G.H. Qin

Systematic Modeling of Workpiece-Fixture Geometric Default and Compliance for the Prediction of Workpiece Machining Error

Control of workpiece machining error (WME) is a key concern in the design of a fixture system. In this paper, source errors, which are categorized into workpiece-fixture geometric default and workpiece-fixture compliance, are systematically investigated to reveal their effects upon the WME. The underlying mechanism is that source errors lead to the workpiece position error (WPE), the workpiece elastic deformations (WED), and the inconsistent datum error (IDE), and all of them will contribute together to the WME. Here, the IDE refers to the dimension deviation of the processing datum from the locating datum once two references do not coincide. An overall quantitative formulation is proposed for the computing of WME in terms of WPE, WED, and IDE for the first time. In detail, the WPE raised in the workpiece-locating and clamping process is evaluated based on the geometric defaults and local deformations of workpiece-fixture in the contact region. The WED relative to the workpiece-clamping process is determined by solving a nonlinear mathematical programming problem of minimizing the total complementary energy of the frictional workpiece-fixture system. Some numerical tests are finally demonstrated to validate the proposed approach on the basis of both theoretical and experimental data given in the references.

New algorithm for calibration of instantaneous cutting-force coefficients and radial run-out parameters in flat end milling

Abstract It is well recognized that the cutter run-out appearing in the milling process will cause an uneven redistribution of the instantaneous uncut chip thickness through the cutter flutes and thereby will generate an irregular distribution of the cutting forces in different tooth periods. This work aims to develop a new approach able to identify the cutter radial run-out and cutting-force coefficients in the flat end milling. It is shown that the total cutting forces can be considered as the sum of a nominal component that is independent of the run-out plus a perturbation component induced by the run-out. Mathematical formulations of both components are developed, accounting for the cutting geometry and the radial run-out parameters. Firstly, to calibrate the cutting-force coefficients, a generic procedure is proposed using the instantaneous value of the nominal component instead of the average value. Secondly, considering the fact that the perturbation component of the cutting force depends non-linearly upon the run-out parameters, the identification of run-out parameters is carried out by solving the linearized equation. In the identification procedure, some key techniques such as the calculation of the immersion boundary at any cutting instant and the reasonable selection of the depth of cut are discussed in detail. Finally, based on simulation and experimental results, the validity of the identification approach is demonstrated.

Analysis and optimal design of fixture clamping sequence

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.

New Cutting Force Modeling Approach for Flat End Mill

Abstract A new mechanistic cutting force model for flat end milling using the instantaneous cutting force coefficients is proposed. An in-depth analysis shows that the total cutting forces can be separated into two terms: a nominal component independent of the runout and a perturbation component induced by the runout. The instantaneous value of the nominal component is used to calibrate the cutting force coefficients. With the help of the perturbation component and the cutting force coefficients obtained above, the cutter runout is identified. Based on simulation and experimental results, the validity of the identification approach is demonstrated. The advantage of the proposed method lies in that the calibration performed with data of one cutting test under a specific regime can be applied for a great range of cutting conditions.

A Machining-Dimension-Based Approach to Locating Scheme Design

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.

A novel approach to fixture design based on locating correctness

In a machining process, since the geometric accuracy of a manufacturing workpiece mainly relies on the relative position of workpiece to the machining tool, fixtures are needed to locate the workpiece. The clear concept of locating correctness is presented based on Venn diagram and a general algorithm is proposed to determine the locator number and layout. On one hand, the theoretical and practical constrained DOFs are formulated as a function of the machining requirement and the locating scheme, respectively. On the other hand, some criteria are concluded to analyse the locating correctness and modify the locating scheme with locating incorrectness.

Locating Correctness Analysis and Modification for Fixture Design

In machining processes, geometric accuracy of a manufacturing workpiece mainly relies on the relative position of workpiece to the machining tool. Fixtures are needed to locate the workpiece relative to the machining tool in order to guarantee the manufacturing quality. Therefore, correct locating is the basic condition for designing a fixture. In addition, computer aided fixture design (CAFD) facilitates more cost effective and efficient fixture design. However, heuristic and case-based methods are still common. This paper firstly presents a general framework to verify the locating correctness so as to soundly determine locator number and positions. Secondly, within the scope of CAFD, a computing tool is developed within the Unigraph system for the correctness analysis of fixture locating schemes. Finally, a detailed discussion is made about the modeling process and its verification by means of practical examples. Analyzing results are consistent with practical ones.