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The processing of precision deep holes on difficult to machine materials has always been one of the difficulties in machining. For a long time, the common way is to adopt the technology of drilling the original expansion hinge. Although there are some tools and methods for deep hole machining such as gun drill, BTA drill and spray drill, the efficiency and quality of machining have been improved. However, problems such as chip breaking, chip removal, and tool wear are still in existence, which still can not meet the requirements of modern industrial production with high precision, high quality, high efficiency and low consumption. The combination of vibration cutting deep hole machining technology and AC frequency conversion speed regulation technology will make an obvious improvement to the efficiency and accuracy of deep hole machining, and the machining quality will be more stable.

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Chinese style 1, frequency converter speed regulation principle

The frequency converter is a product of the comprehensive development of various technologies and theories. It generally includes main circuit, control, detection and protection. The main circuit consists of three parts: rectification, filtering, voltage stabilizing and inverting, and there are interfaces for DC smoothing reactor, braking resistor, braking unit and so on. The control part usually includes digital signal processor and / or high speed microcontroller, which together realize the realization of control algorithm, dynamic adjustment of control, and output of PWM control signal. The monitoring part includes the detection of variables such as voltage, current and temperature, and determines whether it exceeds the set value for system protection. Generally speaking, a frequency converter is a general inverter suitable for Industrial General Motors and general variable frequency motors, and is supplied by general electric network (single phase 220V/50Hz and three-phase 380V/50Hz) for speed control. This kind of frequency converter has become the mainstream of inverter because of its wide application in industry.

It transforms the AC into DC, smoothing the filter, and then converting the DC into different frequencies through the inverter circuit. The voltage, current and frequency required by the motor to achieve stepless speed regulation are obtained. The frequency converter is called AC DC AC converter, and the AC - DC - AC converter is better controlled by bipolar sine wave PWM. This pulsating torque is small, which expands the speed range and improves the speed control performance. Therefore, it has been widely used in the AC drive of CNC machine tools.

Chinese style 2. The basic principle of deep hole machining by vibration cutting.

Vibration drilling deep hole machining is also commonly referred to as vibration drilling. It applies vibratory cutting technology to drilling processing, and adds regular vibration to certain frequencies and amplitudes on the basis of traditional deep hole machining. When cutting tools feed on one side and vibrate at the same time, drilling operations will result in periodic changes of cutting thickness, so as to control the shape and size of cutting. The aim is to effectively solve the problem of chip breaking in deep hole processing, and it is an advanced technology to improve the quality of deep hole processing and improve the processing efficiency. The method is suitable for processing materials and different drilling specifications. As long as the spindle speed n, feed s, vibration amplitude A and frequency f are properly selected and matched, the shape and size of chip can be effectively controlled to meet the requirements of intermittent cutting.

According to this theory, the transformation of deep hole drilling and boring machine for machine tools is carried out. The advanced vibration cutting deep hole machining technology and AC frequency conversion speed regulation technology are effectively combined to form a special precision deep hole processing system, which solves the problems existing in the deep hole processing of the non revolving parts, whose diameter is less than 60mm, the depth of the hole reaches 4000mm, and the aspect ratio L/D is greater than 70.

A kind of (3) requirements for machine tool transformation in deep hole machining with vibration cutting

Because of the different processing materials, Kong Shen and the size of the hole, there are different cutting tools and different cutting speed requirements. Therefore, in order to meet the increasingly extensive processing needs of production enterprises, it is necessary to increase the speed range of spindle motor, and at the same time, in order to meet the requirements of vibration cutting, the speed range of the vibration motor must also be doubled.

In view of the machine tools currently used in our country, we mainly use variable stage multi speed three phase asynchronous motor and electromagnetic clutch to change gears to achieve the purpose of variable speed in the machine tool's headbox. We can't implement high precision constant linearity control, and can't adapt to the machining needs of spindle parts with high speed, high speed, high speed, medium speed and low speed. For some CNC machines with DC speed control, although the stepless speed regulation can be achieved, the commutation brush must be maintained frequently. The cost is high and the maximum speed of the motor is limited. Therefore, we apply the variable frequency speed control technology to the CNC machine tools, and directly drive the motor through the frequency converter. It can achieve stepless speed regulation, reduce the impact on the power network when the large motor starts, improve the power factor, and save energy effectively, and increase the efficiency and accuracy.

In order to use vibration deep hole drilling, it is necessary to carry out proper transformation of deep hole boring and boring machine or general machine tool. If the lathe is to be reformed, it is necessary to add four parts, namely, the vibration device, the oil feeder, the oil supply system and the electric control system on the basis of the original lathe. If the deep hole boring and boring machine is to be reformed, it only needs to add vibration device and control system, and other parts can be changed slightly. Among them, electrical control system is the core part of deep hole drilling.

A kind of 4. The principle of control system.

According to the chip breaking formula of vibration cutting theory, when the feed rate s and the vibration amplitude A are set, it is necessary to establish the formula (1). The vibration frequency f and the spindle speed n must be kept at a specific ratio. The control of frequency conversion ratio is the core of the whole control system. Its performance directly determines the chip breaking effect and the smoothness of the machining process. In order to meet the high precision requirement of deep hole machining in vibration cutting, and ensure that the ratio of frequency to rotation in machining process is relatively stable, it is necessary to use the speed of spindle motor as a quantitative control system, so that the speed of the motor shaft can be used as feedback quantity to control the closed loop speed governing system of motor. Generally speaking, the speed control system adopts the double closed loop system of speed and current. However, the universal inverter itself has limited current protection, so the speed single loop control is adopted, which is not only simple in structure, but also can achieve good comprehensive performance index.

This system uses the control scheme shown in Figure 1, and detects the spindle speed n and the vibration speed F through the speed sensor, compares with the pre established speed ratio (i.e. frequency conversion ratio parameter) k=f/n, through PI adjustment, then compares the difference to PWM voltage signal and sends it to the frequency converter, and controls the speed of the vibration motor through the frequency converter, so that it can follow the spindle motor. The change of rotational speed under different cutting load makes the f/n maintain a set proportion of K, which ensures the machining accuracy and stability of NC lathe.

A kind of 5. Hardware structure of control system.

The whole control system is made up of three important parts: frequency conversion ratio controller (80196KC), frequency converter and AC motor, as shown in Figure 2. In this system, the frequency conversion ratio controls the speed of the vibrating motor by controlling the spindle speed and the vibration speed, and by controlling the external voltage control terminal of the AC motor frequency converter, in order to maintain the set frequency to rotate ratio.

The main motor drives the first stage synchronous belt drive to drive the spindle (cutter) to rotate, and the vibration motor drives the connecting rod mechanism to realize the axial vibration of the spindle (tool). When the main motor is running, the spindle motor and the vibrating motor are controlled by the frequency converter to realize stepless adjustment of spindle speed and vibration frequency, and the closed-loop operation between the vibration motor and the spindle motor is achieved. The frequency of the vibration motor is controlled by the ratio of the controller, and the vibration amplitude is realized by double eccentric mechanism, which can be adjusted according to the size of the selected feed.

According to the requirement of spindle motor maximum speed 1000r/s, the frequency conversion ratio of the control system should be stepless and adjustable between 0 and 5. In the motor drive, in order to maintain the maximum torque of the motor, the system must maintain a constant ratio of frequency to voltage. In case of abnormal conditions, the system must have emergency troubleshooting function and alarm display.

Chinese style 6. Frequency converter type selection.

The choice of frequency converter must be considered comprehensively according to the specific conditions of application and load characteristics, and from the aspects of capacity, output voltage, output frequency, protection structure, V/F (voltage / frequency) mode, grid to inverter switching, instantaneous stop restart, etc., and then select the type and type to meet the requirements. The frequency converter of general general V/F control mode mainly considers output power and current, and chooses the power and current that must be equal to or greater than the drive asynchronous motor. We chose the KVFC series general inverter of Xi'an Chun Electric Appliance Co., Ltd., in which the spindle motor of 11kW chose KVFC-4110 (11kW/400V), and the vibration motor of 1.5kW chose KVFC-417 (1.7kW/400V).

A kind of 7. Function setting of inverter

The frequency converter has designed the full frequency setting function and the frequency control interface. The frequency output of frequency converter can be set by digital keyboard, such as basic frequency, starting frequency, maximum and minimum frequency, jumping frequency and width, multistage running frequency and acceleration and deceleration slope. Another way is to control the operation of the motor by using the input interface terminal, such as the positive control command, the reverse command, reset input, the conversion of acceleration and deceleration time, the selection of the running frequency of multiple stages, and the free operation of the control terminals. At this time, the operating frequency can be set by voltage input 0 - 5V or 0 - 10V, or it can be set with the current signal of (4 - 20) mA.

Because we use single chip microcomputer as controller, its working voltage is 5V. Therefore, using the single chip microcomputer to output 0 - 5V continuous analog signals, the variable frequency stepless speed regulation of spindle motor and vibrating motor is realized through frequency converter VRC terminal control inverter. The main function parameters of the inverter are shown in Table 1.

The setting of frequency converter parameters is also the key to the stable operation of the system. If the speed of the motor is too low and the torque is too small, the loss of the motor will be very large and the heat will be serious because of the load. If it is serious, it will cause the motor to burn down. Therefore, the lower limit frequency of the motor must be set or the function of automatic torque can be set up. The setting of acceleration and deceleration time is also the key term of parameter tuning. If the setting time of the acceleration and deceleration is unreasonable, the frequency converter will overshoot or overvoltage and stop automatically. Therefore, we must limit the rate of increase in frequency to prevent overcurrent when accelerating, and we must limit the rate of decrease in frequency to prevent overvoltage during deceleration.

Chinese style 8. Software design of control system.

The software function is designed on the basis of the performance requirements of the system and the design of hardware. Aiming at the functional requirements of deep hole machining system for vibration cutting, the software system needs to complete the following tasks: main control program, spindle motor speed measurement module, vibration motor speed measurement module, vibration motor control module, servo motor control module, display sub module, setting parameter reading and PI control algorithm, and fault protection module. The vibration motor control module is responsible for real-time output of the PWM motor control signal, and according to the given ratio, it controls the speed of the vibrating motor to keep the f/n K. The main program flowchart is shown in Figure 3.

Chinese style 9, conclusion

In this paper, combined with vibration drilling technology and AC frequency conversion speed regulation technology, the general machine tool is reformed, and the control system of deep hole machining for vibration cutting is designed by using single chip microcomputer as the controller of CNC machine tool. It has been applied in production practice. It is proved that this compound machining technology can effectively solve the main problems of chip breaking and chip removal in deep hole processing, thus simplifying the tool structure, reducing the requirement for cutting fluid system, reducing the space of chip removal, improving the drill pipe strength and the rigidity of deep hole drilling system, and ensuring the stability and quality of machining. Especially for deep drilling of difficult to machine materials, the effect is more obvious. It is the most effective technological measure for deep hole processing on difficult to machine materials.

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