CHINESE JOURNAL OF MECHANICAL ENGINEERING Vol. 26, No. 3, 2013 ·547· DOI: 10.3901/CJME.2013.03.547, available online at www.springerlink.com; www.cjmenet.com; www.cjmenet.com.cn Three-dimensional Tool Radius Compensation for Multi-axis Peripheral Milling CHEN Youdong* and WANG Tianmiao School of Mechanical Engineering and Automation, Beihang University, Beijing 100191, China Received October 26, 2012; revised January 24, 2013; accepted March 5, 2013 Abstract: Few function about 3D tool radius compensation is applied to generating executable motion control commands in the existing computer numerical control (CNC) systems. Once the tool radius is changed, especially in the case of tool size changing with tool wear in machining, a new NC program has to be recreated. A generic 3D tool radius compensation method for multi-axis peripheral milling in CNC systems is presented. The offset path is calculated by offsetting the tool path along the direction of the offset vector with a given distance. The offset vector is perpendicular to both the tangent vector of the tool path and the orientation vector of the tool axis relative to the workpiece. The orientation vector equations of the tool axis relative to the workpiece are obtained through homogeneous coordinate transformation matrix and forward kinematics of generalized kinematics model of multi-axis machine tools. To avoid cutting into the corner formed by the two adjacent tool paths, the coordinates of offset path at the intersection point have been calculated according to the transition type that is determined by the angle between the two tool path tangent vectors at the corner. Through the verification by the solid cutting simulation software VERICUT with different tool radiuses on a table-tilting type five-axis machine tool, and by the real machining experiment of machining a soup spoon on a five-axis machine tool with the developed CNC system, the effectiveness of the proposed 3D tool radius compensation method is confirmed. The proposed compensation method can be suitable for all kinds of three- to five-axis machine tools as a general form. Key words: tool compensation, multi-axis machine tool, offset vector 1 Introduction ∗ Multi-axis peripheral milling is widely used for the manufacture of profiled components in aerospace, automotive and mould/die industries. Functions about 3D tool radius compensation are widely used in CAD/CAM system, but still few is applied to generating executable motion control commands in CNC systems. Once the tool radius is changed, especially in the case of tool size changing with tool wear in machining, a new NC program has to be recreated. HU, et al[1], applied a planar geometry projection method in tool radius compensation for three-axis NC peripheral milling, which is similar to standard 2D tool radius compensation. YANG[2] used space transition to get the cutter position in three dimension space, but it is just used for five-axis spindle-tilting machine tools. WANG[3] presented a movement projection method for 3D tool radius compensation, which is actually similar to the planar geometry projection method. MORETON[4] realized 3D tool compensation by calculating the required position of * Corresponding author. E-mail: chenyd@buaa.edu.cn This project is supported by National Major S&T Program of China (Grant No. 2010zx04008-041), and National Hi-tech Research and Development Program of China (863 Program, Grant No. 2011AA04A104) © Chinese Mechanical Engineering Society and Springer-Verlag Berlin Heidelberg 2013 the tool control point relative to the surface of the workpiece, but it is only used in 3-axis turning. To obtain 3D tool compensation method for five-axis milling in machine controllers, TUNG, et al[5], defined a relationship between the workpiece and the cutter to attain the aim of three-dimensional tool compensation, but it would probably lead to cutting into the corner formed by the two adjacent tool paths in peripheral milling. HONG, et al[6], detailed the algorithm of three-dimensional cutter radius compensation for 5-axis end milling, but it can’t be used for peripaheral milling. CHEN, et al[7], presented a 3D cutter radius compensation method for five-axis peripheral milling, but it is just used for five-axis peripheral milling. Some commercially available CNC systems, such as Sinumerik 840D and Heidenhain iTNC, have provided the feature of a 3D tool compensation method for five-axis peripheral milling. However, the related algorithm is unpublished. Most approaches used in 3D tool radius compensation can only be applicable to specific machine tool configuration. A 3D tool radius compensation algorithm presented here can be suitable for all kinds of three- to five-axis peripheral milling in CNC controllers. The proposed 3D tool radius compensation method can enhance the facility and flexibility of CNC controllers. The offset path can be calculated by offsetting the tool path with a given distance along the direction of the vector that is ·548· CHEN Youdong, et al: Three-dimensional Tool Radius Compensation for Multi-axis Peripheral Milling perpendicular to both the tangent vector of the tool path and the orientation vector of the tool axis relative to the workpiece. To avoid cutting into the corner formed by the two adjacent tool paths, the coordinates of offset path at the intersection point have been calculated according to the transition type that is determined by the angle between two tool path tangent vectors at the corner. 2 Calculation of the Orientation Vector of the Tool Axis Relative to the Workpiece for Generalized Multi-axis Machine Tools 2.1 Generalized kinematics model of multi-axis machine tools Three- to five-axis milling machines have many types of structural configuration. Four-axis machine centers can be classified into two categories: table-tilting type with one rotation on the table, and spindle-tilting type with one rotation on the spindle. KIRIDENA, et al[8], showed the structural configuration of five-axis machine centers and classified them into three categories: table-tilting type with two rotations on the table, spindle-tilting type with two rotations on the spindle, and table/spindle-tilting type with one rotation each on the table and spindle. Denavit-Hartenberg method[9] was adopted to manipulate the orientation and position of the tool relative to the workpiece. RÜEGG, et al[10], presented a generalized kinematic model for three- to five-axis milling machines. Recently, SHE, et al[11–13], proposed a generalized kinematics model of common five-axis machines constructed by combining two rotational degrees of freedom on the fixture table and two rotational degrees of freedom on the spindle. A general kinematic model for three- to five-axis milling was proposed that comprised 3 linear axes, and 4 rotational ones, of which zero, one or two are active for the computation[14]. This method is also applied to calculating the orientation vector of the tool axis relative to the workpiece of multi-axis machine tools. A notation based on Schewerd[15] is used to describe the serial kinematic structure. The general multi-axis machine tools are specified as follows: set of Cartesian vectors. They don’t specify the orientation vector of the tool axis relative to the workpiece. Actually, the orientation vector of the tool axis relative to the workpiece is T[0 0 1 0]. The tool orientation along the X-axis, Y-axis and Z-axis are feasible. The Z-axis is assumed below. Take Schwerd’s notation as a starting point, and the general multi-axis machine tools can be specified as a sequence of transformations of coordinates. To describe the relative position and orientation of the cutting tool with respect to the workpiece, the appropriate coordinate system should be established, as shown in Fig. 1. The coordinate systems for the workpiece and the cutting tool are OwXwYwZw and OTXTYTZT respectively. Coordinate systems OBX BY BZ B, OL1XL1YL1ZL1, OL2XL2YL2ZL2 and OL3XL3YL3Z L3 are attached to the machine base, and X, Y and Z table, respectively. The coordinate systems for the primary and secondary rotations attached to the fixture table are OR4XR4YR4ZR4 and OR3XR3YR3ZR3, respectively. The coordinate systems for the primary and secondary rotations attached to the spindle are OR2XR2YR2ZR2 and OR1XR1YR1ZR1, respectively. The relative orientation and position of the coordinate system OTXTYTZT with respect to the coordinate system OwXwYwZw can be expressed as follows: W AT W AR4 R4 AR3 R3 AL1 L1 AL2 L2 AL3 L3 AR2 R2 AR1 R1 AT AT , (1) where iAj represents the relative transformation matrix of system OjXjYjZj with respect to system OiXiYiZi. W-R4-R3-B -L1-L2-L3 -R2-R1-T, where W—Workpiece, T—Tool, L1, L2, L3—Linear axes, B—Machine base, R1, R2—Rotational axes on the spindle, R3, R4—Rotational axes on the fixture table. 2.2 Calculating the orientation vector of the tool axis relative to the workpiece Machining of workpieces requires the generation of tool paths defined by the motion of the tool with respect to the workpiece[16]. Three-axis tool paths can be represented by a Fig. 1. General multi-axis machine tool with coordinates The orientation vector of the coordinate system OTXTYTZT with respect to the coordinate system OwXwYwZw is just subject to the movement of rotational axes. Therefore, it can be expressed as W OT Rot(k4 , ΦR 4 )Rot(k3 , ΦR3 ) Rot(k2 , ΦR 2 )Rot(k1 , ΦR1 ) AT , (2) where Rot is a 44 homogeneous rotation matrices adopted from Paul’s notation[17], k4, k3, k2, k1 x, y, z. ΦR 4 , ΦR3 , CHINESE JOURNAL OF MECHANICAL ENGINEERING ΦR 2 , and ΦR1 represent the rotation angles for the fixture table and the spindle, respectively. Vector T is the orientation vector of the tool axis relative to the workpiece. If the kinematics model applied to threeaxis machine, and ΦR 4 ΦR3 ΦR2 ΦR1 0, the vector T is expressed as T T 0 0 1 0 . (3) According to the rotational movement characteristics, the four-axis machine tool can be designated as two types, i.e., A and B type. The tool orientation along the X-axis or Y-axis is feasible. Generally, the tool orientation along the Z-axis is used in machine tools. The vector T can be obtained as follows. The A types are chosen as an example. For the spindle-tilting A type, ΦR 4 ΦR3 ΦR1 0 and ΦR 2 x, and the right-hand side of Eq. (1) for vector T is ·549· vector T is T T Rot(k3 , ΦR3 )Rot(k2 , ΦR 2 ) 0 0 1 0 SΦy 3 SΦx CΦy CΦx CΦy T 0 . Algorithm of 3D Tool Radius Compensation 3.1 Basic compensation principle The 3D tool radius compensation method for peripheral milling is illustrated as Fig. 2. The offset vector with a length equal to the tool radius R is perpendicular to the tool path. The tail of the offset vector is on the workpiece side and the head positions on the tool center. The offset path l1 can be calculated by offsetting the tool path l with a given distance R along the direction n (nxinyjnzk) of the offset vector, as follows: l1lRn. T T Rot(k2 , ΦR 2 ) 0 0 1 0 T 0 SΦx CΦx 0 , (8) (9) (4) where “C” and “S” refer to cosine and sine functions, respectively. For the table-tilting A type, ΦR3 ΦR2 ΦR1 0 and ΦR4 x, and the right-hand side of Eq. (1) for vector T is T T Rot(k4 , ΦR 4 ) 0 0 1 0 T 0 SΦx CΦx 0 . (5) According to the rotational movement characteristics, the five-axis machine tool can be designated as six types, i.e., AB, AC, BA, BC, CA and CB type. The tool axis vector along the X-axis and Z-axis are feasible. Ordinarily, the tool orientation along the Z-axis is used in machine tools. The vector T can be obtained as follows. The AB types are chosen as an example. For the spindle-tilting AB type, ΦR 4 ΦR3 0, ΦR 2 x, and ΦR1 y, and the right-hand side of Eq. (1) for vector T is T T Rot(k2 , ΦR 2 )Rot(k1 , ΦR1 ) 0 0 1 0 S Φ y SΦx CΦy CΦx CΦy T 0 . (6) For table-tilting AB type, ΦR2 ΦR1 0, ΦR4 x, and ΦR3 y, and the right-hand side of Eq. (1) for vector T is T T Rot(k4 , ΦR 4 )Rot(k3 , ΦR3 ) 0 0 1 0 SΦy SΦx CΦy CΦx CΦy T 0 . (7) For table/spindle tilting AB type, ΦR4 ΦR1 0, ΦR 2 x, and ΦR3 y, and the right-hand side of Eq. (1) for Fig. 2. Three-dimensional cutter radius compensation for multi-axis peripheral milling With Eq. (9), the tool will cut into the corner formed by the two adjacent tool paths, if it doesn’t take the preventive measures at the corner point. To achieve the accurate workpiece shape, a 3D tool radius compensation method is proposed to make the tool move along corners. Tool radius compensation allows programming of the workpiece contour to be independent of the geometry of the tool. When an angle of intersection created by tool paths specified with movement commands for two blocks is over 180° as shown in Fig. 3(a), it is referred to as the “inner side”. When the angle is between 0° and 180° as shown in Fig. 3(b), it is referred to as the “outer side”. In space, the two straight lines may be coplanar or non-coplanar. When they are coplanar, Compensate C can be easily used for cutter radius compensation. The radius compensation for non-coplanar lines will be discussed here. When the tool path goes around the inner side corner or the outer side corner at an obtuse angle as shown in Fig. 4(a), the transition type is referred to as “Type A”. If the tool moves around the outer side corner at an acute angle, as shown in Fig. 4(b), the transition type is referred to as “Type B”. In Type A, a line that is the common CHEN Youdong, et al: Three-dimensional Tool Radius Compensation for Multi-axis Peripheral Milling ·550· perpendicular to the two paths is inserted since the two tool paths are skew lines which don’t intersect. In Type B, the inserted block is a straight line Q11Q22, as shown in Fig. 4(b). The point Q11 is the offset R along the line P1Q1 to the point Q1. The point Q22 is the offset R along the opposite direction of the line Q2R1 to the point Q2. The vector τ1 (τ1xiτ1y jτ1zk) is the tangent vector of the tool path PQ at the point Q. The vector τ2 (τ2xiτ2y jτ2zk) is the tangent vector of the tool path QR at the point Q. The coordinate system O1X1Y1Z1 is constructed from linearly independent vectors τ1 and τ2 by applying the Gram-Schmidt orthogonalization procedure. It is a Cartesian coordinate system whose vectors are the unit vectors e1, e2, and e3. β1 τ1 , e1 β2 τ 2 1 τ1 (τ 2 , β1 ) β1 , e2 i e3 e1 e2 e1x e2x Fig. 3. Tool path inner side and outer side β1 , β1 j e1y e2y (10) β2 , β2 k e1z . e2z (11) (12) The coordinate conversion matrix M from the coordinate system OXYZ to O1X1Y1Z1 is expressed as e1 M e2 . e 3 (13) (2) The vector τ1 and τ2 in the coordinate system OXYZ are converted to τ1 and τ 2 in the coordinate system O1X1Y1Z1, respectively, by using the following expressions, τ1 Mτ1 , (14) τ 2 Mτ 2 . (15) (3) Determining the transition type: Cθ1 Fig. 4. Transition type of compensation 3.2 Determining the transition type The transition type is determined by the offset direction and the angle between two tangent vectors of tool paths at the corner. In order to get the transition type, firstly, the tangent vector of tool paths are projected to the coordinate system constructed from linearly independent tangent vectors. Secondly, the transition type can be determined by the offset direction and the angles between the projected vectors and the coordinate axis. The transition type is determined as follows. (1) Calculating the coordinate conversion matrix The coordinate system OXYZ is a Cartesian coordinate system whose fundamental vectors are the unit vectors along the X-, Y-, and Z-axis. (τ , e ) (τ 2 , e1 ) , Cθ 2 2 2 . τ 2 e2 τ 2 e1 where θ1 is the angle between e 1 and τ 2 , and θ 2 is the angle between e2 and τ 2 , as shown in Fig. 5. Fig. 5. Angle of intersection CHINESE JOURNAL OF MECHANICAL ENGINEERING According to C θ1 and C θ 2 , the transition type can be obtained, as illustrated in Table 1. Table 1. Cutter compensation Transition type Cθ1 0 Cθ 2 0 Transition type – N Y – Y N Y N N N Y Y A B A A A B Left Right (16) 3.4 Determining the tool paths at the corner The point Q1(x1, y1, z1) is an intersection of the tool offset path and the offset vector at the point Q. The relation between point Q1 and Q is restricted by Q1 Q Rv1, (17) where R is the tool radius, and v1 is the unit offset vector at the point Q. The offset path r that passes through the point Q1 and parallels to the tangent vector τ1 of the tool path at the point Q is rQ1τ1t. (18) The five-axis spindle-tilting AB type and linear interpolation are chosen as an example. The start and end points of a block are P(xp, yp, zp, Ap, Bp), and Q(xq, yq, zp, Aq, Bq) respectively, and the end point of the next block is R(xr, yr, zr, Ar, Br), as shown in Fig. 4. The tangent vector τ1 of the point Q in the line PQ is a τ1 1 , a1 (19) where a1 ( xq xp , yq yp , zq zp ). The orientation vector of the tool axis relative to the workpiece t1 at the point Q is t1 [SBq SAqCBq CAqCBq ]. v1τ1t1. (21) The head position Q1 of the offset vector t1 at the point Q along the straight line PQ can be obtained from Eq. (17). The straight line r1 on which the start point is Q1 that parallels to the tangent vector of the point Q along the straight line PQ is r1Q1τ1t. 3.3 Calculating the offset vector In peripheral milling, the tool axis is perpendicular to both the movement direction and the offset vector. Consequently, the offset vector is perpendicular to both the tangent vector of the tool path τ (τxiτyjτzk) and the orientation vector of the tool axis relative to the workpiece t (txityjtzk). The tail of the vector is on the workpiece side and its head on the tool center path. The offset vector v (vxivyjvzk) is calculated as follows: vτt. ·551· (20) The offset vector v1 at the point Q along the straight line PQ is (22) In the same manner, the head position Q2 (x2, y2, z2) of the offset vector t2 at point Q can be obtained along the straight line QR, and the straight line r2 on which the start point is Q2 that parallels to the tangent vector τ2 of the point Q can be obtained along the straight line QR. 3.4.1 Type A In Type A, the inserted block is a straight line Q11Q22 that is a common perpendicular L of the two line r1 and r2, as shown in Fig. 4(a). The common perpendicular L can be described as x x0 y y0 z z0 , m n l (23) where point c( x0 , y0 , z0 ) is an arbitrary point in the L. Coefficients m , n and l are the direction vector LL of L, at least one of which is non-zero. L meets both lines r1 and r2 and is perpendicular to them. LL is obtained as follows: LL τ1τ2. (24) The connection line between point c and Q1, LL and τ1 are coplanar. Hence, the following equation is valid: x0 x1 m y0 y1 n z0 z1 l 0. τ 1x τ1 y τ 1z (25) The connection line between point c and Q2, LL and τ2 are coplanar. Hence, the following equation is valid: x0 x2 y0 y2 z0 z 2 m n l τ 2x τ 2y τ 2z 0. (26) The point c can be derived as follows from the above two simultaneous equations: x0 0, (τ n τ 2 y m) A (τ1x n τ1 y m) B y0 2 x , C (τ l τ 2 z m) A (τ1x l τ1z m) B z0 2 x , C (27) CHEN Youdong, et al: Three-dimensional Tool Radius Compensation for Multi-axis Peripheral Milling ·552· where A (τ1y l τ1z n) x1 (τ 1x l τ 1z m) y1 (τ 1x n τ 1 y m) z1 , B (τ 2y l τ 2z n) x2 (τ 2x l τ 2z m) y2 (τ 2x n τ 2 y m) z2 , C (τ 2 x l τ 2z m)(τ1x n τ1 y m) (τ1x l τ 1z m)(τ 2x n τ 2y m). The common perpendicular L can be derived from Eqs. (23), (24) and (27). The point Q11 is the intersection point of the common perpendicular L and the straight line r1. The coordinate of Q11 can be obtained as Q11Q1t1τ1. If If If (28) τ1y τ m( z1 z0 ) l ( x0 x1 ) . τ1x l τ1z m τ 1x τ τ1 y m( y1 y0 ) n( x0 x1 ) . τ 1 x n τ 1 y m τ1 y τ τ m( z1 z0 ) l ( y0 y1 ) . τ 1y l τ 1z n τ1x m 1z , then t1 n l 1z , then t1 m l n 1z 1x , then t1 n l m Similarly, the coordinate of point Q22 can be obtained which is the intersection point of the common perpendicular L and the straight line r2. The tool paths at the corner are Q1 Q11 Q22 Q2 , as shown in Fig. 4(a). 3.4.2 Type B In Type B, the inserted block is a straight line Q11Q22, as shown in Fig. 4(b). The point Q11 is the offset radius R along the line P1Q1 to the point Q1. The point Q22 is the offset radius R along the opposite direction of the line Q2R1 to the point Q2. The coordinate of point Q11 is expressed as Q11Q1Rτ1. (29) The coordinate of Q22 is obtained as Q22Q2Rτ2. (30) The tool paths at the corner are Q1 Q11 Q22 Q2 , as shown in Fig. 4(b) 4 Verification and Experiments To illustrate the new approach, the presented compensation method has been applied by the VERICUT and the tool path of the test workpiece. 4.1 VERICUT verification To verify the effectiveness of the proposed methodology, a postprocessor program for multi-axis machine has been developed to generate the offset path. The user configured the kinematics model, and entered the NC code and tool radius. The NC codes with the radius offset were generated accordingly. In order to verify it, the generated NC codes are further input into the solid cutting simulation software VERICUT which can customize or design kinematics of the machine and simulate the NC codes. The VERICUT reads the generated NC codes to perform the cutting actions. Fig. 6 shows the result for simulating the NC data in a three-axis machine tool. The finishing part shapes of simulation are the same with different cutter radiuses as shown in Fig. 6(a), Fig. 6(b) and Fig. 6(c). Note that the cutter radius is 15 mm, 10 mm, and 5 mm, respectively. Fig. 7 shows that a table-tilting type of machine tool is constructed in the software environment and the final shapes of the workpiece with different cutter radiuses are simulated. The cutter radius is 1.5 mm, 1 mm and 0.5 mm as shown in Fig. 7(a), Fig. 7(b) and Fig. 7(c), respectively. Fig. 6 and Fig. 7 demonstrate that the developed 3D radius compensation is highly effective. 4.2 Experimental validation The effectiveness of the presented 3D tool radius compensation algorithm was conducted in an experiment to machine a soup spoon on a spindle-tilting type of machining centre. The CNC controller reads CL data for machining the soup spoon generated by the MasterCam, calculates the cutter compensation, and drives the machine tools. The part shown in Fig. 8 is machined on a five-axis machining center controller with an in-house developed CNC. Fig. 8 shows the machined soup spoon, demonstrating that the proposed algorithm can be successfully applied to the practical five-axis machining. 5 Conclusions (1) A 3D tool radius compensation algorithm is proposed for various three- to five-axis peripheral milling based on generalized kinematics model and relations of two lines in three dimensions. The offset path can be calculated by offsetting the tool path along the direction of the offset vector obtained by the orientation vector of the tool axis relative to the workpiece. (2) To avoid cutting into the corner formed by the two adjacent tool paths, the coordinates of the offset path at the intersection point have been calculated according to the transition type that is determined by the angle between the two tool path tangent vectors at the corner. (3) Although the presented methodology about 3D tool radius compensation can be used in CNC systems, it is just for linear or circular tool paths. Therefore, further research on parametric curves, such as Bezier, B-Spline and NURBS curves for 3D cutter compensation is needed. CHINESE JOURNAL OF MECHANICAL ENGINEERING (a) 15 mm (b) 10 mm (c) 5 mm (d) Screenshot of the VERICUT simulation and verification for the three-axis machine tool Fig. 6. VERICUT simulation and verification for the three-axis machine tool (a) 1.5 mm (b) 1 mm (c) 0.5 mm (d) Screenshot of the VERICUT simulation and verification for the table-tilting type five-axis machine tool Fig. 7. VERICUT simulation and verification for the table-tilting type five-axis machine tool ·553· ·554· CHEN Youdong, et al: Three-dimensional Tool Radius Compensation for Multi-axis Peripheral Milling [8] [9] [10] [11] [12] [13] Fig. 8. Soup spoon machined References [1] HU Zihua, ZHANG Ping, YANG Dongxiang, et al. 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Biographical notes CHEN Youdong, born in 1973, is currently an associate professor at Beihang University, China. He took his PhD degree from Beihang University, China, in 2003. His research interests cover CNC system, mechachonics control, robotics control system and embedded system. Tel: +86-10-82338271; E-mail: chenyd@buaa.edu.cn WANG Tianmiao, born in 1960, is currently a professor at Beihang University, China. He took his PhD degree from Xi’an Jiaotong University, China, in 1990. His research interests include intelligent robotics and mechachonics engineering. Tel: +86-10-82338271; E-mail: wtm_itm@263.net
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