Threads are one of the common geometric features in mechanical engineering and are widely used. There are many thread processing techniques, such as thread rolling and twisting based on plastic deformation, turning, milling, tapping and threading, thread grinding, thread grinding, etc. based on cutting processing. Among them, thread turning is one of the commonly used processing methods for single-piece or small batch production. As a CNC lathe, thread turning is one of its basic functions. Today, the editor will share with you common problems and solutions when turning threads on CNC lathes.
Knife
1. Main reason
(1) The rake angle of the turning tool is too large, and the gap between the X-axis screw rod of the machine tool is large;
(2) The turning tool is installed too high or too low;
(3) The workpiece is not clamped firmly;
(4) The turning tool is too worn;
(5) The cutting amount is too large.
2. Solution
(1) Reduce the rake angle of the turning tool, adjust the X-axis screw gap of the maintenance machine tool, and use the automatic screw gap compensation function of the CNC lathe to compensate the X-axis screw gap of the machine tool.
(2) The turning tool is installed too high or too low: If it is too high, when the cutting tool reaches a certain depth, the flank surface of the turning tool will resist the workpiece, increasing the friction and even bending the workpiece, causing the knife to stick; If it is too low, the chips will not be easily discharged. The direction of the radial force of the turning tool is the center of the workpiece. In addition, the gap between the infeed screw and the nut is too large, causing the cutting depth to continuously and automatically deepen, thereby lifting the workpiece and causing stabbing. At this time, the height of the turning tool should be adjusted in time so that the tool tip is at the same height as the axis of the workpiece (the tailstock tip can be used to set the tool). During rough turning and semi-finishing turning, the tool tip position is about 1% D higher than the center of the workpiece (D represents the diameter of the workpiece being processed).
(3) The workpiece is not clamped firmly: the rigidity of the workpiece itself cannot withstand the cutting force during turning, resulting in excessive deflection, which changes the center height of the turning tool and the workpiece (the workpiece is elevated), resulting in a sudden increase in the depth of cutting. , a stabbing occurs. At this time, the workpiece should be clamped firmly, and the tailstock tip can be used to increase the rigidity of the workpiece.
(4) Excessive wear of the turning tool: causing the cutting force to increase, bending the workpiece, and causing tool sticking. At this time, the turning tool should be sharpened.
(5) The cutting amount (mainly the back cutting amount and cutting speed) is too large: Choose a reasonable cutting amount based on the workpiece lead size and workpiece rigidity.
Random deductions
1. Fault phenomenon
When the screw rotates once, the workpiece does not rotate through an integer rotation.
2. Main reasons
(1) The synchronous transmission belt of the machine tool spindle encoder is worn and the true synchronous speed of the spindle cannot be detected;
(2) The program compiled and input into the host machine is incorrect; the X-axis or Y-axis screw rod is worn.
3. Solution
(1) Spindle encoder synchronous belt wear
Because when a CNC lathe turns threads, the motion relationship between the spindle and the turning tool is controlled by instructions issued by the machine tool host information processing center. When turning threads, the spindle speed is constant. The X or Y axis can be adjusted according to the workpiece lead size and the spindle. The rotation speed is used to adjust the movement speed, so the center must detect the true rotation speed of the spindle in order to issue correct instructions to control the correct movement of the X or Y axis.
If the system cannot detect the true speed of the spindle, it will issue different instructions to X or Y during actual turning. Then when the spindle rotates once, the distance the tool moves is not a lead, and the thread will be lost during the second tool turning. Random buckles. In this case, we only have to repair the machine tool and replace the spindle timing belt.
(2) The program input is incorrect
In order to prevent random buckling when turning threads, it is necessary to ensure that the turning trajectory of the last tool coincides with the turning trajectory of the previous tool. On ordinary cars, we use the reverse turning method to prevent random buckling.
On CNC lathes, we use programs to prevent random buckling. That is, when compiling machining programs, we use programs to control the thread cutter to retract after turning the previous cut, so that the starting point of the next cut coincides with the starting point of the previous cut (equivalent to When turning threads on a general machine, the thread cutter returns to the spiral groove turned by the previous cutter), so that the threads turned out will not be buckled randomly.
Sometimes, due to incorrect lead input in the program (the lead of the next program is inconsistent with the lead of the previous program), random buckling will also occur during turning.
(3) The X-axis or Y-axis screw is seriously worn: Repair the machine tool and replace the X-axis or Z-axis screw.
Incorrect thread pitch
1. Main reason
The data transmitted back to the machine tool system by the spindle encoder is inaccurate; the movement of the X-axis or Y-axis screw and spindle is too large; the program compiled and input is incorrect.
2. Solution
(1) The spindle encoder transmits inaccurate data: repair the machine tool, replace the spindle encoder or synchronous transmission belt;
(2) The movement of the X-axis or Y-axis screw and spindle is too large: adjust the axial movement of the spindle, and the gap of the X-axis or Y-axis screw can be compensated by the system gap automatic compensation function;
(3) Check the program and make sure that the instruction lead in the program is consistent with the drawing requirements.
Incorrect tooth shape
1. Main reason
The turning tool tip is not sharpened correctly; the turning tool is installed incorrectly; the turning tool is worn.
2. Solution
(1) Incorrect turning tool tip sharpening: Correctly sharpen and measure the turning tool tip angle. For thread turning that requires high profile angle accuracy, you can use a standard mechanical clamping thread cutter to turn the thread, or turn the thread Knives are sharpened with a grinder.
(2) The turning tool is incorrectly installed: Use a template to calibrate the tool when installing the tool, or use a dial indicator to align the thread tool holder to install the thread tool.
(3) Wear of turning tools: According to the actual conditions of turning processing, the cutting amount should be reasonably selected and the turning tools should be repaired and sharpened in time.
Failure analysis of large thread surface roughness
1. Main reason
(1) Built-up edge occurs on the tool tip;
(2) The tool handle is not rigid enough and causes vibration during cutting;
(3) The radial rake angle of the turning tool is too large;
(4) When cutting threads at high speed, the cutting thickness is too small or the chips are discharged in an oblique direction, and the roughened flank surface is processed;
(5) The workpiece has poor rigidity and the cutting amount is too large;
(6) The surface roughness of the turning tool is poor.
2. Solution
(1) When cutting with high-speed steel turning tools, the cutting speed should be reduced and the cutting fluid should be correctly selected;
(2) Increase the cross section of the tool holder and reduce the extension length of the tool holder;
(3) Reduce the radial rake angle of the turning tool;
(4) When cutting threads in high-speed steel, the chip thickness of the last cut should generally be greater than 0.1mm, and the chips should be discharged along the vertical axis;
(5) Choose a reasonable cutting amount;
(6) The surface roughness of the cutting edge of the tool should be 2 to 3 grades smaller than the surface roughness value of the parts.
