Threads are one of the common geometric features in mechanical engineering and are widely used. The machining process of threads includes turning, milling, thread tapping and socket threading, thread grinding, thread lapping, etc. of cutting process.

Thread turning is one of the commonly used machining methods for single piece or small batch production. As a CNC lathe, thread turning processing is one of its basic functions. We share with you the common problems and solutions when turning threads on CNC lathe.


The front angle of the turning tool is too large, and the clearance of the X-axis screw of the machine tool is large.

reduce the front angle of the lathe tool, repair the machine tool to adjust the X-axis screw clearance, the use of CNC lathe screw clearance automatic compensation function to compensate for the machine tool X-axis screw clearance.

The lathe tool is installed too high or too low.

Too high, eat the tool to a certain depth, the back face of the lathe tool topped the workpiece, increasing friction, and even the top of the workpiece bending, resulting in the phenomenon of knife; too low, the chip is not easy to discharge, the direction of the radial force of the lathe tool is the center of the workpiece, coupled with the transverse into the screw and nut clearance is too large, resulting in the depth of the tool to eat the depth of the constant automatic tendency to deepen, thereby lifting the workpiece, the emergence of a solid knife. At this time, the height of the tool should be adjusted in a timely manner, so that the tip of the tool and the axis of the workpiece height (can use the tailstock top of the tool). In rough turning and semi-finish turning, the tip of the tool position than the center of the workpiece is about 1% D (D indicates the diameter of the workpiece being processed).

Workpiece clamping is not secure.

The rigidity of the workpiece can not withstand the cutting force of turning, resulting in excessive deflection, changing the center of the tool and the workpiece height (the workpiece is raised), the formation of a sudden increase in the depth of cut, the emergence of the knife, the workpiece should be clamped securely, the use of the tailstock top, etc., in order to increase the rigidity of the workpiece.

Turning tool wear is too large.

Caused by an increase in cutting force, the top bend workpiece, the emergence of solid knife. Should be sharpened lathe.

Cutting amount is too large.

According to the size of the workpiece guide and workpiece rigidity to choose a reasonable cutting dosage.

Incorrect thread pitch

Inaccurate data transmitted back to the machine system from the spindle encoder; excessive runout of the X- or Y-axis screws and spindle; incorrectly prepared and entered programs.


(1) Inaccurate spindle encoder transmission data: repair the machine tool, replace the spindle encoder or synchronous transmission belt;

(2) X-axis or Y-axis screw and spindle runout is too large: adjust the spindle axial runout, X-axis or Y-axis screw clearance can be compensated for with the system gap automatic compensation function.

(3) Review the program, make sure that the instruction guide in the program is consistent with the requirements of the drawing.

Incorrect tooth shape

Incorrect sharpening of the tip of the lathe tool; incorrect installation of the lathe tool; wear of the lathe tool.


(1) Incorrect tip sharpening of turning tool: correctly sharpen and measure the tip angle of the turning tool; for thread turning that requires high precision of tooth angle, use standard mechanically clamped thread turning tool and sharpen the threading tool with a grinder.

(2) Incorrect installation of the tool: use the sample plate to match the tool when installing the tool, and install the threaded tool by finding the correct threaded tool bar with the percentage table.

(3) Wear of turning tool: according to the actual situation of turning processing, choose the appropriate cutting dosage, and repair and grind the turning tool in time.

Failure analysis of large thread surface roughness

The tip of the tool produces a chip tumor
Cutting with high-speed steel turning tool should reduce the cutting speed, and the correct choice of cutting fluid

The tool shank is not rigid enough, and vibration occurs when cutting.
Increase the cross-section of the tool shank and reduce the length of the tool shank.

The radial front angle of the tool is too large
Reduce the radial front angle of the turning tool

When cutting threads at high speed, the cutting thickness is too small or the chips are discharged in an inclined direction, pulling the surface of the processed tooth side.
High-speed steel cutting threads, the last cut of the chip thickness is generally greater than 0.1mm, and make the chip discharged along the vertical axis direction

Workpiece rigidity is poor, and the cutting dosage is too large.
Select reasonable cutting amount

Poor surface roughness of the turning tool.
The surface roughness of the cutting edge of the tool should be 2 ~ 3 grades smaller than the surface roughness value of the parts processing.


Turning threads produced a variety of failure forms, both the reasons for the equipment, but also the tool, the operator and other reasons, in troubleshooting to specific circumstances, through a variety of testing and diagnostic means, to find out the specific factors affecting the adoption of effective solutions.

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