Research on repairing drilling tool threads on CNC lathes
Jan 16, 2024
Petroleum drilling tools are subjected to complex forces and heavy loads during the drilling process, and the threads connected to them are prone to wear and deformation. If not repaired in a timely manner, it can easily lead to drilling tool failure accidents such as detachment and puncture. Due to the excellent tensile and torsional properties of drilling tool pipes and their high purchase price, in order to improve the service life of drilling tools and save costs, drilling tool threads are generally repaired rather than cut off.
1. General methods for thread repair of petroleum drilling tools
Petroleum drilling tools are generally tapered threads (Figure 1). When repairing threads, ① cut off a certain length L of the end face based on the wear of the thread; ② Cut the width L of the step surface accordingly, and machine the cut diameter contour to a conical surface that meets the size requirements; ③ Process the entire thread along the original spiral trajectory to meet the dimensions on the drawing. After such wear and tear
The thread was repaired to new, and after inspection, the drilling tool was put back into use.
1. General methods for thread repair of petroleum drilling tools
Petroleum drilling tools are generally tapered threads (Figure 1). When repairing threads, ① cut off a certain length L of the end face based on the wear of the thread; ② Cut the width L of the step surface accordingly, and machine the cut diameter contour to a conical surface that meets the size requirements; ③ Process the entire thread along the original spiral trajectory to meet the dimensions on the drawing. After such wear and tear
The thread was repaired to new, and after inspection, the drilling tool was put back into use.
Figure 1 Thread Repair
At present, most of the thread repairs for petroleum drilling tools are carried out using ordinary pipe thread lathes. Due to the influence of the work experience of operators and the accuracy of lathes, the dimensions of various parts of the thread, such as taper, tooth shape, and chamfer, are prone to errors; The local dimensional tolerance of some special threads is within 0.2 mm, which is difficult to control manually and affects machining efficiency. Meanwhile, the operator
Bending over to operate the lathe for a long time can easily cause waist fatigue and affect physical health.
2. The principle of machining threads on lathes
The rotational motion of the workpiece driven by the lathe spindle is the main motion, while the movement of the tool is the feed motion. The processing of threads on a regular pipe thread lathe relies on the transmission of power by a change wheel, which drives the screw to rotate and then drives the large support plate to move according to the adjusted pitch. The axial position of the small support plate to move the tool tip is adjusted to achieve tool alignment, thereby completing thread turning. They are achieved through pure mechanical transmission, and in the process of thread machining, if there is cutting, power outage, variable speed, or repairing old threads, it can be relatively easy to achieve tool alignment and reprocessing.
No matter what type of thread the CNC lathe is turning, the main motion and feed motion must maintain a specific relationship, that is, it must accurately achieve one revolution of the workpiece and one pitch of the tool movement. The spindle speed can be adjusted through a program, but the angle position is generally uncontrollable. To achieve the function of cutting threads, it is necessary to use a photoelectric encoder and connect it to the spindle in a 1:1 transmission. When the spindle rotates, the encoder synchronously rotates and sends a series of pulse signals into the CNC system. The CNC system selects a specific angle control motor to start feeding based on the detected spindle rotation signal, achieving the required proportional relationship for turning threads.
3. CNC lathe thread repair
Before each thread feeding on the CNC lathe, the CNC system scans the zero pulse of the photoelectric encoder, and starts feeding when it arrives. Otherwise, it is in a waiting state. This ensures that the spiral trajectory of each feed turning is the same, preventing the occurrence of random threading.
However, when repairing the old thread of the drilling tool, there is a problem of aligning the starting position of the thread with the tool. Due to the possible difference between the zero pulse of the photoelectric encoder and the starting position of the thread to be repaired, when the spindle rotates and the tool performs thread turning feed motion according to the G92 (or G32) command, it will not coincide with the spiral trajectory of the thread to be repaired, resulting in damage to the original thread or even cutting. Therefore, when repairing the thread, it is necessary to first locate and then turn it. Positioning is to use the zero point pulse as a reference and align the starting position of the thread to be repaired with the corresponding tool. After positioning, the tool can move along the spiral trajectory of the thread to be repaired, achieving thread repair. Therefore, the key to thread repair is positioning.
The method of this study is to mark the zero pulse on the surface around the chuck to determine the starting point of thread turning, and use a macro program to locate the tool position: ① When aligning the tool, first move the chuck and turn it to the marked position. At this time, the spindle position is the position of the zero pulse signal. By operating the macro program in a single step, move the tool tip above a certain position of the thread to be repaired; ② Choose a hand cranked pulse generator to move the tool tip to a spiral groove in the middle of the thread, reduce the magnification and fine tune the X and Z values of the tool tip. Observe the tool tip and spiral groove until they coincide. At this time, pay special attention to the position of the tool tip, otherwise it is easy to crush the tool block; ③ Select the program to run in a single step, and read the system variable # 5002 of the Z-axis position information of the tool tip through the common variable # 505, that is, # 505=# 5002, to complete the tool alignment operation; ④ Step by step running the macro program to move the tool tip to a position N pitches away from the thread to be repaired. At this point (starting from X, starting from Z), it is the starting point. Then, specify the R value through a variable expression and use the thread cycle cutting instruction G92 to complete the thread machining. ⑤ During the use of drilling tools, the thread profile sometimes wears out significantly. Even if the tool has been aligned, during the operation of the thread cycle cutting command G92, it may be found that the tool tip is not completely aligned with the spiral line of the thread to be repaired (offset tool). At this time, a value of # 5 variable can be assigned to fine tune the Z-value of the tool tip in the positive or negative direction (Figure 2), achieving the goal of aligning with the spiral line of the thread to be repaired.
At present, most of the thread repairs for petroleum drilling tools are carried out using ordinary pipe thread lathes. Due to the influence of the work experience of operators and the accuracy of lathes, the dimensions of various parts of the thread, such as taper, tooth shape, and chamfer, are prone to errors; The local dimensional tolerance of some special threads is within 0.2 mm, which is difficult to control manually and affects machining efficiency. Meanwhile, the operator
Bending over to operate the lathe for a long time can easily cause waist fatigue and affect physical health.
2. The principle of machining threads on lathes
The rotational motion of the workpiece driven by the lathe spindle is the main motion, while the movement of the tool is the feed motion. The processing of threads on a regular pipe thread lathe relies on the transmission of power by a change wheel, which drives the screw to rotate and then drives the large support plate to move according to the adjusted pitch. The axial position of the small support plate to move the tool tip is adjusted to achieve tool alignment, thereby completing thread turning. They are achieved through pure mechanical transmission, and in the process of thread machining, if there is cutting, power outage, variable speed, or repairing old threads, it can be relatively easy to achieve tool alignment and reprocessing.
No matter what type of thread the CNC lathe is turning, the main motion and feed motion must maintain a specific relationship, that is, it must accurately achieve one revolution of the workpiece and one pitch of the tool movement. The spindle speed can be adjusted through a program, but the angle position is generally uncontrollable. To achieve the function of cutting threads, it is necessary to use a photoelectric encoder and connect it to the spindle in a 1:1 transmission. When the spindle rotates, the encoder synchronously rotates and sends a series of pulse signals into the CNC system. The CNC system selects a specific angle control motor to start feeding based on the detected spindle rotation signal, achieving the required proportional relationship for turning threads.
3. CNC lathe thread repair
Before each thread feeding on the CNC lathe, the CNC system scans the zero pulse of the photoelectric encoder, and starts feeding when it arrives. Otherwise, it is in a waiting state. This ensures that the spiral trajectory of each feed turning is the same, preventing the occurrence of random threading.
However, when repairing the old thread of the drilling tool, there is a problem of aligning the starting position of the thread with the tool. Due to the possible difference between the zero pulse of the photoelectric encoder and the starting position of the thread to be repaired, when the spindle rotates and the tool performs thread turning feed motion according to the G92 (or G32) command, it will not coincide with the spiral trajectory of the thread to be repaired, resulting in damage to the original thread or even cutting. Therefore, when repairing the thread, it is necessary to first locate and then turn it. Positioning is to use the zero point pulse as a reference and align the starting position of the thread to be repaired with the corresponding tool. After positioning, the tool can move along the spiral trajectory of the thread to be repaired, achieving thread repair. Therefore, the key to thread repair is positioning.
The method of this study is to mark the zero pulse on the surface around the chuck to determine the starting point of thread turning, and use a macro program to locate the tool position: ① When aligning the tool, first move the chuck and turn it to the marked position. At this time, the spindle position is the position of the zero pulse signal. By operating the macro program in a single step, move the tool tip above a certain position of the thread to be repaired; ② Choose a hand cranked pulse generator to move the tool tip to a spiral groove in the middle of the thread, reduce the magnification and fine tune the X and Z values of the tool tip. Observe the tool tip and spiral groove until they coincide. At this time, pay special attention to the position of the tool tip, otherwise it is easy to crush the tool block; ③ Select the program to run in a single step, and read the system variable # 5002 of the Z-axis position information of the tool tip through the common variable # 505, that is, # 505=# 5002, to complete the tool alignment operation; ④ Step by step running the macro program to move the tool tip to a position N pitches away from the thread to be repaired. At this point (starting from X, starting from Z), it is the starting point. Then, specify the R value through a variable expression and use the thread cycle cutting instruction G92 to complete the thread machining. ⑤ During the use of drilling tools, the thread profile sometimes wears out significantly. Even if the tool has been aligned, during the operation of the thread cycle cutting command G92, it may be found that the tool tip is not completely aligned with the spiral line of the thread to be repaired (offset tool). At this time, a value of # 5 variable can be assigned to fine tune the Z-value of the tool tip in the positive or negative direction (Figure 2), achieving the goal of aligning with the spiral line of the thread to be repaired.
Figure 2 Knife tip adjustment parameters
Taking the NC50 external thread program as an example.
(1) Program segment 1, tool alignment operation:
In the thread cutting cycle instruction G92 (Figure 3), parameters X and Z represent the coordinate values of the end point of each cutting tool, R represents the radius difference between the starting and ending points of the conical surface cutting, and F represents the thread pitch (NC50 thread pitch is 6.35 mm). Run the variable expression Z [# 505+# 5+63.5] to move the forming tool to a position 10 pitches away from the spiral groove of the tool, which is the starting point. At present, there is no information available to determine the accurate distance of the tool's acceleration segment when the CNC lathe executes G32 or G92 instructions. Therefore, the distance between the starting point and the end face of the thread to be repaired (i.e. N pitches) depends on operating habits. Then assign the parameter R to the variable expression # 4=[# 3- # 505- # 5-56.5]/12, run the thread cycle cutting instruction G92, and complete the thread repair machining.
Figure 3 Thread Cycle Cutting Instruction G92
4. Conclusion
In the field of drilling tool thread maintenance, automation technology represented by CNC machine tools is the direction of future development. The method introduced in this article has low application costs and does not require special modifications to CNC lathes. At the same time, the use of CNC lathes to repair drilling tool threads has improved the quality of drilling tool maintenance. After thread repair, the size is stable, reducing the labor intensity of operators, and providing a new technical means for oil drilling tool thread maintenance.
