Thread Content
1. Concept of machining accuracy: Machining accuracy is primarily used to indicate the quality of product production. Both machining accuracy and machining error are terms used to evaluate the geometric parameters of the machined surface. Processing accuracy is measured by tolerance grades; the lower the grade value, the higher the accuracy ； The machining error is expressed as a numerical value; the higher the value, the greater the error. High machining precision means low machining errors, and vice versa. There are 20 tolerance grades in total, ranging from IT01, IT0, IT1, IT2, IT3 to IT18. IT01 indicates the highest precision in manufacturing of a part, while IT18 denotes the lowest precision. Generally, IT7 and IT8 represent intermediate levels of precision. The actual parameters obtained through any machining method will not be absolutely accurate; from the perspective of the part’s functionality, as long as the machining error remains within the tolerances specified in the part diagram, the machining accuracy is considered to be ensured. The quality of a machine depends on the machining quality of its components as well as the assembly quality of the machine. The machining quality of components includes two main aspects: machining precision and surface quality. Machining accuracy refers to the degree to which the actual geometric parameters (dimensions, shape, and position) of a part after machining match the ideal geometric parameters. The difference between them is called processing error. The magnitude of the machining error reflects the level of machining precision. The greater the error, the lower the machining precision; the smaller the error, the higher the machining precision. 2. Information related to machining accuracy (1) Dimensional accuracy refers to the degree to which the actual size of a part after machining matches the center of its dimensional tolerance band. (2) Shape accuracy refers to the degree of conformity between the actual geometric shape of the surface of the machined part and the ideal geometric shape. (3) Position accuracy refers to the actual difference in position accuracy between relevant surfaces of the part after machining. (4) Interrelationships: When designing machine parts and specifying the precision requirements for their processing, it is important to ensure that shape errors are kept within the limits of positional tolerances, and that positional errors in turn are smaller than dimensional tolerances. That is, for precision parts or the important surfaces of parts, the requirement for shape accuracy should be higher than that for positional accuracy, and the requirement for positional accuracy should be higher than that for dimensional accuracy. 3. Adjustment methods: (1) Adjust the process system; (2) Reduce machine tool errors; (3) Minimize transmission chain errors; (4) Reduce tool wear; (5) Decrease the deformation caused by forces on the process system; (6) Reduce thermal deformation of the process system; (7) Reduce residual stresses. 4. Causes of influence: (1) Errors in the machining principle – These are errors that arise from the use of approximate blade profiles or approximate transmission relationships during machining. Processing principle errors often occur in the machining of threads, gears, and complex surfaces. During processing, approximate machining is generally employed to improve productivity and cost-efficiency, provided that the theoretical errors are sufficient to meet the requirements for precision. (2) Adjustment error: The adjustment error of a machine tool refers to the error that results from inaccurate adjustments. (3) Machine tool errors Machine tool errors refer to the manufacturing errors, installation errors, and wear of the machine tool. These mainly include the guiding error of the machine tool’s guides, the rotational error of the machine tool’s spindle, and the transmission error in the machine tool’s transmission chain. 5. Measurement methods: Different measurement methods are employed depending on the specific requirements regarding processing accuracy and the level of precision needed. Generally, there are the following types of methods: (1) Based on whether the parameter being measured is directly measured, they can be divided into direct measurement and indirect measurement. Direct measurement: The parameter to be measured is directly measured in order to obtain the dimension being assessed. For example, use calipers or comparators for measurement. Indirect measurement: Geometric parameters related to the dimension being measured are determined, and the desired dimension is obtained through calculation. Obviously, direct measurement is more intuitive, while indirect measurement is more cumbersome. Generally, when the dimension to be measured cannot meet the accuracy requirements through direct measurement, indirect measurement has to be employed. (2) Based on whether the reading of the measuring instrument directly represents the value of the dimension being measured, it can be divided into absolute measurement and relative measurement. Absolute measurement: The reading directly indicates the size of the dimension being measured, such as when using a vernier caliper. Relative measurement: The reading indicates only the deviation of the measured dimension from a standard value. When using a comparator to measure the diameter of a shaft, it is necessary to first adjust the zero point of the instrument with gauge blocks, and then proceed with the measurement. The value obtained is the difference between the diameter of the shaft being measured and the size of the gauge block; this is what is known as relative measurement. Generally speaking, relative measurement has a higher accuracy, but it is more complicated to carry out. (3) It is divided into contact measurement and non-contact measurement, depending on whether the surface being measured comes into contact with the measuring head of the measuring instrument. Contact measurement: The measuring head makes contact with the surface to be measured, and a mechanical measuring force is applied. If measuring the part with a micrometer. Non-contact measurement: The measuring head does not come into contact with the surface of the part being measured, and this approach prevents the influence of measuring force on the results. Such as using projection methods, light wave interference methods for measurement, etc. (4) Based on the number of measurement parameters, it is divided into single-parameter measurement and comprehensive measurement. Individual measurement: Each parameter of the part under test is measured separately. Comprehensive measurement: It measures comprehensive indicators that reflect the relevant parameters of a part. When measuring threads with a tool microscope, it is possible to determine the actual pitch diameter of the thread, the error in the half-angle of the thread profile, and the cumulative pitch error, among other parameters. Comprehensive measurement generally offers higher efficiency and is more reliable for ensuring the interchangeability of parts, so it is often used for inspecting finished parts. Individual measurements can determine the error of each parameter separately, and are generally used for process analysis, process inspection, and the measurement of specified parameters. (5) Based on the role played in the processing process, it is divided into active measurement and passive measurement. Active measurement: Measurements are taken of the workpiece during the processing stage, and the results are used directly to control the processing of the part, thereby preventing the generation of defective products in a timely manner. Passive measurement: Measurement carried out after the workpiece has been processed. Such measurements can only determine whether the processed parts are qualified, and are limited to detecting and removing defective items. (6) Based on the state of the part being measured during the measurement process, it is divided into static measurement and dynamic measurement. Static measurement: Measurement is performed at relative rest. Such as measuring the diameter with a micrometer. Dynamic measurement: During measurement, the surface under test and the measuring head move relative to each other in a simulated operating condition. Dynamic measurement methods can reflect the condition of parts near their operating state, and represent the direction of development in measurement technology.