Machining accuracy is primarily used to describe the degree of precision in product manufacturing. Both machining accuracy and machining error are terms used to evaluate the geometric parameters of a machined surface. Machining accuracy is measured by tolerance grades; the smaller the grade value, the higher the accuracy. Machining error is expressed numerically; the larger the value, the greater the error. High machining accuracy means small machining error, and vice versa. There are 20 tolerance grades, ranging from IT01, IT0, IT1, IT2, IT3 to IT18. IT01 indicates the highest machining accuracy for a part, while IT18 indicates the lowest. Generally, IT7 and IT8 represent medium-level machining accuracy.
No machining method can produce absolutely accurate actual parameters. From the perspective of the part's function, as long as the machining error is within the tolerance range required by the part drawing, the machining accuracy is considered to be guaranteed.
The quality of a machine depends on the quality of its parts processing and the quality of its assembly. The quality of parts processing includes two main parts: the precision of the parts processing and the surface quality.
Machining accuracy refers to the degree to which the actual geometric parameters (size, shape, and position) of a machined part conform to its ideal geometric parameters. The difference between them is called machining error. The magnitude of the machining error reflects the level of machining accuracy. The larger the error, the lower the machining accuracy; the smaller the error, the higher the machining accuracy.
I. Methods for Adjusting Machining Accuracy
1. Reduce machine tool error
(1) Improve the manufacturing precision of the spindle components
1) The rotational accuracy of the bearings should be improved:
① Select high-precision rolling bearings;
② High-precision multi-oil wedge hydrodynamic bearings are adopted;
③ High-precision hydrostatic bearings are used.
2) The precision of components配套 with bearings should be improved:
① Improve the machining accuracy of the housing support holes and spindle journals;
② Improve the machining accuracy of the surfaces that mate with the bearing;
③ Measure and adjust the radial runout range of the corresponding components to compensate for or cancel out the errors.
(2) Apply appropriate preload to the rolling bearing.
① Can eliminate gaps;
②Increase bearing stiffness;
③ Homogenized rolling element error.
(3) Ensure that the spindle rotation accuracy is not reflected in the workpiece.
2. Adjust the process system
(1) Trial cutting method adjustment
The process involves trial cutting, measuring dimensions, adjusting the depth of cut, cutting, and then repeating the trial cutting until the desired dimensions are achieved. This method has low production efficiency and is mainly used for single-piece or small-batch production.
(2) Adjustment method
The required dimensions are obtained by pre-adjusting the relative positions of the machine tool, fixture, workpiece, and cutting tool. This method has high productivity and is mainly used for mass production.
3. Reduce tool wear
The tool must be re-sharpened before the wear reaches the stage of rapid wear.
4. Reduce transmission chain errors
(1) Fewer transmission parts, shorter transmission chain, and higher transmission accuracy;
(2) Using speed reduction transmission is an important principle to ensure transmission accuracy, and the transmission ratio should be smaller the closer the transmission pair is to the end;
(3) The precision of the end component should be higher than that of other transmission components.
5. Reduce the stress and deformation of the process system
(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 connecting surfaces;
②Prevent the occurrence of localized low-stiffness components;
③ The structure and cross-sectional shape of the foundation and support components should be selected reasonably.
2) Improve the contact stiffness of the connecting surfaces
① Improve the quality of the mating surfaces between parts in machine tool components;
② Apply a preload to the machine tool components;
③ Improve the accuracy of the workpiece positioning reference surface and reduce its surface roughness value.
3) Adopt reasonable clamping and positioning methods
(2) Reduce the load and its changes
1) Select appropriate tool geometry parameters and cutting parameters to reduce cutting force;
2) Group the blanks to ensure that the machining allowance of the blanks is uniform during the adjustment.
6. Reduce residual stress
(1) Add a heat treatment process to relieve internal stress;
(2) Arrange the process reasonably.
7. Reduce thermal deformation of the process system
(1) Adopt reasonable machine tool component structure and assembly standards
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 gearbox wall uniform and reduce the deformation of the gearbox body;
2) Select appropriate assembly standards for machine tool parts.
(2) Reduce the heat generated by the heat source and isolate the heat source.
1) Use smaller cutting parameters;
2) When high precision is required for parts, separate the roughing and finishing processes;
3) Separate the heat source from the machine tool as much as possible to reduce thermal deformation of the machine tool;
4) For heat sources that cannot be separated, such as spindle bearings, lead screw and nut pairs, and high-speed moving guide rail pairs, improve their friction characteristics from the aspects of structure and lubrication to reduce heat generation or use heat insulation materials;
5) Adopt forced air cooling, water cooling and other heat dissipation measures.
(3) Equilibrium temperature field
(4) Accelerate the attainment of heat transfer equilibrium
(5) Control the ambient temperature
II. Reasons for Machining Accuracy Errors
1. Machining principle error
Machining principle error refers to the error that occurs when an approximate cutting edge profile or an approximate transmission relationship is used in machining. Machining principle error often occurs in the machining of threads, gears, and complex curved surfaces.
For example, in gear hobs used for machining involute gears, Archimedes' basic worm or normal straight-profile basic worm is used instead of the basic involute worm for ease of manufacturing, which introduces errors into the involute tooth profile of the gear. Similarly, when turning module worms, since the worm pitch is equal to the worm wheel's pitch (i.e., mπ), where m is the module and π is an irrational number, but the number of teeth on the lathe's interchangeable gears is finite, π can only be approximated as a fraction (π=3.1415) when selecting interchangeable gears. This leads to inaccuracies in the tool's response to the workpiece's forming motion (helical motion), resulting in pitch errors.
In the machining process, approximate machining is generally adopted. Under the premise that the theoretical error can meet the machining accuracy requirements (<=10%-15% dimensional tolerance), productivity and economy are improved.
2. Adjusting the error
The adjustment error of a machine tool refers to the error caused by inaccurate adjustment.
3. Manufacturing errors and wear of the fixture
The main errors of the fixture refer to:
(1) Manufacturing errors of positioning elements, tool guide elements, indexing mechanisms, fixtures, etc.;
(2) The relative dimensional error between the working surfaces of the above components after the fixture is assembled;
(3) Wear on the working surface of the fixture during use.
4. Machine tool error
Machine tool errors refer to manufacturing errors, installation errors, and wear of machine tools. They mainly include machine tool guideway guiding errors, machine tool spindle rotation errors, and machine tool transmission chain transmission errors.
(1) Machine tool guideway guiding error
1) Guide rail guiding accuracy—the degree to which the actual movement direction of the moving parts of the guide rail pair matches the ideal movement direction. This mainly includes:
① The straightness of the guide rail in the horizontal plane Δy and the straightness (bending) in the vertical plane Δz;
② Parallelism (torsion) of the front and rear guide rails;
③ The parallelism error or perpendicularity error of the guide rail to the spindle rotation axis in the horizontal and vertical planes.
2) The influence of guide rail guiding accuracy on machining
The main consideration is the relative displacement between the tool and the workpiece in the error-sensitive direction caused by guide rail errors. In turning, the error-sensitive direction is horizontal, and the machining error caused by guide errors in the vertical direction can be ignored. In boring, the error-sensitive direction changes with tool rotation. In planing, the error-sensitive direction is vertical, and the straightness of the bed guide rail in the vertical plane causes straightness and flatness errors on the machined surface.
(2) Machine tool spindle rotation error
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 runout, and spindle geometric axis tilt oscillation.
1) The effect of spindle end face runout on machining accuracy:
① It has no effect when machining cylindrical surfaces;
② When turning or boring the end face, errors in the perpendicularity of the end face to the axis of the cylindrical surface or the flatness of the end face will occur;
③ When machining threads, a periodic error in the pitch will occur.
2) The effect of spindle radial runout on machining accuracy:
① If the radial rotation error manifests as a simple harmonic linear motion of its actual axis in the y-axis coordinate direction, the hole bored by the boring machine will be an elliptical hole, and the roundness error will be the radial runout amplitude; while the hole machined by the lathe will have little effect.
②If the geometric axis of the main spindle undergoes eccentric motion, both turning and boring can produce a circle with a radius equal to the distance from the tool tip to the average axis.
3) The effect of spindle geometric axis tilt angle oscillation on machining accuracy:
① The conical trajectory in space with the geometric axis at a certain cone angle relative to the average axis is equivalent to the geometric axis moving eccentrically around the average axis when viewed from each cross section, but the eccentricity value is different at different points when viewed from the axial direction.
② The geometric axis oscillates in a plane. From the perspective of each cross section, this is equivalent to the actual axis making simple harmonic linear motion in a plane, but from the perspective of the axial direction, the amplitude of the oscillation is different at different points.
③ In reality, the tilt angle of the main shaft geometric axis is a superposition of the two above.
(3) Transmission error of machine tool transmission chain
The transmission error of a machine tool transmission chain refers to the relative motion error between the transmission elements at the beginning and end of the transmission chain.
5. Deformation of the process system under stress
The machining system deforms under the influence of cutting forces, clamping forces, gravity, and inertial forces, thereby disrupting the positional relationships between the components of the pre-adjusted machining system, leading to machining errors and affecting the stability of the machining process. The main considerations are machine tool deformation, workpiece deformation, and the total deformation of the machining system.
(1) The effect of cutting force on machining accuracy
Considering only machine tool deformation, for machining shaft-type parts, the deformation caused by the machine tool results in a saddle shape that is thicker at both ends and thinner in the middle, leading to cylindricity errors. Considering only workpiece deformation, for machining shaft-type parts, the deformation caused by the workpiece results in a drum shape that is thinner at both ends and thicker in the middle. However, for machining hole-type parts, considering only the deformation of the machine tool or the workpiece, the shape of the machined workpiece is the opposite of that of the machined shaft-type part.
(2) The effect of clamping force on machining accuracy
When clamping a workpiece, if the workpiece has low rigidity or the clamping force is applied at an improper point, the workpiece will deform accordingly, resulting in machining errors.
6. Manufacturing errors and wear of cutting tools
The impact of tool error on machining accuracy varies depending on the type of tool.
(1) The dimensional accuracy of fixed-size cutting tools (such as drills, reamers, keyway cutters and broaches) directly affects the dimensional accuracy of the workpiece.
(2) The shape accuracy of the forming tools (such as forming lathe tools, forming milling cutters, forming grinding wheels, etc.) will directly affect the shape accuracy of the workpiece.
(3) The cutting edge shape error of generating tools (such as gear hobs, spline hobs, gear shaping tools, etc.) will affect the shape accuracy of the machined surface.
(4) General cutting tools (such as lathe tools, boring tools, and milling cutters) have no direct impact on machining accuracy in terms of manufacturing precision, but they are prone to wear.
7. Environmental impact of the processing site
There are often many tiny metal shavings on the machining site. If these shavings are present on the locating surfaces or holes of the parts, they will affect the machining accuracy. For high-precision machining, even metal shavings so small that they are invisible to the naked eye can affect the accuracy. This influencing factor can be identified, but there is no perfect way to eliminate it, and it often depends heavily on the operator's work techniques.
8. Thermal deformation of the process system
During machining, heat generated by internal heat sources (cutting heat, frictional heat) or external heat sources (ambient temperature, thermal radiation) causes the machining system to deform, thus affecting machining accuracy. In the machining of large workpieces and precision machining, machining errors caused by thermal deformation of the machining system account for 40%-70% of the total machining error.
The effects of workpiece thermal deformation on machined metals include both uniform and non-uniform heating of the workpiece.
9. Residual stress inside 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 process.
Disclaimer: This article is a reprint. If it involves copyright issues, please contact us promptly for deletion (QQ: 2737591964). We apologize for any inconvenience.
