As a robot, dealing with machining every day is inseparable from precision, but do you really understand machining precision? Today, the editor will give you a detailed interpretation of the machining accuracy!
Machining accuracy is the degree to which the three geometric parameters of the actual size, shape, and position of the machined part surface conform to the ideal geometric parameters required by the drawing. The ideal geometric parameters, in terms of size, are the average size; in terms of surface geometry, they are absolute circles, cylinders, planes, cones and straight lines, etc.; in terms of mutual positions between surfaces, they are absolute parallelism , vertical, coaxial, symmetrical, etc. The deviation between the actual geometric parameters of the part and the ideal geometric parameters is called the machining error.
Introduction to Machining Accuracy
Machining accuracy is mainly used to produce products, and both machining accuracy and machining error are terms for evaluating the geometric parameters of the machined surface. The machining accuracy is measured by the tolerance grade, the smaller the grade value, the higher the precision; the machining error is expressed by a 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 represents the highest processing accuracy of the part, and IT18 represents the lowest processing accuracy of the part. Generally speaking, IT7 and IT8 have medium processing accuracy. level.
The actual parameters obtained by any processing method will not be absolutely accurate. From the perspective of 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 results and the true value. The high measurement accuracy means that the systematic error is small. At this time, the average value of the measurement data deviates from the true value less, but the data is scattered, that is, the size of the accidental error is not clear.
2. Precision
Refers to the reproducibility and consistency between the results obtained by repeated measurements using the same spare sample. It is possible to have high precision, but the precision is not exact. For example, the three results obtained by using a length of 1mm for measurement are 1.051mm, 1.053, and 1.052, respectively. Although they have high precision, they are not accurate.
Accuracy means the correctness of the measurement results, precision means the repeatability and reproducibility of the measurement results, precision is the prerequisite for accuracy.
related information
1. Dimensional accuracy
Refers to the degree of conformity between the actual size of the processed part and the center of the tolerance zone of the part size.
2. Shape accuracy
Refers to the degree of conformity between the actual geometric shape of the processed part surface and the ideal geometric shape.
3. Position accuracy
Refers to the difference in actual position accuracy between the relevant surfaces of the machined parts.
4. Interrelationships
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 smaller 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 dimensional accuracy requirements.
Methods of Improving Machining Accuracy
1. Adjust the process system
trial cut adjustment
Trial cutting - measuring size - adjusting the cutting amount of the tool - cutting - cutting again, and so on until the required size is reached. This method has low production efficiency and is mainly used for single-piece and small-batch production.
adjustment method
The required size is obtained by pre-adjusting the relative positions of the machine tool, fixture, workpiece and tool. This method has high productivity and is mainly used for mass production.
2. Reduce machine error
1) Improve the manufacturing accuracy of the main shaft parts
The rotation accuracy of the bearing should be improved:
① Use high-precision rolling bearings;
②Adopt high-precision multi-oil wedge dynamic pressure bearing;
③Using high-precision hydrostatic bearings
The precision of the fittings with the bearing 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 that matches the bearing;
③Measure and adjust the radial runout range of the corresponding parts to compensate or offset the error.
2) Properly preload the rolling bearing
①The gap can be eliminated;
②Increase bearing stiffness;
③ Homogenization of rolling body 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 precision is high;
2) The use of reduced speed transmission (i<1) is an important principle to ensure transmission accuracy, and the closer to the end of the transmission pair, the smaller the transmission ratio should be;
3) The precision of the end piece should be higher than that of other transmission parts.
4. Reduce tool wear
Tool dimensional wear must be resharpened before it reaches the sharp wear stage
5. Reduce the stress and deformation of the process system
Mainly from:
(1) Improve the stiffness of the system, especially the stiffness of weak links in the process system;
(2) Reduce the load and its variation.
Increase system stiffness:
(1) Reasonable structural design
1) Minimize the number of connecting surfaces;
2) Prevent the occurrence of local low stiffness links;
3) The structure and cross-sectional shape of the foundation and support should be selected reasonably.
(2) Improve the contact stiffness of the connection surface
1) Improve the quality of the joint surface between parts in machine tool components;
2) Preload the machine tool components;
3) Improve the accuracy of the workpiece positioning reference plane and reduce its surface roughness value.
(3) Adopt reasonable clamping and positioning methods
Reduced load and its variation:
(1) Reasonably select the geometric parameters and cutting amount of the tool to reduce the cutting force;
(2) Group the blanks, and try to make the processing allowance of the blanks uniform during adjustment.
6. Reduce the thermal deformation of the process system
(1) Reduce the heating of heat sources and isolate heat sources
1) Use a smaller cutting amount;
2) When the precision of parts is required to be high, separate the rough and finish machining 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 inseparable heat sources such as spindle bearings, screw nut pairs, high-speed moving guide rail pairs, etc., improve their friction characteristics from the aspects of structure and lubrication, reduce heat generation or use heat insulating materials;
5) Use forced air cooling, water cooling and other heat dissipation measures.
(2) Equilibrium temperature field
(3) Adopt reasonable machine tool component structure and assembly benchmark
1) Adopting a thermally symmetrical structure—in the gearbox, the shafts, bearings, transmission gears, etc. are arranged symmetrically, which can make the temperature rise of the box wall uniform and reduce the deformation of the box;
2) Reasonably select the assembly datum of machine tool parts.
(4) Accelerate to reach heat transfer equilibrium;
(5) Control the ambient temperature.
7. Reduce residual stress
(1) Increase the heat treatment process to eliminate internal stress;
(2) Arrange the process reasonably.
Factors Affecting Machining Accuracy
1. Processing principle error
Machining principle error refers to the error caused by using an approximate blade profile or an approximate transmission relationship for processing. Processing principle errors mostly appear in the processing of threads, gears, and complex curved surfaces.
For example, the gear hob used for processing involute gears, in order to facilitate the manufacture of hobs, uses Archimedes basic worm or normal straight profile basic worm instead of involute basic worm, so that the gear involute tooth shape can be produced error. Another example is when turning a modulus worm, since the pitch of the worm is equal to the pitch of the worm wheel (ie mπ), where m is the modulus, and π is an irrational number, but the number of teeth of the replacement gear of the lathe is limited, choose the replacement gear When π can only be calculated as an approximate fractional value (π = 3.1415), this will cause the inaccuracy of the tool for the workpiece forming motion (spiral motion), resulting in a pitch error.
In processing, approximate processing is generally used to improve productivity and economy under the premise that the theoretical error can meet the processing accuracy requirements (<=10%-15% dimensional tolerance).
2. Adjustment error
The adjustment error of the 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 guiding error of the machine tool guide rail, the rotation error of the machine tool spindle, and the transmission error of the machine tool transmission chain.
(1) Guidance error of the guide rail of the machine tool
1) Guidance accuracy of the guide rail - the degree of conformity between the actual movement direction of the moving parts of the guide rail pair and the ideal movement direction. mainly include:
① The straightness Δy of the guide rail in the horizontal plane and the straightness Δz in the vertical plane (bending);
② Parallelism (distortion) of the front and rear guide rails;
③ Parallelism error or perpendicularity error of the guide rail to the axis of rotation of the main shaft in the horizontal plane and in the vertical plane.
2) The influence of the guiding accuracy of the guide rail on the cutting process mainly considers the relative displacement between the tool and the workpiece in the error-sensitive direction caused by the guide rail error. During turning, the error-sensitive direction is the horizontal direction, and the machining error caused by the guiding error caused by the vertical direction can be ignored; during boring, the error-sensitive direction changes with the rotation of the tool; during planing, the error-sensitive direction is vertical, and the bed guide rail Straightness in the vertical plane causes errors in straightness and flatness of the machined surface.
(2) Rotation error of machine tool spindle
The rotary error of the machine tool spindle refers to the drift of the actual rotary axis from the ideal rotary axis. It mainly includes the circular runout of the spindle end face, the radial circular runout of the spindle, and the inclination angle swing of the spindle geometric axis.
1) The influence of the runout of the spindle end face on the machining accuracy:
①No effect when processing cylindrical surface;
② When turning and boring the end face, there will be an error in the perpendicularity between the end face and the axis of the cylindrical surface or an error in the flatness of the end face;
③During thread processing, there will be a pitch cycle error.
2) The influence of spindle radial runout on machining accuracy:
①If the radial rotation error is manifested by the simple harmonic linear motion of the 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 produced by the lathe is no effect;
②If the geometric axis of the spindle moves eccentrically, a circle whose radius is the distance from the tool tip to the average axis can be obtained regardless of turning or boring.
3) The influence of the inclination angle swing of the spindle geometric axis on the machining accuracy:
① The conical trajectory of the geometric axis forming a certain cone angle in space relative to the average axis is equivalent to the eccentric movement of the geometric axis around the average axis from the perspective of each section, and the eccentricity values are different from the axial perspective;
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, and the jumping amplitudes are different in different places when viewed from the axial direction;
③In 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 the 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 the fixture
The error of the fixture mainly refers to:
①Manufacturing errors of positioning components, tool guide components, indexing mechanisms, clamp bodies, etc.;
② After the fixture is assembled, the relative size error between the working surfaces of the above various components;
③Abrasion of the working surface of the fixture during use.
2) Manufacturing errors and wear of tools
The impact of tool errors on machining accuracy varies depending on the type of tool.
① The dimensional accuracy of fixed-size tools (such as drills, reamers, keyway milling cutters and round broaches, etc.) directly affects the dimensional accuracy of the workpiece.
②The shape accuracy of forming tools (such as forming turning tools, forming milling cutters, forming grinding wheels, etc.) will directly affect the shape accuracy of workpieces.
③The blade shape error of generated tools (such as gear hobs, spline hobs, gear shaping tools, etc.) will affect the shape accuracy of the machined surface.
④ For general tools (such as turning tools, boring tools, milling cutters), the manufacturing accuracy has no direct impact on the machining accuracy, but the tools are easy to wear.
3) Forced deformation of the process system
The process system will be deformed under the action of cutting force, clamping force, gravity and inertial force, etc., thus destroying the mutual positional relationship between the components of the adjusted process system, resulting in machining errors and affecting the stability of the process sex. Mainly consider the machine tool deformation, workpiece deformation and the total deformation of the process system.
4. The influence of cutting force on machining accuracy
Only considering the deformation of the machine tool, for the processing of shaft parts, the deformation of the machine tool under force makes the processed workpiece have a saddle shape with thick ends and thin middle, that is, cylindricity errors. Only the deformation of the workpiece is considered. For the processing of shaft parts, the workpiece is deformed by force so that the processed workpiece has a drum shape with thin ends and thick middle. For the processing of hole parts, the deformation of the machine tool or workpiece is considered separately, and the shape of the workpiece after processing is opposite to that of the processed shaft parts.
5. Influence of clamping force on machining accuracy
When the workpiece is clamped, due to the low rigidity of the workpiece or improper clamping force, the workpiece will be deformed accordingly, resulting in machining errors.
6. Thermal deformation of the process system
During the processing process, due to the heat generated by internal heat sources (cutting heat, friction heat) or external heat sources (ambient temperature, heat radiation), the process system is heated and deformed, which affects the processing accuracy. In the processing of large workpieces and precision machining, the processing errors caused by thermal deformation of the process system account for 40%-70% of the total processing errors.
The influence of the thermal deformation of the workpiece on the processing of gold includes two types: uniform heating of the workpiece and uneven heating of the workpiece.
7. Residual stress inside the workpiece
Generation of residual stress:
1) Residual stress generated during rough blank manufacturing and heat treatment;
2) Residual stress caused by cold straightening;
3) Residual stress caused by cutting.
8. Environmental impact of processing site
There are often many small metal chips on the processing site. If these metal chips exist on the part positioning surface or the position of the positioning hole, it will affect the machining accuracy of the part. For high-precision machining, 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 methods.
Measurement methods
Processing accuracy According to different processing accuracy content and accuracy requirements, different measurement methods are used. Generally speaking, there are the following types of methods:
1. According to whether to directly measure the measured parameters, it can be divided into direct measurement and indirect measurement.
Direct measurement: directly measure the measured parameters to obtain the measured size. For example, measure with calipers and comparators.
Indirect measurement: measure the geometric parameters related to the measured size, and obtain the measured size through calculation.
Obviously, direct measurement is more intuitive, while indirect measurement is more cumbersome. Generally, when the measured size cannot meet the accuracy requirements by direct measurement, indirect measurement has to be used.
2. According to whether the reading value of the measuring instrument directly represents the value of the measured size, it 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 a vernier caliper.
Relative measurement: The reading value only indicates the deviation of the measured dimension relative to the standard quantity. If you use a comparator to measure the diameter of the shaft, you need to adjust the zero position of the instrument with a gauge block first, and then measure. The measured value is the difference between the diameter of the side shaft and the size of the gauge block, which is relative measurement. Generally speaking, the accuracy of relative measurement 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 tool, it can be divided into contact measurement and non-contact measurement.
Contact measurement: The measuring head is in contact with the surface to be contacted, and there is a mechanically acting measuring force. Such as measuring parts with a micrometer.
Non-contact measurement: The measuring head is not in contact with the surface of the measured part, and non-contact measurement can avoid the influence of measurement force on the measurement results. Such as the use of projection method, light wave interferometry measurement and so on.
4. According to the number of measurement parameters, it can be divided into single measurement and comprehensive measurement.
Single measurement: measure each parameter of the part under test separately.
Comprehensive
Combined measurement: measure the comprehensive index that reflects the relevant parameters of the part. For example, when measuring threads with a tool microscope, the actual pitch diameter of the thread, the half-angle error of the tooth form, and the cumulative error of the pitch can be measured respectively.
Comprehensive measurement is generally more efficient and more reliable for ensuring the interchangeability of parts. It is often used in the inspection of finished parts. Single-item 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 products in time.
Passive measurement: Measurement performed after the workpiece has been machined. This kind of measurement can only judge whether the processed parts are qualified, and is limited to discovering and rejecting waste products.
6. According to the state of the measured part during the measurement process, it can be divided into static measurement and dynamic measurement.
Static measurement: The measurement is relatively static. Like a micrometer to measure diameter.
Dynamic measurement: During the measurement, the measured surface and the measuring head make relative movement in the simulated working state.
The dynamic measurement method can reflect the situation of the parts close to the use state, which is the development direction of the measurement technology.
