Research On Measurement Method And Evaluation Strategy Of Double-faced Parallelism

With the development of high-end equipment manufacturing industry,the manufacturing accuracy of precision parts have achieved the level of micrometer or even sub-micrometer, which proposes strict requirements for the corresponding geometrical tolerances. Geometrical tolerances, defined as the fundamental inspection elements of measured features, are important to mechanical part manufacturing, such as parallelism tolerance adopted as an important criteria for parts. Parallelism tolerance, classified as one kind of orientation tolerance, defined as the allowable variation between the measured feature and its referenced datum in the certain orientation, including line to line, line to plane, plane to line, and plane to plane. And among them, the parallelism tolerance of surface to surface is most widely applied. In the latest Geometrical product Specification(GPS), the geometrical tolerances which have be established with consummate standards include straightness, flatness, roundness, and cylindricity. However, for the parallelism tolerance, the universally accepted standard has not be established yet, which, to some extend, restricts the development of the measurement and manufacturing of precision parts.To solve this problem, in this thesis, a method of plane to plane p arallelism tolerance measurement is presented based on the principal of dual-frequency laser interferometry, and the corresponding mathematical model of measurement is established. Furthermore, an evaluation system for parallelism tolerance is built according to the principle of minimum zone for the orientation tolerance. Then the parallelism tolerance of two planes of an optical flat is measured with the method. Finally, the measurement uncertainty is evaluated based on “Evaluation and Express Uncertainty in Measurement”(GUM). So the main research work is as following:(1) Analysis the method and characteristics of evaluation of direction tolerance measurement in a new generation of GPS system, according to the characteristics of this subject a plane to plane parallelism measurement system is designed based on the principle of dual-frequency laser interferometry, which implements to discretely sample on both of the reference surface and measurement surface synchronously. In this way, mathematical model is established for measurement. And then the parallelism tolerance is evaluated according to the principle of “oriented minimum zone” in accordance with regulations of the GPS system.(2) A two-dimensional moving stage is designed based on the measurement model to make the measured piece moving according to the certain path within range of the 100mm×100mm. And the statics and dynamics performance of support structure is simulated respectively with the finite element analysis, optimizing the structure parameters. Then the coupled displacement introduced by motion machine in Z direction is analysed. To measure and compensate the coupled displacement, the PSD detection circuit based on MSP430 microcontroller is designed Consequently, the displacement resolution of circuit board is calibrated, obtaining the result better than 100 nm.(3) Based on the analytical method specified by “Geometrical product Specifications(GPS)-Geometrical tolerance-Verification prescription”, the evaluation model of parallelism tolerance is built. Then, the reference plane is fitted with the least square principle, the minimum zone principl e, and the maximum and minmum material requirements, respectively. After that, the difference and application object of these principles are comprehensively analysed and compared.(4) A parallelism tolerance measurement system is constructed, and implemented for the parallelism tolerance of two planes of an optical flat. In the measurement, the parasitic displacement in Z direction is real-time detected by a PSD circuit board and compensated by a stage. The parallelism tolerance is then calculated based on the measured data with the four principle stated above, respectively. Taking the least square principle as example, its measurement uncertainty is evaluated with(GUM), measurement error sources are analysed and corresponding model is built, as well as the individual contribution and error propagation coefficient, to synthetically calculate the expanded uncertainty Ut=2.2μm, k=2. And the final measurement result of parallelism tolerance is t=(3.8±2.2)μm.In summary, this paper proposes a method for parallelism measurement based on the principal of dual-frequency laser interferometry, which is adaptable for pieces with optically reflect surface. Furthermore, a complete model of measuring and evaluating is established, which promotes unity.in standard of orientation tolerance.