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148 Reverse Engineering – Recent Advances and Applications classification. In third level TEs are classified to external and internal ones as the achievement of internal tolerances usually results to higher accuracy costs. The fourth TE classification level distinguishes between plain and complex TEs depending on the absence or presence of additional feature details (grooves, wedges, ribs, threads etc). They do not change the principal TE geometry but they indirectly contribute to the increase of the accuracy cost. In the final fifth level, the involvement of the TE size to the accuracy cost is considered. TEs are classified, to the nominal size of the chain dimension, into six groups by integrating two sequential ISO 286-1 size ranges. Fig. 6. Tolerance Elements five-class hierarch system (Dimitrellou et al., 2007a) Based on the TE-method the actual machining accuracy capabilities and the relative cost per TE-class of a particular machine shop are recorded through an appropriately developed Database Feedback Form (DFF). The latter includes the accuracy cost for all the 96 TE-classes in the size range 3-500 mm and tolerances range IT6-IT10. DFF is filled once, at the system commissioning stage, by the expert engineers of the machine shop where the assembly components will be manufactured and can then be updated each time changes occur in the shop machines, facilities and/or expertise. The DFF data is then processed by the system through the least-square approximation and the system constructs and stores a costtolerance relationship of the power function type, per TE-class. 5.1 Tolerance chain constrains In a n-member dimensional chain the tolerances of the individual dimensions D1,D2,…,Dn, control the variation of a critical end-dimension D0, according to the chain, D � f( D , D ,..., D ) 0 1 2 where f(D) can be either a linear or nonlinear function. To ensure that the end-dimension will be kept within its specified tolerance zone, the worst-case constrain that provides for 100% interchangeability has to be satisfied, � i j � f D � D �D �t max min 0 0 n (34) (35)
A Systematic Approach for Geometrical and Dimensional Tolerancing in Reverse Engineering �D D � D t � f � 0 � 0 imin jmax where t0 is the tolerance of the end-dimension D0. For an (i+j)-member dimensional chain dimensions Di constitute the positive members of the chain while dimensions Dj constitute its negative members. In RE tolerancing, preferred alternatives for nominal sizes and dimensional tolerances that are generated from the analysis of Section 3, for each dimension involved in the chain are further filtered out by taking into consideration the above tolerance chain constraints. 5.2 Minimum machining cost A second sorting out is applied by taking into account the accuracy cost for each combination of alternatives that obtained in the previous stage. Cost-tolerance functions are provided by the machine shop DFF and the total accuracy cost is thus formulated as, total n � � i � n � i i ki i i�1 i�1 C � C t � � �A �B / t � ��min where C(t) is the relative cost for the production of the machining tolerance ±t and A, B, k are constants. The combination of alternatives that corresponds to the minimum cost is finally selected as the optimum one. 6. Application examples and case studies In order to illustrate the effectiveness of the proposed method three individual industrial case studies are presented in this section. All necessary input data measurements were performed by means of a direct computer controlled CMM (Mistral, Brown & Sharpe-DEA) with ISO 10360-2 max. permissible error 3.5μm and PC-DMIS measurement software. A Renishaw PH10M head with TP200 probe and a 10mm length tip with diameter of 2mm were used. The number and distribution of sampling points conformed with the recommendations of BS7172:1989, (Flack, 2001), (9 points for planes and 15 for cylinders). 6.1 Application example of RE dimensional tolerancing For a reverse engineered component of a working assembly (Part 2, Figure 7) assignment of dimensional tolerances was carried out using the developed methodology. The case study assembly of Figure 7 is incorporated in an optical sensor alignment system. Its’ location, orientation and dynamic balance is considered of paramount importance for the proper function of the sensor. The critical assembly requirements that are here examined are the clearance gaps between the highlighted features (D1, D2, D3) of Part 1 – Shaft and Part 2- Hole in Figure 8. Four intact pairs of components were available for measurements. The analysis of section 3 was performed for all three critical features of Figure 8 individually. However, for the economy of the chapter, input data and method results are only presented for the D2 RE-feature, in Tables 2 and 3 respectively. The selected ISO 286 fits, Figure 9, produced in 12min (10min CMM-measurements + 2min Computer aided implementation) were experimentally verified and well approved by fitting reconstructed components in existing and in use assemblies. 149 (36) (37)
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REVERSE ENGINEERING - RECENT ADVANC
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Contents Preface IX Part 1 Software
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Preface Introduction In the recent
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Preface XI and con’s related to t
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Preface XIII the step-by-step appli
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Part 1 Software Reverse Engineering
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4 Reverse Engineering - Recent Adva
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10 Reverse Engineering - Recent Adv
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58 2. Model driven architecture: An
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Integrating Reverse Engineering and
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Part 3 Reverse Engineering in Medic
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238 5. Summary and discussions Reve
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268 Fig. 6. A closed polygon mesh r
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272 3. Results Reverse Engineering
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