An integrated fixture planning system for minimum tolerances

Fixture planning is an important aspect of process planning. The steps involved in automatic fixture planning are manufacturing feature recognition, setup planning and fixture configuration planning. In the present work, an integrated setup and fixture planning system is developed for minimum tolerances at critical regions using a data exchanged part model as an input. A platform-independent STEP-based automatic feature recognition system that can recognize both design and manufacturing features, including intersecting features is implemented. An automatic setup planning module is developed for generating setup plans for complete machining of a given component. A fixture planning module is developed applying the criteria of uniqueness, stability, accessibility and tolerance minimization. A case study is presented to demonstrate the capabilities and integration between the various modules of the system.     

Selection of machining datum and allocation of tolerance through tolerance charting technique

Tolerance charting is an effective tool to determine the optimal allocation of working dimensions and working tolerances such that the blueprint dimensions and tolerances can be achieved to accomplish the cost objectives.The selection of machining datum and allocation of tolerances are critical in any machining process planning as they directly affect any setup methods/machine tools selection and machining time.This paper mainly focuses on the selection of optimum machining datums and machining tolerances simultaneously in process planning.A dynamic tolerance charting constraint scheme is developed and implemented in the optimization procedure.An optimization model is formulated for selecting machining datum and tolerances and implemented with an algorithm namely Elitist Non-Dominated Sorting Genetic Algorithm(NSGA-II).The computational results indicate that the proposed methodology is capable and robust in finding the optimal machining datum set and tolerances.     

Dimensional and geometrical tolerance balancing in concurrent design

In conventional design, tolerancing is divided into two separated sequential stages, i.e., product tolerancing and process tolerancing. In product tolerancing stage, the assembly functional tolerances are allocated to BP component tolerances. In the process tolerancing stage, the obtained BP tolerances are further allocated to the process tolerances in terms of the given process planning. As a result, tolerance design often results in conflict and redesign. An optimal design methodology for both dimensional and geometrical tolerances (DGTs) is presented and validated in a concurrent design environment. We directly allocate the required functional assembly DGTs to the pertinent process DGTs by using the given process planning of the related components. Geometrical tolerances are treated as the equivalent bilateral dimensional tolerances or the additional tolerance constraints according to their functional roles and engineering semantics in manufacturing. When the process sequences of the related components have been determined in the assembly structure design stage, we formulate the concurrent tolerance chains to express the relations between the assembly DGTs and the related component process DGTs by using the integrated tolerance charts. Concurrent tolerancing which simultaneously optimizes the process tolerance based on the constraints of concurrent DGTs and the process accuracy is implemented by a linear programming approach. In the optimization model the objective is to maximize the total weight process DGTs while weight factor is used to evaluate the different manufacturing costs between different means of manufacturing operations corresponding to the same tolerance value. Economical tolerance bounds of related operations are given as constraints. Finally, an example is included to demonstrate the proposed methodology.     

基于工序加工能力的并行公差优化设计

提出一种基于工序加工能力的并行工序公差优化设计方法。在产品的初步结构设计阶段，通过相配零件的加工工艺规划把装配功能公差表示为零件的工序公差，建立以加权制造总成本最小为目标，以并行公差链、标准化的工序公差系数、机床最大经济极限公差为约束的非线性并行公差优化设计模型，求解该模型得到最佳的工序公差。最后给出了并行公差优化设计的一个工程实例，结果表明，所提的方法具有比传统串行等精度方法更合理、工序公差数值更大的优点。     

An improved tolerance charting technique using an analysis of setup capability

The tolerance charting method enables the calculation of working tolerances in machining process planning. The method has been used as a basic tool for analysing process plans for many decades. Process capability in tolerance charting is modelled using the tolerances of the working dimensions. The literature shows that machining process capability can be analysed from the point of view of surface position errors. During setups, it is possible to perform decomposition into two surface position tolerances: a datum surface position tolerance and a machining surface position tolerance. This type of analysis has the advantage of producing simplified tolerance chains. This paper provides an adaptation of the tolerance charting technique that uses a capability model based on datum and machining surface position tolerance. The results show an improvement in the working tolerance stackup that reduces the capability required for productive resources. As a result, reductions in manufacturing costs can be achieved. The proposal is valid for manual or computer-assisted techniques.     

Integrated setup planning and operation sequencing (ISOS) using genetic algorithm

Process planning is an essential component for linking design and manufacturing process. Setup planning and operation sequencing are two most important functions in the implementation of CAD/CAPP/CAM integration. Many researches solved these two problems separately. Considering the fact that the two functions are complementary, it is necessary to integrate them more tightly so that performance of a manufacturing system can be improved economically and competitively. This paper presents a generative system and genetic algorithm (GA) approach to process plan the given part. The proposed approach and optimization methodology analysis constraints such as TAD?(tool?approach?direction), tolerance relation between features and feature precedence relations to generate all possible setups and operations using workshop resource database. Tolerance relation analysis has a significant impact in setup planning for obtaining the part accuracy. Based on technological constraints, the GA algorithm approach, which adopts the feature-based representation, simultaneously optimizes the setup plan and sequence of operations using cost indices. Case studies show that the developed system can generate satisfactory results in optimizing the integrated setup planning and operation sequencing in feasible condition.     

Review: geometric and dimensional tolerance modeling for sheet metal forming and integration with CAPP

The focus of this publication is a review of the state of the art in tolerance analysis, synthesis, and transfer for geometric and dimensional tolerances in sheet metal forming and the integration solutions with computer-aided process planning systems. In this context, the general tolerance methods are first described. Then, the mathematical models for sheet metal tolerance analysis and synthesis are examined in detail. To address the CAPP modeling concerns, the paper is then followed up with a brief review of past research works related to feature-based process planning. Finally, those imperative future research areas are identified.     

Computer aided manufacturing planning for mass customization: part 2, automated setup planning

Setup planning plays a crucial role in CAPP to ensure product quality while maintaining acceptable manufacturing cost. The tasks of setup planning include identifying manufacturing features and corresponding manufacturing processes, determining the number of setups, part orientation, locating datum and process sequence in each setup, and selecting machine tools and fixtures. An automated setup planning technique and system has been developed based on not only the tolerance analysis, but also the manufacturing resource capability analysis. The automated setup planning is divided into two levels: setup planning in single part level and in machine station level. Algorithms for setup generation and process sequencing have been developed and a case study of setup planning is presented.     

Prediction and analysis of size tolerances achievable in peripheral end milling

Size tolerance is the most critical parameter requiring attention for ensuring dimensional repeatability of manufactured component parts, yet very little research has been reported on this important topic. This paper presents a method for predicting size tolerances of component parts machined through peripheral end milling. The method makes use of prototype software based on previously reported cutting-force and surface-generation models in which the end mill is modelled as a cantilever beam rigidly gripped by the tool holder. It also takes into account the effect of size variation for the cutting tool. The method is validated through several cutting experiments. For further analysis, the method is employed for predicting the size tolerances of a prismatic component by varying one controllable variable at a time and then monitoring the relationship between size tolerance and the variable. When a distinct relationship is noted it is verified both analytically and experimentally. The results indicated that whilst the average size variation, which contributed to the variation of the basic size of component parts, is always proportional to the metal removal rate, the range of size variations that contributed to the size tolerance is not. Therefore, there is scope for increasing the metal removal rate without sacrificing the size tolerance. The knowledge acquired through this research can be applied for selecting an optimum cutting condition using the developed method when the size tolerances of component parts are specified.     

Optimal Parameter and Tolerance Design with a Complete Inspection Plan

The need to remain competitive has led manufacturing sectors to consider tolerances as the key to achieving low cost and high quality. To produce quality products at low cost in today’s manufacturing industry, an integration of product design and process planning is essential. Process tolerance is one of the most important parameters that link product design and process planning. The process mean is also a critical parameter for further quality improvement and cost reduction under the permissible process setting adjustment within design tolerance limits. This study discusses an approach to integrate the product and process design via the optimisation of process mean and process tolerance.     

Modelling of tolerances and manufacturing dimensions

To obtain imposed dimensional and geometrical specifications for any mechanical piece, production tolerances must be calculated. So a simulation of workpiece behavior when it is machined, has permitted calculations of deviations on machined surfaces.The method of deviation calculation is based on a comparison between imposed functional tolerance and the tolerance calculated in relation to deviations on two machined surfaces or between a machined surface and the operational datum.The one direction modelling of deviations inquires practical inputs such as the planning process, the operational datum, the deviations on rough surfaces, and deviations on surface datum for any stage of machining. The developed method allows determining the deviations on machined surfaces. Then, tolerances on production dimensions were calculated in three directions. These results have permitted to define average production dimensions, which may be used for NC machine programming and to prepare an optimal rough piece configuration.The developed method has been applied for the machining of a fixing screw.     

Automated tolerance analysis for CAPP system

Various computer-aided process planning (CAPP) systems can produce different process plans, but few of them can deal with the specified design tolerances that have a significant influence on the process selection, machine selection, set-up and datum selection, operation sequencing, cutting condition decisions, and time and cost calculations; nor can they provide exact information on fixture/datum and the coordinates of the tool movements that are the necessary data for automated NC programming. The focus of this paper is on the discussion of an automated tolerance based analysis approach of CAPP systems.     

A model for evaluating the quality of the fixturing of the part from a machining process planning perspective

Achieving the required tolerance of positioning between two features is not obvious when features are produced in multiple set-ups where the fixture/workpiece interface plays a critical role. From a computer-aided process planning perspective, the suitability of a fixturing feature to achieve the required tolerances has to be evaluated. A model of an indicator for a locating quality is proposed here based upon the distribution of a small displacement of the workpiece compatible with the required tolerance onto the candidate fixturing surfaces.     

A tolerancing optimisation method for product design

Although extensive research has been carried out in the area of tolerancing techniques for product design, concurrent engineering is still very seldom used in this context. This paper introduces a unique tolerancing method which applies the concept of concurrent engineering. The proposed method essentially allocates the required functional assembly tolerances to the component tolerances by formulating the tolerancing problem into a mathematical model and solving the model using a linear programming approach. The component tolerances are first represented in terms of the process tolerances, assigned by process planners at an early stage of the product design. The objective function of the mathematical model, which is to maximise the residual tolerances of the processes, is then established and the constraints formulated based on the assembly requirements and process constraints. The model is subsequently solved using a linear programming approach. Finally, the proposed method is tested on a practical example.     

Optimal tolerance allocation for precision machine tools in consideration of measurement and adjustment processes in assembly

The high geometric accuracy requirement of precision machine tools represents a challenge for tolerance design and assembly process planning that guarantee the final assembly accuracy. Component tolerances should be allocated in association with assembly processes. However, tolerance design and assembly process planning are usually considered separately and lack quantitative analysis. In this paper, to integrate the geometric tolerance of components and variation propagation in assembly process, a state space model is developed. The measurement and adjustment process are expressed as observation matrix and control inputs. An optimal control problem is formulated to determine the adjustment process in consideration of the loss of final assembly accuracy and costs of remachining adjustment process. Tolerances of components can be optimally allocated based on the variation propagation in this deterministic assembly process. The generality and effectiveness of this approach are validated by applying the model on a four-axis horizontal machining center.     

Cost optimization of process tolerance allocation-a tree based approach

Tolerance transfer techniques are used extensively for allocation of tolerances for each machining operation in the process sequence, and hence are used to coordinate the process planning and the design activities in the evolution of a new product. This paper deals with an extended approach to allocate process tolerances based on a tree topology called technologically and topologically related surfaces (TTRS) and its subsequent extension to cost optimization. The benefit of this approach is that equations are generated explicitly with regard to tolerance transfer. These have been exploited by implementing the cost function in the generated explicit equations and subsequent optimization. The TTRS approach has been extended which scores over the limitations of some of the already existing techniques in that, it is optimal with respect to two factors namely, cost of producing the required tolerances, as also the process capability of the machines involved in producing the part. This technique has been proved practically feasible as it has been implemented in an industry. To enable a better view of the advantages of the approach, a comparative study with the existing techniques has been carried out in addition to a simulation using the Monte Carlo method.     

Concurrent Optimisation of Parameter and Tolerance Design via Computer Simulation and Statistical Method

This study optimises component parameters and component tolerances simultaneously via computer simulation and response surface methodology (RSM). The approach first generates a set of experimental data through computer simulation, then the data are converted to a total cost as a response value before applying RSM for statistical analysis and mathematical optimisation. The response value (total cost) includes quality and related costs which reflect the combined effect of the parameter and tolerance values being assigned. The results provide designers with the optimal component design values, the critical components, and the response function of a product or process design, which are very important to know during design activities as they give designers information about repeated applications, accurate feedback and appropriate suggestions, particularly under uncertain design conditions. Three examples are provided: They are mechanical assembly design, machining process planning, and electronic circuit design.     

Algorithms for computer-aided generative process planning

Process planning is the function within a manufacturing facility that establishes the machining processes and parameters to be used so as to convert a piece-part from its initial form to the final form predetermined on an engineering drawing. Computer-aided process planning (CAPP) has become a major focus of manufacturing automation as it forms the interface between computer-aided design (CAD) and computer-aided manufacturing (CAM). Issues in CAPP include part representation, process selection, alternative process-plan generation, intermediate surface and tolerance determination, and operation sequencing. This paper focuses on quantitative models for determining cutting dimensions and tolerances for intermediate surfaces, and on a heuristic for sequencing cutting operations.