Grinding is a final finishing method of precision machining or super precision machining. The grinding methods can be classified into a loose abrasive grinding, a fixed abrasive grinding, a magnetic grinding, a vibration grinding, an electrolytic grinding, a mechanical-chemistry grinding, a magnetic fluid grinding and the like. The grinding methods can also be classified into a plane grinding and a curved surface grinding according to a surface shape of a workpiece. The plane grinding is further divided into one side grinding and two sides grinding. With respect to the two sides grinding, the workpiece is placed between an upper grinding plate and a lower grinding plate with two sides of the workpiece being grinded at the same time, resulting in a high grinding efficiency. The present invention relates to a computer numerical control machine tool for grinding two sides of a plane by fixed abrasives (that is, the abrasive and the grinding tool are combined as a grinding plate).
Requirements for the grinding principle include: 1) the grinding movement track should continuously change at each instant (that is, the grinding movement track should be time-variant), and should not repeat as possible, so as to ensure forming uniform grinding stripes on the surface of the workpiece without any leading direction; 2) the relative movement speed between the grinding plate and the workpiece should be distributed as uniform as possible, because the speed is very important for the uniform grinding of the grinding plate, the uniform grinding of the workpiece and the surface shape precision of the grinding plate and the workpiece; 3) a high grinding speed should be adopted in order to improve a machining efficiency, whereas the surface residual stress will increase as the grinding speed increases, so does the hardening extent of the surface, and thus, the surface residual stress and the hardening limit the increase of the grinding speed.
With respect to the fixed abrasive plane grinding, it requires that the workpiece should be self-rotatable while the grinding plate is rotating so as to guarantee the tracks of the grinding movement (the relative movement between the grinding plate and the workpiece) unrepeated to satisfy the time-variation requirement of the grinding movement. The existing double surface grinding machines mostly adopt a planetary mechanism to drive a separation plate to perform a planetary motion, so as to bring the workpiece to self-rotate. However, the structure of the planetary mechanism is relatively complex and the stress conditions of the separation plate (planet gear) is rather bad, resulting in a heavy wear. The distance (radius) between the self-rotation center of the workpiece and the center of the grinding plate is constant, so that the rotation may be referred to as a constant distance self-rotation. One of the defects of the constant distance self-rotation lies in that the self-rotation speed of the workpiece at its self-rotation center is zero, which degrades the time-variation property. Another defect lies in that the grinding speed at each grinding point on the surface of the workpiece equals to the sum of the rotation speed of the grinding plate and the linear velocity of the self-rotation speed of the workpiece at the grinding point, and the distribution uniformity of the grinding speed is deteriorated, because the speed at the self-rotation center of the workpiece is zero. A certain double surface plane grinding machine operates in a manner that the separation plate performs an eccentric transitional motion, the upper grinding plate and the lower grinding plate are fixed, and the workpiece is clamped in the separation plate to perform a transitional motion along therewith (brought by a sine mechanism). This kind of double surface grinding machines can partially overcome the defects of the bad track time-variation and bad uniformity of grinding speed distribution in the former grinding method, while there are still defects as follows: the sine mechanism is complex in structure, the stress condition of the separation plate is bad and the abrasion is serious, especially, the upper and lower grinding plates are fixed and only the workpiece performs a planar motion, so that the grinding speed is low, and thus the processing efficiency is low. The grinding contact pressure of a general grinding machine tool for grinding two sides of a plane is controlled by a hydraulic device. At present, there is no computer numerical control machine tools for grinding two sides of a plane by shifting self-rotation in which not only the workpiece can self-rotate along therewith (i.e. self-rotate without being driven by the partition plate), but also a self-rotation position of the workpiece can be automatically controlled through a numerical control.
The ultrasonic vibration grinding includes a loose abrasive ultrasonic vibration grinding and a fixed abrasive ultrasonic vibration grinding. With respect to the fixed abrasive ultrasonic vibration grinding, an ultrasonic vibration apparatus is used to drive the fixed abrasives to generate a high frequency vibration. The fixed abrasive ultrasonic vibration grinding differs from the normal grinding in that: during the general grinding, if the abrasives move once relative to the workpiece, then the abrasive cutting edge of the normal grinding tool only performs cutting operation once. However, during an ultrasonic vibration grinding, each of the abrasive cutting edges in the fixed abrasives grinding tool vibrates at a frequency of twenty thousands to fifty thousands times per second and conducts a fine cutting (grinding), accordingly the processing efficiency is high, and uniform, fine and dense cutting tracks are generated on the workpiece with a low surface roughness and a high machining precision. The fixed abrasive ultrasonic vibration grinding is adapted to a small mass fixed abrasive application (such as an outer circumference grinding), but hardly implemented on the double side grinding plate for grinding two sides of a plane. Till now, there is no computer numerical control machine for grinding two sides of a plane in which the upper and lower grinding plates are driven by an ultrasonic vibration apparatus to vibrate at a high frequency.