Machining accuracy is mainly used to measure the degree of production. Machining accuracy and machining error are 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 machining accuracy. The actual parameters obtained by any machining method will not be absolutely accurate. From 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 a machine depends on the processing quality of the parts and the assembly quality of the machine. The processing quality of the parts includes two parts processing accuracy and surface quality.
Machining accuracy refers to the degree to which the actual geometric parameters (size, shape and position) of the parts after processing are consistent with the ideal geometric parameters. The difference between them is called processing error. The size of the processing error reflects the level of processing accuracy. The larger the error, the lower the processing accuracy, and the smaller the error, the higher the processing accuracy.
(1) Improve the manufacturing accuracy of spindle components
1) 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.
2) The accuracy of accessories with bearings should be improved:
① Improve the machining accuracy of the box support hole and 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). Appropriately preload the rolling bearings
① Eliminate the gap;
② Increase the bearing stiffness;
③ Even out the rolling element error.
(3). Make the spindle rotation accuracy not reflected on the workpiece.
(1) Trial cutting method adjustment
Trial cutting - measuring the size - adjusting the cutting depth of the tool - cutting - trial cutting again, repeating until the required size is reached. This method has low production efficiency and is mainly used for single-piece small batch production.
(2) 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 large-scale mass production.
The tool must be sharpened before the tool size wear reaches the stage of rapid wear.
(1) Fewer transmission parts, shorter transmission chain, and higher transmission accuracy;
(2) The use of reduced speed transmission is an important principle to ensure 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 piece should be higher than that of other transmission parts.
(1) Improve the stiffness of the system, especially the stiffness of the weak links in the process 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 stiffness 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
(2) Reduce load and its changes
1) Rationally select tool geometry parameters and cutting parameters to reduce cutting force;
2) Group blanks to make the blank machining allowance uniform during adjustment.
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 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).
The adjustment error of machine tools refers to the error caused by inaccurate adjustment.
The error of fixtures mainly refers to:
(1) Manufacturing error of positioning elements, tool guide elements, indexing mechanism, fixture base, etc.;
(2) Relative dimensional error between the working surfaces of the above various components after the fixture is assembled;
(3) Wear of the working surface of the fixture during use.
Machine tool error refers to the manufacturing error, installation error and wear of the machine tool. It mainly includes machine tool guide rail guidance error, machine tool spindle rotation error, and machine tool transmission chain transmission error.
(1) Machine Tool Guide Rail Guidance Error
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. Mainly include:
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 verticality 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 processing
Mainly consider the relative displacement of the tool and the workpiece in the error-sensitive direction caused by the guide rail error. In turning processing, the error-sensitive direction is the horizontal direction, and the processing error caused by the guidance error in the vertical direction can be ignored; in boring processing, the error-sensitive direction changes with the rotation of the tool; in planing processing, the error-sensitive direction is the vertical direction, and the straightness of the bed guide rail in the vertical plane causes the straightness and flatness errors of the machined surface.
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 surface;
② When turning or boring end face, vertical error between end face and cylindrical axis or end face flatness error will be generated;
③ When machining thread, pitch period error will be generated.
2) The influence of spindle radial circular runout on machining accuracy:
① If the radial rotation error is manifested as 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 radial circular runout amplitude; while the hole turned by the lathe has no influence;
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. From the perspective of each cross section, it is equivalent to the geometric axis center moving eccentrically around the average axis center, while the eccentricity values at different locations are different from the axial direction;
② The geometric axis swings in a certain plane. From the perspective of each cross section, it is equivalent to the actual axis center moving in a plane in a simple harmonic linear motion, while the amplitude of the runout at different locations 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.
(3) Transmission error of the 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.
The process system will deform under the action of cutting force, clamping force, gravity and inertia force, thereby destroying the relative position relationship of the components of the adjusted process system, resulting in machining errors and affecting the stability of the machining process. Mainly consider the deformation of the machine tool, the deformation of the workpiece and the total deformation of the process system.
(1) The influence of cutting force on machining 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 saddle-shaped with thick ends and thin in the 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 drum-shaped with thin ends and thick in the middle after machining. For machining hole parts, considering the deformation of the machine tool or the workpiece alone, the shape of the workpiece after machining is opposite to that of the machined shaft parts.
(2) 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 point, the workpiece will produce corresponding deformation, resulting in machining error.
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 hob, spline hob, gear shaping tool, 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) has no direct impact on the machining accuracy, but the tools are easy to wear.
There are often many small metal chips at the processing site. If these metal chips are in contact with the positioning surface or positioning hole of the part, it will affect the machining accuracy of the part. For high-precision machining, some metal chips that are too small to be seen will affect the accuracy. This influencing factor will be identified, but there is no very effective method to eliminate it, and it is often highly dependent on the operator's operating skills.
During the machining process, the process system is heated and deformed due to the heat generated by internal heat sources (cutting heat, friction heat) or external heat sources (ambient temperature, thermal radiation), thereby affecting the machining accuracy. In large-scale workpiece machining and precision machining, the machining error caused by thermal deformation of the process system accounts for 40%-70% of the total machining error.
The influence of workpiece thermal deformation on the processed metal includes two types: uniform heating of the workpiece and uneven heating of the workpiece.
Generation of residual stress:
(1) Residual stress generated during blank manufacturing and heat treatment;
(2) Residual stress caused by cold straightening;
(3) Residual stress caused by cutting.
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