Chunsik Yi

Subassembly stability and reorientation

The authors propose the stability analysis of subassemblies which has several important benefits in assembly planning. Some of the cost criteria such as reorientation, parallelism, and fixturing are identified. Incorrect assembly sequences can be eliminated by identifying unstable subassemblies. A more efficient planner can be built by reducing the search space by pruning out unstable subassemblies early in the planning steps. A more complete stability analysis of subassemblies is presented. Three necessary stability criteria are considered: subassembly-stability, pose-stability, and mating-stability. Force-based reasoning is introduced in the analysis. Algorithms to solve the stability criteria are described.<<ETX>>

Tolerance analysis for multi-chain assemblies with sequence and functionality constraints

We present an approach for evaluating the assemblability of a product based on tolerances and adjustable displacements. An adjustable displacement is a functionally permitted free space between two mating features, which can be used for compensating tolerances. The assemblability of a product is computed in term of a multi chain by incrementally solving the parallel chains. We consider both the functionality and assembly sequence constraint in the evaluation. The result of the computation is a statistical measure of the product assemblability which can be used by the designer to evaluate and to optimize the tolerance allocation. In addition, this measure can be used for evaluating assembly sequences. The algorithms and simulation results are given.

Assemblability evaluation based on tolerance propagation

The specification of tolerances is an integral part of product design since tolerances directly affect the assemblability, as well as the functionality, manufacturability, and cost-effectiveness of the product. However, there has been little research on the assemblability evaluation based on tolerance propagation which can play an important role in assembly planning. Assembly planners in the past have assumed nominal dimensions for product components for generating the feasible assembly sequences. These sequences, however, may not be assemblable in practice due to propagations and accumulations of tolerances. In this paper, we present a new approach to assemblability evaluation based on tolerance propagation. For the assemblability evaluation, we introduce an important concept of clearance. Clearance represents the possible free space between two mating features. In the assemblability evaluation, clearances are used to compensate for tolerances.

Statistical representation and computation of tolerance and clearance for assemblability evaluation

The design of tolerances should take the functionality, manufacturability, assemblability and cost effectiveness of a product into consideration. This paper presents a method of analyzing and evaluating the assemblability of a product under given tolerance specifications in order to provide a tool to aide for tolerance design. Introduced is the range of feasible displacement of a part of an assembly as a result of the accumulation of clearances between mating parts along the serial as well as parallel chain of assembly, which can play a role of compensating for the pose error of the corresponding part due to the accumulation of tolerances. Assemblability is determined by the probability that all the parallel as well as serial chains of the assembly can be f ormed successfully under given statistical properties of tolerances. This requires to numerically compute the probability that a pair of mating parts forming a parallel chain can align themselves within the error margin defined by their mating clearance, accounting for the possible compensation of the ranges of pose errors associated with the mating parts due to tolerance accumulations by the ranges of feasible displacements of the mating parts due to clearance accumulations. Simulation results are shown.

Statistical tolerance and clearance analysis for assembly

In this paper, we present statistical tolerance and clearance analysis for the assembly. Tolerances work against the assemblability of a product since they can propagate and accumulate in the product, making it more difficult or impossible to assemble. Clearances work for the assemblability since they can be used to compensate for tolerances. The poses of parts in an assemble may be adjusted within the permitted clearances to assemble the parts. Monte-Carlo method is used in the analysis, with Gaussian distribution, Gaussian-Sigmoid distribution, and Chi-Square error reduction scheme to approximate tolerances and clearances. Then, algorithms to compute the propagation of tolerances and clearances are proposed. Our proposed work is expected to make the following contributions: 1) to help the designers to evaluate products for assemblability, 2) to provide a new perspective to tolerance problems, and 3) to provide a tolerance analysis tool which can be incorporated into a CAD or solid modeling system.

Assemblability analysis with adjusting cost evaluation based on tolerance and adjustability

Tolerances affect the assemblability of a product, which in turn affects the cost of the product because of the scrap cost, and wasted time and energy. In this paper, we propose a method to analyze the product in terms of assemblability based on tolerances and adjustability of the parts. More specifically, the product is considered assembled if the real parts can be fit regardless of the deviations the real parts may have from their nominal, because some parts can be adjusted in position after it is assembled due to clearances between the parts. Moreover, there is a cost associated with adjusting the parts because of the adjusting time, fixturing costs, and additional operations. A method is proposed to evaluated the adjusting cost by counting the minimum number of parts that must be moved or adjusted in order to complete the assembly.

Assembly planning for modular fixtures

Fixtures are devices used in manufacturing to immobilize a part far operations such as assembly, inspection or machining. Modular fixtures are configurations of standard components. The goal of a automatic modular fixture design is to find algorithms to determine a set of fixturing contact points automatically, given a set of modular components and the fixturing requirements. This paper considers the problem of generating valid assembly sequences for a given modular future configuration. This is called assembly planning for modular fixtures. Because the application domain is known, we can devise an efficient assembly planning system taking advantage of the characteristics of the fixture domain. In this paper, we propose a fastener-based approach which considers the functionality of fasteners (to hold the fixture components together) to achieve both the efficiency in assembly planning and the stability of the generated assembly sequences. We also describe an accessibility measure that can be used to select preferred assembly sequences. This work complements automatic fixture design such that complete automation can be achieved from the design of a fixture to its assembly, which can be performed manually or with the aid of robots.

Force-based reasoning in assembly planning

Assembly planning based on force-based reasoning of decomposable subassemblies, clusters of parts that are naturally broken apart from an assembly by decomposition forces is presented. The proposed force-based approach does not depend on combinatorial cut-set enumerations, but it generates only the minimal cut-sets of force-flow networks through a max-flow/min-cut algorithm, which incurs only polynomial time complexity. The key features of the proposed approach are as follows: (1) force-flow networks are constructed from the liaison graph of an assembly to represent the maximum amount of forces that individual parts can deliver to their mating parts for the chosen disassembly directions, fixtured and grasped parts, and removable connectors, (2) decomposable subassemblies are identified from the minimal cut-sets of force- flow networks based on a max-flow/min-cut algorithm, (3) the assemblability of the identified decomposable subassemblies are evaluated based on a force-based stability analysis, and (4) planning is done by the recursive decomposition of an assembly into subassemblies and verification of its reversibility.

Evaluation of assemblability based on statistical analysis of tolerance propagation

In this paper we present a statistical approach to assemblability evaluation based on tolerance and clearance propagations. Tolerance affect assemblability of an assembly sequence, however, clearance can compensate for tolerance. Assembly planners in the past have assumed nominal dimensions for product components for generating feasible assembly sequences. These sequences, however, may not be assemblable in practice due to propagations and accumulations of tolerances. With this concept of clearance, approaches to compute statistical tolerance and clearance propagations for serial, parallel, and multi-loop chains are proposed. The proposed method can be a valuable tool for CAD/CAM since early detection of assembly problems can significantly reduce the product cost.

An analytic approach to assemblability analysis

The assemblability, which is the probability of successfully assembling the product parts, is directly related to the product cost. Especially, we consider the assembly failures due to tolerances. Because clearances provide the adjustability to part poses in an assembly, the assemblability analysis must be analyzed not only in terms of tolerances but also clearances. In this paper, we propose an analytic approach to the assemblability analysis. Represented by ellipsoids in a deviation space, the propagations of tolerances and clearances are calculated using Jacobian and sweep operations. The simulation results are given using a 2D assembly. The expected contribution of this paper is that the proposed method allows to compute the assemblability of a product in real time.