The difference between accuracy and precision: 1. Accuracy refers to the degree of closeness between the measurement results obtained and the true value. Measurement of high accuracy means that the systematic error is small, when the average value of the measurement data deviates less from the true value, but the data dispersion, that is, the size of the chance error is not clear.
2, precision refers to the reproducibility and consistency between the results obtained by repeated measurements using the same kind of alternate samples. It is possible to have high precision, but the accuracy is inaccurate. For example, using a length of 1mm for the determination of the three results obtained were 1.051mm, 1.053, 1.052, although their precision is high, but it is inaccurate.
Accuracy indicates the correctness of the measurement results, precision indicates the repeatability and reproducibility of the measurement results, and precision is a prerequisite for accuracy.
Related contents 1. Dimensional accuracy refers to the degree of conformity between the actual size of the part after machining and the center of the tolerance band of the part size.
2. Shape accuracy refers to the degree to which the actual geometry of the machined part surface conforms to the ideal geometry.
3. positional accuracy refers to the actual difference in positional accuracy between the surfaces of a machined part.
4. Interrelationships usually in the design of machine parts and parts machining accuracy, should pay attention to the shape of the error control in the position tolerance, position error should be less than the size tolerance. That is, precision parts or parts of the important surface, its shape accuracy requirements should be higher than the positional accuracy requirements, positional accuracy requirements should be higher than the size accuracy requirements.
Methods to improve machining accuracy
1. Adjustment of the process system test cutting method adjusted by test cutting - measurement of the size - adjust the tool's draft - go cutting - and then test cutting, and so on until it reaches the required size. This method of low productivity, mainly used for single-piece small batch production.
Adjustment method by pre-adjusting the machine tool, fixture, workpiece and tool relative position to obtain the required size. This method of high productivity, mainly for mass production.
2. Reduce machine tool error 1) improve the manufacturing accuracy of spindle parts should improve the rotary accuracy of the bearing: ① choose high-precision rolling bearings; ② use high-precision multi-oil wedge dynamic pressure bearings; ③ use high-precision static pressure bearings should be improved with the precision of the bearing fittings: ① to improve the case support holes, the machining accuracy of the spindle journals; ② to improve the machining accuracy with the surface of the bearing; ③ measuring and adjusting the corresponding piece of the radial runout range, make the error Measure and adjust the radial runout range of the corresponding parts, so that the error compensation or offset.
(2) Appropriate preload on the rolling bearing ① can eliminate clearance; ② increase bearing stiffness; ③ equalization of rolling body error.
(3) So that the spindle rotary accuracy is not reflected on the workpiece.
3. Reduce the transmission chain transmission error 1) the number of transmission parts is small, the transmission chain is short, the transmission accuracy is high; 2) the use of reduced speed transmission (i < 1), is to ensure the transmission accuracy of the important principle, and the closer to the end of the transmission vice, the smaller the ratio should be; 3) the end of the parts should be higher than the accuracy of the other transmission parts.
4. Reduce tool wear in the tool size wear to reach the sharp wear stage before the tool must be resharpened
5. Reduce the deformation of the force of the process system mainly from: (1) improve the rigidity of the system, especially to improve the rigidity of the weak links in the process system; (2) reduce the load and its changes.
Improve the stiffness of the system: (1) reasonable structural design 1) minimize the number of connecting surfaces; 2) to prevent the emergence of local low stiffness links; 3) should be a reasonable choice of foundation parts, support parts of the structure and cross-section shape.
(2) Improve the contact stiffness of the connecting surfaces 1) Improve the quality of the bonding surface between the parts in the machine tool components; 2) Give the machine tool components to pre-load; 3) Improve the accuracy of the workpiece positioning reference surface and reduce its surface roughness value.
(3) Adopt reasonable clamping and positioning methods.
Reduce the load and its changes: (1) reasonable selection of tool geometry parameters and cutting dosage to reduce the cutting force; (2) embryo grouping, try to make the adjustment in the embryo machining allowance uniform.
6. Reduce the thermal deformation of the process system (1) to reduce the heat source of heat generation and isolation of the heat source 1) using smaller cutting dosage; 2) parts with high precision requirements, the roughing and finishing processes will be separated; 3) as far as possible, the heat source will be separated from the machine tool to reduce the thermal deformation of the machine tool; 4) on the spindle bearings, screw nut sub, high-speed movement of the guide rail sub and other heat sources can not be separated from its friction characteristics, from the structure, lubrication, etc., to improve its friction characteristics (5) the use of forced air-cooled, water-cooled and other heat dissipation measures.
(2) Equalize the temperature field (3) Adopt reasonable machine tool parts structure and assembly benchmarks 1) Adopt thermo-symmetric structure - in the gearbox, the shaft, bearings, transmission gears, etc. symmetrically arranged, which can make the box wall temperature rise uniformly, box deformation is reduced; 2) Reasonable selection of machine tool parts assembly benchmarks.
(4) Accelerate to achieve heat transfer equilibrium; (5) Control the ambient temperature.
7. Reduce residual stress (1) Increase the heat treatment process to eliminate internal stress; (2) Reasonable arrangement of the process.
Factors affecting machining accuracy
1. machining principle error machining principle error refers to the use of an approximate blade profile or an approximate transmission relationship for machining and the resulting error. Machining principle error occurs in threads, gears, complex surface processing.
For example, the processing of involute gear with gear hob, in order to make the hob manufacturing convenience, the use of Archimedes basic worm or normal straight contour of the basic worm instead of the basic involute worm, so that the gear involute tooth shape has produced errors. Another example is when turning a modulus worm, because the pitch of the worm is equal to the circumference of the worm wheel (i.e., mπ), where m is the modulus and π is an irrational number, but the number of teeth of the mating gears of the lathe is limited, and the selection of the mating gears can only be computed by reducing π to an approximate fractional value (π = 3.1415), which will cause inaccuracy of the tool for the workpiece shaping motion (helical motion), resulting in the pitch error.
In machining, the general use of approximate processing, in the theoretical error can meet the requirements of machining accuracy under the premise (= 10%-15% size tolerance), to improve productivity and economy.
2. adjustment error machine tool adjustment error refers to the error arising from inaccurate adjustment.
3. machine tool error machine tool error refers to the machine tool manufacturing error, installation error and wear and tear. Mainly includes machine tool guide error, machine tool spindle rotation error, machine tool transmission chain transmission error.
(1) machine tool guideway guidance error 1) guideway guidance accuracy - guideway sub-movement of the actual direction of movement and the degree of conformity of the ideal direction of movement. Mainly includes: ① guideway in the horizontal plane straightness Δy and vertical plane straightness Δz (bending); ② before and after the parallelism of the two rails (distortion); ③ guideway to the spindle axis of rotation in the horizontal plane and vertical plane parallelism error or verticality error.
(2) guideway guiding accuracy on the impact of cutting mainly consider the guideway error caused by the relative displacement of the tool and the workpiece in the error-sensitive direction. Turning processing error-sensitive direction for the horizontal direction, the vertical direction caused by the guiding error generated by the processing error can be ignored; boring processing error-sensitive direction with the tool rotation and change; planing processing error-sensitive direction for the vertical direction, the straightness of the bed guide in the vertical plane caused by the machining surface straightness and flatness error.
(2) machine tool spindle rotary error machine tool spindle rotary error refers to the actual rotary axis for the ideal rotary axis drift. Mainly includes spindle face runout, spindle radial runout, spindle geometry axis tilt swing.
(1) spindle face runout on the machining accuracy of the impact: ① no impact on the processing of cylindrical surfaces; ② turning, boring end face will produce end face and cylindrical axis perpendicularity error or end face flatness error; ③ processing of threads, will produce pitch cycle error.
(2) spindle radial runout on the impact of machining accuracy: ① if the radial rotary error manifested as its actual axis in the y-axis coordinates of the direction of simple harmonic linear motion, boring machine boring out of the hole for the elliptical hole, the roundness error for the radial runout amplitude; and the lathe turned out of the hole nothing to affect; ② if the spindle geometry axis of the eccentricity of the movement, regardless of the car, the boring can be obtained for the tip of the tool to the average distance from the axis of the radius of a circle.
(3) spindle geometric axis tilt swing on the impact of machining accuracy: ① geometric axis relative to the average axis in space into a certain cone angle of the conical trajectory, from the cross-section equivalent to the geometric axis around the average axis of the eccentric movement, and from the axial point of view of the eccentricity of the value of the different; ② geometric axis in a plane for the pendulum, from the cross-sectional point of view equivalent to the actual axis of the plane for the simple harmonic straight-line motion, and from the axial point of view of the different value of the Jump amplitude from the axial point of view is different; ③ actually the spindle geometric axis tilt swing for the superposition of the above two.
(3) machine tool transmission chain transmission error machine tool transmission chain transmission error refers to the transmission chain in the first and last end of the relative motion error between the transmission components.
(1) fixture manufacturing error and wear fixture error mainly refers to: ① positioning elements, tool guiding elements, indexing mechanism, clamping specific manufacturing error; ② fixture assembly, the above components of the relative size error between the working surface; ③ fixture in the use of the process of wear and tear of the working surface.
(2) tool manufacturing error and wear tool error on the impact of machining accuracy according to the different types of tools. ① fixed-size tools (such as drills, reamers, keyway milling cutters and circular broaches, etc.) the dimensional accuracy directly affects the dimensional accuracy of the workpiece. ② molding tool (such as molding cutter, molding milling cutter, molding wheel, etc.) the shape accuracy will directly affect the shape accuracy of the workpiece. ③ The shape error of the cutting edge of the spreading tool (such as gear hob, spline hob, gear inserting tool, etc.) will affect the shape accuracy of the machined surface. ④ general tools (such as turning tools, boring tools, milling cutters), the manufacturing accuracy of the machining accuracy does not have a direct impact, but the tool is easy to wear.
(3) process system force deformation process system in the cutting force, clamping force, gravity and inertia force, etc. will produce deformation, thereby destroying the process system has been adjusted to the mutual position of the components of the position of the system, resulting in the generation of machining errors, and affect the stability of the machining process. Mainly consider the deformation of the machine tool, workpiece deformation and the total deformation of the process system.
4. The effect of cutting force on machining accuracy
Considering only the deformation of the machine tool, for the processing of shaft parts, the deformation of the machine tool force so that the processing of the workpiece is thick at both ends, the middle of the thin saddle, that is, resulting in cylindricity error. Considering only the deformation of the workpiece, for the machining of shaft parts, the deformation of the workpiece under force makes the machined workpiece to be thin at both ends and thick in the middle, i.e. the cylindricity error is generated. As for the processing of hole parts, considering the deformation of the machine tool or workpiece alone, the shape of the processed workpiece is the opposite of the processed shaft parts.
5. clamping force on the impact of machining accuracy
Workpiece clamping, due to the workpiece rigidity is low or clamping force focus point is not appropriate, so that the corresponding deformation of the workpiece, resulting in machining errors.
6. thermal deformation of the process system in the processing process, due to internal heat sources (cutting heat, friction heat) or external heat sources (ambient temperature, thermal radiation) heat to make the process system heat and deformation, thus affecting the machining accuracy. In large workpiece processing and precision machining, the processing error caused by thermal deformation of the process system accounts for 40% -70% of the total processing error.
The effect of thermal deformation of the workpiece on the machining of gold, including the workpiece uniform heat and uneven heat of the workpiece two kinds. 7.
7. Residual stress within the workpiece residual stress generated by: 1) embryo manufacturing and heat treatment process of residual stress; 2) cold straightening residual stress; 3) residual stress brought about by the cutting process.
8. Processing site environmental impact of the processing site there are often many small metal shavings, these metal shavings, if the presence of parts with the positioning surface or positioning hole location will affect the accuracy of parts processing, for high-precision machining, some of the metal shavings so small that they can not be seen visually will affect the accuracy of the shavings. This influence factor will be recognized but there is no very in place method to eliminate, often on the operator's work practices are highly dependent on.
Measurement Methods
Processing accuracy according to different processing accuracy content as well as accuracy requirements, using different measurement methods. Generally speaking, there are the following categories of methods: 1. According to whether to directly measure the measured parameters, can be divided into direct measurement and indirect measurement. Direct measurement: direct measurement of the measured parameters to obtain the measured size. For example, with calipers, comparator measurements. Indirect measurement: measure the geometric parameters related to the measured size, after calculation to obtain the measured size. Obviously, direct measurement is more intuitive, indirect measurement is more cumbersome. Generally when the measured size or with direct measurement can not meet the accuracy requirements, will have to use indirect measurement.
2, according to the reading value of the gauge gauge whether the value of the measured size directly, can be divided into absolute measurement and relative measurement. Absolute measurement: the reading value directly indicates the size of the measured size, such as measuring with vernier calipers. Relative measurement: the reading value only indicates the deviation of the measured size relative to the standardized quantity. Such as the comparator to measure the diameter of the shaft, you need to use the amount of block to adjust the zero position of the instrument, and then measured, the measured value is the side of the shaft diameter relative to the size of the difference between the block, which is the relative measurement. Generally speaking, the accuracy of relative measurement is higher, but the measurement is more troublesome.
3, according to the measured surface and gauge gauge head contact, divided into contact measurement and non-contact measurement. Contact measurement: the measuring head and the contact surface contact, and there is a mechanical role of the measurement force exists. Such as micrometer measuring parts. Non-contact measurement: the measuring head is not in contact with the surface of the measured part, non-contact measurement can be avoided to measure the impact of force on the measurement results. Such as the use of projection method, light wave interference method of measurement.
4, according to the number of parameters measured at a time, divided into a single measurement and comprehensive measurement. Single measurement: each parameter of the measured part is measured separately. Comprehensive measurement: measurement reflects the comprehensive indicators of the relevant parameters of the part. Such as measuring threads with a tool microscope, can be measured out of the actual diameter of the thread, tooth half-angle error and pitch cumulative error.
Comprehensive measurement is generally more efficient, more reliable to ensure the interchangeability of parts, commonly used in the inspection of completed parts. Individual measurements to determine the error of each parameter, generally used for process analysis, process inspection and the measurement of the specified parameters.
5, according to the role played by the measurement in the process, divided into active and passive measurement. Active measurement: the workpiece in the process of measurement, the results are directly used to control the parts of the process, so as to prevent and control the generation of scrap in a timely manner. Passive measurement: measurement after the workpiece processing. This measurement can only determine whether the processed parts are qualified, limited to the discovery and elimination of scrap.
6, according to the measured parts in the measurement process in the state, divided into static measurement and dynamic measurement. Static measurement: the measurement is relatively static. Such as micrometer diameter measurement. Dynamic measurement: measurement of the measured surface and measuring head simulation work in relative motion. Dynamic measurement methods can reflect the parts close to the use of the state of the situation, is the direction of development of measurement technology.
