Grinder surface grinder is a common machine tool for machining. It has high requirements for motion stability, commutation accuracy and commutation frequency. At present, the surface grinding machine is developing towards large-scale and high-speed. The reciprocating speed of the worktable of some surface grinding machines has reached 40 n []. The motion inertia of large-scale high-speed surface grinder is very large. When it is reversed, it will lead to a sharp increase in back pressure, resulting in reversing impact, which will have a disastrous impact on the machine tool. Therefore, the problem of reversing stability has become an important factor restricting the improvement of grinder working speed and machining accuracy.

The commutation mode and commutation control parameters in the system play a decisive role in the commutation impact. The surface grinder adopts hydraulic transmission. It is urgent to design a new hydraulic commutation system. The commutation impact of the grinder is discussed from the perspective of commutation method and control strategy.

1. Mechanism of commutation impact

In its hydraulic system, when the hydraulic transmission surface grinder is reversing, the valve port of the reversing valve is closed instantaneously and the oil circuit is suddenly disconnected, so that the oil in the oil return chamber cannot be drained.

The larger the M and V, it can be seen that the greater the kinetic energy, the greater the reversing impact. For the surface grinder with large inertia and high-speed operation, the reversing impact is huge, which not only affects the machining accuracy of the machine tool, but also hinders the normal operation and service life.

People all hope that the machine tool can realize the ideal commutation. The so-called ideal commutation means that, generally, under any working condition, the speed of the machine tool can be reduced smoothly without sudden change according to an ideal curve. At the moment when the valve is closed, the speed just decreases to zero, that is, all kinetic energy is converted into thermal energy and lost. The ideal commutation process is impact free

2 Analysis of common reversing methods of hydraulic transmission

A simple analysis and comparison is made below. There are many hydraulic transmission reversing methods currently applied to surface grinding machines. >

2.1 reversing method using stroke reversing valve

A pull-out rod is connected on the reversing valve core to use the stroke reversing valve for reversing. The pull-out rod is pushed by the stroke stop on the worktable to realize automatic reversing. When the worktable moves slowly, when the reversing valve reaches the middle position, whether the left and right chambers of the hydraulic cylinder are connected with pressure oil, return oil, or closed, at this time, there is no hydraulic pressure to push the two chambers of the hydraulic cylinder, which will make the work The movement of the table stops, so the directional valve can not reach the other end, which is the so-called "dead center" In addition, when the worktable moves at high speed, the stop pushes the pull rod to change the direction of the reversing valve very quickly, and the pressure in one chamber of the hydraulic cylinder suddenly decreases from the working pressure P to 0, and the pressure in the other chamber suddenly rises from 0 to P, which leads to a great reversing impact. At present, this system is widely used in small grinding machines.

2.2 reversing method using electromagnetic reversing valve

The travel switch is pushed by the travel stop to send the reversing signal, and the travel reversing valve is changed into the reversing method of the electromagnetic reversing valve. Making the electromagnet act to push the spool valve to reverse can prevent the "dead center". However, it is a switch type hydraulic valve, which is opened or closed instantaneously according to the command, that is, the oil return channel is connected or cut off instantaneously. Such a hydraulic reversing system will have a great impact during reversing.

2.3 reversing method using electro-hydraulic reversing valve

Then the control oil pushes the main valve to change direction. The pilot valve does not change forward, and the electro-hydraulic change-over valve replaces the solenoid change-over valve to form a new change-over method. The electro-hydraulic change-over valve is composed of the solenoid slide valve of the pilot valve and the hydraulic slide valve of the main valve. This system changes the control oil circuit through the change-over of the pilot valve. The oil flow direction of the control oil circuit does not change, the change-over valve always remains at the original end, and the main oil is replaced If the direction of the main oil circuit does not change, the workbench can always move forward. Once the control oil circuit changes the direction, the valve core of the main valve will move to another working position according to the preset speed. If the direction of the main oil circuit changes, the workbench will reverse movement to prevent the "dead point" of the reversing

In this way, the kinetic energy of the large inertia worktable can be converted into heat energy and consumed through throttling. The size of the control oil port of the main valve of the electro-hydraulic directional valve is adjustable, that is, the commutation time △ t can be extended. It can effectively reduce the commutation impact, so this commutation method is dominant for a long time. However, its commutation parameters can only be set in advance and cannot be changed according to the change of working conditions For the system whose working conditions change at any time, it is impossible to realize the ideal commutation. With the ideal control curve, the surface grinder adopts the hydraulic transmission commutation system of electro-hydraulic proportional valve. It realizes the purpose of actively reducing the flow to reduce the speed, and finally achieve the purpose of smooth and impact free commutation. An intelligent control commutation system can also realize the ideal commutation when the working conditions change. The hydraulic transmission reversing system realizes a good intelligent control of the output displacement or speed, which makes the reversing process controllable, so it has a very wide range of applications

The commutation impact mechanism is analyzed. Abstract: the commutation process of surface grinder is briefly discussed, and the advantages and disadvantages of several common commutation methods are compared. Finally, the intelligent control commutation system is introduced, and the concept of ideal control curve is proposed.