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Common faults and solutions of turning thread

Threads are continuous protrusions and grooves with the same profile formed along a helix on the surface of a cylindrical workpiece. In the machine manufacturing industry, threaded parts are widely used. Thread processing by turning is a commonly used processing method at present.

 

On the horizontal lathe (such as CA6140), four standard threads can be turned: metric, inch, modulus and radial, no matter which thread is turned, a strict motion relationship must be maintained between the lathe spindle and the tool: that is, every rotation of the spindle (i.e., one rotation of the workpiece), the tool should move evenly the distance of one (workpiece) lead. Their motion relationship is ensured in such a way that the spindle rotates with the workpiece, and the movement of the spindle is transmitted to the feed box through the hanging wheel; After changing the speed from the feed box (mainly to obtain various pitches), it is then transmitted to the lead screw; The tool holder is driven to move in a straight line by the opening and closing nut on the screw and the slide box, so that the rotation of the workpiece and the movement of the tool are realized by the drive of the spindle, so as to ensure the strict motion relationship between the workpiece and the tool. In the actual turning of threads, due to various reasons, the movement between the spindle and the tool, there is a problem in a certain case, causing a failure when turning the thread, affecting the normal production, which should be solved in time. The common faults and solutions when turning threads are as follows:

1.   Gnawing cutter
Failure analysis and solution: The reason is that the turning tool is installed too high or too low, the workpiece is not clamped firmly or the turning tool is worn too much.

A. If the turning tool is installed too high or too low or too high

when the tool is reach to a certain depth, the flank surface of the turning tool will resist the workpiece, increase the friction, and even bend the workpiece, resulting in the phenomenon of Gnawing cutter; If it is too low, the chips are not easy to discharge, and the direction of the radial force of the turning tool is the center of the workpiece, and the gap between the transverse lead screw and the nut is too large, resulting in the depth of the cutter automatically tending to deepen, so as to lift the workpiece and Gnawing cutter， At this time, the height of the turning tool should be adjusted in time to make its tip equal to the axis of the workpiece (the tailstock center can be used to set the tool). In rough and semi-finishing, the position of the cutting edge is about 1% higher than the center of the workpiece (D indicates the diameter of the workpiece to be machined).

B. The workpiece clamping is not firm

the rigidity of the workpiece itself can not withstand the cutting force during turning, so it produces excessive deflection, changes the center height of the turning tool and the workpiece (the workpiece is raised), forms a sudden increase in cutting depth, and the edge was broken.

C. The cutting force increases due to excessive wear of the turning tool

 and the workpiece is bent and break edge. At this point, the turning tool should be resharpened.

2.Unjustly thread

Analysis and solution:

The reason is that when the lead screw rotates once, the workpiece does not turn the whole number and turns.

A. When the ratio of the pitch of the lead screw of the lathe to the pitch of the workpiece is not an integer multiple, if the opening and closing nut is used to shake the saddle to the starting position when the cutter is withdrawn, then, when the opening and closing nut is closed again, the tip of the turning tool is not in the spiral groove turned out by the previous tool, so that the buckle occurs.

The solution is to use the positive and negative turning method to retract the tool, that is, at the end of the first stroke, do not lift the opening and closing nut, after the cutter is withdrawn along the radial direction, the spindle is reversed, so that the turning tool is returned along the longitudinal direction, and then the second stroke is carried out, so that in the reciprocating process, because the transmission between the spindle, the lead screw and the tool holder has not been separated, the turning tool is always in the original spiral groove, and there will be no random buckle.

B. For the threaded workpiece and the lead screw of the turning lathe screw pitch and the ratio of the workpiece distance are integer multiples,

after lifting the opening and closing nut, at least wait for the lead screw to turn once, before the opening and closing nut can be re-closed, so that when the leading screw turns once, the workpiece is rotated by integer times, and the turning tool can enter the spiral groove that the previous tool turns out, and there will be no random buckle, so that the opening and closing nut can be used to manually retract the knife. This is a quick retract, which is conducive to improving productivity and maintaining the accuracy of the lead screw, and at the same time the lead screw is safer.

3.incorrect pitch. Failure analysis and solution of incorrect pitch:

    A. Incorrect thread length

The reason is that the hanging wheel is improperly matched or the handle position of the feed box is wrong, you can re-check the handle position of the feed box or check the hanging wheel.

B. localities are incorrect

The reason is that due to the local error of the pitch of the lathe lead screw itself (generally caused by wear), the lead screw can be replaced or locally repaired.

C. Uneven pitch on the whole length of thread

The reason is:

Axial movement of the lead screw.

Axial movement of the main shaft.

The opening and closing nut of the slide box is not coaxial with the lead screw, resulting in poor meshing.

The dovetail guide rail of slide box is worn, which causes the opening and closing nut to be unstable when closed.

The gap of the hanging wheel is too large.

Through detection:

 if it is caused by the axial channeling of the lead screw, the adjustment round nut at the connection between the lead screw of the lathe and the feed box can be adjusted to eliminate the axial clearance of the thrust ball bearing at the connection.

If it is caused by axial channeling of the spindle, the rear adjustment nut of the spindle can be adjusted to eliminate the axial clearance of the rear thrust ball bearing.

If it is caused by poor meshing caused by different shafts between the opening nut and the lead screw of the slide box, the opening nut can be trimmed and the gap between the opening and closing nuts can be adjusted.

If the dovetail rail is worn, the dovetail rail and inserts can be formulated to meet the correct fit requirements. If the gap between the hanging wheels is too large, you can readjust the gap between the hanging wheels.

The reason for the occurrence of slub pattern is that there is a periodic error in the gear transmission from the spindle to the lead screw, such as the gear in the hanging wheel box, the gear in the feed box due to itself, manufacturing error, or local wear, or the gear installation eccentricity on the shaft, etc., resulting in a low center of rotation, which causes the periodicity of the screw rotation to be uneven, and the periodicity of driving the tool to move is uneven, resulting in the appearance of slub pattern, and the gear with error or wear can be repaired.

4. the pitch diameter of the incorrect

fault analysis and solution: the reason is that the knife is too large, the dial is not accurate, and the measurement is not in time. The solution is to check whether the dial is loose in detail when finishing, the allowance for finishing is appropriate, and the cutting edge of the turning tool should be sharp and measured in time.

5. Thread surface rough fault analysis and solution:

the reason is that the cutting edge of the turning tool is not polished, the cutting fluid is not appropriate, the cutting speed and workpiece material are not suitable, and the vibration generated during the cutting process is successful.

The solution is: correct dressing of the grinding wheel or grinding the tool with a stone; Select the appropriate cutting speed and cutting fluid; Adjust the inlays of the lathe saddle pressure plate and the dovetail guide rail of the medium and small slide plates to ensure the accuracy of the clearance of each guide rail and prevent vibration during cutting.

In short, there are various forms of faults when turning threads, both the reasons of the equipment, the tools and the operator, etc., and the specific situation should be analyzed in the troubleshooting, and the specific influencing factors should be found out through various detection and diagnostic means, and effective solutions should be taken.