Knife drop refers to high speedCNC vertical latheIn the process of processing, the tool rest ram automatically slides down instantly. The knife drop will cause the tool tip to crack, the surface of the workpiece pierced pits and other hazards, is a kind of equipment failure. So what causes this?
fault cause analysis
The lifting of the ram of the vertical tool holder is mainly driven by the servo motor through the ball screw directly connected with the reduction gearbox, and the weight of the ram is 11t. The ram vertical guide rail is precisely guided by 12 rolling guide rail blocks with minimal friction loss. Considering the heavy weight of the ram itself, the AC servo motor driving the ram is equipped with a brake device. At the same time, in order to ensure the Z-axis reverse speed, action sensitivity and positioning accuracy, and reduce the load of the motor and the ball screw transmission pair, the equipment is equipped with a ram hydraulic balance system.
The ram hydraulic balance system uses an axial piston variable displacement pump to supply oil to the ram balance hydraulic cylinder. The hydraulic balance pressure (about 10MPa) is set in advance by calculation. During the working process, after reaching the set pressure, the oil pump automatically adjusts to reduce the oil supply to 0 and keep the pressure constant. When the ram moves up, the oil supply rises as required, but the pressure remains constant. When the ram moves down, the oil discharged from the oil return tank by the safety valve. At the same time in the design, the system also takes into account the turret spindle box shift.
In order to ensure the balance ram weight, so that the motor load reduction, to meet the rapid motion response, the need to always keep the balance cylinder pressure constant. Although the high-speed CNC vertical lathe system is simple to configure and can achieve basic functions, it also has the following shortcomings:
(1) The axial plunger variable displacement pump is the core of the system. The system has extremely high requirements for the operation of the variable displacement pump. Problems with the variable displacement pump can easily lead to a sudden drop in pressure. When the ram rises and falls rapidly, the flow changes greatly and the pressure fluctuates easily.
(2) In addition to achieving ram balance, the hydraulic system also supplies oil to the tool holder spindle box for shifting. The two parts of the oil circuit are not isolated, the reliability is reduced, and there is mutual influence, especially when the spindle box is shifting at the moment, or when the shifting oil circuit is connected to the oil return due to problems, it is easy to cause a sudden drop in pressure. The pressure of the hydraulic balance cylinder is not stable, resulting in large fluctuations in the output of the motor, which is easy to damage the motor and cause knife failure.
In addition, in the electrical control of high-speed CNC vertical lathe, the design is mainly based on the normal operating conditions, and no relevant braking measures are taken for the sudden situation of knife drop, and there is no prevention mechanism before the knife drop fault occurs and no error correction mechanism after the occurrence.