A Study on Tolerance Design, Analysis and Distribution of a Closed-type Hydrostatic Slideway

Abstract

Hydrostatic slideways are widely employed in precision machine tools as it provides high stiffness, high damping and low friction. The design of its tolerances plays an important role in the performance of such a slideway. Based on the stiffness design, this study investigates the design, analysis and distribution of tolerances for a closed-type hydrostatic slideway. The study started with deciding the dimensioning and tolerancing of clearance of such a slideway based on the analysis of clearance and stiffness under the conditions that the structure and capillary restrictors of each bearing pads remained the same. Furthermore, by considering critical dimensions related to both precision and performance of the slideway, dimensional tolerance distribution and analysis are conducted through tolerance network of the slideway under the error budget constraint. While the resultant tolerances distributed by the equal tolerance, the equal dimensional ratio and the equal precision approaches are too small and not acceptable. An alternate approach named 'negative tolerancing' is then applied to compensate tolerance accumulation. By doing so, a relatively practical tolerance range is reached which can be implemented compromising with the error budget and design target. The major contribution of this research is to develop an approach of tolerance distribution which fulfills the requirements of capacity and precision of a closed-type hydrostatic slideway. This approach combines knowledge in both tolerance engineering and stiffness design of hydrostatic slideway. In the meantime, the negative tolerancing approach is employed to reach a more feasible tolerance specification which not only achieve precision and stiffness requirements, but also lower the fabrication cost.

液靜壓滑軌具有高剛性、高阻尼與低摩擦之優點，適用於對於精度要求嚴苛的工具機，其公差設計分析與配置對滑軌性能具有關鍵之影響，本研究針對一閉式液靜壓滑軌，由該滑軌之剛性設計，探討其公差設計分析與較佳之配置。 研究首先在滑軌封油面之結構參數以及節流器之參數不變的條件下，由剛性及軌道間隙分析決定合適的軌道間隙及其允差範圍。同時考量工作台精度與性能相關之關鍵尺寸，透過該滑軌之公差網路，進行尺寸公差配置與分析。關鍵尺寸及其目標公差經以等公差、等公差比例及等精度三種不同方法配置後，公差規格過於嚴苛難以達成；研究進一步應用負公差配置法，以吸收組裝的精度變異，配置出可達成設計目標且能維持元件可實際達成的尺寸公差。 本研究之主要貢獻在於結合液靜壓滑軌性能設計與公差工程兩領域，探討閉式液靜壓滑軌之性能與精度要求下之公差配置，並應用負公差配置方式，配置合適可行之公差規格，除達成精度與剛性要求之外亦降低加工成本。