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In the drilling process, the accuracy of the machined hole's position is influenced by various factors. One significant factor is the tool setting error in the drilling fixture. To meet the required positional accuracy, the traditional extreme value method is commonly used for calculating these errors. However, this method often leads to overly strict manufacturing tolerances because the extreme conditions it considers rarely occur during actual machining. As a result, the cost and precision requirements for jig manufacturing increase unnecessarily.
Moreover, the extreme value method does not account for assembly variations, making it less aligned with real-world scenarios. In this paper, we propose a more practical approach that takes into account the effects of the interference fit between the drill sleeve and the clamp, as well as the actual distribution of manufacturing deviations in different components. This new method, known as the real-state analysis method, offers a more accurate estimation of tool setting errors in drilling fixtures.
When an interference fit is applied, the inner diameter of the drill sleeve experiences a theoretical shrinkage due to the radial pressure at the mating surface. The shrinkage depends on factors such as the material properties, the size of the interference, and the geometry of the components involved. Using principles from elastic mechanics, we derive equations that describe how the interference affects the diameter of the cylindrical surface of the drill sleeve.
During assembly, the microscopic roughness of the mating surfaces can reduce the effective interference, which must be accounted for in calculations. By considering both the theoretical and actual interference, we can more accurately predict the shrinkage of the inner diameter of the drill sleeve.
For the calculation of the maximum probability of interference, we assume that the size deviations follow a Rayleigh distribution, which is common in small-batch production. This allows us to estimate the most probable deviation values for the outer diameter of the drill sleeve and the inner diameter of the clamp's fitting hole.
Once the actual dimensions are determined, we calculate the diameter of the inner hole of the drill sleeve under real assembly conditions. This helps in determining the gap between the drill bit and the drill sleeve, which directly affects the positioning accuracy of the machined hole.
Considering the normal distribution of drill bit sizes due to mass production, we calculate the maximum probability deviation of the drill bit. This deviation, along with the gap between the drill bit and the sleeve, is used to determine the actual tool offset error.
The displacement of the hole position caused by the gap is calculated using a formula that involves the workpiece thickness, the height of the drill sleeve, and the chip removal space. This provides a clear understanding of how the tool setting error affects the final machining result.
Through theoretical analysis and practical application, it has been demonstrated that the proposed real-state analysis method offers a more realistic and accurate approach compared to the traditional extreme value method. It accounts for real-world factors such as interference fits and manufacturing variations, leading to better performance and lower costs in the production of drilling fixtures.
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