(I) Spindle Rotation Error
Spindle rotation error refers to the variation between the actual instantaneous rotation axis of the spindle and its average rotation axis. The main causes of radial spindle rotation error include: coaxiality errors of the spindle’s journal sections, various errors of the bearings themselves, coaxiality errors between bearings, and spindle deflection. Appropriately improving the manufacturing accuracy of the spindle and housing, selecting high-precision bearings, enhancing the assembly precision of spindle components, balancing high-speed spindle components, and preloading rolling bearings can all improve the rotation accuracy of the machine tool spindle.
(II) Guideway Error
Guideways are the reference on a machine tool that determines the relative positional relationship of various machine components, and are also the benchmark for machine motion. The accuracy requirements for lathe guideways mainly include the following three aspects: straightness in the horizontal plane; straightness in the vertical plane; and parallelism (twist) of the front and rear guideways. Besides manufacturing errors of the guideways themselves, uneven wear and installation quality of the guideways are also significant factors causing guideway errors.
(III) Transmission Chain Error
Transmission error of the transmission chain refers to the relative motion error between the transmission elements at the beginning and end of an internally connected transmission chain. Transmission errors are caused by manufacturing and assembly errors of each link in the transmission chain, as well as wear during the usage process.
(IV) Tool Geometric Error
Any tool will inevitably experience wear during the cutting process, which in turn causes changes in the dimensions and shape of the workpiece. Correctly selecting tool materials and using new, wear-resistant tool materials, reasonably choosing tool geometric parameters and cutting parameters, and correctly using coolant can all minimize tool dimensional wear. If necessary, compensation devices can also be used to automatically compensate for tool dimensional wear.
(V) Positioning Error
- Datum Non-coincidence Error:The datum used to determine the dimensions and position of a certain surface on a part drawing is called the design datum. The datum used on a process sheet to determine the dimensions and position of the machined surface for that specific operation is called the operational datum. When machining a workpiece on a machine tool, several geometric features of the workpiece must be selected as positioning datums. If the selected positioning datum does not coincide with the design datum, a datum non-coincidence error occurs.
- Inaccurate Manufacturing Error of Locating Element Pairs:The locating components on a fixture cannot be manufactured absolutely accurately to their nominal dimensions; their actual dimensions (or positions) are allowed to vary within specified tolerances. The workpiece locating surface and the fixture locating component together form the locating element pair. The maximum positional variation of the workpiece caused by the inaccurate manufacturing of the locating element pair and the clearance fit between them is called the inaccurate manufacturing error of the locating element pair.
(VI) Error Caused by Deformation of the Technological System Under Force
- Workpiece Stiffness: If the stiffness of the workpiece is relatively low compared to the machine tool, cutting tool, and fixture within the technological system, the deformation of the workpiece due to insufficient stiffness under the action of cutting forces will have a significant impact on machining accuracy.
- Tool Stiffness: An external turning tool has high stiffness in the direction normal (y) to the machined surface, and its deformation can be neglected. When boring a small-diameter inner hole, the boring bar has very low stiffness, and its deformation under force significantly affects the hole machining accuracy.
- Machine Tool Component Stiffness: Machine tool components consist of many parts. To date, there is no simple and suitable calculation method for the stiffness of machine tool components; it is primarily determined experimentally. The relationship between deformation and load is non-linear. The loading curve and unloading curve do not coincide; the unloading curve lags behind the loading curve. The area enclosed between the two curves represents the energy dissipated during the loading-unloading cycle, consumed by the work done by friction forces and contact deformation work. After the first unloading, the deformation does not return to the starting point of the first loading, indicating residual deformation exists. After multiple loading-unloading cycles, the starting point of the loading curve coincides with the end point of the unloading curve, and the residual deformation gradually decreases to zero.
(VII) Error Caused by Thermal Deformation of the Technological System
The impact of thermal deformation of the technological system on machining accuracy is significant, especially in precision machining and large-part machining. Machining errors caused by thermal deformation can sometimes account for up to 50% of the total workpiece error. The machine tool, cutting tool, and workpiece are affected by various heat sources, their temperatures gradually rise, and they also dissipate heat to the surrounding matter and space through various heat transfer methods.
(VIII) Adjustment Error
In each operation of mechanical machining, some form of adjustment of the technological system is always required. Because adjustment cannot be absolutely accurate, adjustment errors occur. In the technological system, the mutual positional accuracy between the workpiece and the cutting tool on the machine tool is ensured by adjusting the machine tool, cutting tool, fixture, or workpiece. When the original accuracy of the machine tool, cutting tool, fixture, workpiece blank, etc., meets process requirements and dynamic factors are not considered, the influence of adjustment error plays a decisive role in machining accuracy.
(IX) Measurement Error
When measuring a part during or after machining, the measurement method, measuring instrument accuracy, as well as the workpiece, subjective and objective factors, all directly affect measurement accuracy.
