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About processing accuracy
By: www.plusminusthree.com  Time2018-03-21
Machining accuracy is mainly used to produce product levels. Machining accuracy and machining error are terms used to evaluate the geometric parameters of the machined surface. The processing accuracy is measured by the tolerance level. The smaller the level value is, the higher the precision is. The machining error is represented by a numerical value. The larger the value is, the greater the error is. High machining accuracy means little machining error and vice versa. There are 20 tolerance levels from IT01, IT0, IT1, IT2, IT3, IT18 to IT18. ITIT means that if the part has the highest machining accuracy, IT18 means that the machining accuracy of the part is the lowest. In general IT7 and IT8 are the processing accuracy of medium precision. level.
 
The actual parameters obtained by any processing method will not be absolutely accurate. From the point of view of the function of the part, as long as the machining error is within the tolerance range required by the part drawing, it is considered that the machining accuracy is guaranteed.
 
The quality of the machine depends on the processing quality of the part and the assembly quality of the machine. The processing quality of the part includes the two parts of the machining precision and the surface quality of the part.
 
Machining accuracy refers to the extent to which the actual geometrical parameters (size, shape, and position) of the part are consistent with the ideal geometrical parameters. Intuitively learn the mechanical micro signal you are paying attention to. The difference between them is called processing error. The size of the machining error reflects the level of machining accuracy. The larger the error is, the lower the machining accuracy is, and the smaller the error is, the higher the machining accuracy is.
 
 cnc machining related information
 
Dimensional accuracy
The actual size of the machined part and the tolerance of the part size match the center.
 
Shape accuracy
It refers to the degree of correspondence between the actual geometry of the machined part surface and the ideal geometry.
 
Position accuracy
Refers to the difference in the actual positional accuracy between the relevant surfaces of the parts after machining.
 
Relationships
 
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 dimensional tolerance. That is, the critical surface of precision parts or parts, the shape accuracy requirements should be higher than the position accuracy requirements, the position accuracy requirements should be higher than the dimensional accuracy requirements.
 
Adjustment method
 
Key 01: Adjusting Process System
 
Trial adjustment
 
Pass the test cut - measure the size - adjust the amount of knife to eat - cut the knife - re-cut, so repeated until it reaches the desired size. This method has low production efficiency and is mainly used for single batch small batch production.
 
Adjustment method
 
Obtain the required size by pre-adjusting the relative positions of the machine, fixture, workpiece and tool. This method has high productivity and is mainly used for mass production.
 
Focus 02, reduce machine tool error
 
1) Improve the manufacturing accuracy of the spindle components
 
Should improve the bearing rotation accuracy:
 
1 Select high-precision rolling bearings;
 
2 using high-precision multi-oil dynamic pressure bearing;
 
3Using high-precision hydrostatic bearings,
 
Should improve the precision with the bearing phase fittings:
 
1 Improve the machining accuracy of the box support holes and spindle journals;
 
2 Improve the machining accuracy of the mating surface with the bearing;
 
3 Measure and adjust the radial jumping range of the corresponding parts so that the error can be compensated or cancelled out.
 
2) Preload the rolling bearing properly
 
1 can eliminate gaps;
 
2 increase the bearing stiffness;
 
3 Homogenization rolling element error.
 
3) The spindle rotation accuracy is not reflected on the workpiece
 
Focus 03, reduce transmission chain transmission error
 
1) The number of transmission parts is small, the transmission chain is short, and the transmission accuracy is high;
 
2) The use of reduced speed transmission (i < 1) is an important principle for ensuring the transmission accuracy, and the closer to the end of the transmission, the smaller the transmission ratio should be;
 
3) The accuracy of the end pieces should be higher than other transmission parts.
 
Focus 04, reduce tool wear
 
Re-sharpening must be done before tool wear reaches rapid wear
 
Key 5. Reduce the stress deformation of the process system
 
1) Improve system stiffness
 
1 Reasonable structural design
 
Minimize the number of connecting surfaces; prevent the occurrence of localized low-stiffness links; reasonably select the structure and cross-sectional shape of the basic parts and supports.
 
2 to improve the contact surface of the contact stiffness; improve the quality of the joint between the parts in the machine parts; preload the machine parts; improve the accuracy of the workpiece positioning reference surface and reduce its surface roughness.
 
3 using a reasonable clamping and positioning
 
2) Reduce the load and its changes
 
1 Reasonably select tool geometry parameters and cutting amount to reduce cutting force;
 
2 The grouping of the embryos is as far as possible to make the trimming margin even.
 
Focus 06, reduce the thermal deformation of the process system
 
1 reduce heat source heat and insulation heat source
 
Use a smaller amount of cutting;
 
When the precision of the parts is high, the rough finishing process is separated;
 
As far as possible, the heat source is separated from the machine tool to reduce the thermal deformation of the machine tool;
 
For the main shaft bearings, screw nut pairs, high-speed guide rails and other non-separable heat sources, from the structure, lubrication and other aspects to improve its friction characteristics, reduce heat or heat insulation materials;
 
Use forced air cooling, water cooling and other cooling measures.
 
2 equilibrium temperature field
 
3 Adopt reasonable machine component structure and assembly standard
 
The use of a symmetrical structure in the heat - in the gearbox, symmetrical arrangement of the shaft, bearings, transmission gears, etc., can make the wall temperature rise evenly, the box deformation decreases;
 
Reasonably select the assembly standards for machine tool components.
 
4 Acceleration to achieve heat balance
 
5 control the ambient temperature
 
Focus 07, reduce residual stress
 
1 increase the heat treatment process to eliminate internal stress;
 
2 Reasonably arrange the process.
 
Influence reason
 
Focus 01 Processing principle error
 
Machining principle error refers to the error caused by processing using an approximate blade profile or approximate transmission relationship. Machining principle errors occur in the processing of threads, gears, and complex surfaces.
 
For example, for the machining of involute gear hobs, the Archimedes basic worm or the normal straight basic worm is used in place of the involute basic worm in order to make the hob easier to produce, so that the gear involute tooth shape is generated. The error. Another example is when turning a modular worm, since the pitch of the worm is equal to the circumference of the worm gear (ie, mπ), where m is the modulus and π is an irrational number, the number of teeth of the changing gear of the lathe is limited. At this time, only π can be calculated as an approximate fractional value (π = 3.1415). This will cause inaccuracies in the tool's forming motion (spiral motion) and pitch error.
 
In processing, generally approximate processing is used, and the theoretical error can meet the processing accuracy requirements (“=10%-15% dimensional tolerance”) to improve productivity and economy.
 
Focus 02 Adjustment Error
 
The adjustment error of the machine tool refers to the error due to inaccurate adjustment.
 
Key 03 machine tool error
 
Machine tool error refers to the manufacturing error, installation error and wear of the machine tool. Including machine tool guide error, machine tool spindle rotation error, machine tool transmission chain transmission error.
 
Key 04 Machine Tool Guide Error
 
1. Track Guide Accuracy - The degree of conformity between the actual movement direction of the guide rail and the ideal movement direction.
 
1 The straightness Δy of the guideway in the horizontal plane and the straightness Δz (bending) in the vertical plane;
 
2 Parallelism between two guide rails (twist);
 
3 Error of parallelism or squareness error between the guide rail and the axis of rotation of the main shaft in the horizontal and vertical planes.
 
2. The influence of the guideway guide precision on the cutting processing mainly considers the relative displacement of the tool and the workpiece in the error-sensitive direction caused by the guide rail error (welcome attention to WeChat: visually intuitive machine).
 
The error-sensitive direction during turning is the horizontal direction, and the machining error caused by the guide error caused by the vertical direction can be neglected; the error-sensitive direction during boring processing changes with the tool rotation; the error-sensitive direction during the planing process is the vertical direction, the bed guide rail Straightness in the vertical plane causes errors in straightness and flatness of the machined surface.
 
Key 05 Machine Tool Spindle Rotation Error
 
Spindle rotation error of a machine tool refers to the drift of the actual axis of rotation to the ideal axis of rotation. Including the main axis of the spindle runout, the radial runout of the spindle, the main axis of the tilt axis tilt.
 
1. The effect of the spindle end surface runout on the machining accuracy: 1 No effect when machining the cylindrical surface; 2 When the end face of the vehicle and the crucible is perpendicular to the cylinder surface, the error of the squareness of the axis of the end surface and the cylindrical surface, or the flatness error of the end surface; 3 When the thread is machined, the pitch will be generated. Cycle error.
 
2. Effect of spindle radial runout on machining accuracy:
 
1 If the radial rotation error is represented by the straight line moving in the y-axis coordinate direction of the actual axis, the hole in the boring machine is an oval hole, the roundness error is the radial circle jump amplitude, and the lathe hole is out of the lathe. No effect;
 
2 If the spindle's geometric axis is eccentrically moved, a circle with a radius from the tip to the average axis distance can be obtained for both the car and the raft.
 
3, the influence of the spindle axis inclination angle swing on the machining accuracy:
 
1 The conic trajectory of the geometric axis with respect to the average axis in a certain cone angle in space, from the perspective of each section is equivalent to the eccentric movement of the geometric axis around the average axis, and the eccentric value is different from the axial direction;
 
2 The geometrical axis oscillates in a certain plane. From the perspective of each section, it is equivalent to the actual axis in a plane for simple harmonic motion, and from the axial point of view, the jump amplitude is different everywhere. 3 Actually, the inclination of the main axis of the geometric axis For the above two kinds of superposition.
 
Key 06, transmission error of machine tool chain
 
The transmission error of the machine tool transmission chain refers to the relative motion error between the transmission components at the end and end of the transmission chain.
 
Focus 07, Manufacturing Errors and Wear of Fixtures
 
Fixture errors mainly refer to: a) manufacturing errors of positioning elements, tool guide elements, indexing mechanisms, clamps, etc.; b) relative dimensional errors between the working surfaces of the above components after the fixtures are assembled; c) fixtures are in use Work surface wear.
 
Key 08, tool manufacturing errors and wear
 
The effect of tool error on machining accuracy varies depending on the type of tool.
 
a) The dimensional accuracy of sizing tools (such as drills, reamers, keyway cutters, and round broaches) directly affects the dimensional accuracy of the workpiece.
 
b) The accuracy of the shape of a forming tool (such as a turning tool, forming cutter, forming wheel, etc.) will directly affect the shape accuracy of the workpiece.
 
c) Shape error of developed tools (such as gear hobs, spline hobs, pin cutters, etc.) will affect the shape accuracy of the machined surface.
 
d) General tools (such as turning tools, boring tools, and milling cutters), whose manufacturing accuracy has no direct influence on the machining accuracy, but the tool is easy to wear.
 
Focus 09, process system deformation
 
Process system will produce deformation under the action of cutting force, clamping force, gravity and inertia force, thus destroying the mutual positional relationship among the components of the adjusted process system, resulting in the production of machining errors and affecting the stability of the processing process. Sex. The main considerations are machine tool deformation, workpiece deformation, and total deformation of the process system.
 
Focus 10, the impact of cutting force on machining accuracy
 
Considering only the deformation of the machine tool, the machine tool is deformed by force to make the workpiece to have a saddle shape with two ends that are thick and thin at the middle, which results in a cylindricity error. Considering only the deformation of the workpiece, in the case of machining of shaft parts, the workpiece is deformed by force so that the workpiece after processing is in the shape of a drum with two ends that are fine and the middle is thick. In the case of machined-hole parts, the machine tool or workpiece deformation is considered separately, and the shape of the machined workpiece is the reverse of that of the machined shaft.
 
Focus 11, the impact 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 deforms accordingly and the machining error is caused.
 
Focus 12, thermal deformation of the process system
 
During processing, the heat generated by the internal heat source (cutting heat, frictional heat) or external heat source (ambient temperature, heat radiation) causes the process system to become heated and deform, thereby affecting the machining accuracy. In large-scale workpiece machining and precision machining, the machining error caused by the thermal deformation of the process system accounts for 40%-70% of the total machining error.
 
The effect of workpiece thermal deformation on the processing of gold includes uniform heating of the workpiece and uneven heating of the workpiece.
 
Focus 13, the residual stress inside the workpiece
 
Residual stress generation: 1) Residual stress generated during manufacture and heat treatment of the blank; 2) Residual stress due to cold straightening; 3) Residual stress due to cutting.
 
Focus 14. Environmental impact at the processing site
 
There are often many small metal shavings on the processing site. If these metal shavings are present and the position of the component positioning surface or positioning hole affects the machining accuracy of the component, for precision machining, some small to unreadable metal chips will affect the accuracy. This influencing factor will be identified but there is no way to put an end to it, often relying on operators' operating practices.
 
Measurement methods
 
Processing accuracy Different measurement methods are used according to different processing accuracy contents and accuracy requirements. In general there are the following types of methods:
 
Point 01 press directly measured parameters, can be divided into direct measurement and indirect measurement
 
Direct measurement: Directly measure the measured parameter to obtain the measured size. For example, it is measured with a caliper or a comparator.
 
Indirect measurement: Measure the geometric parameters related to the measured dimensions and calculate the measured dimensions.
 
Obviously, direct measurement is more intuitive and indirect measurement is more tedious. Generally when the measured size or direct measurement fails to meet the accuracy requirements, indirect measurement must be used.
 
Key 02: The reading of the measuring instrument directly indicates the measured value. It can be divided into absolute measurement and relative measurement.
 
Absolute measurement: The value of the reading directly indicates the size of the measured size, as measured by a cursor caliper.
 
Relative measurement: The reading only indicates the deviation of the measured dimension from the standard. If you use a comparator to measure the diameter of the shaft, you need to adjust the instrument's zero position first, and then measure. The measured value is the difference of the diameter of the side axis relative to the size of the gauge block. This is the relative measurement.
 
In general, relative measurement accuracy is higher, but measurement is more troublesome.
 
Key 03 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 a mechanically acting measuring force exists. Use a micrometer to measure parts.
 
Non-contact measurement: The measuring head is not in contact with the surface of the part under test. Non-contact measurement can avoid the influence of measuring force on the measurement result. Such as the use of projection method, light interference interferometry measurement.
 
The key 04 is divided into single items and integrated ones according to the number of measured parameters.
 
Single measurement; each parameter of the part under test is measured separately.
 
Comprehensive measurement: Measure comprehensive indicators that reflect the relevant parameters of the part. If a tool microscope is used to measure the thread, the actual thread diameter, half-angle error, and pitch error can be measured separately.
 
Comprehensive measurements are generally more efficient and more reliable in ensuring the interchangeability of parts. They are often used for inspection of finished parts. Individual measurement can determine the error of each parameter separately, generally used for process analysis, process inspection and measurement of specified parameters.
 
Focus 05 is divided into active measurements according to the role of measurement in the processing
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