Chipping, scoring? Those failures in thread processing, let's see how the lathe technician solves them!
Published Time:
2020-02-13
A thread is a continuous raised and grooved profile along a helix on a cylindrical working surface. In actual thread turning, due to various reasons, problems may arise in the kinematic relationship between the spindle and the tool, resulting in defective products and affecting normal processing production and economic benefits. I. Analysis of tool digging (biting) faults: ⑴. The cause is that the lathe tool is installed too high or too low: If the lathe tool is installed too high, the actual back rake angle of the tool is reduced. When the tool cuts to a certain depth, the back surface of the tool rests against the workpiece, increasing friction and even bending the workpiece, causing tool digging. The lathe tool
Threads are continuous raised portions and grooves of the same profile formed along a helix on a cylindrical working surface. During actual thread turning, due to various reasons, problems may occur in the kinematic relationship between the spindle and the tool, causing defects in the turned threads, affecting normal processing production and economic benefits.
I. Analysis of tool digging (biting) failures:
⑴ The cause is that the tool is installed too high or too low: If the tool is installed too high, the actual back rake angle of the tool is reduced. When the tool cuts to a certain depth, the back surface of the tool rests against the workpiece, increasing friction and even bending the workpiece, causing tool digging; if the tool is installed too low, the actual front rake angle of the tool is increased, making it difficult for the chips to be discharged, causing tool digging.
⑵ The workpiece or tool is not clamped firmly: The rigidity of the workpiece itself cannot withstand the cutting force during turning, resulting in excessive deflection, changing the center height of the tool and the workpiece (the workpiece is raised), resulting in a sudden increase in the cutting depth and tool digging.
⑶ Severe tool wear: Increased friction increases the cutting force, causing tool digging.
⑷ Improper cutting method and feed rate: When processing large pitches, both cutting edges of the tool participate in cutting simultaneously, and excessive feed rate results in excessive cutting force, causing tool digging.
Solutions:
⑴ When installing the tool, use the tailstock center or measurement method to make the tool tip level with the axis of the workpiece. [During rough turning and semi-finishing, the tool tip position is 1%D higher than the center of the workpiece, approximately 0.2~0.5mm, D = workpiece diameter]
⑵ The workpiece and tool must be clamped firmly. Accessories such as tailstock center, center rest, and follow rest can be used to increase the rigidity of the workpiece.
⑶ Correctly and timely sharpen the geometric parameters of the tool. As shown below:
⑷ When processing large pitches, use the left and right cutting method or oblique feeding method. The tool is single-edged cutting, and both the cutting force and friction are relatively small, so tool digging is not easy to occur. In actual work, a self-made elastic tool holder (as shown in the figure) is used. When the cutting force exceeds a certain value, the tool can automatically let the cutter, keeping the cutting thickness appropriate, effectively avoiding tool digging.
2. Analysis of thread misalignment failures: The cause is that when the lead screw rotates one turn, the workpiece does not rotate an integer turn.
Solutions:
⑴ When the ratio of the lathe lead screw pitch to the workpiece pitch is not an integer multiple: If the tool is retracted by opening the half-nut and rocking the saddle to the starting position, when the half-nut is closed again, the tool tip will not be in the helical groove cut by the previous cut, resulting in misalignment. The solution is to use the positive and negative cutting method to retract the tool, that is, at the end of the first stroke, do not lift the half-nut, retract the tool along the radial direction, then reverse the spindle, and retract the tool along the longitudinal direction, and then perform the second stroke. In this back-and-forth process, because the transmission between the spindle, lead screw, and tool holder has not been separated, the tool is always in the original helical groove, and misalignment will not occur.
The movement has not been separated, and the tool is always in the original helical groove, so there will be no misalignment.
⑵ For turning threads where the ratio of the lathe lead screw pitch to the workpiece pitch is an integer multiple: Both the workpiece and the lead screw are rotating. After lifting the half-nut, wait for at least one turn of the lead screw before closing the half-nut again. In this way, when the lead screw rotates one turn, the workpiece rotates an integer multiple, and the tool can enter the helical groove cut by the previous cut, so there will be no misalignment.
3. Incorrect pitch
Failure analysis: The pitch is incorrect over the entire length or part of the thread, the pitch is uneven over the entire length of the thread, or bamboo joint marks appear on the thread.
Solutions:
⑴ Incorrect pitch over the entire length of the thread: The cause is improper gear selection or incorrect feed box handle position. Recheck the feed box handle position or recalculate the gear ratio.
⑵ Local incorrectness: The cause is local errors in the pitch of the lathe lead screw itself (usually caused by wear). Replace the lead screw or repair it locally.
⑶ Uneven pitch over the entire length of the thread: Causes include axial movement of the lead screw, axial movement of the spindle, poor meshing between the half-nut of the apron and the lead screw, wear of the apron dovetail guide causing instability when the half-nut is closed, excessive gear backlash, etc. Detection: If it is caused by axial movement of the lead screw, adjust the adjusting nut at the connection between the lathe lead screw and the feed box to eliminate the axial clearance of the thrust ball bearing at the connection. If it is caused by axial movement of the spindle, adjust the spindle rear adjusting nut to eliminate the axial clearance of the rear thrust ball bearing. If it is caused by poor meshing due to misalignment between the half-nut of the apron and the lead screw, repair the half-nut and adjust the half-nut clearance. If it is due to wear of the dovetail guide, replace the dovetail guide and insert strip to achieve the correct fit. If there is excessive backlash in the gear, readjust the gear backlash.
Large, readjust the gear backlash.
⑷ Bamboo joint marks: The cause is periodic errors in the gear transmission from the spindle to the lead screw, such as the gears in the gear box, the gears in the feed box, due to manufacturing errors, local wear, or eccentric installation of the gears on the shaft, causing the center of rotation to be low, resulting in periodic uneven rotation of the lead screw, and periodic uneven movement of the tool, leading to the appearance of bamboo joint marks. Replace the gears with errors or wear.
4. Incorrect pitch diameter
Failure analysis: The cause is excessive cutting depth, inaccurate dial, and failure to measure carefully and promptly.
Solution: During finishing, carefully check whether the dial is loose, the finishing allowance should be appropriate, the cutting edge of the tool should be sharp, and the correct measurement method and reasonable measuring tools should be used for careful measurement.
5. Roughness of threaded surface
Fault Analysis: The cause is due to the dull cutting tool edge, improper cutting fluid, inappropriate cutting speed and workpiece material, and vibrations during the cutting process.
Solutions: Correctly dress the grinding wheel or use an oilstone to precisely sharpen the tool; Select appropriate cutting speed and cutting fluid to avoid built-up edge; Use a small feed rate to control the chip discharge along the vertical axis to avoid scratching the processed surface; Adjust the lathe bed saddle pressure plate and the gap between the dovetail guides of the medium and small slide plates to ensure the accuracy of the gaps between the guides and prevent vibration during cutting; Select a tool holder with sufficient rigidity, and the tool holder should not extend too long.
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