Design and Research of Large Metallographic Sample Cutting Machine

1 Introduction

The metallographic sample cutting machine is mainly used for the interception of metallographic samples and the cutting of various materials, incisions, etc. It is widely used in metallurgy, automotive, aerospace and other manufacturing industries. At present, there is a lack of large-scale metallographic sample cutting machines in China. It is difficult to directly sample large parts such as automobile crankshafts. According to the basic requirements of the metallographic structure before and after sample cutting, the cutting efficiency of various parts is satisfied. In this paper, a large-scale metallographic sample cutting machine was designed and studied. The maximum cutting diameter can reach 150mm, and the longitudinal cutting range can reach 300mm, which can meet the interception and blanking of larger parts.

Unlike metal cutting in the general machinery industry, cutting metallographic specimens is relatively strict in maintaining the original structure state, that is, it must be ensured that the metallographic structure of the intercepted sample is consistent with the original part structure, which requires the process of intercepting the sample. The sample is heated and subjected to external forces as little as possible because both conditions cause certain tissue changes. Therefore, when the metal specimen is mechanically cut, the specimen temperature must not exceed 150°C so that the incision does not burn, and the authenticity of the specimen is good. The metallographic sample cutting method can be flexibly selected according to the size of the sampled part, the properties of the material, and the actual conditions at the site. Although there are many ways to cut the sample, the requirements for cutting samples in the metallographic laboratory are: the cutting surface is relatively smooth, the verticality of the section and the axis is good, and the abrasive cutting machine has strong applicability, and can cut soft metal parts. Such as copper and aluminum and hard metal parts such as quenched carbon steel, high-speed steel; and can be cut superhard materials such as carbide, ceramics and so on. In addition, it is also required that the cutting speed is high, the labor intensity is low, the operation is simple and flexible, the sample is cut accurately, the size is suitable, and the cutting cost is low. According to the above requirements, combined with the characteristics and advantages of existing cutting machines at home and abroad, we started to develop large-scale cutting machines.

2 Large Metallographic Cutting Machine Design

2.1 The overall structural design of the metallographic sample cutting machine

This design analyzes and designs the mechanical main part of the metallographic sample cutting machine through the idea of ​​the mechatronics system design. The mechanical and electrical integration technology enables the traditional mechanical design and transformation to have new thinking methods and technical means; and the information Processing technology is an indispensable part of the mechatronics technology. A mechanical system is computer controlled and has information processing capabilities to automate the production process. The organic integration of electronic devices replaces complex mechanical devices, so that the designed metallographic sample cutting machine is simpler in structure, easier to operate, with higher reliability and stability, and a higher degree of automation than the previous cutting machine in meeting the requirements of the work. high productivity.

The cutting machine is a vertical structure, and the mechanical equipment is mainly composed of four parts: a power transmission device, an actuator and a fixture, a cooling system, and a body. According to the movement of the workpiece and the movement of the grinding wheel, the metallographic cutting machine can be divided into two types, one is a cutting machine where the workpiece moves and the grinding wheel is fixed, and the other is a cutting machine where the workpiece is fixed and the grinding wheel moves. The motion is divided into two forms: curve swing and linear motion. Taking into account the force of the grinding wheel and the weight of the workpiece during cutting, the cutting machine is designed in such a way that the grinding wheel oscillates and the workpiece reciprocates when cutting a large workpiece; when cutting a small workpiece, the grinding wheel oscillates and the workpiece is fixed The immobile form.

During the cutting process, the tool can be swung up and down to achieve cutting feed. The worktable can realize both horizontal and vertical two-way movement to facilitate cutting. The grinding wheel is fixed on the cutting shaft, and the output speed of the motor is driven by the secondary V-belt drive to achieve high-speed rotation of the grinding wheel. The axis of the grinding wheel is connected with the No. 2 shaft through the cross beam. The stepping motor drives the cross beam to swing up and down to drive the cutting and feeding movement, and the cutting feed distance can be adjusted. Considering that the own weight of the grinding wheel shaft will increase the weight of the worm wheel and cause the vibration in the cutting process to affect the cutting quality, the counterweight is balanced by setting the counterweight on the clamping plate. The main motor is a three-phase asynchronous motor, which realizes stepless speed regulation through the inverter. The table is driven by the horizontal screw and the vertical screw, and the horizontal screw moves the table through the guide block movement. Both ends of the screw are equipped with stepper motors and handwheels to meet different cutting requirements.

Sliding doors on the box cover make it easy to pick and place the machined workpieces, and open the perspective glass on the front to observe the cutting process. During cutting, a large amount of heat is generated due to grinding resistance, so it is necessary to add a cooling liquid to ensure sufficient cooling of the sample. To facilitate lifting the entire cutting machine, install lifting rings on both sides of the base. Below the workbench, two screens are installed to receive cuttings, and a bottom-mounted, built-in water tank is used to provide coolant.

2.2 Transmission Mechanism Design

The transmission adopts V-belt drive. The belt drive is a kind of flexible transmission mechanism. It is especially suitable for occasions with large shaft spacing. Belt transmission is a commonly used transmission method in the mechanical industry. It can buffer and absorb vibration. It operates smoothly, without noise and is easy to apply. In the cutting machine system, the function of the main motor is to drive the cutting blade to rotate at a high speed. Since the belt drive has the above advantages, the system uses a belt drive to increase the speed to increase the cutting blade rotational speed. The two-pole transmission guarantees no change in center-to-center distance when the swing arm rotates.

2.3 Design of cooling system

In order to ensure that the metallographic structure of the sample before and after cutting is not changed, the sample must be cooled during the cutting process. The cooling system consists of a water tank, a cooling pump, a water outlet pipe, a return pipe, and a filter net. After the power is turned on, the cooling pump starts to work, the coolant in the water tank is pumped out, and is sprayed on the grinding wheel through the water outlet pipe. The cooling liquid flows to the cutting part of the sample under the action of the centrifugal force of the high-speed rotating grinding wheel. This allows the grinding wheel and sample to cool at the same time. After the cooling fluid passes through the filter, the chips are filtered and returned to the tank from the return pipe, saving water resources and ensuring the cutting quality.

2.4 Design of feed mechanism

Because the large sample cutting machine has two cutting modes, the feed also includes two parts. One is that the grinding wheel swings to the lower limit, the table moves longitudinally, and the workpiece feeds, so that the workpiece is held at a constant speed. Cutting. The second is that the grinding wheel swings the arm to the cutting position and performs uniform cutting according to the requirements. When the workpiece size is large, the grinding wheel arm can be used for intermittent motion, the longitudinal feed of the table can be reciprocated, and the cutting size of large parts can be enlarged. The feed of the grinding wheel is that the stepping motor drives the beam to oscillate up and down through the worm and worm to realize the feeding of the grinding wheel.

2.5 fixture design

During the cutting process, due to the radial force and friction force of the grinding wheel, the fixture of the specimen should be clamped reliably. Otherwise, the verticality of the specimen cannot be ensured during cutting, and the grinding wheel is easily damaged. In addition, the fixture should have a self-locking function for easy mounting. According to the above requirements, a fast clamping fixture is designed. The working process is as follows: the workpiece is clamped by the handle to push the workpiece, and then the handle is rotated, so that the fixture generates a force acting on the workpiece, thereby realizing the self-locking of the fixture. The fixture clamps on the work surface and moves along with the feed of the worktable. When the cutting force is balanced, the sample can prevent the generation of burrs after cutting. Moreover, the operation is simple and the clamping is convenient, the labor intensity is greatly reduced, and the cutting efficiency is improved.

Workshop cutting machine

3 Hardware Design of Control System for 4-axis Large Metallographic Sample Cutting Machine

3.1 Determination of overall plan of control system

The cutting machine control system is mainly based on sequential control. Most of the requirements are on-off control. The core of the control system adopts a programmable logic controller (PLC). This is because: the working environment of the cutting machine is very bad, when cutting the workpiece , There will be strong vibration, noise, and due to the need to cool the workpiece, the working environment is wet, due to the PLC's input / output system features complete, reliable performance, can adapt to a variety of forms and properties of the switch and analog signals Input and output; PLC's hardware structure adopts modular structure, can adapt to different size and scale, function complexity and various field environment control requirements, providing a convenient and fast way for the control system hardware design. Reduce the overall system design, production, commissioning cycle, saving system investment; PLC maintenance is easy to operate, easy to expand, has good performance and high reliability, can run normally in harsh industrial environments, the average time between failures can be Over tens of thousands of hours, PLC has a higher performance/price ratio. In addition, PLC also has a more complete self-diagnosis and self-test function. Therefore, the use of programmable controllers to achieve system control.

3.2 wheel speed adjustment

Since the rotation speed of the grinding wheel has a great influence on the quality of the cut sample, the grinding wheel should have different rotation speed when cutting different samples. For example: when cutting a large hardness specimen, the grinding wheel should have a higher rotation speed; and when cutting a relatively soft specimen such as copper or aluminum, the rotation speed of the grinding wheel is low. In order to reduce the rotational speed of the grinding wheel, the asynchronous motor must be frequency-controlled, so that the speed range of the grinding wheel is increased. The speed of the grinding wheel can be adjusted from 0 to 3400 r/min through the variable frequency motor.

3.3 Speed ​​Adjustment of Working Platform's Lateral Feed and Longitudinal Feed

The feed power is mainly driven by the stepper motor to rotate the screw, so as to drive the front, back, or left and right of the table. Stepper motor control is achieved by the PLC, PLC generates two signals, one for the step pulse signal CP, stepper motor driver receives a pulse signal CP every time, it drives the stepper motor to rotate step angle, pulse signal CP The frequency is proportional to the rotation speed of the stepper motor. The number of pulse signals CP determines the rotation angle of the stepper motor. The other is the direction level signal DIR. When DIR is high, the stepper motor rotates clockwise. Conversely, when DIR is low, the stepper motor rotates counterclockwise. In this way, the speed of the longitudinal feed can be precisely controlled by mediating the PLC output signal. By adjusting the output signal of the PLC, the feed position of the table can be controlled to achieve an accurate positioning effect. The schematic diagram of the feed mechanism is shown in Figure 2.

4 Structure of the control system

Because there is one asynchronous motor and three stepping motors in this control system. A single PLC can be used to control multiple objects. As long as the high-performance PLC is properly selected, the system's control functions can be fully realized. Therefore, the control structure shown in Figure 3 is used: In this system, PLC uses sensors to collect information on various types of objects such as monitoring motors and frequency converters. After processing the input information, the PLC synthesizes the information obtained by the collector and feedbacks it. Control the monitoring object. On the other hand, through the analysis and judgment of the input and operation commands, and the comprehensive processing, the control signals are output to realize the control of the cutting machine. At the same time, information collected by the PLC is used to perform fault alarms and parameter display on the monitored objects. It has the cutting mode selection control, system start and stop control, cutting status display, cutting speed display, accident alarm display and so on.

In the system, when choosing the cutting mode of three back-in ones, PLC will send control signals to the frequency converter and the stepper motor driver that drives the cantilever to move up and down, and automatically adjust the frequency of the frequency converter according to the signal collected by the sensor to make the speed of the grinding wheel. Can be changed according to the force of the grinding wheel when cutting. When choosing the layer-by-layer cutting mode, the PLC not only sends control signals to the inverter and the stepper motor driver that drives the cantilever to move up and down, but also sends a control signal to the stepper motor driver that drives the X-direction and Y-direction movement to realize the work. The reciprocating motion of the table. Among them, the inverter can receive the analog signal output from the PLC to control the output frequency to adjust the rotation speed of the grinding wheel. The inverter also has perfect self-protection and motor protection functions. It can feed back its own working status signal to the PLC when a fault occurs. When the alarm signal to the PLC.

5 Software Design of Control System

The realization of the function of the cutting machine is inseparable from the software of the control system, and the software is the soul of the control system. The software design of the control system mainly includes the design of the main program of the system and the design of each function module subroutine. The design of each function module mainly includes the design of the communication subroutine between the host computer and the PLC, the inverter control program design, and the stepper motor. Control program design, AC low speed motor design, etc.

5.1 Control System Main Program Design

The function of the main program includes system initialization and system control. Because the cutting machine needs to control more objects, the control part of the system is more complicated. After receiving the PLC command to set the system, it also needs to complete the input according to the requirements of the cutting process. Signal cycle monitoring, and output different signals to control the various parts of the system to make it work in coordination to complete the workpiece cutting. The main program flow chart of the system is shown as in Fig. 4.

5.2 Frequency Converter Control Program

The frequency converter controls the stepless speed regulation of the grinding wheel and automatically adjusts the speed according to the cutting status. Alarms can occur when an exception occurs. The flow chart of the system is shown in Figure 5. When cutting the workpiece, first, set a frequency value for the inverter. If the frequency value exceeds 50Hz, the motor will not start. If the frequency value is in the range of 0-50Hz, the motor starts to rotate at this frequency. If there is an abnormality in the cutting process, Will alarm, otherwise it will stop until the cutting is completed.

6 Conclusion

(1) The cutting machine system has three cutting methods: straight cut, triple back cut and layered cut. This enables the cutting machine to efficiently cut different sizes of parts and enhance the range of use. The lower part of the grinding wheel is selected for cutting. When the size of the grinding wheel is certain, the cutting capacity of the cutting machine is maximized.

(2) The use of frequency control technology to control the speed of the grinding wheel to achieve automatic adjustment of the grinding wheel speed, ensuring the quality of the sample cutting.

(3) Adopting an advanced automatic mechanism for advancing and retracting the knife, it is convenient to realize manual operation and automatic function. At the same time, the feed speed can be directly adjusted by controlling the speed of the stepper motor, which reduces the complicated mechanical deceleration device, simplifies the system mechanism, and improves the transmission efficiency.

(4) Develop a fixture that is easy to operate, which makes the clamping of the workpiece reliable, convenient, and flexible, which greatly reduces the labor intensity of the operator.

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