There are many types of threaded products that can be seen everywhere in people's daily lives, such as bolts, screws, lead screws, screws, nuts, and plugs, which are closely related to people's clothing, food, housing, and transportation. Threads can be divided into connecting threads and transmission threads according to their purposes, and can be classified into triangular, rectangular, circular, trapezoidal, and serrated threads according to their tooth types. There are many processing methods for threaded products, and external threads such as bolts and screws are mostly processed using turning methods. For screws with small thread diameters, rolling or rubbing can improve processing efficiency during mass production. Internal thread machining is generally done using a tap, and larger internal threads can be threaded using a lathe.

With the development of machining technology, CNC machine tools have been widely used in factories, and threading with CNC lathes is one of the commonly used methods in thread machining. It can process both ordinary threads and complex shaped irregular threads through program control. The threads processed by CNC lathes have high precision, high product consistency, fast processing speed, good surface quality, and easy debugging. Car threads can produce various defects, which are caused by factors such as machine tools and equipment, as well as cutting tools and operators. Now analyze the common adverse phenomena and corresponding measures in thread machining from the following aspects.

1. Large burrs on the external thread end face or internal thread hole

When turning external threads such as bolts and screws, the outer diameter of the bar material is usually turned to the larger diameter of the thread, and then the end face is chamfered. If not chamfered, the beginning of the thread is prone to outward rotation, resulting in significant burrs. Such burrs are easy to prick hands, which is not conducive to processing operations and can also affect measurement and subsequent assembly. The size of the chamfer can also affect the effectiveness of removing burrs. When the chamfer is large, it affects the appearance of the thread and the effective length of the thread; When chamfering is small, burrs may appear. The chamfer size for turning external threads is generally the size of the thread pitch. For example, when machining M10 screws, the chamfer size of C1.5 is more suitable because the standard pitch of M10 is 1.5mm. Chamfering of internal threads to the larger diameter of the thread, such as machining M10 threaded holes, first drill the bottom hole of the thread with a φ 8.5 drill bit, and then chamfer with a drill bit that is two pitches larger than the diameter of the bottom hole by about φ 14. After chamfering, the threads will no longer have burrs at the beginning of the threads.

2. The thread has irregular teeth and buckles

When turning threads on a regular lathe, the wheel is hung according to the pitch (lead) of the thread. When feeding, the spindle rotates in the forward direction, and when retracting, the spindle rotates in the reverse direction. A strict motion relationship must be maintained between the spindle and the tool, that is, for every revolution of the workpiece driven by the spindle, the tool should move uniformly by a constant distance, which is the pitch (or lead) of the thread. This way, the cutting point will be the same every time without causing tooth misalignment.

The reason why there is no need to reverse the tool when machining threads on a CNC lathe and there will be no misalignment is that a photoelectric encoder is installed on the spindle of the CNC lathe. The spindle, which rotates with the workpiece, is transmitted to the spindle encoder through a synchronous belt. After the spindle encoder detects the speed of the spindle, the information is fed back to the machine tool's numerical control system. The numerical control system then sends instructions to strictly control the distance of the tool moving one pitch (lead) per revolution of the spindle according to the programmed pitch (lead) size, and ensures the position of each feed point. Even if the spindle speed is fast, it is easy to find each feed point, so that there will be no misalignment during thread machining.

When machining threads with a CNC lathe, sometimes there may be defects such as messy or rotten teeth, which may be caused by the following reasons: (1) damage to the photoelectric encoder. The photoelectric encoder is usually installed at the end of the lathe spindle, and can be replaced by simply opening the protective cover on the side of the lathe spindle box. (2) The synchronous belt teeth are severely worn. The wear of the synchronous belt can cause the encoder and spindle transmission to be out of proportion, affecting the spindle speed and the pitch (lead) relationship formed between the tools, resulting in disorderly turning teeth. Synchronous belts are fragile components that connect encoders and spindles, making them easy to disassemble and install. (3) The spindle of the CNC lathe moves axially and there is a gap. Just adjust the clearance in the axial screw nut. If the gap is small, the system gap automatic compensation function can be used to modify parameters for compensation; If the gap is too large and maintenance is difficult, the screw nut must be removed and a washer of corresponding thickness must be added to the nut according to the amount of play. (4) There are issues with the program developed by the operator. When programming, the main task is to determine the positioning points. When preparing layered machining for CNC turning threads, it is important to ensure that the axial positioning points remain consistent each time, which can effectively avoid tooth misalignment. For example, when processing M20 screws with an effective thread length of 50mm, the programming is as follows:

Every time you retract the tool, you should also pay attention to the radial retraction distance. If the diameter remains the same during retraction, and the retraction is still based on the original diameter or the distance is too small, the tool tip will damage the processed tooth profile or flatten the tooth tip, resulting in waste. Especially for beginners machining threads, this phenomenon often occurs.

Due to the need for multiple cutting of the car thread, the Z-axis must be positioned the same each time, otherwise there will be misalignment during machining. Nowadays, most systems have composite loop instructions, and once the positioning point is determined, there is no need to reset it for each layer processing in the future. The single turning cycle instructions G92 and G76 for threading belong to such preparation function instructions.

3. Unstable pitch phenomenon at the beginning and end of thread processing

The positioning point for each thread machining must be the same, whether it is set by the programmer for the G32 code or guaranteed by the internal parameter values of the machine tool system for the composite instruction G92. During the threading stage, it is necessary to ensure that the spindle drives the workpiece to rotate once, and the cutting tool moves one pitch, so that the processed parts do not have broken teeth or messy teeth. However, in the initial stage of thread machining, due to the fast speed of rotation and tool movement, when the tool reaches the surface of the workpiece, it is not yet possible to ensure the constant values of spindle speed and tool movement (pitch or lead), often resulting in unstable pitch at the beginning of the thread, with larger pitches being smaller than smaller, making it difficult to screw in the nut during assembly. When the thread machining is about to end, the spindle speed and tool movement speed will decrease, and the phenomenon of pitch instability will also occur. In order to overcome this phenomenon during machining, the Z-value distance at the beginning of turning is set to be longer during each programming, and the unstable stage of machining is used for tool idle running until it stabilizes before turning begins. As in the above program, the Z value can be set to a distance of 5mm or even longer from the right end face of the workpiece. For the end section, there is often a backlash groove in the thread structure, and the stage of unstable thread pitch is in the backlash groove. This structure effectively solves the problem of unstable thread pitch in the end section.

The programming example is as follows:

4. Knife stabbing

The phenomenon of tool piercing is often encountered in machining, which is closely related to the installation height and grinding angle of the cutting tool. If the installation of the threading tool is too high, when the cutting depth reaches a certain value, the back face of the tool will press against the workpiece, increasing friction and even bending the workpiece. If the installation of the turning tool is too low, the chips are not easily discharged. The direction of the radial force of the turning tool is at the center of the workpiece. In addition, the clearance between the horizontal lead screw and the nut is too large, causing the cutting depth to automatically increase, thereby lifting the workpiece and even causing blade breakage. The workpiece clamping is not firm enough to withstand the cutting force during turning, resulting in excessive deflection and changing the center height between the cutting tool and the workpiece. The workpiece is raised, causing a sudden increase in cutting depth. Excessive rake angle and tool wear can also lead to tool stabbing. The general methods to avoid stabbing include:

(1) Adjust the height of the cutting tool in a timely manner to ensure that its tip is at the same height as the axis of the workpiece. The usual method is to use the tailstock to align the tool. During rough and semi precision turning, the position of the cutting tip should be about 1/100 higher than the center of the workpiece diameter;

(2) Timely grinding and reducing the rake angle of the threaded turning tool, repairing and adjusting or automatically compensating to reduce the clearance between the X-axis screw, are also common practices to avoid tool sticking;

(3) When machining threads, do not choose too much back cutting amount and cutting speed. Choose a reasonable cutting amount based on the thread pitch (lead) size and workpiece rigidity.

5. Inaccurate dental shape

Sometimes the processed thread profile may deform, which is mainly manifested as a larger or smaller tooth tip angle, or an asymmetric tooth profile that deviates to one side. The main reasons for inaccurate tooth shape are as follows.

The angle deviation of tool grinding is relatively large. The tip angle of a regular triangular thread tool is 60 °, and the tip angle of a trapezoidal thread tool is 30 °. When grinding, the tool angle template should be used for measurement. If the angle requirement is not met, re grinding is required. Cutting tools with high precision requirements can be sharpened at different angles on a tool grinder.

The lathe tool is not installed correctly. The centerline of the left and right edges of the thread should be perpendicular to the axis of the lathe spindle during tool alignment, which means that the main and secondary angles of the tool are equal, both at 60 °. If the centerline of the tool is not perpendicular to the axis during tool installation, the machined thread will be skewed to one side, making it impossible for the thread gauge to pass through. If the tool is machined further down, the thread shape will become thinner, affecting the strength of the threaded product. So when installing threaded cutting tools, it is necessary to use a threaded backing plate or a dial gauge to find alignment. First, tighten one bolt on the tool holder that is used to secure the tool. Apply a little force and adjust the angle of the tool before tightening the other bolt on the tool holder. Observe the angle of the tool and alternate between the two bolts to prevent the tool from rotating during tightening.

Tool wear. Most machining tools are made of hard alloy, and thread turning tools are no exception. This is because hard alloy tools have high hardness, wear resistance, high strength, and good toughness. By adjusting the appropriate cutting parameters according to different processing conditions, the durability of the cutting tool will increase. However, any cutting tool will experience wear and tear after prolonged use. Especially when the thread turning tool is sharp and wears out faster, the size of the processed thread will change. At this time, the tool should be removed and re ground or replaced with a new tool in a timely manner.

6. Poor surface quality of threads

The main reasons for poor surface quality and high surface roughness values of threads are as follows.

(1) The handle or workpiece is relatively thin. The handle of the tool extends for a long time, and the handle or workpiece is relatively thin, resulting in poor rigidity. If the cutting amount is too large, vibration will inevitably occur during cutting, causing vibration marks on the surface of the machined thread, resulting in poor surface quality. When cutting threads at high speed, if the cutting thickness is too small or the chips are discharged in an inclined direction, the machined tooth side surface may become rough, resulting in a larger roughness value on the thread surface. Therefore, the cross-section of the handle should be increased as much as possible to reduce the length of the handle extension. The selection of appropriate cutting parameters has a significant impact on surface quality.

(2) There is an issue with the angle of grinding the cutting edge of the turning tool, with an excessively large radial rake angle.

If the radial rake angle is large, or if there is a large gap between the screw nut of the middle sliding plate, it is easy to cause knife stabbing, resulting in vibration marks. The solution is to reduce the radial rake angle of the turning tool. When cutting threads with high-speed steel, the thickness of the chips in the last cut should generally be greater than 0.1mm, and the chips should be discharged along the vertical axis direction without damaging the quality of the processed surface.

(3) Threaded turning tools can produce chip deposits on the tip of the tool. As processing progresses, debris tumors continue to form, grow, and detach. At the same time, due to the embedding of some debris and nodules on the surface of the workpiece, hard points are formed on the surface of the workpiece, which will seriously affect the surface roughness of the thread. The common method to avoid the formation of chip lumps is to increase or decrease the cutting speed, increase the back angle and edge inclination angle appropriately during tool grinding, and select the correct cutting fluid according to the material.

7. Conclusion

There are various reasons for product defects during thread processing, in addition to the influence of factors such as machine tools, equipment, cutting tools, and operators mentioned above, there are also other comprehensive factors. Therefore, the elimination of faults caused by defects should be analyzed on a case by case basis, using various detection and diagnostic methods, supplemented by work experience, to identify specific influencing factors and adopt reasonable and effective solutions.