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Research on tool compensation algorithm for multi axis Machining NC program

tool wear compensation is an important part of numerical control machining. In the past, when 2D plane contour milling is carried out, the G code (g41/g42) provided by the controller can be used to calculate the compensated machining path by the controller for workpiece cutting, which is the basic function of the low-order controller. However, the tool compensation function provided by the high-order controller should be used to compensate the three-dimensional space. The purpose of this study is to explore the mathematical model of tool compensation vector path in three-dimensional space, and develop it into a numerical control program post-processing system for multi axis machining. The correctness of the tool compensation algorithm proposed in this paper is verified by the solid simulation cutting software

key words: numerical value even if both sides are color steel plate control of non combustible grade a steel plate, post-processing, tool compensation

i, preface

the geometric shape design of modern aviation parts and molds is becoming more and more complex, and the requirements for product accuracy and production efficiency are also constantly improving. The five axis tool machine can provide two additional degrees of freedom of rotation, making the tool feed at different angles, providing important advantages in manufacturing. Therefore, It has been widely used in industry

the five axis machining program is closely related to the tool setting. Once the tool diameter design size or tool length changes, the original program cannot be used. In particular, the tool will be worn during the machining process, resulting in size variation. Therefore, the use of the program is greatly limited. In terms of numerical control machining, whenever there is a change in the geometric shape of the tool, the tool path file should be generated again, and then converted to the actual NC program through the post-processing program. The tool wear caused by machining can also be regarded as the tool geometry change (the tool diameter becomes smaller), and the correct machining program can be obtained through the above methods. However, this will make it difficult to maintain the program, and the relative production efficiency cannot be improved. If the replacement of new tools is considered, the cost of spare tools will be increased

for the machining of two and a half axes of plane contour, g41/g42 code is often used to correct the tool radius, which can be completed by using the lower order controller. However, if it is to be applied to the function of tool compensation in three-dimensional space, a higher-order controller must provide vector mode compensation, such as FANUC 15-ma (FANUC, 1994), Cincinnati micron acramatic 950 (Cincinnati, 1990), etc, However, the known studies have not been further published on this function. On the other hand, for the general-purpose post-processing sold on the market, such as intellipost (intellipost, 1994), only the two-dimensional tool vector compensation format is provided, but the three-dimensional tool vector compensation mode is not provided. At present, only the original manufacturers selling five axis tool machines, such as Cincinnati, Shin Nippon Koki, SNK, etc., have special post-processing programs for their individual five axis tool machines, Only in this way can the mode output of multi axis three-dimensional tool vector compensation be provided, but its price is very expensive. Therefore, it is necessary to make further discussion and breakthrough in this field

ii. Research methods

2.1 research process

when the tool size is changed, if you want to get the correct machining surface, the tool center must be offset appropriately. For example, if the tool size is small, it must be offset inward. Otherwise, if the tool size is large, it should be offset outward. The offset is compensated in vector mode by subtracting the difference between the actual tool size and the correct tool size. This compensation mode, All high-order controllers are provided. FANUC controller is represented by ijk code, while Cincinnati controller is represented by PR code

before three-dimensional tool vector compensation, the mathematical model of two-dimensional tool vector compensation must be established. The author of this paper has published relevant research (she Zhenhua, chenzhengtang, 2000), but this is only applicable to the case of tool axial fixation. For multi axis machining, because the tool axis changes in each moving single section, it is necessary to derive a generalized three-dimensional tool compensation mode. In this paper, the projection method is used to project three points in three-dimensional space along the normal vector of the middle point to a plane perpendicular to the normal vector, that is, expressed in the coordinates represented by the plane, and then the tool path offset method (offset) is used to obtain the tool compensation vector in two-dimensional space, and then the inverse operation is performed to make the coordinates of the three points expressed in the surface coordinates, In this way, the final three-dimensional tool compensation vector can be obtained

secondly, this paper will develop a post-processing system, which can read the cutter location file (CLF) generated by general-purpose cad/cam and convert it into an NC program with tool compensation vector. Finally, the correctness of the converted NC program is verified by the solid simulation cutting software (VERICUT, 2001). The research flow of this paper is shown in Figure 1

Figure 1 research flow chart

2.2 mathematical derivation of multi axis tool error compensation vector calculus

we first select three points in the tool path file as P1P2P3, then define a plane with the point coordinates and their normal vectors, and then project the normal vectors along the three points of P1P2P3 to the plane as three points of p'1p'2p'3, as shown in Figure 2. Then, according to the lower left or right instructions in the cutcom instructions in the tool path file, Offset one unit to the left or right, As shown in Fig. 2 and Fig. 3, the offset is followed by the three-point coordinates of P "1p" 2p "3.

the three-point projection of the tool path in Fig. 2 to the first plane schematic diagram

the three-point projection offset of the tool path in Fig. 3 (offset) Schematic diagram

in the tool path, it is assumed that the normal vectors of the second point are ax, ay and AZ, and the two vectors that are perpendicular to each other are NX, NY, NZ and ox, oy and oz. these three vectors constitute a coordinate system, and the plane defined by the coordinate system must pass through point P2. The distance between the coordinate system and the ground coordinate system is Px, py and PZ

among them, three mutually perpendicular vectors can be expressed as the homogeneous coordinate transformation matrix t of the coordinate system formed by the three vectors and the base coordinates can be expressed as formula (4)

according to the expression of vector scalar product (or dot product), see (5), (6) and (7)

according to the vector orthogonality rule, if the included angle of two vectors is 90 °, that is, the dot product of two vectors is zero, that is, cos θ= 0 is substituted into equations (5) and (6) and (7), and

can be obtained. Therefore, it can be obtained that:

if nx=az is substituted into equation (8), it is known that

according to the characteristics of the unit vector, the unit vector is a free vector with a size equal to 1 unit, so

according to equations (11) and (12), it can be known that

is substituted into equation (11), and it is known that (16)

on the other hand, According to the vector outer product, we can know that

after that, this paper verifies the rule that the three vectors' dot products are zero through the vector inner product, and we know that the dot products of the three vectors are zero and are indeed perpendicular to each other. Therefore, this paper assumes that nx=az can be established

define that the three points selected in the tool path file are P1, P2 and P3, and their coordinate values are respectively expressed as (21)

(22)

(23)

according to formula (22), this paper defines

where x1, Y1, Z1, X2, Y2, Z2, x3, Y3 and Z3 represent the X, y and Z axis components of the coordinates of P1, P2 and P3 in the base coordinate system. The coordinates of P1, P2 and P3 are projected onto a plane composed of the coordinate values Px, py and PZ of P2 and their normal vectors ax, ay and AZ, and two vectors that are perpendicular to each other, NX, NY, NZ and ox, oy and oz

after the three points P1, P2 and P3 are projected onto the plane, they are p'1, p'2 and p'3, and the Z values of these three points are the same because they are projected on the same plane. Then, according to the left or right under the cutcom command in the tool path file, it is assumed to offset one unit to the right. The three points can be substituted into the equations (30) and (31) to obtain the X and The values of Y are as follows (she Zhenhua and Chen Zhengtang, 2000):

therefore, the coordinates of P "1, P" 2 and P "3 are respectively

p" 1, P "2 and P" 3. The back projection of the three points p'"1 and p'" 2 can be obtained by projecting the normal vector of the second point to the original base coordinate system P "'3 coordinate value:

iii. results and discussion

3.1 post processing program conversion

according to the derived compensation vector, this paper develops a set of post-processing conversion program, which can automatically generate the required five axis numerical control program. In the program interpretation tool path file, the data in goto is three-axis or five axis, if it is three-axis, the converted tool compensation code is only P and O code, otherwise if it is five axis, the converted tool compensation code is only P and O code The compensation code has a POR code

the system sets three commonly used tool path files, namely, the NCL program source code and the tool path format generated by two commercial cad/cam systems, SURFCAM and CATIA, to convert them into NC programs with por, as shown in Table 1 to table 3. The NCL format uses $as the annotation in the tool path file. In the tool path file of SURFCAM, "$" is used to indicate that the next line is the same line of instructions as this line, In the tool path file of CATIA, "PT" is used to indicate the line number of the next line. Table 1 tool path file in NCL format

tool path file format generated by SURFCAM in Table 2

tool path file format generated by CATIA in Table 3

basically, the above three tool path file formats are different. Therefore, in order to make the conversion more flexible and efficient, this paper develops a universal post-processing program to convert the tool path files of these three formats. The results are shown in Table 4 to table 6, The results in the table show that the converted multi axis NC program contains the required tool path compensation vector value (POR). Table 4 NC program after post-processing and conversion of NCL file in this document table 5 NC program after post-processing and conversion of SURFCAM tool path file in this document table 6 NC program after post-processing and conversion of CATIA tool path file in this document

3.2 virtual tool machine solid simulation cutting verification

in order to verify the NC program code converted in this document, we have designed a processing workpiece, as shown in Figure 4, The tool path file required for machining is generated by the commercial software Edgecam (Table 7), and then converted into the NC program with tool compensation code of por (Table 8) through the post-processing program developed in this paper. Secondly, we input the converted NC program into VERICUT, a commercial solid cutting simulation software, for machining simulation. The simulation results are shown in Figure 5. The size of the workpiece after cutting can be used to carry out the solid section through VERICUT's section function, and then converted into IGES files with the export model function. After reading it through Edgecam, the size of the workpiece can be detected to be 79.988826mm (Figure 6), This can be compared with the relevant size of 80mm in Figure 4. At the same time, it can also confirm the correctness of the tool compensation vector algorithm established in this paper

Figure 4 workpiece outline dimension figure 5 VERICUT simulation cutting Figure 6 measured dimension after workpiece profile Figure 7 tool path file generated by Edgecam Table 8 NC program converted from post-processing program developed in this paper

iv conclusion

this paper uses the principle of tool path offset to establish an analytical model of tool compensation vector for multi axis machining, and develops it into a post-processing system, Tool path files generated by universal cad/cam software can be

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