Numerical control machining of arc cam with large center distance
Globoidal cams can only be processed through two rotating coordinates and cannot be converted into rectangular coordinates due to their non-expandable space characteristics. Whether it is a special Cnc Machine tool or a general-purpose multi-coordinate CNC machine tool for machining cambered cams, the two rotating axes should not only meet the motion requirements of meshing transmission, but also ensure that their center distance remains unchanged. For a specific machine tool, it can be The center distance parameter range of machining cams is generally limited. When the center distance parameter exceeds the adjustable range of the machine tool, it cannot be processed.
Most of the special CNC machine tools for machining arc cams have two rotation coordinates, and the distance between the two rotation centers is adjustable to meet the requirements of cam processing with different center distances. This special machine tool has a simple structure, good rigidity, and cost. Low, but the processing range is limited and can only be processed by Fan Cheng method. The general five-axis CNC machine tool is restricted by the machine tool motion structure and tooling. The center distance of the cambered cam is guaranteed by the algorithm when machining the camber ; that is, two linear coordinates are required for the rotary motion required by the camber cam. Position compensation to ensure that the tool axis always passes through the theoretical center of rotation and coincides with the center of swing. Therefore, its processing range is almost unlimited. It can be processed by normal method or by knife method. However, the cost of programming and calculation of complex equipment is high. The special five-axis CNC machine tool combines the advantages of the above two types of machine tools in the structural design, so in terms of function and performance The above has been greatly improved. This article will take the machine tool as the object to discuss the machining method and related problems of the arc cam with large center distance. 2 The machine tool structure and coordinate system is a schematic diagram of the structure and coordinate system of a five-axis CNC machine tool for cams.
The machine tool adopts a horizontal structure, in which the XY and Z axes of the linear axis move in the same way as a general horizontal CNC milling machine. The strokes are respectively: 850,700 and 700mm. The B axis is a rotary table structure and is located on a worktable composed of XY axes. It adopts double-lead worm gear drive and large diameter, high-strength rolling bearing and high-precision grating encoder. The movement stroke is ±65°. The A-axis is located on the B-axis turntable, and two rotary indexing heads with different center heights of 250mm and 160mm are used. . The adjustment mechanism and digital display device called W-axis are installed between A-axis and B-axis, which are used to adjust the distance between the two axes. The adjustable range is: 40~280mm. Due to the machine tool structure and tooling design, the center of the workpiece and the B-axis are guaranteed. The center passes through the spindle axis, so the Z-axis feed is only used to control the groove depth of the machining cam and has nothing to do with other parameters.), male, native of Zigong, Sichuan, associate professor, postdoctoral fellow at Northwestern Polytechnical University; thus making the operation and programming easier. The disadvantage of this structure is that when the cam center distance is large, the overhang of the main shaft is longer, which reduces the rigidity of the main shaft. It is the simple coordinate transformation when machining by the knife method. These characteristics are very beneficial to reduce the design and manufacturing cost, improve the processing efficiency and accuracy, and quickly change the tool. 3 Coordinate transformation and programming are shown in the figure. The cnc maching method of tool swing is used for research. O is the actual swing center of the tool, and O is the theoretical swing center of the driven disk. And is the actual machining radius of the tool, R is the theoretical radius of gyration. When the tool swings over the B angle, the corresponding machining position is point b, and the actual tool position is point a. Comparing the two points ab, we can see that the distances in the X and Z directions are x and z respectively, that is, when the tool is compensated according to the work and z values, a cam with a small center distance can be machined with a large center distance.
It can be derived from the geometric relationship in the figure: it can be seen that the compensation value is only related to the cam center distance parameter G machine center distance C and swing angle B, and has nothing to do with the cam rotation angle A. In practical applications, CC is a known quantity, and the tool swing angle B is determined by the law of motion B=B(A), and it is also a known quantity during processing. Therefore, through the four-axis linkage control of two rotating coordinates and two linear coordinates, it is theoretically possible to realize the machining of cams with any center distance. Not only can the cams be machined with a small machine center distance, but also a small center can be machined with a large machine center distance. The distance cam uses the above algorithm as a special module and puts it into the cam automatic programming software kDoctorCAM1.0 cam automatic programming system developed by the author to automatically realize the conversion of coordinates. This conversion does not affect the existing tool support in the system. Algorithm, eccentric algorithm, feed speed correction, cam surface modification and calculation error control functions 4 application examples The existing globular cam model to be processed is: SH350. 8, its parameters are: center distance 350mm, division number 8, dynamic and static ratio 1351 /225°, I type, left-handed, groove depth 48mm, roller diameter 90mm, volute radius 197mm, cam width 250mm. Since the cam center distance is greater than the maximum adjustable range of the machine tool, it cannot be processed directly.
When the center distance of the machine tool is greater than 200mm, the length from the tool to the end face of the king shaft will be greater than 300mm, the overhang length of the king axis will be greater than 500mm, and the rigidity of the machine tool spindle will be significantly worse, which is very unfavorable for processing.
Considering the rigidity of the spindle, adjust the center distance of the machine tool to C’350mm at 200mm under the premise of no motion interference. The compensation value x and z at any time can be calculated according to the formula. The actual cutting results show that the method can be used for Expanding the original processing range of the machine tool can also be used to adjust the machine tool parameters during processing; as the spindle overhang is shortened, the rigidity of the machine tool is improved, and the processing accuracy and processing efficiency have been significantly improved (page 70), which ensures more accuracy The realization of the fermentation process provides a basis for improving the process; saving investment: the actuator used in the human-like intelligent control method is a pneumatic butterfly valve, the price is about 2000 yuan; while the PID control uses a pneumatic control valve, the price is about 6000~10000 yuan, Calculated based on the three-point control of 60 large tanks, the investment can be saved from 12 to 1.44 million yuan. The application of this system frees workers from heavy manual labor. At the same time, it reduces the wine loss (120,000 yuan per year) caused by manual temperature measurement, thereby improving economic benefits.
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