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.     

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.     

A Graph Representation Scheme for Process Planning of Machined Parts

A new generic graph representation scheme for process planning is presented. It has been specifically developed to facilitate the planning and control of geometric dimensions and tolerances. Relationships between cut, location, and datum surfaces and machining operations are represented by a graph consisting of connected boxes. A box is a multi-terminal node that corresponds to a machine set-up. Machining operations are depicted as arrows inside the box. These connect input nodes, representing datum and location surfaces, to output nodes, representing machined surfaces. External connectors join boxes together to form a directed graph. The main purpose of these connectors is to indicate the surfaces (from prior set-ups) that are used as location and datum surfaces for later set-ups. The representation covers cutting operations and non-cutting operations, such as surface finishes and coatings. The paper concludes with an example that illustrates the application and power of the scheme.     

A matrix approach to tolerance charting

This paper presents a unique method of deriving the dimensions and tolerances during tolerance charting. It is simple and practical. Unlike other methods, it does not require any representation of the machining sequence. The dimensions/tolerances between workpiece surfaces are organised into an input matrix. The input matrix then undergoes a series of operations to yield the final matrix. The final matrix contains the resultant dimensions/tolerances between any two surfaces of the workpiece. This method is easy to understand and implement on a microcomputer. An example is used to illustrate the method.     

Three-dimensional modelling of manufacturing tolerancing using the ascendant approach

This paper proposes a three-dimensional approach towards manufacturing tolerancing that uses a strategy to strictly consider the definition of a datum system imposed by ISO standards. This approach, called three-dimensional manufacturing tolerancing (TMT), is based on the small displacement torsor, which describes the possible deviations between machined surfaces and nominal surfaces of the part model. Every requirement of the definition drawing is treated separately with its own calculation model. The iterative ascending analysis allows one to establish the influence of the most recent phase and determine the influential parameters from previous phases. The nominal part model is defined on the datum systems of the requirement, and deviations are expressed with respect to both the machining process and the chosen machine adjustment method. The result takes the form of a formula that yields the variation in characteristics specified in the requirement based on quantifiable parameters identified on the machine.     

Computer-Aided Fixture Design Verification. Part 2. Tolerance Analysis

Tolerance analysis is the most important issue in computer-aided fixture design (CAFD), and it is an important part of computer-aided fixture design verification (CAFDV). This study presents a new approach for fixture tolerance analysis that is more generalised and can be used to assign locator tolerances based on machining surface tolerance requirements. The tolerance analysis is also generalised to handle any type of fixture design, workpiece, datum feature, and machining feature tolerance. Locator tolerance assignment distributes tolerances to locators based on a sensitivity analysis.     

Dimensioning of the intermediate states of the machined phases “DISMP” approach

In manufacturing practice for each machining operation, transfer techniques are extensively used for the allocation of manufacturing specification tolerance type and value. This paper focuses more on tolerance type which is the basis for a coherent and complete tolerancing process. We developed an algorithmic method called “DISMP” in order to generate the necessary types of manufacturing specifications that guarantee the respect of the functional requirements. A geometric variation model, based on the invariants degrees of freedom (DOFs) of the datum reference frames (DRF) and the toleranced surfaces serves to specify the two extremities of the tolerance chain. The identification of the controlled DOFs (Cont-DOFs) of the positioning reference frames in each machining phases and also the constrained DOFs (Cons-DOFs) of each machined surface contributes to the development of the way linking between the functional toleranced surface and its functional DRF. To generate the appropriate manufacturing geometric specifications, our method can be divided in five steps. We started by the translation of the ISO functional specification using the topologically and technologically related surfaces rules “TTRS” (Step 1: M1). Mapping the manufacturing process (Step 2: M2 and M3) requires the identification of the Cons-DOFs of the machined surfaces and those controlled by the positioning surfaces “Cont-DOFs” in each phase. The search of all chain links, which constitute the tolerance chain, is realized on Step3 (M4 and M5). Then, in step 4 (M6), we generate ISO standardized manufacturing specification. The fifth step (M7) draft technological constraints related to the manufacturing process, i.e., the case where there are inversion in the order between the toleranced surfaces and its DRF. Finally, we present the manufacturing process phases with ISO standards requirements.     

A novel machining of multi irregular surface using improved noncircular turning method

The backplane processing is an important part of the manufacture of electronic communication equipment; the current processing of backplane surface, especially the rough machining, is still using milling; and the production efficiency is quite low. This paper presents a new method to turn with a cylinder for the rough machining of multi backplane surfaces, which is based on a conventional horizontal lathe and uses the improved method of noncircular machining. The performance requirements of fast tool servo (FTS) device and control model for the machining of noncircular surface are analyzed firstly; the position following control strategy is used instead of traditional position interpolation, according to the multi backplane surfaces machining technology and control strategy; and a novel algorithm, which is combined UG (Unigraphics NX) software surface fitting with cubic spline curve fitting of multi backplanes’ space, is proposed for computer-aided manufacturing (CAM) software design. Finally, the proposed processing method and CAM model algorithm are proved to be effective through the simulation and actual verifications, the products meet the processing requirements and the processing time of each piece is reduced from 8 to 2 min, and they also greatly improve the production automation and achieve energy saving and emission reduction.     

Using tolerance maps to validate machining tolerances for transfer of cylindrical datum in manufacturing process

Process planers typically utilize different datum features than designers use when specifying tolerances. Datum features used in process plans are chosen to simplify setups and achieve desired geometric accuracy. Meanwhile, proper machining tolerances are required to be reassigned to satisfy design requirements. However, existing methods to transfer geometric tolerances directly and accurately are still missing due to incompatible mathematical models of tolerances. Also, the affection of material conditions on datum and partial constraint situations have not been deeply considered yet. Since cylindrical features are often used as datum features, this paper describes the use of tolerance maps (T-Maps) (patent no. 6963824) and manufacturing maps (M-maps) to establish analytical relationship among all relevant design and machining tolerances for transfer of cylindrical datum. Firstly, a parametric model of datum transfer is proposed to describe factors involving the process. Next, based on spatial and geometric parameters, as well as tolerances information, variation analysis among features is implemented to formulate transformed T-Maps, sum of which constructs M-Map. Then, distinct bounding boxes of cross-sections in M-Map are extracted through computing vertex coordinates of their boundaries due to complete and partial constraint scenarios. Thereafter, by virtue of bounding boxes, relationship among design and machining tolerances are obtained through fitting M-Maps into T-Maps. Finally, an example is introduced to verify feasibility of the proposed model and method.     

Maximization of process tolerances using an analysis of setup capability

This paper presents a method for tolerance balancing in machining process planning. The surface position tolerance (SPT) method offers several advantages in comparison with the tolerance charting technique. These advantages arise from the separate consideration of the datum and machining surface position capabilities. To solve the problem of tolerance balancing, the mathematical model used in the literature is adapted to the SPT method. When solving the problem, the objective of maximizing process tolerances was chosen. The proposal was applied to a problem that has been used by many authors in the literature and so a wide comparison can be made of the results. The solution enables the tolerances in the process to be improved and the solution can also be optimized with respect to the capability of the fixture.     

SPECKLE METHOD EVALUATION OF THE ROUGHNESS OF ELECTRODISCHARGE MACHINED SURFACES

Abstract

This paper describes the applicability of the speckle method to evaluate the roughness of surfaces produced by non-traditional machining processes such as EDM, ECM or USM, where material removal occurs randomly. The method is based on the digital correlation of two speckle images produced by interference phenomena arising when a coherent light beam is incident on a rough surface. The basic principle of the speckle pattern correlation is presented and the theoretical analysis based on a hypothesis concerning the morphology of the machined surface is reported. An experiment has been set-up in order to assess the feasibility of applying the speckle method to evaluate the roughness of machined surfaces. In the experimental tests, two speckle patterns produced from the same rough surface under two different illumination conditions have been correlated. The two different conditions have been obtained by varying the angle of incidence of coherent light on the surface being analysed. The roughness of electrodischarge machined surfaces, as measured by the speckle method, is found to be in good agreement with that of a stylus instrument.     

基于几何与公差信息的加工特征识别方法

为实现计算机辅助设计与计算机辅助工艺规划系统的有效集成,提出了一种同时利用几何信息和公差信息的加工特征识别新方法。建立了加工资源、加工表面和加工方法三类信息模型。提出了切削模式的概念及以之为基础的表面加工方法生成原理和过程。建立了表面加工方法优化选择模型。采用多目标模糊优化结合蚁群算法求解该模型,为每个加工表面选择最优加工方法,并将在同次装夹中采用同一刀具类型和加工条件进行加工的表面聚为加工特征。最后,通过实例测试,验证了该方法的正确性和有效性。     

Machine capability index evaluation of machining center

Recently, there has been an increasing need to produce more precise products, with only the smallest deviations from a defined target value. Machine capability is the ability of a machine tool to produce parts within the tolerance interval. Capability indices are a statistical way of describing how well a product is machined compared to defined target values and tolerances. Currently, there is no standardized way to acquire a machine capability value. This paper describes how machine capability indices are evaluated in machining centers. After the machining of specimens, straightness, roundness and positioning accuracy were measured using CMM(coordinate measuring machine). These measured values and defined tolerances were used to evaluate the machine capability index. It will be useful for the industry to have standardized ways to choose and calculate machine capability indices.     

至正轴盖10156001加工工艺方法

阐述了零件的加工工艺方法,加工工艺过程的合理安排,为保证零件符合图纸的尺寸与公差要求、表面粗糙度要求、表面镀覆要求,最终生产出合格的、具有较高质量的产品。     

Five-Axis NC Machining of Sculptured Surfaces

This paper presents a new method for the determination of tool position and orientation for NC milling of sculptured surfaces with a five-axis NC machine. This method uses an analysis of local curvatures of the surface and the tool geometry. Based on matching these local properties, optimal tool axis orientations are selected so that the gouging error in machining is eliminated or controlled within a specific tolerance. An accurate technique is also proposed for the computation of cusp height of the machined surface. By eliminating gouging and controlling scallops within a specified tolerance, it is expected that the time-consuming manual grind-ing process could be greatly reduced or completely eliminated in the finish machining of complex die or mould surfaces.     

Optimized sequential design of two-dimensional tolerances

Tolerancing has great impact on the cost and quality of a product. Previous research essentially focussed on one-dimensional tolerancing where sized tolerances accumulate only in one direction. When sized and angular tolerances are considered simultaneously, tolerances accumulate, however, in two different directions in the given view plane. We utilize related tolerance zones to illustrate the accumulation processes of sized and angular tolerances. The orientational tolerances are converted into the equivalent sized or angular tolerances in terms of their engineering semantics. We establish the sequential linear optimization models to maximize the 2D sized, angular, and orientational working tolerances of a 3D machined part based on the process capabilities. At any completion stage of operations, we measure the processed sized dimensions and then substitute them into the dimensional chains to dynamically re-calculate the mean working dimensions for remaining operations. We also re-evaluate the working tolerances for remaining operations using sequential optimization models. This approach can release the working tolerances, reduce manufacturing costs, and enhance the acceptance rate of machined parts when we manufacture the complex, low-volume, and high-value-added parts. The approach is finally illustrated with a practical example.     

Grind-free tool path generation for five-axis surface machining

This paper discusses automatic tool path generation for five-axis filleted end mill finish-surface machining. A new method of automatic five-axis tool path generation is introduced called Grind-Free (GF) tool path generation. GF surfaces result from tool paths that avoid gouging and have scallops that are within the surface profile or waviness tolerances. New algorithms are presented for determining tool forward step and tool path step-over that produce a GF surface. Gouge-free tool paths can be generated directly from CAD data based solely on local and global machining constraints. The proposed methodology for GF tool path generation has been implemented in the C language using the CODE/Robline system. Surfaces were machined on a Boston Digital 505 five-axis milling machine to confirm this method.     

新型并联机床数控系统插补算法的研究

提出一种用于新型并联机床的粗、精插补策略和算法 ,粗插补采用直接对加工曲面进行插补的方法。以NURBS曲线为例 ,运用合理计算方法推导出粗插补计算公式 ,并对粗、精插补进行了误差分析。插补实例结果表明 ,该插补算法具有恒速进给、加工表面轮廓误差易于控制等特点     

矩阵法求解尺寸及公差

提出了一种在机械加工工艺过程中用于计算尺寸及公差的方法,矩阵法。它只需要按照简单规则进行相应加减计算,并依照既定的方法进行追踪,最后在矩阵表中布置即可。不必作图,也不必进行尺寸链的解算,便可得到加工后各表面的尺寸及公差。详述了此法的原理及过程,并用拖拉机上的差速器做了详尽说明。与传统的图表追踪法、图论法相比,矩阵法在处理轴向尺寸链解算方面更胜一筹。由此可知,此法对今后尺寸链的解算具有一定推广意义。     

Characterisation of the workpiece dilatation phenomenon during machining using the neural network method. Application to NC turning

Dilatation of workpieces during machining is a major source of defects. With the current trend for re-treatment of cutting and cleaning fluids becoming compulsory, lubrication by a stream of oil and dry machining are becoming more widely used in aluminium alloy machining. Indeed, this makes it easier to recycle chippings and greatly simplifies the cleaning and grease removal phases for workpieces that are compulsory before any finishing surface treatment. However, the workpiece’s deformation during machining must be taken into account. This is especially true for NC turning of machining diameters with very tight tolerances. Here we propose a method based on the use of a neural network intended to model changes in the workpiece’s dimensions to correct tool paths. This study covered machining of workpieces made of 2017 T4 aluminium alloy during the turning phase. We first conducted preliminary tests on a workpiece to highlight workpiece dilatation. We then implemented a neural network to predict this deformation to be able to compensate for it. The results of a first test campaign gave us knowledge of the network then a second test campaign was used to validate that network. To finish off, we machined a test workpiece in order to test and analyse network performance.