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This paper presents an investigation of fixel design alternatives for active (dynamic) fixturing to be incorporated into mesoscale manufacturing systems. Using simple compliant mechanisms and components, fixels exhibiting mechanically adjustable stiffness characteristics are achievable. Via manual or automated stiffness adjustments, these fixels provide functionality for enabling greater control of the dynamic response of the workpiece subject to vibrations and/or variations in contact forces at the tool-work piece fixture interface. To quantify the fixel functionality, this paper presents theoretical models of the stiffness characteristics expressed as a function of the mechanical variable(s), thus forming a basis for exploring the adjustability in stiffness achievable for each fixel design. Also presented are results of the dynamic behavior of the active fixturing implemented in a milling process based on a "regenerative force, dynamic deflection model" augmented with the active fixturing variable stiffness model and inclusion of tool runout. These simulation results indicate the expected performance of the active fixturing upon its implementation in actual fixturing for the creation of micron features on micro- and macroparts. [ABSTRACT FROM AUTHOR]

This paper presents a novel model for workpiece positioning analysis. Existing fixturing models may underestimate the positioning error due to neglect of the curvature of one or both contacting bodies. By using surface-to-surface signed distance function, a two-sided quadratic model for fixture locating analysis is developed. A system of quadratic sensitivity equations, which relate locators' source errors to the resulting workpiece localization error, are derived. The second-order geometric properties of both workpiece and locators are taken fully into account. The present model is more accurate than the existing linear model and one-sided quadratic model, which is verified through numerical examples. It has potential applications in fixture design, fault diagnosis, and tolerance analysis. [ABSTRACT FROM AUTHOR]

In a free state, flexible parts mar have different shapes compared to their computer-aided design (CAD) model. Such parts may likewise undergo large defornzations depending on their space orientation. These conditions severely restrict the feasibility of inspecting flexible parts without restricting the deformations of the part and therefore require dedicated and expensive tools such as a conformation jig or a fixture to maintain the integrity of the part. To address these challenges, this paper proposes a new inspection method, the iterative displacement inspection (IDI) algorithm, that evaluates profile variations without the need for specialized fixtures. This study examines 32 models of simulated manufactured parts to show that the IDI algorithm can iteratively deform the meshed CAD model until it resembles the scanned manufactured part, which enables their comparison. The method deforms the mesh in such a manner so as to ensure its smoothness. This way, neither surface defects nor the measurement noise of the scanned parts are concealed during the matching process. As a result, the case studies illustrate that the method's error essentially only represents the scanned part's measurement noise. The inspection results, therefore, solely reflect the effect of variations from the manufacturing process itself and not the deformation of the part. [ABSTRACT FROM AUTHOR]

This paper presents an algorithm for optimal grasp planning and fixture synthesis. The object surface is depicted by a number of discrete points, which are used as the candidate contact points. This algorithm consists of two procedures. The first procedure selects a minimal subset of contacts from the candidates so as to construct a form -closure grasp or fixture, which is to determine a minimal subset of contact wrenches such that their convex hull forms a simplex containing the origin of the wrench space as an interior point. The second procedure adjusts the contact positions or alters the selected contact wrenches to maximize the minimum distance from the origin to the facets of the simplex, which enhances the capability of the grasp or fixture to immobilize and balance the object. As this algorithm does not employ any general optimization techniques and utilizes recursive formulas for most computations, it runs very fast. Its effectiveness and efficiency are demonstrated with illustrative examples. [ABSTRACT FROM AUTHOR]

Reconfigurable assembly systems enable a family of products to be assembled in a single system by adjusting and reconfiguring fixtures according to each product. The sharing of fixtures among different products impacts their robustness to fixture variation due to trade offs in fixture design (to allow the accommodation of the family in the single system) and to frequent reconfigurations. This paper proposes a methodology to achieve robustness of the fixture layout design through an optimal distribution of the locators in a multistation assembly system for a product family. This objective is accomplished by (1) the use of a multistation assembly process model for the product family, and (2) minimizing the combined sensitivity of the products to fixture variation. The optimization considers the feasibility of the locator layout by taking into account the constraints imposed by the different products and the processes (assembly sequence, data scheme, and reconfigurable tools' workspace). A case study where three products are assembled in four stations is presented. The sensitivity of the optimal layout was benchmarked against the ones obtained using dedicated assembly lines for each product. This comparison demonstrates that the proposed approach does not significantly sacrifice robustness while allowing the assembly of all products in a single reconfigurable line. [ABSTRACT FROM AUTHOR]

Fixtures control the positions and orientations of parts in an assembly process. Inaccuracies of fixture locators or nonoptimal fixture layouts can result in the deviation of a workpiece from its design nominal and lead to overall product dimensional variability and low process yield. Major challenges involving the design of a set of fixture layouts for multistation assembly system can be enumerated into three categories: (1) high- dimensional design space since a large number of locators are involved in the multistation system, (2) large and complex design space for each locator since the design space represents the area of a particular part or subassembly surfaces on which a locator is placed, (here, the design space varies with a particular part design and is further expanded when parts are assembled into subassemblies), and (3) the nonlinear relations between locator nominal positions and key product characteristics. This paper presents a new approach to improve process yield by determining an optimum set of fixture layouts for a given multistation assembly system, which can satisfy (1) the part and subassembly locating stability in each fixture layout and (2) the fixture system robustness against environmental noises in order to minimize product dimensional variability. The proposed methodology is based on a two-step optimization which involves the integration of genetic algorithm and Hammersley sequence sampling. First, genetic algorithm is used for. design space reduction by estimating the areas of optimal fixture locations in initial design spaces. Then, Hammersley sequence sampling uniformly samples the candidate sets of fixture layouts from those predetermined areas for the optimum. The process yield and part instability index are design objectives in evaluating candidate sets of fixture layouts. An industrial case study illustrates and validates the proposed methodology. [ABSTRACT FROM AUTHOR]

In traditional and modern manufacturing systems, how to plan the workpiece holding is the first issue of the machining operation to be confronted for the fixture design. To ensure the machining accuracy of specific dimensions, it is necessary to develop a proper fixture locating scheme to constrain correctly the degrees of freedom (DOFs) of a workpiece with respect to the cutting tool. In this paper a machining-dimension-based locating scheme design approach is developed and partitioned into two main parts for the study. First, the relationship is established between the machining dimensions and the DOFs to be constrained theoretically. Second, the fixture locating scheme is, established to characterize the practical constrained DOFs of a workpiece in terms of the known locator number and positions. As a result, judgment criteria are formulated quantitatively for the first time to verify not only the locating scheme correctness but also to identify the cause of locating scheme incorrectness. Finally, two test examples are illustrated to show the proposed locating scheme. [ABSTRACT FROM AUTHOR]

Considering the great impacts of the application sequence of multiclamps on the workpiece machining accuracy, this paper analyzes and optimizes clamping sequence. A new methodology that takes into account the varying contact forces and friction force during clamping is presented for the first time. A new analysis model is established to capture the effect of clamping sequence on contact force distributions and workpiece machining accuracy. It reveals that the historical accumulation of clamping steps influences heavily the final distribution of contact forces in the workpiece-fixture system. Therefore, the present contact forces in each clamping step are solved incrementally in terms of contact forces of the precedent step by means of the principle of the total complementary energy. Furthermore, based on the fact that the variation of contact forces from one step to another results in different workpiece deformations and position, a novel design model is formulated to select optimally the clamping sequence in order to minimize the workpiece deformation and position errors. Workpieces of low stiffness and high stiffness are investigated separately in order to simplify the modeling of clamping sequence optimization. Some numerical tests are finally demonstrated to validate the proposed model and method. Computational results are discussed and compared with experimental results available in the reference. [ABSTRACT FROM AUTHOR]

Fixtures are used to locate and constrain workpieces in machining. Fixture properties, such as accuracy and locator layout schemes, directly affect the resultant geometric variation on the machined workpiece surfaces. N-2-1 fixturing schemes (N>3) are often used for large or compliant workpieces in machining. However, the impact of N-2-1 fixturing on product geometric variation is not well understood because of the uncertainty of contact under the overconstraint condition. This paper presents a modeling methodology to study the machining geometric variation in a 4-2-1 fixturing scheme. The model integrates the fixture-workpiece interactions resulted from the rigid-body variation in fixtures, workpieces, and machine tools, and the static deformation under external forces. The uncertainty of the fixture-workpiece contact condition due to the 4-2-1 constraint condition is modeled using the minimum potential energy theory. A numerical example of face milling is presented to demonstrate the methodology. [ABSTRACT FROM AUTHOR]

In a number of manufacturing processes—tooling installation, calibration, and maintenance--guarantee the precision of fixtures and play important roles toward the overall quality of products. Recently, a new type of measurement equipment called a ‘laser tracker’ was developed and utilized for assembly fixture calibration to shorten calibration time and improve the accuracy of the currently used theodolite systems. Though calibration of the assembly fixture is critical for product quality, as such, calibration time creates a significant burden for productivity of multistation assembly processes. In order to shorten calibration lead time, the number of necessary setups, determined by visibility analysis, needs" to be minimized. This paper presents a screen space transformation-bated visibility analysis that allows minimizing the number of setups. The screen space transformation is applied to transform the visibility problem from three- to two-dimensional space, thus, efficiently solving the visibility problem. A case study illustrates the procedure and verifies the validity of the proposed methodology. This methodology can be applied not only for manufacturing processes, such us in-line fixture calibration, but also toward analysis and optimization of AGVs, robot navigation systems, and building security. [ABSTRACT FROM AUTHOR]

This paper presents a systematic finite element model to predict the fixture unit stiffness by introducing nonlinear contact elements on the contact surface between fixture components. The contact element includes three independent springs: two in tangential directions and one in the normal direction of the contact surface. Strong nonlinearity is caused by possible separation and sliding between two fixture components. The problem is formulated by the penalty function method and is solved by the Newton­Raphson procedure. The model was validated by two cases of analysis of a linear cantilever beam and a simple fixture unit with two components. The results are in agreement with the corresponding analytical solution of beams and the previous experimental results for fixture in the literature. [ABSTRACT FROM AUTHOR]

This paper presents a new approach to multiple fault diagnosis for sheet metal fixtures using designated component analysis (DCA). DCA first defines a set of patterns based on product/process information, then finds the significance of these patterns from the measurement data and maps them to a particular set of faults. Existing diagnostics methods has been mainly developed for rigid-body-based 3-2-1 locating scheme. Here an N-2-1 locating scheme is considered since sheet metal pans are compliant. The proposed methodology integrates on-line measurement data, pan geometry, fixture layout and sensor layout in detecting simultaneous multiple fixture faults. A diagnosability discussion for the different type of faults is presented. Finally, an application of the proposed method is presented through a computer simulation. [ABSTRACT FROM AUTHOR]

Analysis and characterization of workpiece-fixture contact forces are important in fixture design since they define the fixture stability during clamping and strongly influence workpiece accuracy during manufacturing. This paper presents a method for predicting and analyzing the normal and frictional contact forces between workpiece-fixture contacts. The fixture and workpiece are considered to be rigid bodies, and the model solution is solved as a constrained quadratic optimization by applying the minimum norm principle. The model reveals some intricate properties of the passive contact forces, including the potential of a locator release and the history dependency during a sequence of clamping and/or external force loading. Further, a notion of passive force closure is considered to characterize the passive nature of the fixture forces. Geometric conditions for two types of passive force closure (concordant and discordant closures) are provided, showing a complication of released locator under clamping with a limited role in force closure. Model predictions are shown to be in good agreement with known results of an elastic-contact model prediction and experimental measurements. The passive force closure conditions are illustrated with examples. This presented method is conceptually simple and compurationally efficient. It is particularly useful in the early stages of fixture design and process planning. [ABSTRACT FROM AUTHOR]