Integration of Process-Oriented Tolerancing and Maintenance Planning in Design of Multistation Manufacturing Processes

Abstract

Manufacturing systems are inherently imperfect both statically and dynamically. Tolerance and maintenance design are two major tools to address the static and dynamic imperfection of manufacturing processes (i.e., inherent process imperfection and tooling deterioration, respectively). Yet, traditionally, tolerance and maintenance designs have been studied separately to address these two critical areas of manufacturing systems. This paper presents an integrated framework of tolerance and maintenance design for multistation manufacturing processes. Two nonlinear optimization problems are formulated to minimize the overall average production cost in the long run, which includes the tolerance cost of tooling fabrication, maintenance cost, and the overall loss of quality (as a part of the objective function or as a constraint function). The proposed methodology is illustrated, analyzed, and further discussed in the context of a multistation automotive body assembly process. Extensive numerical analyses are conducted to demonstrate the efficiency of the developed methodology. Given various cost components and time horizons, the integrated design scheme is compared with traditional design schemes in terms of cost efficiency, offering new insights into the interrelation between manufacturing process maintenance and tolerancing in the context of the product life cycle.Note to Practitioners-With intensified competition as a result of economic globalization, quality and cost have become crucial factors to the success of any manufacturing industry. Decisions in the process design phase, such as process tolerance assignment and maintenance planning, play a substantial role for overall manufacturing quality and costs. Tolerance of process variables determines the inherent variation level of a manufacturing process. Preventive maintenance oversees and controls process degradation and its resulting deterioration on product quality. Significant tooling and operational costs result from both tolerancing and maintenance activities. Traditionally, tolerancing and maintenance decision-making have been studied separately. Tolerancing was mainly conducted during the design stage; while maintenance policy was often determined after a manufacturing system was designed and installed. However, tolerancing of process variables and maintenance decision-making policy are interconnected in modern manufacturing systems. Intuitively, tight initial tolerances specified on process variables are able to reduce the frequency of conducting maintenance during production, since the process can accommodate more deterioration to reduce maintenance cost; but they take a toll on tolerance cost. On the other hand, loose initial tolerances specified on process variables can lower design cost but increase the frequency of maintenance during production. Hence, there is a critical need to strike a balance between the tolerance cost of tooling fabrication and the maintenance cost of tooling replacement. This paper presents a new framework to integrate tolerance ...

Process-oriented tolerancing using the extended stream of variation model

“…Considering reciprocal functions as cost-tolerance functions, the optimal tolerance of KCCs with the minimum manufacturing cost was allocated by solving a constrained optimization problem. Similar problem was described by Chen et al [14], who expanded Ding's work to integrate the process-oriented tolerancing with the fixture maintenance planning. Tool fabrication cost, fixture maintenance cost and quality loss functions were considered together to optimize the process tolerance allocation and the frequency of fixture maintenance operations in MAPs.…”

“…In this paper, we assume a fixture layout at each station based on the common 3-2-1 locating principle applied in machining processes [26]. For this fixture layout, the cost function for the jth fixture component (locator) installed at station k is chosen to be a reciprocal function as [14,16] …”

“…Based on SoV models, many quality improvement activities have been conducted on MMPs, such as process diagnosis [6], sensor placement for in-process inspection [7,8], quality prediction [9][10][11] and dimensional quality control [12,13]. However, only few works have been focused on process-oriented tolerancing [3,[14][15][16]. In this specific field, Ding et al [3] applied the process-oriented tolerancing approach to allocate product and process tolerances in a multi-station assembly process (MAP).…”

Manufacturing variation models in multi-station machining systems

“…In the literature, the SoV model has been applied for a large number of applications such as part quality estimation and process planning [9][10][11], manufacturing fault identification [12][13][14][15][16][17][18][19][20], dimensional quality control [21][22][23][24][25][26] and process-oriented tolerancing [27,28].…”

“…In these research works, the tolerances of process variables in a MMP are optimally allocated by solving a non-linear constrained optimization problem defined by: cost functions, the SoV model, a process degradation model of fixture components, a tolerance accumulation model, and several constraints related to part specifications (tolerances). Chen et al [27] expanded the work in [28] to integrate process-oriented tolerancing with maintenance planning in multi-station assembly processes. They incorporated tool fabrication cost, fixture maintenance cost and quality loss functions to optimize the tolerance allocation of manufacturing process variables and the frequency of fixture maintenance operations.…”