Grinder belongs to the metal workpiece surface precision machining machine. The basic principle of the grinding machine is to perform shallow depth micro-cutting on the surface of the part with a grinding wheel or oil stone (cutting tool). The depth of cut during grinding is small, and the metal layer that can be removed in one stroke is thin. Abrasive tool rotation is the main movement, and the movement of the workpiece or grinding tool is the feed motion. Grinding machine with high precision, surface roughness Ra value is small. Grinding can obtain higher machining accuracy and a small surface roughness value. Grinding can not only process soft materials such as unhardened steel, cast iron and non-ferrous metals, but also can process hard materials such as ceramics and hard alloys that can not be processed by hardened steel and other tools.
The CNC grinding machine belongs to the type of grinding machine and is mainly used for grinding tap tools such as tap drill bits and carbide dies. It belongs to relatively inconvenient special mechanical processing equipment. It is not used in large quantities, and the number of manufacturers is relatively small. . Because of the high requirements on the control system and the difficulty of the CNC grinding machine, this project has almost used all the important functions of the Emerson CT EC20H PLC for high-speed motion control. The numerical control grinding machine outline see Figure 1.
Figure 1 CNC grinding machine
2 Conceptual Design of CNC Grinding Machine
2.1 System Requirements Analysis
(1) The workpiece feed index control. When processing the workpiece feeding, the indexing axis rotation indexing mainly has two process requirements. First, it is not allowed to change the indexing direction during the processing, and only one-way rotation can be used to eliminate the back-line error of the mechanical equipment; There are special requirements for the processing sequence of grooves during indexing, and the number of edges is even or odd.
Figure 2 Even division
The even number uses an 8-bladed milling cutter as an example. The processing sequence for each groove is 1,5,2,6,3,7,4,8. That is, after the first edge is finished, the difference between the starting point of the first edge and the starting point of the next edge is A. After the next edge is finished, the angle difference between the starting point of this edge and the starting point of the following edge is For B, and so on until the processing to the final edge is completed, as shown in Figure 2. .
Figure 3 Odd indexing
The odd number uses a 5-blades milling cutter as an example. The processing sequence of each groove is 1, 3, 5, 2, and 4, as shown in Fig. 3.
(2) Grinding wheel incremental feed control. The depth of the groove is a process that gradually becomes shallow during the processing of the workpiece. Gradual increase refers to the height of the grinding wheel when the workpiece is fed into 100mm, as shown in Figure 4.
Figure 4 Incremental grinding wheel
(3) Grinding wheel dressing compensation control. Grinding wheel dressing is due to the needs of the processing process, and the grinding wheel's cross-section trimmed into the required shape, commonly used grinding wheel bearing surface shape, there are two straight lines, a straight line, two arcs, three arcs, as shown in Figure 5 .
Figure 5 wheel bearing surface shape
2.2 System Function Design
(1) Product processing function: The feed axis is used to control the feed of the product, and the rotary axis achieves the synchronous rotation with the feed axis through the linear interpolation function to produce fixed pitch and lead angle products, and the grinding wheel feed axis Through the electronic gear function, the fixed ratio of the feed shaft is increased.
(2) Trimming wheel function: In order to improve the product quality, different users will trim the grinding wheel into different cross-section shapes. This function achieves this function through linear or circular interpolation of the Y-axis and Z-axis.
2.3 System Concept Design
According to the degree of automation can be divided into three kinds of mechanical and electrical combination automation program.
(1) Pure hydraulic control type: PLC only provides logic control, and the production of the product and dressing of the grinding wheel are entirely performed by hydraulic and mechanical.
(2) Ordinary PLC+hydraulic control: PLC controls a rotary axis (A axis) to realize the indexing function of the workpiece. The functions such as feed and grinding of the workpiece are completed by hydraulic and mechanical.
(3) NC (CNC Machining Center) system: For example, Mitsubishi's NC E60 system controls three axes (X+Y+A) to realize the production of the product, which mainly realizes the workpiece feeding and workpiece indexing function, and the grinding wheel Trimming is entirely accomplished by hydraulic systems and is one of the most automated solutions.
3 Emerson CT EC20H Solution
3.1 Principle Design
This project applies the Emerson CT EC20H PLC for high-speed motion control to realize the full-axis servo control of X+Y+Z+A axis, achieving higher functionality than the CNC machining center. The project added grinding wheel dressing and wheel wear compensation, which is also the most difficult part of the project. The system principle based on Emerson CT EC20H PLC solution is shown in Figure 6.
Figure 6 Emerson CT PLC All-Axis Servo System Block Diagram
3.2 Electronic Control System Design
Electronic control system configuration shown in Figure 7: Touch screen HMI for parameter settings, device status display and other functions. The servo is used to control the feed axis, rotary axis, grinding wheel feed axis and grinding head axis. Permanent magnet brushless motors are used to control the rotation of the grinding head. Ordinary motors are used to control the rotation of the grinding wheel.
Figure 7 Electronic Control System
3.3EC20H servo control design
(1) Individual control of each axis: Realize the manual control function of each axis of X, Y, Z, and A, allowing the user to individually control the operation of each axis. It is mainly used for tool setting, manual debugging and other functions. Use DRVI instruction to realize this function.
(2) Interpolation functions: X-axis and A-axis: linear interpolation, used to achieve product pitch and lead angle; pitch and lead angle are two important technical parameters of processed products, through the X-axis and A-axis Linear interpolation achieves this function. The X axis advances at a certain speed. The A axis rotates the workpiece through the turbine vortex rod and rotates at a certain angular velocity. This allows the workpiece to be operated at the set pitch and lead angle.
This function is implemented by the LIN instruction.
(3) X-axis and Y-axis: Electronic gears are used to reduce the depth of the grinding groove. In the grinding process, the depth of the grinding groove is linearly decremented with the length of the feeding material; this function is realized through the GEARBOX command.
(4) Y-axis and Z-axis: linear and circular interpolation, according to the user's requirements, can achieve a different cross-sectional shape
The grinding wheel; first import the section shape by AUTOCAD and then replace the corresponding coordinates with the D element so that the user can set it through the touch screen. The LIN and CCW instructions are mainly used.
(5) OPR: When the device is initially powered on, a round-point regression is performed to achieve the same initial state when the device starts to operate. At the same time, all coordinates are zeroed. Use the DSZR instruction to implement this function.
At present, the main functions of the project have been realized. Compared with the CNC machining center, the dressing function of the grinding wheel carrier has been increased, and the outstanding technical features of the EC20H operation control PLC have been demonstrated. For applications that require multi-axis interpolation, electromechanical integration products based on Emerson CT PLCs will exhibit strong technical advantages.
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