Some problems on optimal design of NC machining to

2022-08-01
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Several problems about optimizing the design of NC machining tool path

for a part with high complexity, high precision requirements and the need to be machined by NC machine tools, how to turn the design drawing generated by CAD into an actual physical product through cam is the technical goal of cad/cam. One of the important technical links is how to design the motion path of NC tool according to the CAD design drawing and the relevant parameters of specific machining tools. The quality of tool path design will directly affect the machining quality and cost of parts. This paper studies and solves several optimization problems in tool path design, and has been well applied in NC tool path design of cad/cam system developed and designed by ourselves

1 error source in NC tool path design

before the computer automatically compiles the NC program, the corresponding tool path must be generated according to the contour of the part. The size of the tool is different, and the tool path generated by the design is also different. Therefore, the error will be caused by the size of the tool and the design algorithm of the tool path

1. The influence of tool size

is mainly caused by taking the nominal size of the tool as the design parameter of the tool path. In the calculation of tool path design, the size of the tool should be the actual size of the tool, so that the error caused by the wrong size of the tool can be avoided

2. The influence of tool path design algorithm

is mainly caused by the careless consideration of the algorithm designer:

a. when the tool cuts along the normal direction of the part contour, the tool mark error caused by the motion inertia of the machine tool

b. when machining the contour surrounding surface of a part, in order to make the machining continuous and avoid the overlap of intersections at the starting point of contour machining, which makes the computer unable to determine the next step in NC programming, the method of separating the starting point and the end point of the machining contour is often adopted. However, when the method of simply breaking the contour surface curve is adopted, the contour machining error will occur

c. for the machining at the sharp corners of the external corners of the sharp corners of the part contour, in order to keep the sharp corners well, if the sharp corners of the contour are strictly designed, the resulting tool path will consume a lot of time for meaningless movement, and it is likely to interfere with other parts of the parts, resulting in the scrapping of the machined parts

d. when using compound path machining, how to design a more optimized tool path, which not only ensures the machining quality, but also makes the machining path shortest, is a problem worth studying

Several optimization problems in tool path design

1 Solution to tool mark error

the solution to reduce the tool mark error caused by different cutting directions is to try to avoid cutting along the normal direction of the part contour and try to cut along the tangential direction of the part contour. For some parts with special machining starting point requirements, blindly pursuing tangential cutting may cause interference. Therefore, in the design of the entry point, we adopt the method of treating separately. For cylinder, the function of automatic optimization as tangential cutting is set; For other contours, with the help of computer, the optimization entry point is set by means of man-machine dialogue. This not only avoids new problems in pursuit of a certain goal, but also gives full play to the respective advantages of computers and people

2. Machining of parts

when machining the contour surrounding surface of parts, it is necessary to ensure that all contours are machined completely, and avoid overlapping intersections in the machining process, so that the machining can be carried out continuously. In order to achieve this goal, we have done a positive gas treatment on the contour of the part: first cut off the contour curve, and then lengthen the contour curve according to the design of the original contour curve, so that the tool path passes through the starting point, and then advance about 0.5mm along the contour curve to reach the end point. In this way, the designed end point can be smoothly connected with the next machining path of the tool, and the contour surface can also be processed completely and smoothly

3. Automatic optimization design at sharp corners of acute corners

a. analysis at sharp corners of external corners

situation at sharp corners

as shown in the figure, the distance DS from the center of the tool path to the actual contour (sharp corners) of the part is:

ds=ro ÷ sina/2

where ro is the tool radius and a is the sharp angle

the relationship between distance and sharp angle is shown in Table 1. It can be seen that when the sharp angle is less than 60 ° (ds=2ro), the increase of DS distance begins to accelerate. At this time, the technology of optimizing tool path will have a good effect

b. introduction to the algorithm

step 1: find out the connection point pT1 of "line to line" in the tool path

in the second step, measure the distance DS between the contact pT1 and the "line to line" contact PS1 on the part contour. When ds=ro (tool radius), return to the first step and continue to look down until the entire machining tool path is found, and then go to the seventh step

step 3: when ds>ro, check the angle pt23a>60 °, and return to step 1

step 4: make a circle with pT1 as the center and ds-ro as the radius to truncate and delete the line segments in the circle

step 5: connect the two breakpoints with a straight line to form an optimized tool path

step 6: check whether the whole machining tool path has been found. If not, return to step 1

step 7: stop working

4. The optimization design of compound tool path

the so-called compound tool path refers to that when machining the contour and the surrounding surface (there can be protected objects in the surrounding surface, such as bosses, etc.), in order to ensure the contour quality and complete machining in the contour surface, the FRP/elastic element fatigue life testing machine first processes the contour, and then processes the surface surrounded by the contour according to the parallel path. In order to design the optimal path with the shortest distance, we first discrete each simulation quantity: for example, 1 cup number tool path, and then use the optimization algorithm to connect and design the optimal tool path. The optimization algorithm is:

1 Starting from the starting point of the connection, check the discrete tracks that need to be connected, and find out that the most limited oxygen index from the starting point reaches ~ 32.8% (Fig. 1); At the same time, this kind of epoxy resin is near the end of the trajectory in the combustion experiment

2. Connect the starting point and the nearest end point of the track with a straight line, and set the other end point of the track as the new starting point

3 Judge whether there are discrete tracks to be connected. If yes, return to a to continue. Otherwise, end the optimization design

3 conclusion

by analyzing the error in the design of NC machining tool path, the optimization of NC tool path is discussed and studied. The method to reduce and eliminate the design error is found out, the algorithm and application program to optimize the tool machining path are established and developed, and embedded in the developed cad/cam system as a strategic supplier providing innovative material solutions, which provides convenience for the automatic generation of tool machining path. It has a good reference value for the realization of cad/cam technology. (end)

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