminate it altogether and integrate the seal with the wire as shown in Figure 2.36. The wire is then positively located with the seal in the cover hole. Clearly, the risks associated with the cover assembly have been reduced Figure 2.36 Cover assembly redesign analysis 94 Designing capable components and assemblies [...]... is to be moulded into the bobbin and the pole face is considered to be part of a mould related dimension The bobbin is injection moulded using 30% ®lled polybutylene terephthalate (PBT) The tolerance assigned to the bobbin dimension is Æ0:035 mm Figure 2.42 Variability risks analysis for the solenoid end assembly initial design 10 1 10 2 Designing capable components and assemblies The pole has a characteristic... components to face B on the plunger seal 99 Figure 2. 41 Assembly sequence diagram for the solenoid end assembly design 10 0 Designing capable components and assemblies Case studies The body is impact extruded from a cold forming steel The characteristic dimension to be analysed in the tolerance stack is the base thickness of 3 mm (on a 12 0 mm bore) and this dimension has been assigned a tolerance of Æ0:02... result in user injury High losses of the order of those calculated above for the particular failure mode, including legal costs, were incurred A number of alternative designs are possible, and one which does not involve the above problems is included with its Conformability Matrix in Figure 2.38 95 96 Designing capable components and assemblies Figure 2.37 Telescopic lever assembly analysis for Design A... used as directed by the customer This model assumes that each component tolerance is at its maximum or minimum limit and that the sum of these equals the assembly tolerance, given by equation 2 . 16 (see Chapter 3 for a detailed discussion on tolerance stack models): n ˆ i 1 ti ta …2 : 16 † where: ti ˆ bilateral tolerance for ith component characteristic ta ˆ bilateral tolerance for assembly stack: Figure... they are not taken into account in the ®nal costs of failure In conclusion, the process capabilities Figure 2.44 Solenoid end assembly redesign 10 3 10 4 Designing capable components and assemblies of several characteristics in this tolerance stack are inadequate and will not meet the customer's requirements consistently Solenoid end assembly redesign This is similar to the initial design, but involving... a high degree of interaction with the user 2.7.4 Solenoid end assembly The following case study determines the manufacturing and assembly variability risks for a solenoid end assembly design, shown in Figure 2.39, and projects the potential 97 98 Designing capable components and assemblies Figure 2.39 Solenoid end assembly costs of failure associated with the capability of an assembly tolerance stack... dimensional tolerance on the fuel port block of 12 Æ 0:05 mm which is set by the Case studies Figure 2.40 Solenoid end assembly initial design supplier) Also shown is a table describing the process used to manufacture each component and an assembly sequence diagram is given in Figure 2. 41 Referring back to Figure 2.40, the tolerance stack starts at face A on the fuel port and accumulates through the individual... Matrix in Figure 2.43 The tolerance for the brass tube base thickness has no risk and is an acceptable design Figure 2.43 Conformability matrix for the solenoid end assembly initial design Case studies The associated cost of failure for the solenoid end assembly is calculated to be over £3 million for a product cost of £7 .66 and production volume of one million units This ®gure is for the tolerance stack... further design development 2.7.3 Telescopic lever assembly Consider the telescopic lever assembly, Design A in Figure 2.37, which is part of a stretcher, and hence safety critical The assembly has an FMEA Severity Rating …S† ˆ 8, and is used in a product having a cost of 15 0 It is estimated that 5000 units are produced per annum The assembly is subjected to bending in operation, with the maximum bending...Case studies following the elimination of one positionally unstable component and its integration with another Again, with an FMEA …S† ˆ 5, and referring to the Conformability Map, isocosts for each assembly variability risk can be evaluated and the total failure cost is calculated to be £7000 Comparing this value with the initial design's high potential cost . (automated process) A 1. 0 B (medium skill level) B 1. 3 C (poor access) C 1. 6 D (pressure used for deformation) D 1. 2 E (no heat applied) E 1. 0 F (one stage operation) F 1. 0 For the bending. initial design 10 2 Designing capable components and assemblies The associated cost of failure for the solenoid end assembly is calculated to be over £3 million for a product cost of £7 .66 and production. 2.32 Conformability matrix symbols and their quanti®cation 86 Designing capable components and assemblies Figure 2.33 Conformability matrix example Objectives, application and guidance for an analysis