Factors Affecting Machining Accuracy of CNC Machining Center Machine Tool
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1. Error in processing principle
The error of the machining principle of the CNC machining center refers to the error that occurs when the approximate blade outline or the approximate transmission relationship is used for processing. Errors in the processing principle are mostly found in the processing of threads, gears, and complex surfaces.
In the processing of CNC machining centers, approximate processing is generally used to improve productivity and economy under the premise that theoretical errors can meet the requirements of processing accuracy (<=10%-15% standard service).
2. Adjustment errors
The adjustment error of the CNC machining center machine tool refers to the error that occurs due to inaccurate adjustment.
3. CNC machining center machine tool error
CNC machining center machine tool errors refer to the production errors, device errors and wear of the machine tools. It mainly includes the guide error of the machine tool guide rail, the reverse error of the machine tool spindle, and the transmission error of the machine tool transmission chain.
(1) CNC machining center machine tool guide rail guide error
1) Guide accuracy of the guide rail—the degree of conformity between the actual movement direction of the guide rail pair moving parts and the desired movement direction. Mainly include:
① The straightness of the guide rail in the horizontal plane Δy and the straightness in the straight plane Δz (zigzag);
②The parallelism (distortion) of the front and rear guide rails;
③ The parallelism error or straightness error of the guide rail to the spindle reversal axis in the horizontal plane and the straight plane.
2) The influence of guide rail guide accuracy on cutting processing mainly considers the relative displacement of the tool and the workpiece in the error-sensitive direction caused by the guide rail error. When turning, the sensitive direction of the error is the horizontal direction, and the machining error caused by the guide error caused by the straight direction can be ignored; when the boring process is performed, the sensitive direction of the error changes with the reversal of the tool; when the planing process is performed, the sensitive direction of the error is the vertical direction. The straightness of the guide rail in the straight plane causes errors in the straightness and flatness of the machined surface.
(2) CNC machining center machine tool spindle reversal error
Machine tool spindle reversal error refers to the drift of the actual reversal axis relative to the desired reversal axis. It mainly includes the circular runout of the end face of the main shaft, the radial circular runout of the main shaft, and the inclination of the geometric axis of the main shaft.
1) The influence of the circular runout of the spindle end face on the machining accuracy:
①No effect when machining cylindrical surface;
②When turning or boring the end face, the straightness error between the end face and the cylindrical surface axis or the flatness error of the end face will occur;
③ When machining threads, a pitch cycle error will occur.
2) The influence of the radial runout of the spindle on the machining accuracy:
①If the radial reversal error is manifested as a simple harmonic linear motion of the actual axis in the y-axis coordinate direction, the hole drilled by the boring machine is an elliptical hole, and the roundness error is the amplitude of the radial circular runout; holes have no effect;
②If the geometric axis of the spindle moves eccentrically, no matter turning or boring, a circle with a radius of the tool tip to the uniform axis interval can be obtained.
3) The influence of the inclination angle of the spindle geometric axis on the machining accuracy:
①Conical trajectory in which the geometric axis is related to the uniform axis and forms a certain cone angle in space. Viewed from each section, it is equivalent to the eccentric movement of the geometric axis around the uniform axis, but the eccentric value is different everywhere when viewed from the axial direction;
(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 when viewed from each section, but the amplitude of the beating is different when viewed from the axial direction;
③In practice, the inclination swing of the geometric axis of the main shaft is the superposition of the above two.
(3) Transmission error of CNC machining center machine tool transmission chain
The transmission error of the CNC machining center machine tool transmission chain refers to the relative movement error between the first and last two transmission elements in the transmission chain.
②After the fixture is assembled, the relative size error between the working surfaces of the above various components;
③The wear of the working surface of the fixture during use.
2) Manufacturing errors and wear of tools
The influence of tool error on machining accuracy varies depending on the type of tool.
① The dimensional accuracy of fixed-scale tools (such as drills, reamers, keyway milling cutters and circular 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 the workpiece.
③ The shape error of the blade of the generating tool (such as gear hob, spline hob, gear shaping tool, etc.) will affect the shape accuracy of the machined surface.
④ For general tools (such as turning tools, boring tools, milling tools), the production accuracy has no direct impact on the machining accuracy, but the tools are easy to wear.
3) The process system is deformed by force
The process system will deform under the effects of cutting force, clamping force, gravity and inertial force, thus destroying the mutual positional relationship of the components of the adjusted process system, resulting in the occurrence of machining errors and affecting the stability of the machining process. sex. Mainly consider the deformation of the machine tool, the deformation of the workpiece and the total deformation of the process system.
4. Influence of CNC machining center cutting force on machining accuracy
Only considering the deformation of the CNC machining center machine tool, for the machining of shaft parts, the machine tool is deformed by force, so that the machined workpiece is in a saddle shape with two thick ends and a thin center, that is, a cylindricity error occurs. Only considering the deformation of the workpiece, for the processing of shaft parts, the workpiece is deformed by force, so that the workpiece is in the shape of a drum with thin ends and a thick center. For machining hole parts, considering the deformation of the machine tool or the workpiece separately, the shape of the workpiece after machining is opposite to that of the machined shaft parts.
5. Influence of clamping force on machining accuracy
When the workpiece is clamped, due to the low rigidity of the workpiece or the improper application of the clamping force, the workpiece is deformed accordingly, resulting in processing errors.
6. Thermal deformation of the process system
During the machining process, the process system is heated and deformed due to the heat generated by the internal heat source (cutting heat, friction heat) or the external heat source (ambient temperature, heat radiation), thus affecting the machining accuracy. In the machining of large workpieces and fine machining, the machining errors caused by thermal deformation of the process system account for 40%-70% of the total machining errors.
The influence of the thermal deformation of the workpiece on the machining gold includes two kinds of uniform heating of the workpiece and uneven heating of the workpiece.
7. Residual stress inside the workpiece
Onset of residual stress:
1) Residual stress generated in the process of blank making 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 fine metal shavings on the machining site. If these metal shavings exist in the positioning surface or positioning hole of the part, it will affect the machining accuracy of the part. For high-precision machining, some metal shavings that are too small to be seen will affect the accuracy. This influencing factor will be identified but there is no adequate method to eradicate it, and it is often highly dependent on the operator's working method.