Machining accuracy refers to the degree of conformity between the three geometric parameters of the actual size, shape, and position of the surface of the part after machining and the ideal geometric parameters required by the drawing. The ideal geometric parameters, for size, are the average size; for surface geometry, they are absolute circles, cylinders, planes, cones, and straight lines; for the relative positions between surfaces, they are absolute parallelism, verticality, coaxiality, symmetry, etc. The deviation between the actual geometric parameters of the part and the ideal geometric parameters is called machining error.
Introduction to machining accuracy
Machining accuracy is mainly used to measure the degree of production products. Machining accuracy and machining error are both terms for evaluating the geometric parameters of the machining surface. Machining accuracy is measured by tolerance grade. The smaller the grade value, the higher the accuracy; machining error is expressed by numerical value. The larger the numerical value, the greater the error. High machining accuracy means small machining error, and vice versa.
There are 20 tolerance grades from IT01, IT0, IT1, IT2, IT3 to IT18. Among them, IT01 indicates that the part has the highest machining accuracy, and IT18 indicates that the part has the lowest machining accuracy. Generally, IT7 and IT8 are medium-level machining accuracy.
The actual parameters obtained by any processing method will not be absolutely accurate. From the function of the part, as long as the processing error is within the tolerance range required by the part drawing, the processing accuracy is considered to be guaranteed.
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The difference between accuracy and precision: 1. Accuracy refers to the degree of closeness between the obtained measurement result and the true value. High measurement accuracy means that the system error is small. At this time, the average value of the measured data deviates less from the true value, but the data is scattered, that is, the size of the accidental error is unclear.
2. Precision refers to the reproducibility and consistency between the results obtained by repeated measurements using the same spare samples. It is possible to have high precision but inaccurate accuracy. For example, the three results obtained by measuring with a length of 1mm are 1.051mm, 1.053, and 1.052 respectively. Although they have high precision, they are inaccurate.
Accuracy indicates the correctness of the measurement result, and precision indicates the repeatability and reproducibility of the measurement result. Precision is a prerequisite for accuracy.
Related content 1. Dimensional accuracy refers to the degree of conformity between the actual size of the part after processing and the center of the tolerance band of the part size.
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 difference in actual position accuracy between the relevant surfaces of the machined part.
4. Relationship Usually when designing machine parts and specifying the machining accuracy of parts, attention should be paid to controlling the shape error within the position tolerance, and the position error should be less than the size tolerance. That is, for precision parts or important surfaces of parts, the shape accuracy requirements should be higher than the position accuracy requirements, and the position accuracy requirements should be higher than the size accuracy requirements.
Methods to improve machining accuracy
1. Adjust the process system. The trial cutting method is adjusted by trial cutting - measuring the size - adjusting the cutting amount of the tool - cutting - trial cutting again, and repeating until the required size is reached. This method has low production efficiency and is mainly used for single-piece small batch production.
The adjustment method obtains the required size by pre-adjusting the relative positions of the machine tool, fixture, workpiece and tool. This method has high productivity and is mainly used for large-scale mass production.
2. Reduce machine tool errors 1) Improve the manufacturing accuracy of spindle components. The rotation accuracy of bearings should be improved: ① Select high-precision rolling bearings; ② Use high-precision multi-oil wedge dynamic pressure bearings; ③ Use high-precision static pressure bearings. The accuracy of accessories with bearings should be improved: ① Improve the machining accuracy of the box support hole and the spindle journal; ② Improve the machining accuracy of the surface matching with the bearing; ③ Measure and adjust the radial runout range of the corresponding parts to compensate or offset the errors.
2) Appropriate pre-tightening of rolling bearings ① can eliminate the gap; ② Increase the stiffness of the bearing; ③ Equalize the rolling element error.
3) Make the spindle rotation accuracy not reflected on the workpiece.
3. Reduce the transmission error of the transmission chain 1) The number of transmission parts is small, the transmission chain is short, and the transmission accuracy is high; 2) The use of speed reduction transmission (i<1) is an important principle to ensure the transmission accuracy, and the closer the transmission pair is to the end, the smaller the transmission ratio should be; 3) The accuracy of the end parts should be higher than that of other transmission parts.
4. Reduce tool wear. Before the tool size wear reaches the rapid wear stage, the tool must be sharpened again.
5. Reduce the stress deformation of the process system mainly from: (1) improve the rigidity of the system, especially improve the rigidity of the weak links in the process system; (2) reduce the load and its change. Improve the rigidity of the system: (1) Reasonable structural design 1) Minimize the number of connection surfaces; 2) Prevent the occurrence of local low rigidity links; 3) The structure and cross-sectional shape of the base and support parts should be reasonably selected.
(2) Improve the contact rigidity of the connection surface 1) Improve the quality of the joint surface between parts in the machine tool components; 2) Preload the machine tool components; 3) Improve the accuracy of the workpiece positioning reference surface and reduce its surface roughness value.
(3) Use reasonable clamping and positioning methods
Reduce the load and its change: (1) Reasonably select the tool geometry parameters and cutting amount to reduce the cutting force; (2) Group the blanks to make the blank machining allowance uniform during adjustment.
6. Reduce thermal deformation of the process system (1) Reduce the heat generation of heat sources and isolate heat sources 1) Use smaller cutting amount; 2) When the precision requirements of parts are high, separate the rough and fine processing processes; 3) Separate the heat source from the machine tool as much as possible to reduce the thermal deformation of the machine tool; 4) For heat sources that cannot be separated, such as spindle bearings, screw nut pairs, and high-speed guide rail pairs, improve their friction characteristics from the aspects of structure and lubrication, reduce heat generation or use heat insulation materials; 5) Use forced air cooling, water cooling and other heat dissipation measures.
(2) Balance the temperature field (3) Use reasonable machine tool component structure and assembly datum 1) Use thermal symmetric structure - in the gearbox, symmetrically arrange the shaft, bearings, transmission gears, etc., which can make the box wall temperature rise uniform and reduce the box deformation; 2) Reasonably select the assembly datum of machine tool parts.
(4) Accelerate the achievement of heat transfer equilibrium; (5) Control the ambient temperature.
7. Reduce residual stress (1) Add a heat treatment process to eliminate internal stress; (2) Reasonably arrange the process.
Factors affecting machining accuracy
1. Machining principle error Machining principle error refers to the error caused by using an approximate blade profile or an approximate transmission relationship for machining. Machining principle errors often occur in the machining of threads, gears, and complex curved surfaces.
For example, the gear hob used to machine involute gears uses Archimedean basic worms or normal straight profile basic worms instead of involute basic worms to facilitate hob manufacturing, which causes errors in the involute tooth shape of the gear. For another example, when turning a modulus worm, since the pitch of the worm is equal to the pitch of the worm wheel (i.e., mπ), where m is the module and π is an irrational number, the number of teeth of the replacement gear of the lathe is limited. When selecting the replacement gear, π can only be converted into an approximate fractional value (π = 3.1415) for calculation, which will cause the tool to be inaccurate in the forming motion (spiral motion) of the workpiece, resulting in pitch error.
In machining, approximate machining is generally used to improve productivity and economy, provided that the theoretical error can meet the machining accuracy requirements (<=10%-15% dimensional tolerance).
2. Adjustment error The adjustment error of a machine tool refers to the error caused by inaccurate adjustment.
3. Machine tool error Machine tool error refers to the manufacturing error, installation error and wear of the machine tool. It mainly includes the guide error of the machine tool guide rail, the spindle rotation error of the machine tool, and the transmission error of the machine tool transmission chain.
(1) Guide rail guidance error of machine tools 1) Guide rail guidance accuracy - the degree of conformity between the actual movement direction of the guide rail pair moving parts and the ideal movement direction. It mainly includes: ① The straightness Δy of the guide rail in the horizontal plane and the straightness Δz (bending) in the vertical plane; ② The parallelism (twist) of the front and rear guide rails; ③ The parallelism error or perpendicularity error of the guide rail to the spindle rotation axis in the horizontal plane and the vertical plane.
2) The influence of guide rail guidance accuracy on cutting machining mainly considers the relative displacement of the tool and the workpiece in the error-sensitive direction caused by the guide rail error. The error-sensitive direction in turning is the horizontal direction, and the machining error caused by the guide error in the vertical direction can be ignored; the error-sensitive direction in boring changes with the tool rotation; the error-sensitive direction in planing is the vertical direction, and the straightness of the bed guideway in the vertical plane causes the straightness and flatness errors of the machined surface.
(2) Machine tool spindle rotation error The machine tool spindle rotation error refers to the drift of the actual rotation axis relative to the ideal rotation axis. It mainly includes spindle end face circular runout, spindle radial circular runout, and spindle geometric axis inclination swing.
1) The influence of spindle end face circular runout on machining accuracy: ① No influence when machining cylindrical surfaces; ② When turning or boring end faces, an error in the perpendicularity between the end face and the cylindrical axis or an error in the end face flatness will be generated; ③ When machining threads, a pitch period error will be generated.
2) The influence of the radial circular runout of the spindle on the machining accuracy: ① If the radial rotation error is manifested as the simple harmonic linear motion of its actual axis in the y-axis coordinate direction, the hole bored by the boring machine is an elliptical hole, and the roundness error is the amplitude of the radial circular runout; while the hole turned by the lathe has little effect; ② If the geometric axis of the spindle moves eccentrically, a circle with a radius equal to the distance from the tool tip to the average axis can be obtained regardless of turning or boring.
3) The influence of the inclination swing of the geometric axis of the spindle on the machining accuracy: ① The geometric axis forms a conical trajectory with a certain cone angle relative to the average axis in space, which is equivalent to the eccentric motion of the geometric axis around the average axis from the perspective of each section, while the eccentricity values at different locations are different from the axial direction; ② The geometric axis swings in a certain plane, which is equivalent to the simple harmonic linear motion of the actual axis in a plane from the perspective of each section, while the runout amplitudes at different locations are different from the axial direction; ③ In fact, the inclination swing of the geometric axis of the spindle is the superposition of the above two.
(3) Transmission error of machine tool transmission chain The transmission error of machine tool transmission chain refers to the relative motion error between the transmission elements at the first and last ends of the transmission chain.
1) Manufacturing error and wear of fixtures The error of fixtures mainly refers to: ① Manufacturing error of positioning elements, tool guide elements, indexing mechanism, fixture base, etc.; ② Relative size error between the working surfaces of the above components after the fixture is assembled; ③ Wear of the working surface of the fixture during use.
2) Manufacturing error and wear of tools The influence of tool error on machining accuracy varies according to the type of tool. ① The dimensional accuracy of fixed-size tools (such as drills, reamers, keyway milling cutters and circular broaches, etc.) directly affects the dimensional accuracy of the workpiece. ② The shape accuracy of forming tools (such as forming turning tools, forming milling cutters, forming grinding wheels, etc.) will directly affect the shape accuracy of the workpiece. ③ The blade shape error of the developing tool (such as gear hobs, spline hobs, gear shaping tools, etc.) will affect the shape accuracy of the machined surface. ④ The manufacturing accuracy of general tools (such as turning tools, boring tools, milling cutters, etc.) has no direct effect on machining accuracy, but the tools are prone to wear.
3) Process system deformation under force The process system will deform under the action of cutting force, clamping force, gravity and inertia force, thereby destroying the mutual position relationship of the components of the adjusted process system, resulting in processing errors and affecting the stability of the processing process. Mainly consider the deformation of the machine tool, the deformation of the workpiece and the total deformation of the process system.
4. The influence of cutting force on processing accuracy
Only considering the deformation of the machine tool, for machining shaft parts, the deformation of the machine tool under force makes the machined workpiece appear in a saddle shape with thick ends and thin middle, that is, cylindricality error occurs. Only considering the deformation of the workpiece, for machining shaft parts, the deformation of the workpiece under force makes the workpiece appear in a drum shape with thin ends and thick middle after machining. For machining hole parts, the deformation of the machine tool or the workpiece is considered separately, and the shape of the workpiece after machining is opposite to that of the machined shaft parts.
5. The influence of clamping force on machining accuracy
When the workpiece is clamped, due to the low rigidity of the workpiece or the improper clamping force application point, the workpiece is deformed accordingly, resulting in machining errors.
6. Thermal deformation of the process system During the processing, the process system is heated and deformed due to heat generated by internal heat sources (cutting heat, friction heat) or external heat sources (ambient temperature, thermal radiation), thus affecting the processing accuracy. In large-scale workpiece processing and precision processing, the processing error caused by thermal deformation of the process system accounts for 40%-70% of the total processing error.
The impact of thermal deformation of the workpiece on the processed metal includes two types: uniform heating of the workpiece and uneven heating of the workpiece.
7. Residual stress inside the workpiece Residual stress generation: 1) Residual stress generated during blank manufacturing and heat treatment; 2) Residual stress caused by cold straightening; 3) Residual stress caused by cutting.
8. Impact of the processing site environment There are often many small metal chips at the processing site. If these metal chips exist in the positioning surface or positioning hole position of the part, it will affect the processing accuracy of the part. For high-precision processing, some metal chips that are so small that they cannot be seen will affect the accuracy. This influencing factor will be identified, but there is no very effective method to eliminate it, and it often relies heavily on the operator's operating skills.
Measuring method
The machining accuracy adopts different measuring methods according to different machining accuracy contents and accuracy requirements. Generally speaking, there are the following methods: 1. According to whether the measured parameters are measured directly, it can be divided into direct measurement and indirect measurement. Direct measurement: directly measure the measured parameters to obtain the measured dimensions. For example, measure with a caliper or a comparator. Indirect measurement: measure the geometric parameters related to the measured dimensions, and obtain the measured dimensions after calculation. Obviously, direct measurement is more intuitive, and indirect measurement is more cumbersome. Generally, when the measured dimensions or direct measurement cannot meet the accuracy requirements, indirect measurement has to be used.
2. According to whether the reading value of the measuring instrument directly represents the value of the measured dimension, it can be divided into absolute measurement and relative measurement. Absolute measurement: the reading value directly represents the size of the measured dimension, such as measuring with a vernier caliper. Relative measurement: the reading value only represents the deviation of the measured dimension relative to the standard. If the diameter of the shaft is measured with a comparator, the zero position of the instrument must be adjusted with a gauge block first, and then the measurement is performed. The measured value is the difference between the diameter of the side shaft and the size of the gauge block, which is a relative measurement. Generally speaking, the relative measurement accuracy is higher, but the measurement is more troublesome.
3. According to whether the measured surface is in contact with the measuring head of the measuring instrument, it is divided into contact measurement and non-contact measurement. Contact measurement: The measuring head is in contact with the contacted surface, and there is a mechanical measuring force. For example, using a micrometer to measure parts. Non-contact measurement: The measuring head does not contact the surface of the measured part. Non-contact measurement can avoid the influence of the measuring force on the measurement result. For example, using projection method, light wave interference method, etc.
4. According to the number of parameters measured at one time, it is divided into single measurement and comprehensive measurement. Single measurement: Each parameter of the measured part is measured separately. Comprehensive measurement: The measurement reflects the comprehensive indicators of the relevant parameters of the part. For example, when measuring the thread with a tool microscope, the actual mean diameter of the thread, the tooth profile half-angle error and the cumulative pitch error can be measured separately.
Comprehensive measurement is generally more efficient, more reliable to ensure the interchangeability of parts, and is often used for the inspection of finished parts. Single measurement can determine the error of each parameter separately, and is generally used for process analysis, process inspection and measurement of specified parameters.
5. According to the role of measurement in the processing process, it is divided into active measurement and passive measurement. Active measurement: The workpiece is measured during the processing, and the results are directly used to control the processing of the parts, so as to prevent the generation of waste in time. Passive measurement: The measurement is performed after the workpiece is processed. This type of measurement can only determine whether the processed part is qualified, and is limited to discovering and eliminating waste.
6. According to the state of the measured part during the measurement process, it is divided into static Measurement and dynamic measurement. Static measurement: measurement is relatively static. For example, micrometer measures diameter. Dynamic measurement: during measurement, the measured surface and the measuring head simulate relative motion in the working state. The dynamic measurement method can reflect the condition of the parts close to the use state, which is the development direction of measurement technology.
