How to choose a reference system

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In the design of non-standard machinery, the hole system and the shaft system are two common choices for reference systems. Choosing the proper base hole system or base shaft system can improve the rationality of the design and the performance of the product. The following will explore how to select the base hole system and the base shaft system from several aspects. 1. Understand the concepts of the hole-based system and the shaft-based system. The hole-based system refers to a system in which the hole is used as the primary positioning reference; its characteristic is that the centerline of the hole serves as the basis for dimensions and geometric tolerances. The base axis system, on the other hand, is a system that uses an axis as the primary positioning reference; its characteristic is that the center line of the axis serves as the basis for dimensions and geometric tolerances. 2. Factors to consider when selecting a datum system: The structural characteristics of the part: For parts with multiple holes, using the hole-based datum system can reduce the number of adjustments during processing and improve efficiency. For parts with the axis as the main element, the base axis system is more appropriate. Processability: During the machining process, using the base hole system facilitates the control of hole size and positional accuracy, as machining holes is generally easier to control than machining shafts. Furthermore, the keyhole system can also reduce the number of clamping operations during processing, thereby improving efficiency. Economics: From an economic perspective, for products produced in large quantities, using the base hole system can reduce production costs. Because once the size of the hole is determined, different plug inserts can be used to meet the requirements of various products, eliminating the need for separate shaft machining for each product. Degree of standardization: In the field of mechanical manufacturing, there is a high degree of standardization for holes, with a large number of standard holes available for use. Therefore, adopting the base hole system facilitates standardization and reduces production costs. 3. The hole-based system is a commonly used reference system, primarily applied in the following scenarios: hole machining in mechanical manufacturing. The hole-based system uses the centerline of the hole as the benchmark for dimensions and geometric tolerances, and it is widely used in the machining and fabrication of various holes. For example, when machining various fasteners such as bolts and screws, the base hole system is generally used to ensure the accuracy and positioning requirements of the holes. Applications with a clearance fit: The hole-based system is suitable for applications that require a clearance fit, such as bearings and housing components. In these situations, the pitch system can provide sufficient flexibility to meet the motion requirements between different components. Preferred datum system: In mechanical manufacturing, in order to simplify the manufacturing process, reduce production costs, and improve efficiency, the hole system is usually preferred as the datum system. This is because machining holes is easier to control compared to machining shafts, and fixing the tolerance zone for holes allows for a reduction in the number of tooling and measuring devices required. Special application areas: In some special application fields, such as aerospace, precision instruments, and medical devices, the base hole system is also widely used. In these fields, extremely high requirements are placed on the precision and stability of products, and the base hole system can provide reliable positioning and assembly accuracy. 4. When there are clear economic benefits, adopting the base axis system can be a suitable choice; the following are some possible scenarios: Standardization and interchangeability: The base axis system facilitates standardization and interchangeability. When a company needs to produce a large quantity of a standard component, the base axis system can reduce production costs, improve interchangeability, and simplify inventory management. Since the shaft dimensions in the base axis system are fixed, companies can produce standard shafts in bulk and then replace them with different plug inserts as needed to meet the requirements of various products. This reduces the number of custom cutting tools and plug gauges, thereby lowering production costs. Reduce machining difficulty: In some cases, using the base axis system can reduce the complexity of machining. For example, for some parts with a long axis, using the hole-based system may make it difficult to ensure the machining accuracy of the holes, whereas using the shaft-based system allows for better control over the machining accuracy of the shafts. Furthermore, for some special materials or processing methods, such as precision casting and precision forging, high requirements are placed on dimensional and form/tolerance specifications, and the base axis system can also help meet these precision requirements more effectively. Flexibility: The base axis system offers good flexibility, allowing it to meet various assembly and maintenance requirements. For some components that require frequent replacement or repair, the base axis system enables easy adjustment of the positional accuracy between components, thereby reducing maintenance costs.