Low Roughness Grinding includes precision grinding, ultra precision grinding and mirror grinding, which can obtain workpiece shape, position and dimensional accuracy with high machining surface roughness. Compared with manual grinding, it has high productivity, easy to realize automatic measurement, and a wide range of processing. However, this processing technology is also relatively demanding for machine tools.
The key problems of low roughness precision grinding depend on the selection and dressing of grinding wheels, the selection of grinding parameters and the accuracy and requirements of machine tools. On the special high-precision grinding machine, it can generally meet the grinding requirements, while the general m1432, M131 and other ordinary grinding machines can be used as long as they can ensure the spindle accuracy, the workbench has no jitter, and there is no creeping phenomenon at slow speed. Otherwise, ordinary machine tools must be adjusted and refitted to meet the requirements of low roughness grinding.
Geometric accuracy of grinding machine
1. Rotation accuracy of grinding wheel spindle
The rotation accuracy of the grinding wheel spindle is required to be high, and the radial runout and axial displacement of the spindle during rotation shall not be greater than 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 rotation accuracy. Therefore, the spindle should be ultra precision ground to improve the spindle accuracy.
(2) Choose a reasonable bearing. Common bearings include short three pad or long three pad 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 to 0.01-0.015mm. Hydrostatic bearings can obtain high spindle rotation accuracy, and their circular runout should be less than 0.05mm. With three short new bearing shells, the rotation accuracy can reach about 0.001mm under the conditions of small clearance (0.005-0.01mm) and small load.
It is important to adjust the clearance properly. The clearance is small, and the runout of the main shaft is small, but the clearance is too small, which may cause bearing block phenomenon.
2. Geometric accuracy of workbench
Among them, the straightness and parallelism of the longitudinal guide rail of the bed, the straightness of the transverse guide rail of the bed, the parallelism between the center line of the head and tailstock and the moving direction of the workbench, and the parallelism between the center line of the grinding wheel spindle and the moving direction of the workbench should all meet the factory technical requirements of ordinary precision grinding machines.
For example, the accuracy of the traverse mechanism is not high, and it is difficult to control the micro feed when dressing the grinding wheel, which cannot meet the high requirements such as micro blade. During grinding, the workpiece will be burned due to too large transverse feed, or the grinding pressure between the grinding wheel and the workpiece will be small due to too small transverse feed, which will not achieve the effect of friction polishing.
2、 Stability of worktable at low speed
Low speed dressing grinding wheel is used for low roughness grinding. It is required that there is no crawling and impact phenomenon when the workbench is at low speed (10mm/min). The reasons for creeping are the poor rigidity of the drive system, large friction resistance and large changes in friction. Therefore, to eliminate creeping, the following measures can be taken.
1. Exhaust. If air is mixed in the oil circuit of the hydraulic system, an air release valve can be added to the hydraulic cylinder of the workbench to remove the air.
2. Improve the lubrication effect of guide rail. The guide rail of the workbench is generally lubricated by pressure. If the lubricating oil pressure is too high or the oil quantity is too much, the workbench will float when moving. Avoiding floating can change the structure of the oil groove on the guide rail surface, so that the oil groove is connected with the atmosphere, or add a three-way valve in each of the two guide rail lubrication pipelines of the workbench to return the lubricating oil to the oil pool, so that the guide rail of the workbench can realize pressureless oil supply lubrication.
3. The oil pressure fluctuation should be small. When the gear pump is used in the hydraulic system, its instantaneous flow is uneven, which causes oil pressure pulsation. Vane pump or screw pump can be used instead of gear pump.
4. The reversing of the workbench should be stable. The oil pressure fluctuation will be caused when the worktable is reversed, especially for the machine tool with quick jump action when the worktable is reversed, there is hydraulic shock during the quick jump, so the quick jump reversing should be cancelled.
3、 Reduce machine tool vibration
1. Static balance grinding wheel. Generally, two static balances are required. If possible, dynamic balancing or automatic grinding wheel balancing device can be used.
2. Eliminate motor vibration. The vibration of the grinding wheel frame motor will be directly transmitted to the grinding wheel, which has the greatest impact on grinding.
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