Common faults and solutions in thread machining wi

2022-08-23
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Common faults and solutions when machining threads with CNC lathe

threads are continuous protrusions and grooves with the same profile and specified tooth shape formed along the spiral line on the surface of a cylinder or cone. In various mechanical products, parts with threads are widely used. It is mainly used as connecting parts, fastening parts, transmission parts, measuring parts and so on. Machining thread on CNC lathe is one of the commonly used machining methods. Machining thread on CNC lathe has the advantages of high machining accuracy, good product uniformity and wide machining range, which plays an increasingly important role in machining. Starting from the practical application, this paper expounds the faults that are easy to occur in the cutting process due to the reasons of equipment, tools or operators when machining threads on the NC lathe, and the solutions

with the development of science and technology, CNC lathes are becoming more and more popular. Turning threads on CNC lathes is more and more used in machining. CNC lathes are playing a more and more important role in machining because of their advantages such as high machining accuracy, good product identity, wide machining range, convenient debugging (especially that they can accurately process some special surface parts that are difficult to process on ordinary cars). Common faults and solutions when turning threads on lathes with high degree of NC automation are:

1 Cutting tool

1.1 main reasons

(1) the front angle of the turning tool is too large, and the clearance of 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) Excessive wear of turning tool; (5) The cutting amount is too large

1.2 solutions

(1) reduce the rake angle of the trolley, repair the machine tool, adjust the screw clearance of the X axis, and use the screw clearance automatic compensation function of the NC lathe to compensate the screw clearance of the X axis of the machine tool

next, Jinan Shijin, the manufacturer of Jinan experimental machine, will introduce to you what kind of 8 (2) turning tools are installed too high or too low: if it is too high, when the knife reaches a certain depth, the rear face of the turning tool will support the workpiece, increase the friction, and even bend the workpiece, causing the phenomenon of cutting; If it is too low, the chip is not easy to be discharged. The direction of the radial force of the turning tool is the center of the workpiece. In addition, the clearance between the transverse lead screw and the nut is too large, resulting in the cutting depth continuously and automatically tends to deepen, thus lifting the workpiece and cutting. 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 tool can be adjusted by using the center of the tailstock). During rough turning and semi fine turning, the position of the tool tip is about 1% d higher than the center of the workpiece (d represents the diameter of the workpiece to be machined)

(3) the workpiece is not clamped firmly: the rigidity of the workpiece itself cannot bear the cutting force during turning, resulting in excessive deflection, which changes the center height of the turning tool and the workpiece (the workpiece is raised), resulting in a sudden increase in cutting depth and knife pricking. At this time, the workpiece should be clamped firmly, and the tailstock center can be used to increase the rigidity of the workpiece

(4) excessive turning tool wear: it causes the increase of cutting force, bending the workpiece, and knife pricking. At this time, the turning tool should be polished

(5) the cutting amount (mainly the back cutting amount and cutting speed) is too large: select a reasonable cutting amount according to the lead size of workpiece 5 and the rigidity of workpiece

2. Disorderly screwing

2.1 fault phenomenon

when the lead screw rotates, the workpiece does not rotate by an integer

2.2 main reason

the synchronous drive belt of the machine tool spindle encoder is worn, and the synchronous real speed of the spindle cannot be detected; The program of compiling and inputting the host is incorrect; The x-axis or y-axis screw is worn

2.3 solution

(1) the synchronous belt of the spindle encoder is worn: when turning threads on a CNC lathe, the motion relationship between the spindle and the turning tool is controlled by the command issued by the information processing center of the machine host. When turning threads, the spindle speed is constant, and the moving speed of the X or Y axis can be adjusted according to the size of the workpiece lead and the spindle speed, so the center must detect the real synchronous speed of the spindle, To send the correct command to control the correct movement of the X or Y axis. If the system cannot detect the true rotation speed of the spindle, it will send different instructions to X or Y during actual turning. At this time, when the spindle rotates, the distance of tool movement is not a lead, and the thread will be screwed disorderly during the second tool turning. This kind of plastic experimental machine can stretch, shrink, zigzag, tear, 90 ° peel, 180 ° peel, shear, adhesive force, pull-out force, elongation and other experiments on plastics and films. In this case, we only have to repair the machine tool and replace the spindle synchronous belt

2) the input program is incorrect: in order to prevent disordered thread cutting, we must ensure that the turning path of the next tool coincides with the turning path of the previous tool. On ordinary cars, we use the reverse turning method to prevent disordered thread cutting. On CNC lathes, we use programs to prevent disorderly screwing, that is, when preparing processing programs, we use programs to control the thread cutter to retreat after turning the previous cutter, so that the starting point position of the latter cutter coincides with the starting point position of the previous cutter (equivalent to that when cutting threads on ordinary cars, the thread cutter returns to the spiral groove turned by the previous cutter), so that the threads turned out will not be disorderly screwing. Sometimes, due to the incorrect lead of the program input (the lead of the next program is inconsistent with the lead of the previous program), there will also be disorderly deduction during turning

(3) serious wear of x-axis or y-axis screw rod: repair the machine tool and replace the x-axis or z-axis screw rod

3 incorrect pitch

3.1 main reason

the data transmitted back to the machine tool system by the spindle encoder is inaccurate; Excessive movement of x-axis or y-axis screw rod and spindle; The program compiled and entered is incorrect

3.2 solution

(1) the transmission data of spindle encoder is inaccurate: repair the machine tool, replace the spindle encoder or synchronous transmission belt

(2) excessive movement of x-axis or y-axis screw rod and main shaft: adjust the axial movement of main shaft, and the clearance of x-axis or y-axis screw rod can be compensated by the automatic clearance compensation function of the system

(3) check the procedure and make sure that the instruction lead in the procedure is consistent with the drawing requirements

4 incorrect tooth profile

4.1 main reason

incorrect sharpening of turning tool tip; Incorrect installation of turning tool; The turning tool is worn

4. Through simple connection 2 solution

(1) the sharpening of the turning tool tip is incorrect: sharpen and measure the angle of the turning tool tip correctly. For thread turning with high accuracy requirements for tooth angle, you can use standard mechanical clamping thread cutter for turning, or use a grinder to sharpen the thread cutter

(2) incorrect installation of turning tool: when installing the tool, set the tool with the template, or install the screw thread cutter by aligning the screw thread cutter rod with the dial indicator

(3) turning tool wear: according to the actual situation of turning, reasonably select the cutting amount and repair the turning tool in time

5 major fault analysis of thread surface roughness

5.1 main causes

chip buildup on the tool tip; The rigidity of the tool handle is not enough, causing vibration during cutting; The forward angle of turning tool diameter is too large; When cutting threads at high speed, the cutting thickness is too small or the chips are discharged in an inclined direction, and the machined tooth side surface is roughened; The rigidity of the workpiece is poor, and the cutting amount is too large; The surface roughness of turning tool is poor

5.2 solutions

(1) when cutting with high-speed steel turning tool, reduce the cutting speed and correctly select the cutting fluid

(2) increase the cross-section of the tool handle and reduce the extension length of the tool handle

(3) reduce the forward angle of the trolley tool diameter

(4) when cutting threads with high-speed steel, the chip thickness of the last knife is generally greater than 0.1mm, and the chip is discharged along the vertical axis

(5) select reasonable cutting parameters

(6) the surface roughness of the cutting edge of the tool should be 2-3 grades lower than the surface roughness value of the part

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