Analysis of the causes of five common failures in thread processing and their solutions

Failure analysis:

⑴. The reason is that the turning tool is installed too high or too low: the turning tool is installed too high, which reduces the actual clearance angle of the tool. When the tool reaches a certain depth, the flank face of the turning tool bears against the workpiece, increasing friction If the turning tool is installed too low, the actual rake angle of the tool will increase, and the chips will not be easily discharged, which will cause the tool to be pierced.

⑵. The workpiece or turning tool is not firmly clamped: the rigidity of the workpiece itself cannot withstand the cutting force during turning, so excessive deflection is generated, which changes the center height of the turning tool and the workpiece (the workpiece is raised), resulting in a sudden increase in the cutting depth , The phenomenon of piercing the knife appears.

⑶, the turning tool wear is serious: the increased friction increases the cutting force and causes the tool to stick.

⑷. Improper selection of feed method and feed rate: When machining with large pitch, both edges and one tip of the tool participate in cutting at the same time. If the feed rate is too high, the cutting force will be too large and the tool will be stuck.

Solution:

⑴ When installing the turning tool, use the tailstock center to set the tool or use the measurement method to make the tool tip and the axis of the workpiece equal. [During rough turning and semi-finishing turning, the position of the tool tip is 1% higher than the center of the workpiece. D is about 0.2~0.5mm, and D=the diameter of the workpiece. ]

⑵. The workpiece and the tool must be clamped firmly, and accessories such as the center frame of the tailstock and the tool holder can be used to increase the rigidity of the workpiece.                                  

⑶. Correctly sharpen the geometrical angle parameters of the tool in time. As shown below:

⑷. The left and right cutting method or the oblique method is adopted in the large pitch machining. The turning tool is single-edge cutting, and the cutting force and friction force are relatively reduced, so the phenomenon of tool sticking is not easy to occur. In actual work, a self-made elastic toolholder (as shown in the figure) is used. When the cutting force exceeds a certain value, the turning tool can automatically let the tool to keep the cutting thickness at an appropriate thickness, effectively avoiding the phenomenon of knife sticking.

Failure analysis:

The main reason for the random buckle failure is that when the lead screw rotates one revolution, the workpiece does not turn over an integer.

Solution:

⑴. When the ratio of the screw pitch of the lathe to the workpiece pitch is not an integral multiple: if the opening and closing nut is used to swing the saddle to the starting position when the tool is retracted, turning will occur when the opening and closing nut is closed again. The tip of the knife is not in the spiral groove cut out of the previous knife, so random buckles appear. The solution is to use the forward and reverse turning method to retract the tool, that is, at the end of the first stroke, do not lift the opening and closing nut, and after the tool is retracted in the radial direction, the main shaft is reversed, so that the turning tool is retracted in the longitudinal direction, and then the first stroke is performed. secondary stroke, so that a reciprocating process, due to transfer between the spindle, tool holder spindle and the movable separation had not, always turning the original spiral groove will not appear Labeling.

⑵. For the thread with the ratio of the screw pitch of the turning lathe to the workpiece pitch in an integral multiple: the workpiece and the screw are rotating. After lifting the opening and closing nut, at least wait for the screw to turn around before re-closing the opening and closing nut. In this way, when the lead screw is rotated once, the workpiece is rotated by an integer multiple, and the turning tool can enter the spiral groove that was turned out by the previous tool, and there will be no random buckling.

Failure analysis:

The thread is not correct in full length or partly, the pitch of the thread is uneven or the slub pattern appears on the thread.

Solution:

⑴. The thread length is incorrect: the reason is improper matching of the change wheel or the wrong position of the feed box handle. You can recheck the position of the feed box handle or check the change wheel.

⑵ Partially incorrect: the reason is the partial error of the screw pitch of the lathe screw itself (generally caused by wear), the screw can be replaced or partially repaired.

⑶. The reason for the uneven thread pitch on the full length of the thread is: the axial movement of the screw, the axial movement of the spindle, the opening and closing nut of the slide box and the lead screw are not in the same axis, resulting in poor meshing, and the slide box dovetail guide rail. Wear and tear causes the opening and closing nut to be unstable when closing, and the hanging wheel gap is too large. Pass detection: If it is caused by the axial movement of the screw, the adjustment round nut at the connection between the screw of the lathe and the feed box can be adjusted to eliminate the axial gap of the thrust ball bearing at the connection. If it is caused by the axial movement of the main shaft, adjust the nut after the main shaft to eliminate the axial clearance of the rear thrust ball bearing. If it is caused by poor meshing caused by the different axis of the opening and closing nut of the slide box and the lead screw, the opening and closing nut can be trimmed and the opening and closing nut gap can be adjusted. If the dovetail rail is worn, dovetail rails and inserts can be prepared to meet the correct matching requirements. If the gap between the hanging wheels is too large, readjust the gap between the hanging wheels.

⑷. Slub pattern: the reason is that the gear transmission from the main shaft to the lead screw has periodic errors, such as the gear in the gear box, the gear in the feed box is due to its own manufacturing error, or partial wear, or gear The installation of eccentricity on the shaft causes the rotation center to be low, which causes the periodic uneven rotation of the lead screw, and the uneven periodicity of the tool movement, resulting in the appearance of slub patterns, and the error or worn gears can be repaired.

Failure analysis:

The reason is that the knife is too big, the dial is inaccurate, and the measurement is not carried out in time.

Solution: When finishing turning, check in detail whether the dial is loose, the fine turning margin must be appropriate, the cutting edge of the turning tool must be sharp, and the correct measurement method and reasonable measurement tool must be selected in time to carefully measure.

Failure analysis:

The reason is that the cutting edge of the turning tool is not polished and clean, the cutting fluid is not appropriate, the cutting speed and the workpiece material are appropriate, and the vibration during the cutting process is caused.

Solution:

Correctly dress the grinding wheel or use oilstone to fine grind the tool; select the appropriate cutting speed and cutting fluid to avoid the generation of built-up edge; use a small feed to control the vertical axis of the chip to be discharged to avoid straining the machined surface; adjust the lathe saddle pressure plate And the insert gaps of the dovetail guide rails of the medium and small slide plates, etc., to ensure the accuracy of the gaps of the guide rails and prevent vibration during cutting; choose a tool holder with sufficient rigidity, and the tool holder cannot extend too long.

In short, there are many types of failures in thread turning, including equipment reasons, tools, operators, etc. When a problem occurs, the specific situation should be analyzed in detail, and the specific problems can be found through various detection and diagnosis methods. Influencing factors, take effective solutions.