In order to reduce the maintenance cost and drilling cycle of PDC drill bits, based on laser cladding remanufacturing technology, it is proposed to use robots to remanufacture drill bits. Based on reverse engineering, data collection of PDC drill bits is carried out, and then the acquired drill bit point cloud data is processed and three-dimensionally reconstructed to construct a three-dimensional model identical to the solid PDC drill bit. The defective part of the workpiece is obtained by Boolean operation of Geomagic software, and the isometric plane family Γ is used in the software NX1899 to intersect with the drill bit repair part to realize the path planning of the curved surface parts. The trajectory of the PDC drill bit repaired by the robot end welding gun is simulated, and the position of the workpiece relative to the robot in the working environment is simulated by the software PQart to optimize the robot trajectory. The posture of the robot end welding gun is adjusted to improve the surface performance of the repaired PDC drill bit. The feasibility of the method is verified, which provides a reference for the repair of complex curved surfaces by laser cladding remanufacturing technology.
At present, most key oilfield development areas are facing difficulties such as poor drillability and complex development site structure, which aggravates the PDC The wear of drill bits[1-2]. Traditional methods of repairing or directly scrapping damaged drill bits will affect work efficiency and increase costs. Laser cladding additive remanufacturing technology has been industrialized and has achieved certain results. It is a technology that performs reverse modeling of worn workpieces, extracts and layers failed parts, and plans paths. It intelligently controls heat sources such as laser beams, electron beams, and plasma beams to complete the process accumulation of damaged parts of workpieces, and restores and improves the size and performance of worn workpieces[3]. Repairing PDC drill bits using laser cladding remanufacturing technology can not only reduce maintenance cycles and save costs, but also better maintain the performance of the drill bits, bringing economic benefits to oilfield development. In terms of PDC drill bit repair, relevant scholars have analyzed and evaluated the degree of drill bit wear, and developed a set of sintering repair solutions for the drill bits. The performance of the repaired drill bits reaches 80%~90% of that of new drill bits, while the cost is only 30% of that[4]. As for laser cladding repair of PDC There is little research on drill bits. Using robots for selective laser cladding repair can save materials, and its performance can better meet the requirements of poor working environment. Therefore, it is necessary to study the robot laser cladding repair of PDC drill bits. Compared with traditional manual teaching programming, robot offline programming can greatly improve processing efficiency and accuracy [5]. Li Jinhua et al. corrected the robot motion path through visual simulation to ensure safety while improving work efficiency [6]. Before using the robot to repair the workpiece, the posture of the cladding gun head can be observed through robot path simulation, and the optimization of the processing trajectory can obtain better repair effect [7].
Based on laser cladding remanufacturing, this paper reversely models the damaged PDC drill bit, further performs path planning, and simulates the robot laser cladding. Combined with the simulation results, the feasibility of path planning for remanufacturing repair of PDC drill bits is verified, and a higher quality cladding layer is obtained by timely adjusting the welding gun posture during the processing process. It provides a certain reference for laser cladding remanufacturing repair of PDC drill bits and other complex curved workpieces.
1 Modeling and path planning
1.1 PDC Drill bit reverse modeling
Before scanning the drill bit, black circle marks are affixed to the workpiece to be repaired. The distance between two adjacent marks should be greater than 5 mm. The affixed marks are not on the same line. There are 30 marks in total, as shown in Figure 1.
After the marks are positioned, the HandySCAN 3D scanner is used to obtain the point cloud data of the drill bit surface features, as shown in Figure 2. During the scanning process, when the scanning laser scans the workpiece surface only once, the point cloud data collection of the workpiece surface will be incomplete, and multiple scans of the workpiece surface will obtain too much unnecessary point cloud data. Therefore, the original point cloud data obtained by the scanner needs to be pre-processed before the workpiece reverse modeling can be performed. The scanner used in the study can realize the automatic splicing of scattered point clouds. For the three-dimensional point cloud map generated by automatic splicing, Geomagic Studio is used to process the point cloud data into patch data. On this basis, the patch data with relatively complete point cloud data is selected, and a complete PDC drill bit model is generated by extending the surface, cutting the surface, and splicing and fitting the surface, as shown in Figure 2b. As shown.
1.2 Path planning for laser cladding remanufacturing of PDC drill bits
The defective part of the workpiece obtained by using Geomagic Boolean operation is shown in Figure 3a. The processed PDC drill bit 3D model is converted into stl format and imported into the software NX1899, as shown in Figure 3b.
The plane family Γ of a certain thickness is intersected with the target repair position of the model to be repaired to obtain a slice and generate a laser cladding path. The direction of the slice is generally perpendicular to the cladding path. Figure 4 shows the target repair position slice diagram. The distance between two adjacent planes of the plane family Γ is the distance δ between the cladding paths. δ is mainly affected by the cladding overlap rate. The height and width of a single cladding track are measured, and the plane spacing δ[8] is further calculated and deduced, as shown in formula (1) in the figure.
Where: ε is the width of a single cladding layer, and h is the height of the cladding layer.
The point cloud slice is shown in Figure 4b. The slice point cloud representation of different slices is Di = {d1, d2, d3, ··· ,dn｝ (2) See formula (2) in the figure
This is the machining trajectory of the cladding gun head, and the machining trajectory is finally output in NC code format.
2 Terminal welding gun posture adjustment
2.1 6-DOF robot kinematic model
The research adopts the SA1400 model 6-axis robot, whose D-H coordinate system is shown in Figure 5. 0 is the robot’s base coordinate system, 1~6 are the 6 coordinate origins of the robot’s mechanical arm, and the origin of the robot’s end coordinate system is 6. The D-H parameters of each joint of the robot are shown in Table 1. When the D-H parameters of each joint of the robot are known, the robot’s end position expression for the base coordinate can be obtained [11-12]: See formula (3) in the figure
According to Table 1, the transformation matrix of each joint of the robot is obtained: See formula (4)-(9) in the figure
In the above matrix, = , = . According to pieper According to the robot angle expression, the robot inverse kinematics has the following simple algorithm [13]: see formula (10) in the figure. According to the equation, the robot’s six joint angles θ1 ∼ θ6 are calculated respectively: see formula (11)-(16) in the figure. Where: e = oxD1 +oyB1, f = nx + nyB1. According to the robot angle expression, the robot inverse kinematics has multiple sets of solutions. The selected angle should be within the robot’s motion range, and a smaller joint angle value is selected in the same set of solutions to achieve continuous and rapid operation and improve the efficiency of repairing PDC drill bits.
2.2 Expression of welding gun posture
As shown in Figure 6, the length of the welding tool cladding welding gun is set to , the origin of the tool coordinate system is 7, and the rotation angle of the tool relative to the coordinate axis of the robot end manipulator is θ. The transformation matrix l 6T7 of the tool relative to the origin of the end coordinate system can be expressed as: See formula (17) in the figure
2.3 Multi-layer cladding trajectory welding gun posture planning
The posture of the welding gun has a very important influence on the quality of each layer of the repair trajectory of the PDC drill bit target position. Therefore, in the process of repairing the PDC drill bit, it is necessary to adjust the posture of the welding gun in time according to each layer of known cladding trajectory, so as to obtain higher processing quality [15]. The posture of the multi-layer trajectory welding gun is shown in Figure 7, and are the movement of the welding gun along the y direction and the z direction with O as the reference.
The offset of the welding gun in the horizontal direction and the vertical direction are: See formula (18) and (19) in the figure
Where: , are the horizontal offset and vertical offset of the jth track of the i-th layer in the trajectory; n is the number of trajectory layers; is the total number of f tracks in the i-th layer; is the cladding area of the i-th layer trajectory; is the groove angle.
φX is the rotation angle of the cladding gun head around the X axis. The offset of the welding gun along the X direction will have a direct impact on the depth and width of the cladding trajectory, so it will be given on site. According to the above formula, the welding gun posture matrix of each layer of cladding trajectory is obtained as follows: See formulas (20) and (21) in the figure
3 Laser cladding repair drill bit simulation
The simulation process of the PDC drill bit repair robot is shown in Figure 8. Before the simulation, a unified drill bit model and a coordinate system for the design of the repair drill bit path are established. In this way, after importing PQart, the position of the drill bit workpiece to be repaired is guaranteed to coincide with its corresponding trajectory. The drill bit to be processed is shown in Figure 9 As shown.
3.1 Processing trajectory optimization
During processing, the drill workpiece should be as close to the cladding welding gun as possible to avoid inaccessible points at the position of the workpiece to be repaired, and avoid axis overrun and singular points of the robot. Axis overrun means that there are points on the surface of the workpiece to be repaired that are inaccessible within the range of motion of the robot joint axis; singular points mean that when the robot’s end effector reaches a certain point on the robot’s surface to be repaired, two of the robot’s joints are on the same axis, for example, the 3rd and 5th axes are on the same axis. According to the knowledge of the inverse solution of kinematics, it can be known that θ3 and θ5 will have multiple solutions, and rotating θ3 or θ5 can reach the specified point. At this time, the joint axis of the robot arm will not be able to continue to operate, and this point is called a singular point. In the process of adjusting the position of the workpiece, avoid these problems and realize the normal operation of the robot. The robot processing trajectory optimization is shown in Figure 10, and the robot is within the working range.
3.2 Adjusting the welding gun posture
From formula (21), it can be concluded that the welding gun’s posture is always in a state of continuous change during the repair of the PDC drill bit. Keeping the gun head perpendicular to the processing surface can improve the performance after repair. As shown in Figure 11, the posture of the welding gun at a certain point in the repair process is perpendicular to the processing surface. Adjusting the welding gun posture is unified with the gun head posture used at this point.
3.3 Simulation
In order to ensure that the robot’s machine errors are reduced, the robot’s movement should be mastered before the robot is actually operated. The imported trajectory should be simulated. As shown in Figure 12, there are no problem points in the trajectory and each point on the trajectory, and each joint of the robot is within the range of motion.
4 Conclusion
(1) Based on reverse engineering, the method of combining laser scanning and reverse modeling is adopted to realize the point cloud data acquisition and surface reconstruction of complex surface parts, and establish a three-dimensional model of the PDC drill bit.
(2) The defective part of the PDC drill bit is obtained by Boolean operation, and the isometric plane family Γ is used to obtain the defective part of the PDC drill bit. Intersecting with the drill bit repair part, the planning of the PDC drill bit laser cladding remanufacturing path is completed.
(3) A 6-DOF robot kinematic model is established, and the posture of the robot cladding welding gun is expressed using the homogeneous transformation matrix, and the multi-layer trajectory cladding welding gun posture matrix for repairing the PDC drill bit is determined.
(4) Through the simulation of laser cladding repair drill bit, it is found that when the robot repairs according to the specified trajectory, the welding gun posture is in a changing process, and the welding gun posture is unified at one point to optimize the robot’s processing trajectory. The laser cladding repair of the complex curved surface workpiece is realized.
