Common faults and solutions when machining threads in CNC lathes processing
2021-10-05

Common faults and solutions when machining threads on CNC lathes

Threads are continuous protrusions and grooves with the same cross-section and prescribed tooth profile formed along the spiral line on the cylindrical or conical surface. Among various mechanical products, parts with threads are widely used. It is mainly used as connecting parts, fastening parts, transmission parts, measuring parts, etc.

This article describes the faults in the cutting process due to equipment, tools, or operators when machining threads in CNC lathe and the solutions.

With the development of science and technology, CNC lathes are becoming more and more popular, and turning threads are used more and more in CNC lathe processing. Numerical control lathes are playing an increasingly important role in machining with the advantages of high machining accuracy, good product identity, wide machining range, and convenient debugging. (Especially it can precisely process some special surface parts that are difficult to process on general cars)

Common faults and solutions when turning threads in CNC lathes mainly include:

Pierce a knife

(1) The rake angle of the turning tool is too large, and the X-axis screw gap of the machine tool is large

Reduce the rake angle of the turning tool, repair the machine tool to adjust the screw gap of the X-axis, and use the automatic compensation function of the screw gap of the CNC lathe to compensate the screw gap of the machine tool X-axis.

(2) The turning tool is installed too high or too low

Too high is because when the tool is eaten to a certain depth, the flank face of the turning tool is against the workpiece, increasing the friction force and even bending the workpiece, causing the phenomenon of the knife sticking.

The installation of the turning tool is too low, which makes it difficult for the chips to be discharged. In addition, the gap between the traversing screw and the nut is too large, which causes the depth of the tool to be continuously and automatically deepened so that the workpiece is lifted and the tool is stuck.

When this happens, we should adjust the height of the turning tool in time to make the tip of the tool equal to the axis of the workpiece (the tip of the tailstock can be used to calibrate the tool). In rough turning and semi-finishing turning, we need to adjust the tooltip's position to about 1% D higher than the center of the workpiece (D represents the diameter of the workpiece to be processed).

(3) The workpiece is not firmly clamped

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 raised), resulting in a sudden increase in the depth of cut and the occurrence of cutting.

At this time, we should clamp the workpiece firmly, and we can use the tailstock center to increase the rigidity of the workpiece.

(4) Excessive wear of the turning tool

The cutting force will increase, the workpiece will be bent, and the knife will appear. At this time, we should grind the turning tool.

(5) The amount of cutting (mainly the amount of back cutting and cutting speed) is too large

We need to choose a reasonable cutting amount according to the workpiece lead's size and the workpiece's rigidity.

Random deduction

1. Symptoms

When the lead screw rotates one revolution, the workpiece does not turn over an integer.

2. Main reasons

The synchronous transmission belt of the spindle encoder of the machine tool is worn, and the real synchronous speed of the spindle cannot be detected; the program input to the host is incorrect; the X-axis or Y-axis screw is worn.

3. Solution

(1) Wear the synchronous belt of the spindle encoder

When the CNC lathe is turning the thread, the movement relationship between the spindle and the turning tool is controlled by the instruction issued by the information processing center of the machine tool host. When turning the thread, the spindle speed is constant. The X or Y axis can adjust the moving speed according to the workpiece lead and the spindle speed. Therefore, the center must detect the real synchronous speed of the spindle to issue the correct command to control the X or Y axis to move correctly. If the system cannot detect the real speed of the spindle and will issue different commands to X or Y during actual turning, then the spindle rotates once, and the distance of the tool movement is not a lead, and the thread will be in the second tool turning. Random deduction. In this case, we only have to repair the machine tool and replace the main shaft timing belt.

(2) The input program is not correct

To prevent random threading when turning threads, we must ensure that the next cut's turning path coincides with the previous cut's turning path. In ordinary cars, we use the reverse turning method to prevent random buckles, and in CNC lathes, we use programs to prevent random buckles. That is, when compiling the processing program, we use the program to control the thread cutter to retract the tool after turning the previous one so that the starting point of the next tool coincides with the starting point of the previous tool (equivalent to the thread tool returning to the previous one when turning the thread on a general lathe in the spiral groove of the machine) so that the thread of the machine will not be buckled indiscriminately. Sometimes, due to the incorrect lead input of the program (the latter part of the program lead is inconsistent with the last part of the program lead), random buckles will also occur during turning.

(3) X-axis or Y-axis screw rod is severely worn

Repair the machine tool and replace the X-axis or Z-axis screw.

Incorrect pitch

1. Main reasons

The data sent back by the spindle encoder to the machine tool system is inaccurate; the X-axis or Y-axis screw and spindle movement are too large; the programmed and input program is incorrect.

2. Solution

(1) The data transmitted by the spindle encoder is inaccurate: repair the machine tool, replace the spindle encoder or synchronous transmission belt.

(2) X-axis or Y-axis screw and spindle move too much: adjust the axial spindle movement, the X-axis or Y-axis screw clearance can be compensated by the system clearance automatic compensation function.

(3) Check the program and make sure that the instruction lead in the program is consistent with the requirements of the drawing.

Incorrect tooth shape

1. Main reasons

The tip of the turning tool is not sharpened correctly; the turning tool is installed incorrectly; the turning tool is worn.

2. Solution

(1) Incorrect sharpening of the tip of the turning tool

Correctly sharpen and measure the tip angle of the turning tool. For thread turning that requires high profile angle accuracy, you can use a standard mechanical clamping thread tool to turn or thread the thread. The knife is sharpened with a grinder.

(2) The turning tool is not installed correctly

when installing the tool, use the template to set the tool, or install the normal thread tool by using a dial indicator to find the normal thread toolbar.

(2) Turning tool wear

According to the actual situation of turning, the cutting amount should be selected reasonably, and the turning tool should be repaired in time.

Failure analysis of large thread surface roughness

1. Main reasons

A built-up edge is generated on the tooltip; the tool holder is not rigid enough, and vibration occurs during cutting. The radial rake angle of the turning tool is too large. When high-speed thread cutting, the cutting thickness is too small, or the chips are discharged in an oblique direction, and the machined flank surface is napped. The rigidity of the workpiece is poor, and the cutting amount is too large. The surface roughness of the turning tool is poor.

2. Solution

(1) When cutting with a high-speed steel turning tool, the cutting speed should be reduced, and the cutting fluid should be selected correctly;

(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 high-speed steel is cutting threads, the chip thickness of the last cut is generally greater than 0.1mm, and the chips are discharged along the vertical axis;

(5) Choose a reasonable cutting amount;

(6) The surface roughness of the tool's cutting edge should be 2-3 grades smaller than the surface roughness value of the part machined.


In short, various types of failures occur when turning threads in CNC lathe processing, including equipment reasons, tools, operators, and so on. When troubleshooting, we need to analyze the specific situation, find out the specific influencing factors through various detection and diagnosis methods, and adopt effective solutions.

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