Centerless grinding machine processing method
Centerless Grinding Machine (Centerless Grinding Machine) is a kind of grinding machine that does not need to use the axial positioning of the workpiece. It is mainly composed of grinding grinding wheel, adjusting wheel and workpiece support. The grinding wheel actually works as grinding. The adjusting wheel controls the rotation of the workpiece and controls the feeding speed of the workpiece. As the workpiece holder supports the workpiece during grinding, the three kinds of parts can have several matching methods, except for stopping the grinding, the principle is the same.
When machining cylindrical parts such as valves and pins, the centerless grinding machine is often used to machine the outer diameter. However, due to the different outer diameters of the parts, the speed of the guide wheels is required to be variable. Nowadays, a simple single-phase thyristor speed control device is often used to adjust the speed of the guide wheel.
Due to the small diameter of the machined parts, the speed of the guide wheels is also required to be low in order to reduce the output voltage and load capacity of the thyristor. In actual production, due to the excess of blank or other reasons, the grinding amount per unit time increases, and the grinding force generated by grinding the workpiece by the grinding wheel will change the rotation speed of the guide wheel. This phenomenon is called “reverse dragging”. ". Since the guide wheel is “upside down”, when the speed of the guide wheel is adjusted to 20r/min, the actual speed of the guide in the machining will increase to 10r/min with the grinding wheel grinding speed. When the grinding wheel exits grinding, the rotation speed of the guide wheel Drop 0. The fluctuation of speed is up to 200%, which seriously affects product quality and safe production. Measured motor current: During the grinding process, the current drops from normal 1.5A to 0A and then rises to 1.5A. It can also be seen by observation with an oscilloscope that the thyristor has been turned off during the grinding process of the grinding wheel, and the motor is completely out of control.
After inspection, all the links of the speed control system work normally, and the speed of the guide wheel is continuously adjustable at light load. The reason for the thyristor being turned off during the grinding process is that the rotation speed rises due to the "drag" of the motor, and the current drops, so that the given voltage of the thyristor device is lower than the voltage of the motor terminal, causing the thyristor to be turned off. At this point, the motor actually runs in the generator state.
In order to eliminate the "dumping" effect, the machine tool factory has specially designed the load device, which requires adding components to the machine tool, and the structure becomes complicated. According to the above analysis, the motor is in the running state of the generator when "downward dragging". If the reversible system works, the current channel is provided at this point to reverse the armature current. Therefore, the motor can be braked at a higher speed than the given speed (refer to the speed of the lead-in motor during normal grinding, usually when the diameter of the grinding part is below 25mm, the motor speed is below 50r/min), and the stable speed can be achieved. the goal of. We use a method of paralleling the two ends of the DC motor to make the brake wheel supply the braking current to the motor in the generator state when the speed is higher than the given speed, limiting the speed rise, and effectively eliminating the motor. The effect of reverse dragging keeps the speed basically stable. In addition, we connect the switch in the resistance loop to ensure that the resistor is only connected when the guide wheel is ground at low speed to prevent the resistor from consuming energy at other times.
By adopting this measure, not only can the speed of the guide wheel be kept stable, but also the processing capability of the machine tool (increasing the amount of grinding) can be improved. Therefore, for DC motors operating at low speeds, this method can be used to eliminate “drag”. Since the resistor is connected in parallel at the two ends of the motor armature winding, that is, directly connected to the thyristor rectified voltage output terminal, the braking effect cannot be simply pursued to reduce the resistance, which causes the resistance shunting to be too large during normal operation. The resistance value should be adjusted from large to small. We use RX-20, power 10W resistor, the shunt value is 1A during normal operation, and has little effect on 20A thyristor; the motor current during braking is also 1A, The motor has no effect. This circuit works well for machining shaft parts with a diameter of 25mm or less. The diameter of the parts is large. If the speed of the guide wheel is high, the resistance is high and it is not suitable for use.
