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140 Reverse Engineering – Recent Advances and Applications tolerancing systems are not fully compatible, they both define position geometrical tolerance as the total permissible variation in the location of a feature about its exact true position. For cylindrical features such as holes or bosses the position tolerance zone is usually the diameter of the cylinder within which the axis of the feature must lie, the center of the tolerance zone being at the exact true position, Figure 2, whereas for size features such as slots or tabs, it is the total width of the tolerance zone within which the center plane of the feature must lie, the center plane of the zone being at the exact true position. The position tolerance of a feature is denoted with the size of the diameter of the cylindrical tolerance zone (or the distance between the parallel planes of the tolerance zone) in conjunction with the theoretically exact dimensions that determine the true position and their relevant datums, Figure 2. Datums are, consequently, fundamental building blocks of a positional tolerance frame in positional tolerancing. Datum features are chosen to position the toleranced feature in relation to a Cartesian system of three mutually perpendicular planes, jointly called Datum Reference Frame (DRF), and restrict its motion in relation to it. Positional tolerances often require a three plane datum system, named as primary, secondary and tertiary datum planes. The required number of datums (1, 2 or 3) is derived by considering the degrees of freedom of the toleranced feature that need to be restricted. Change of the datums and/or their order of precedence in the DRF results to different geometrical accuracies, (Kaisarlis et al., 2008). Fig. 2. Cylindrical tolerance zone and geometric true position tolerancing for a cylindrical feature according to ISO 1101 (Kaisarlis et al, 2008) The versatility and economic benefits of true position tolerances are particularly enhanced when they are assigned at the Maximum Material Condition (MMC). At MMC, an increase in position tolerance is allowed, equal to the departure of the feature from the maximum material condition size, (ISO, 1988; ASME, 2009). As a consequence, a feature with size beyond maximum material but within the dimensional tolerance zone and its axis lying inside the enlarged MMC cylinder is acceptable. The accuracy required by a position tolerance is thus relaxed through the MMC assignment and the reject rate reduced. Moreover, according to the current ISO and ASME standards, datum features of size that are included in the DRF of position tolerances can also apply on either MMC, Regardless of Feature Size (RFS) or Least Material Condition (LMC) basis. Position tolerances mainly concern clearance fits. They achieve the intended function of a clearance fit by means of the relative positioning and orientation of the axis of the true
A Systematic Approach for Geometrical and Dimensional Tolerancing in Reverse Engineering geometric counterpart of the mating features with reference to one, two or three Cartesian datums. The relationship between mating features in such a clearance fit may be classified either as a fixed or a floating fastener type, (ASME, 2009; Drake, 1999), Figure 3. Floating fastener situation exists where two or more parts are assembled with fasteners such as bolts and nuts, and all parts have clearance holes for the bolts. In a fixed fastener situation one or more of the parts to be assembled have clearance holes and the mating part has restrained fasteners, such as screws in threaded holes or studs. The approach that is here presented deals with both the floating and fixed fastener cases by integrating the individual case methodologies published by (Kaisarlis et al. 2007; 2008). Fig. 3. Typical floating and fixed fasteners and worst case assembly conditions (Drake, 1999) Basic issues of the assignment of a Position Tolerance in RE are included in Table 1. Limited number of reference components that does not allow for statistical analysis, availability or not of the mating parts and the presence of wear may affect the reliability of the RE results. Moreover, datum selection and the size of the position tolerance itself should ensure, obviously, a stress-free mechanical mating. The analytic approach presented in this section deals with the full range of these issues in order to produce a reliable solution within realistic time. i. The number of available RE components that will be measured. The more RE parts are measured, the more reliable will be the extracted results. Typically, the number of available RE components is extremely limited, usually ranging from less than ten to a single one article. ii. Off the shelf, worn or damaged RE components. Off the shelf RE components are obviously ideal for the job, given that the extent of wear or damage is for the majority of cases difficult to be quantified or compensated. iii. Accessibility of the mating part (-s). iv. Existence of repetitive features in the RE component that may have the same function (group or pattern of features). v. Type of assembly (e.g. floating or fixed fasteners). vi. The size and the form (cylindrical, circular, square, other) of the geometrical tolerance zone. vii. Candidate datums and datum reference frames. Depending on the case more possible DRFs may be considered. viii. Precedence of datum features in DRFs. ix. Theoretical (basic) dimensions involved. x. Assignment of Maximum Material and Least Material Conditions to both the RE-feature and RE datum features. xi. Measurement instrumentation capabilities in terms of final uncertainty of the measurements results. Measurements methods and software. Table 1. Issues of Geometrical Tolerance Assignment in RE 141
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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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62 Fig. 1. Model, metamodels and tr
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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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