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Interference inspection and tool position correction in NC machining of integral impeller Abstract: in order to independently develop cad/cam software for NC machining of impeller, an algorithm of interference inspection and tool position correction is proposed. By analyzing the relative position relationship between the cutter and the impeller surface, the interference is divided into three types. The corresponding inspection and correction methods are proposed for different interference types. Finally, a simulation example is given to verify the effectiveness of the algorithm

1 introduction

impeller is the key component of turbine machinery, and its NC machining experimental machine has always been the focus of research. At present, there is no mature and adaptable cad/cam system in China. Therefore, most manufacturers of impellers still rely on imported software to complete their multi coordinate NC machining. Imported software is not only expensive and costs a lot of foreign exchange, but also the software is a closed operation module. It is difficult to obtain the technical data of its difficulties and some key problems. Therefore, it cannot be modified and developed according to the actual situation, which seriously hinders the development of new products

in recent years, with the rapid development of machinery, aerospace and aviation industry, the requirements for improving the machining quality and accuracy of impeller parts are becoming more and more urgent. It is imperative to develop independent cad/cam software. For this reason, many researchers have carried out this research work, but little information has been found on the key problem of interference inspection in logarithmic control machining. The doctor's thesis geometric design and NC machining of impeller and blade parts by Wuming, an engineering student of Beijing University of Aeronautics and Astronautics, adopts the method of discrete triangle to carry out interference analysis. The amount of calculation is large, and the research is not comprehensive. The interference between adjacent blades and cutters is not analyzed. In the NC machining process of integral impeller, the working space of the cutter is strictly limited by the impeller structure, and the interference phenomenon is much more complex than machining free-form surface. According to the characteristics of the integral impeller and based on the analysis of interference types, a set of interference inspection method suitable for multi coordinate NC machining is proposed in this paper

2 interference type

integral impeller means that the hub and blade are on a blank substrate. When machining the impeller blade surface, in addition to the interference between the tool and the blade to be machined, the tool is very easy to interfere with the adjacent blade due to the small space between the adjacent blades. In practical machining, the blade surface is given in the form of coordinate points, and the surface needs to be discretized when calculating the tool position. Because the ball end taper milling cutter is generally used, the surface can be discretized into a row of points. The information of each point will be given in the form of structure, including the sequence number of the arrangement, the coordinates of the points, the unit surface normal vector of the point, and the points will be given in the form of a single Necklace table

the interference discussed in this paper refers to the gnawing interference, that is, the part that should be reserved when the tool cuts into the surface. In machining blades with milling cutters, the orientation of the cutter on the cutting plane of the surface at the contact point is taken as the initial position of the cutter. As shown in Figure 1, the milling cutter processes the integral impeller. From the perspective of the relative position relationship between the cutter and each blade surface, the interference can be divided into three types: the first is the interference between the milling cutter and its own blade tip; the second is the interference between the milling cutter and the adjacent blade tip; the third is the interference between the milling cutter and the adjacent points around the machined area. If these three types of interference checks are passed, it is not necessary to verify the points of the entire blade surface, thus greatly reducing the amount of calculation

1. Hub 2 Blade 3 Tool

Fig. 1 overall impeller processing

3 interference inspection and tool position adjustment

1) interference between the tool and its own blade tip

the impeller surface can be expressed by s (U, V) ={x (U, V), y (U, V), Z (U, V)} after being discretized by NURBS method, u ∈ (0,1), V ∈ (0,1). Therefore, this kind of interference only needs to judge the distance E between each point on the parameter line of v=0 corresponding to the blade top of the blade surface and the tool axis after 2020, as shown in Figure 2. If the distance is greater than the effective radius of the tool, no interference occurs: if the distance is less than the effective radius of the tool, interference occurs. The so-called effective tool radius refers to the tool radius relative to the blade tip. Distance E is

, where a (x0, Y0, Z0) is the coordinate of the ball center point of the tool; B (x1, Y1, z1) is the coordinate of the blade tip parameter point: (ax, ay, AZ) is the three coordinate components of the unit tool axis vector a

the effective radius of the tool is

r0= r0+ |ab|cosqtanb (2)

where: R0 is the radius of the tool ball head: B is the half cone angle of the tool: cosq=ab a/|ab|

2) interference between the tool and the tip of adjacent blades

the judgment of this type of interference is similar to that of the first interference. The parameter curve of adjacent blade tip can be obtained by rotating the blade tip parameter point of the machined blade clockwise or counterclockwise α ( α= 360m, M is the number of blades)

3) interference between the tool and the adjacent points around the machined area

theoretically, this type of interference should detect the existence of the intersection point between the conical surface where the tool is located and the surface in the area covered by the tool. However, this calculation is more complex than green packaging, which is the general trend of the development of the world packaging industry, and the amount of calculation is also large. For simplicity, the following algorithm is used

as shown in Figure 2, make the cutting plane of the tool so that the intersection angle between the cutting plane and the tool axis vector is the half cone angle of the tool, and obtain two cutting planes F1 and F2. Select one plane F1 that contacts the blade surface, calculate the directional distance d between this plane and the discrete point covered by the tool, and judge the symbol of D to judge whether the tool interferes with the blade surface

Fig. 2 the unit normal vector N1 (a, B, c) of plane F1 made by the third interference detection

can be obtained by winding the cutter axis vector a around (a × n) Rotation is not difficult to see (90 °)- β) The angle is obtained, that is,

n1= cos (90 ° - b) a+ sin (90 ° - b) n

then the equation of plane F1 is ax+ by+ cz= 1 (4)

the directional distance d from any point (XS, ys, ZS) on the blade surface to the tangent plane can be obtained from equation (5)

d= axs+ bys+ czs-1

the symbol of D can be obtained through equation (5). If D has the same sign and is greater than zero, there is no interference; if D has different sign, there is interference. Ds+ bys+ czs- 1

to eliminate interference, the tool should rotate Q angle around the tool ball center point in the initial cutting plane to make the tool axis deviate more from the machined blade surface. Q angle is

q= arcsin (max|d|/d1)

where D1 is the distance between the maximum interference point and the tool ball center point

the adjusted tool axis vector is

anew=acosq ± nsinq

in the above formula, if Q angle is around (a × n) Clockwise rotation takes "+" sign, and counterclockwise rotation takes "-" sign

the adjustment method of the first type of interference is the same as that of the third type of interference, and the cutter axis should be more deviated from the surface. For the second kind of interference, the cutter axis should be close to the surface. After the cutter axis angle is adjusted, recheck the interference until no interference occurs

the interference inspection method proposed above has been used in the "special impeller cad/cam software" system developed by us. Figure 4 shows two simulation examples. The tool position in FIG. 4A is calculated by the tool position, and the dark color represents the interference region. Fig. 4B shows the tool position after interference is eliminated by the above algorithm

Figure 4 interference simulation example

author unit: Harbin Institute of Technology (end)

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