Keywords: machining accuracy knowledge

Machining accuracy refers to the degree of conformity between the actual size, shape and position of the surface of the machined parts and the ideal geometric parameters required by the drawings. The ideal geometric parameter, in terms of size, is the average size; For surface geometry, it is absolute circle, cylinder, plane, cone and straight line, etc. For the mutual position between the surface, it is absolutely parallel, vertical, coaxial, symmetric, etc. The deviation between the actual geometric parameters and the ideal geometric parameters is called machining error.

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1.Brief introduction of machining accuracy is mainly used for the degree of production of products. Machining accuracy and machining error are terms for evaluating the geometric parameters of machining surface. Machining accuracy is measured by tolerance grade, the smaller the grade value, the higher the accuracy; The machining error is expressed by numerical value, the larger the value is, the larger the error is. High machining precision means small machining error and vice versa. There are 20 tolerance grades from IT01, IT0, IT1, IT2, IT3 to IT18, among which IT01 means that the machining accuracy of the part is the highest, IT18 means that the machining accuracy of the part is the lowest, generally IT7, IT8 is the medium level of machining accuracy. The actual parameters obtained by any machining method are not absolutely accurate. From the perspective of the function of the parts, as long as the machining error is within the tolerance range required by the parts drawing, it is considered that the machining accuracy is guaranteed.

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2.Dimensional accuracy: refers to the degree of conformity between the actual size of the processed part and the tolerance zone center of the part size. Shape accuracy: refers to the degree of conformity between the actual geometric shape and the ideal geometric shape of the processed parts surface. Position accuracy: refers to the actual position accuracy difference between the relevant surfaces of the machined parts. Mutual relation: usually in the design of machine parts and the processing accuracy of specified parts, attention should be paid to the shape error control within the position tolerance, the position error should be less than the size tolerance. That is, the precision parts or the important surface of the 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.

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Adjustment method

1.Adjust the process system by trial cutting: through trial cutting — measuring size — adjusting the cutting amount of the tool — cutting — trial cutting again, and so on until the required size is reached. This method has low production efficiency and is mainly used for single small batch production. Adjustment method: obtain the required size by adjusting the relative position of machine tool, fixture, workpiece and cutting tool in advance. This method has high productivity and is mainly used for mass production.

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2.Reduce machine tool error 1) to improve the manufacturing accuracy of spindle parts should improve the rotary accuracy of bearings: ① Choose high precision rolling bearings; Adopt high precision multi oil chipper dynamic pressure bearing; (3) Using high precision hydrostatic bearing should improve the accuracy of bearing accessories: (1) improve the processing accuracy of the box support hole and the spindle journal; ② Improve the machining accuracy of the surface matching with the bearing; Measure and adjust the radial runout range of the corresponding parts, so that the error compensation or offset. 2) Appropriate pretightening of rolling bearings can eliminate clearance; ② Increase bearing stiffness; (3) Homogenized rolling body error. 3) the spindle rotation accuracy is not reflected on the workpiece


3.Reduce transmission chain transmission error 1) Fewer transmission pieces, short transmission chain, high transmission accuracy; 2) The use of reduced speed transmission (I <1), is an important principle to ensure the transmission accuracy, and the closer to the end of the transmission pair, the transmission ratio should be smaller; 3) The accuracy of the end parts should be higher than that of other transmission parts.


4.Reduce tool wear The tool must be resharpened before the tool size wear reaches the stage of rapid wear


5.To reduce the force and deformation of the process system, the main aspects are: 1) to improve the stiffness of the system, especially the stiffness of the weak link in the process system; 2) Reduce the load and its variation to improve the system stiffness ① Reasonable structural design 1) reduce the number of connection surfaces as far as possible; 2) Prevent local low stiffness link; 3) The structure and section shape of the foundation and support should be reasonably selected. (2) Improve the contact stiffness of the connecting surface (1) improve the quality of the joint surface between the parts of the machine tool parts; 2) Preload machine parts; 3) Improve the accuracy of the workpiece positioning datum and reduce its surface roughness value. (3) Using reasonable clamping and positioning methods to reduce the load and its variation (1) reasonable selection of tool geometric parameters and cutting parameters to reduce the cutting force; (2) Grouping of hair embryos, try to make the adjustment of hair embryos processing allowance uniform.


6.Reduce the thermal deformation of the process system. 1) Reduce the heat source and isolate the heat source. 2) When the precision of parts is high, separate the rough and finish machining processes; 3) Separate the heat source from the machine tool as far as possible to reduce the thermal deformation of the machine tool; 4) To the main shaft bearing, screw nut pair, high-speed motion guide rail pair and other heat source can not be separated, from the structure, lubrication and other aspects of improving the friction characteristics, reduce heat or heat insulation material; 5) Adopt forced air cooling, water cooling and other heat dissipation measures. Metal processing wechat, good content, worth paying attention to. 1) Using thermal symmetry structure — in the gearbox, the shaft, bearing, transmission gear and other symmetrical arrangement, can make the box wall temperature rise evenly, the box deformation is reduced; 2) Reasonable selection of machine tool parts assembly datum. ④ Accelerate to achieve heat transfer balance ⑤ control the ambient temperature


7.Reduce residual stress 1. Increase the heat treatment process to eliminate internal stress; 2. Arrange the process reasonably.

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Cause of influence

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① Machining principle error Machining principle error refers to the error produced by using the approximate blade profile or approximate transmission relationship for processing. Machining principle error mostly occurs in thread, gear, complex surface processing. For example, the gear hob used in the processing of involute gear, in order to make the hob manufacturing convenient, the basic worm of Archimedes or the normal straight profile basic worm is used instead of the basic worm of involute gear, so that the involute tooth shape has errors. Such as turning the worm module, because the worm pitch is equal to the circumference of the worm gear (mπ), where M is the module, and π is an irrational number, but the number of teeth of the gear of the lathe is limited, the choice of gear change can only be π as an approximate fractional value (π =3.1415) calculation, This will cause the tool to the workpiece forming motion (spiral motion) is not accurate, resulting in pitch error. In the processing, the general use of approximate processing, in the theoretical error can meet the requirements of machining accuracy (” =10%-15% dimensional tolerance), to improve productivity and economy.


②Adjustment error The adjustment error of the machine tool refers to the error caused by the inaccurate adjustment.


③ Machine tool error machine tool error refers to the manufacturing error, installation error and wear of machine tools. It mainly includes the guiding error of the machine tool guide rail, the rotary error of the machine tool spindle and the transmission error of the transmission chain of the machine tool. 1. Guide guide precision — the degree of conformity between the actual movement direction and the ideal movement direction of the moving parts of the guide pair.


Mainly include:

Straightness of guide in horizontal plane δ Y and straightness in vertical plane δ Z (bending);


 The parallelism of the front and back guide rail (distortion);


the parallelism error or verticality error of the guide rail to the spindle axis of rotation in the horizontal plane and the vertical plane. 2. The influence of guide guide precision on machining mainly considers the relative displacement of tool and workpiece in the error-sensitive direction caused by guide guide error. When turning, the error sensitive direction is horizontal, and the machining error caused by the steering error in the vertical direction can be ignored. The sensitive direction of the error varies with the tool rotation during the boring process. When planing, the error sensitive direction is vertical, and the straightness of the bed guide in the vertical plane causes the straightness and flatness error of the machining surface. Machine tool spindle rotation error the machine tool spindle rotation error refers to the actual axis of rotation to the ideal axis of rotation drift.


Mainly includes the spindle face runout, the spindle radial runout, the spindle geometric axis inclination swing.

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Influence of spindle end face circular runout on machining accuracy:

There is no influence when processing the cylinder surface;

When turning and boring the end face, the end face will be perpendicular to the axis of the cylinder or the end face flatness error;

When machining threads, the pitch period error will be generated.


Influence of spindle radial runout on machining accuracy:

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(1) If the radial rotation error is shown as the actual axis moves in a linear harmonic motion in the Y-axis coordinate direction, the boring holes are elliptic holes, and the roundness error is the amplitude of radial runout; And the hole that the lathe turns doesn’t matter;

(2) If the spindle geometric axis of the eccentric movement, no matter the car, boring can get a radius of the tool tip to the average axis distance of the circle.


Influence of inclination swing of geometric axis of spindle on machining accuracy:

(1) The geometric axis relative to the average axis in space into a certain cone Angle cone trajectory, from each section is equivalent to the geometric axis around the average axis for eccentric motion, and from the axial eccentricity values are different;

(2) The geometric axis oscillates in a plane, which is equivalent to the harmonic linear motion of the actual axis in a plane from each section, and the amplitude of the runout is 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 kinds. Transmission error of machine tool transmission chain the transmission error of machine tool transmission chain refers to the relative motion error between the first and the end of the transmission elements in the transmission chain.

④The manufacturing error of fixture and the error of wear fixture mainly refers to:

1) The manufacturing error of positioning element, tool guiding element, dividing mechanism, clamping concrete, etc.;

2) Relative size error between the above components working face after fixture assembly;

3) The wear and tear of the working surface during the use of the fixture. Metal processing wechat, good content, worth paying attention to.

4) The impact of the manufacturing error of the tool and the wear tool error on the machining accuracy varies according to the type of the tool.


 The dimensional accuracy of the fixed size cutter (such as drill, reamer, keyway milling cutter and circular broach, etc.) directly affects the dimensional accuracy of the workpiece.

The shape accuracy of the forming tool (such as forming turning tool, forming milling cutter, forming grinding wheel, etc.) will directly affect the shape accuracy of the workpiece.

The blade shape error of the generating tool (such as gear hob, spline hob, gear shaper, etc.) will affect the shape accuracy of the machining surface.

The general tool (such as turning tool, boring tool, milling cutter), the manufacturing precision has no direct impact on the processing accuracy, but the tool is easy to wear.

The process system stress deformation process system in the cutting force, clamping force, gravity and inertia force under the action of deformation, which has destroyed the adjustment of the process system of each part of the mutual position relationship, leading to the production of processing error, and affect the stability of the processing process. The deformation of machine tool, workpiece and the total deformation of process system are mainly considered. Effect of cutting force on machining accuracy.

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Only considering the deformation of machine tool, for the processing of shaft parts, the deformation of machine tool makes the workpiece to be thick at both ends and fine in the middle of the saddle shape, that is, cylindrical error. Only consider the deformation of the workpiece, for the processing of shaft parts, the deformation of the workpiece force makes the workpiece after processing is thin at both ends, thick in the middle of the drum. For machining hole parts, consider the deformation of the machine tool or the workpiece alone, the shape of the workpiece is opposite to the shaft parts processed after machining. Effect of clamping force on machining accuracy


When the workpiece is clamped, due to the low stiffness of the workpiece or the improper clamping force point, the workpiece is deformed and the machining error is caused.


The thermal deformation of the process system in the process of processing, due to the internal heat source (cutting heat, friction heat) or external heat source (environmental temperature, thermal radiation) heat production process system heated deformation, thus affecting the processing accuracy. In large workpiece machining and precision machining, machining errors caused by thermal deformation of process system account for 40%-70% of the total machining errors. The influence of thermal deformation of workpiece on the processing of gold includes two kinds of uniform heating and uneven heating


The production of residual stress inside the workpiece:

1) Residual stress generated during the manufacturing and heat treatment of the blank;

2) Residual stress caused by cold straightening;

3) Residual stress caused by machining.

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Processing site environment impact processing site often has many small metal chips, these metal chips if there is a positioning surface or positioning hole location parts will affect the accuracy of the parts, for high precision processing, some small to invisible metal chips will affect the accuracy. This influence can be identified but there is no adequate way to prevent it, often relying on the operator’s skill.


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