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In the drilling process, the accuracy of the machined hole position is influenced by various factors. Among these, the tool setting error of the drilling fixture plays a significant role. To meet the required positional accuracy, the traditional extreme value method is commonly used to calculate this error. However, in practice, the extreme conditions assumed by this method rarely occur, leading to unnecessarily high manufacturing precision requirements and increased processing costs. Additionally, the extreme value method fails to consider the effects of assembly, making it less aligned with real-world scenarios.
This paper proposes a more practical approach for calculating tooling errors by taking into account the impact of interference fits between the drill sleeve and the clamp, as well as the actual distribution of manufacturing deviations across different components. This method, referred to as the real-state analysis method, provides a more accurate representation of tool setting errors in drilling fixtures.
When an interference fit is applied, the inner diameter of the cylindrical surface of the drill sleeve undergoes shrinkage due to the radial pressure generated at the mating interface. The theoretical shrinkage can be calculated using equations based on elastic mechanics principles. These equations incorporate parameters such as the interference amount, material properties, and geometric dimensions.
During assembly, the microscopic roughness of the mating surfaces is flattened, reducing the actual interference compared to the theoretical value. This adjustment must be considered when calculating the real shrinkage of the drill sleeve’s inner diameter. Formulas are provided to account for this effect, ensuring more accurate predictions of the final dimensions after assembly.
The calculation of the maximum probability of the actual interference between the drill bush and the clamp involves considering the statistical distribution of size deviations. For small batch production, deviations follow a Rayleigh distribution, which allows for a more realistic estimation of the interference amount. This approach helps in determining the most probable deviation values for both the outer diameter of the drill bush and the inner diameter of the clamp hole.
Under real assembly conditions, the inner diameter of the drill sleeve is determined by subtracting the maximum probability shrinkage from the nominal dimension. This calculation ensures that the actual state of the component is accurately represented, improving the reliability of the tool setting error analysis.
For the drill bit, which is typically produced in mass quantities, the size distribution follows a normal distribution. The maximum probability deviation is calculated as the average of the upper and lower tolerances. Using this information, the gap between the drill bit and the drill sleeve can be determined, which directly affects the alignment and positioning of the drilled hole.
Due to this gap, the centerline of the drill bit may shift, causing a displacement in the position of the machined hole. This displacement, known as the real tool offset error, is calculated based on the geometry of the workpiece, the drill sleeve, and the chip removal space. A detailed formula is provided to quantify this error, allowing for better control over the final machining result.
The proposed real-state analysis method offers a more accurate and practical alternative to the traditional extreme value method. By incorporating real-world factors such as interference fits and actual manufacturing variations, it provides a more reliable basis for calculating tool setting errors in drilling fixtures. This approach not only improves the accuracy of the machining process but also reduces unnecessary cost and complexity.
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