Low roughness value grinding includes precision grinding, ultra precision grinding, and mirror grinding, which can obtain workpiece shape, position, and dimensional accuracy with high surface roughness. Compared with manual grinding, it has high productivity, is easy to achieve automatic measurement, and has a wide processing range. However, this processing technology also has relatively strict requirements for machine tools.
The key issues in low roughness precision grinding depend on the selection and dressing of the grinding wheel, the selection of grinding parameters, and the precision and requirements of the machine tool. On specialized high-precision grinding machines, the grinding requirements can generally be met, while ordinary grinding machines such as M1432 and M131 can be used as long as they can ensure spindle accuracy, no shaking of the worktable, and no crawling phenomenon at slow speeds. Otherwise, ordinary machine tools need to be adjusted and modified to achieve low roughness grinding requirements.
1. Geometric accuracy of grinding machine
1.1 Rotation accuracy of grinding wheel spindle
The precision requirement for the rotation of the grinding wheel spindle is high, and the radial runout and axial displacement during spindle rotation should not exceed 0.001mm. The following measures can be taken:
(1) Improve the machining accuracy of the spindle itself. The spindle accuracy has a direct impact on the rotational accuracy, therefore, the spindle should undergo ultra precision grinding to improve its accuracy.
(2) Choose appropriate bearings. Common bearings include short or long three tile oil film bearings, integral multi oil wedge sliding bearings, and hydrostatic bearings. The above bearings can meet the requirements of low roughness grinding. When using sliding bearings, the clearance must be adjusted reasonably to 0.01-0.015mm. Static pressure bearings can achieve high spindle rotation accuracy, and their circular runout should be less than 0.05mm. Using three short new bearing shells, the rotation accuracy can reach about 0.001mm under small clearance (0.005-0.01mm) and small load conditions.
It is important to adjust the gap reasonably. The gap is small, and the spindle's runout is small, but if the gap is too small, it may cause the phenomenon of tile holding.
1.2 Geometric accuracy of workbench
The straightness and parallelism of the longitudinal guide rails of the bed body, the straightness of the transverse guide rails of the bed body, the parallelism between the centerline of the head and tail frames and the direction of movement of the worktable, and the parallelism of the centerline of the grinding wheel spindle to the direction of movement of the worktable should all meet the technical requirements for ordinary precision grinders when they leave the factory.
If the precision of the lateral feed mechanism is not high, it is difficult to control the micro feed amount when dressing the grinding wheel, and it cannot meet the high performance requirements such as micro blade. During grinding, the workpiece may be burned due to excessive lateral feed, or the grinding pressure between the grinding wheel and the workpiece may be low due to insufficient lateral feed, which cannot achieve the friction polishing effect.
2. Stability of low-speed movement of workbench
Low roughness grinding uses low-speed dressing grinding wheels, and it is required that there is no crawling or impact phenomenon when the worktable is at low speed (10mm/min). The reason for crawling is the poor rigidity of the driving system, high frictional resistance, and large changes in friction force. Therefore, to eliminate crawling, the following measures can be taken.
2.1 Exhaust. If air is mixed into the hydraulic system oil circuit, an air release valve can be added to the hydraulic cylinder of the workbench to remove the air.
2.2 Improve the lubrication effect of the guide rail. The workbench guide rail is generally lubricated by pressure. If the lubricating oil pressure is too high or the oil volume is too large, it will cause the workbench to float during movement. Avoiding floating can change the structure of the oil groove on the guide rail surface, allowing the oil groove to communicate with the atmosphere, or adding a three-way valve to each of the lubrication pipelines of the two guide rails on the workbench to allow the lubricating oil to return to the oil pool and achieve pressure free oil supply lubrication for the workbench guide rail.
2.3 The oil pressure fluctuation should be small. When a gear pump is used in a hydraulic system, its instantaneous flow rate is uneven, which causes oil pressure pulsation. Blade pumps or screw pumps can be used instead of gear pumps.
2.4 The workbench should be smoothly reversed. When the worktable changes direction, it can cause fluctuations in oil pressure, especially for machine tools that have fast jumping movements during worktable changing direction. There is hydraulic shock during fast jumping, so fast jumping direction should be cancelled.
3. Reduce machine tool vibration
3.1 Static balance grinding wheel. Usually, two static balances are required. If conditions permit, dynamic balancing or automatic grinding wheel balancing devices can be used.
3.2 Eliminate motor vibration. The vibration of the grinding wheel holder motor will be directly transmitted to the grinding wheel, which has the greatest impact on grinding.
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