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Inadequate machining thread on CNC lathe and its countermeasures

There are many kinds of thread products, which are widely seen in people's daily lives. Bolts, screws, screws, screws, nuts, and plugs are closely related to people's clothing, food, housing, and transportation. The thread can be divided into the connection thread and the transmission thread according to the purpose. The thread type can be divided into triangular, rectangular, circular, trapezoidal and zigzag threads. There are many methods for the processing of threaded products. The external threads such as bolts and screws are mostly machined by turning methods. For screws with small thread diameters, rolling or twisting can be used for mass production to improve processing efficiency. Internal threading is generally tapped with a tap, and larger internal threads can be threaded with a lathe.


With the development of machining technology, CNC machine tools have been widely used in factories. Using CNC lathe thread is one of the commonly used methods in thread processing. It can process both ordinary threads and special-shaped thread with complex shapes through program control. The thread produced by the CNC lathe has high precision, high product consistency, high processing speed, good surface quality and easy commissioning. Threads can create a variety of defects, both for machine tools and equipment, as well as for tools and operators. From now on, analyze the common defects and corresponding measures in thread processing.


1, the burr is larger at the end of the external thread or the internal thread hole


When external threads such as bolts and screws are turned, the outer diameter of the bar is usually turned to the major diameter of the thread, and then the end face is chamfered. If you do not chamfer, the starting point of the thread is easy to evert and there is a large burr. Such burrs are easy to puncture and are not only detrimental to the machining operation but also affect the measurement and subsequent assembly. The size of the chamfer will also affect the deburring effect. When the chamfer is large, it affects the appearance of the thread and the effective length of the thread; when the chamfer is small, burrs occur. The chamfer size of the turning external thread is generally the size of the thread pitch. For example, when machining an M10 screw, the M10 standard pitch is 1.5mm, so the chamfer size is C1.5 is more appropriate. The chamfering of the internal thread to the major diameter of the thread, such as the M10 threaded hole, first use the φ8.5 bit to drill the bottom hole of the thread, and then chamfer with two bits larger than the diameter of the bottom hole by about φ14. Threads are machined after chamfering and no burrs are produced at the beginning of the thread.


2, thread chaos, disorderly buckle phenomenon


Ordinary lathe turning thread, according to the thread pitch (lead) hanging round, when the spindle feed forward rotation, spindle reversal when retracting, spindle and tool must maintain a strict relationship between the movement, that is, the spindle drive the workpiece every turn The tool should move evenly over a constant distance. This constant distance is the pitch (or lead) of the thread. In this way, the same point of entry will occur every time without turbulence.


The CNC turning machine does not need to reverse the retracting knife when processing the thread, and the reason for not causing turbulence is that an optical encoder is installed on the spindle of the numerically controlled lathe. With the workpiece rotating with the spindle, the movement of a state sent by a synchronous belt to the spindle encoder, the spindle encoder detects the spindle speed, the information will be fed back to the machine control system, the NC system according to the program Pitch (lead) size, issued instructions to strictly control the spindle every revolution, the tool moves a pitch (lead) distance, and to ensure the position of each feed point, even if the spindle speed is faster, you can still easily find each time Feeding point, so that there will be no turbulence when machining threads.


When machining a thread on a CNC lathe, problems such as spurs or rotten teeth may occur. The reasons for this may be as follows: (1) Damage to the photoelectric encoder. Optical encoders are usually installed at the end of the spindle of the lathe. The encoder can be replaced by opening the protective cover on the side of the headstock of the lathe. (2) Synchronous belt wear is serious. The wear of the timing belt will cause the encoder and spindle drive to be out of proportion, affecting the relationship between the spindle speed and the pitch (lead) formed between the tools and causing turbulence in turning. The timing belt is a fragile item. It is a transmission element that connects the encoder and the spindle. It is very convenient for disassembly and installation. (3) Spindle axial movement of CNC lathe, there is a gap. Just adjust the gap in the axial screw nut. If the gap is small, you can use the system gap automatic compensation function to modify the parameters to compensate; if the gap is too large, maintenance is more troublesome, you must remove the screw nut, according to the amount of turbulence in the nut and the corresponding thickness of the gasket. (4) There is a problem with the operator's program. When programming, the positioning point is mainly determined. When CNC lathe threading is programmed in a layered process, attention must be paid to ensuring that each axial alignment point is consistent and that turbulence can be effectively avoided. For example, when machining an M20 screw, the effective thread length is 50mm. 



Every time the knife is retracted, pay attention to the radial retract distance. If the diameter is the same when retracting the knife, it still exits or the distance is too small according to the original diameter. At this time, the cutting edge will damage the processed tooth profile or tip. Che Ping, causing waste products. Especially for beginners, threading often occurs.


Since the thread of the car needs to be divided by multiple times, the Z-axis must be positioned the same each time, otherwise machining will occur. However, most systems now have compound loop instructions. Once the positioning point is determined, each subsequent layering process does not have to be reset. The single turning cycle instructions G92 and G76 threads are such preparatory function instructions.


3, screw pitch instability at the beginning and end of thread processing


Each positioning point of thread processing must be the same, whether the positioning point of the G32 code is set by the programmer, or the compound command G92 is guaranteed by the internal parameter values of the machine tool. The thread processing stage must ensure that the spindle drives the workpiece one rotation per turn, and the turning tool can take a pitch, so that the machined parts can not be scraped or confuse. However, at the beginning of the threading process, due to the faster rotation speed and moving speed of the turning tool, when the turning tool reaches the surface of the workpiece, the fixed value (pitch or lead) of the spindle rotation speed and the turning movement of the turning tool will often occur. The thread pitch at the beginning of the thread is not very stable and the pitch is mostly too small, making it difficult for the nut to screw in during assembly. At the end of the threading process, the spindle speed and the tool movement speed will decrease, and pitch instability will also occur. In order to overcome this phenomenon during processing, the Z distance at the beginning of turning is set to be longer in each programming, and the unstable stage of machining is used for the idle movement of the tool. After the stabilization, the turning is started. As in the above procedure, the Z value can be set to a distance of 5 mm or more from the right end of the workpiece. For the end section, there is often an undercut groove in the thread structure, and the thread pitch is unstable in the undercut groove. This structure solves the instability of the pitch in the end section.



4, tie knife


The knives phenomenon often encountered in the car processing, which has a great relationship with the installation height and sharpening angle of the turning tool. If the thread cutting tool is installed too high, the cutting surface of the turning tool will resist the workpiece when the depth of cut reaches a certain value, which increases the friction and even bends the workpiece. When the turning tool is installed too low, the chips are not easy to discharge. The direction of the radial force of the turning tool is the center of the workpiece, and the gap between the horizontal screw and the nut is too large, so that the depth of the knife is automatically deepened and the workpiece is lifted. It even caused a chipping. The workpiece is not firmly clamped so that its own rigidity can not withstand the cutting force when turning, resulting in excessive deflection, changing the center height of the turning tool and the workpiece, the workpiece is raised, resulting in a sharp increase in cutting depth. Excessive tool rake angles and tool wear can also occur. The methods for avoiding knives are roughly:


(1) Adjust the turning tool height in time so that the tool tip and workpiece axis are at the same height. The usual practice is to use the tailstock* to set the tool. When the roughing and semi-finishing, the tool tip position is higher than the center of the workpiece. About 1/100 of the diameter of the processed part;


(2) Sharpening and reducing the rake angle of the thread turning tool in time, repairing or automatically compensating to reduce the gap of the X-axis screw, is also a common practice to avoid the phenomenon of twisting knife;


(3) Do not select the amount of backing tool and cutting speed when machining the thread. Select a reasonable cutting amount according to the thread pitch (lead) size and workpiece rigidity.


5, dental inaccurate


Sometimes the processed thread profile will be deformed. This deformation is mainly manifested in the large or small angle of the cusp, or the asymmetry of the shape of the teeth. The reasons for the inaccurate shape of teeth mainly include the following aspects. (1) The tool sharpening angle has a large deviation. Ordinary triangular thread corner angle is 60°, trapezoidal thread corner angle is 30°, and tool angle measurement is used for sharpening. If it does not meet the angle requirement, it needs to be sharpened again. A high-precision turning tool can be sharpened on a tool grinder, as shown in Fig. 1.


Figure 1 Thread positioning and path planning


(2) The turning tool is not installed correctly. The centerline of the left and right two edges of the thread should be perpendicular to the spindle axis of the lathe when setting the tool, ie the primary and secondary angles of the tool are equal to 60°. If the center line of the tool is not perpendicular to the axis when the tool is installed, the processed thread teeth will be twisted to one side, so that the thread passing rules cannot pass. If the thread is processed further, the thread profile will be thin, affecting the strength of the thread products. . Therefore, when installing a threaded tool, be sure to use a threaded back plate or dial indicator to correct. Twist a bolt on the tool holder to fix the tool, use a little bit of force, adjust the angle of the tool, and then twist the other bolt on the tool holder to see the angle of the tool. When the two bolts are screwed together, they will not be tightened. When the tool rotates, as shown in Figure 2.


Figure 2 Ordinary triangular thread sharpening and installation angle diagram


(3) Tool wear. Machining tool materials are mostly carbide, thread cutter is no exception, because carbide tools have high hardness, wear resistance, high strength and good toughness. According to different processing conditions, adjusting the proper cutting amount will increase the tool's durability. However, any tool wears after a long period of use. In particular, the thread turning tool is more pointed and wears more quickly, and the thread size produced may change. In this case, remove the tool in time to re-grind or replace the new tool. Figure 3 shows a typical wear curve for a carbide turning tool.


Figure 3 Typical Wear Curves of Cemented Carbide Turning Tools


6, thread surface quality is poor


The reason for the poor surface quality of the thread and the large surface roughness is mainly due to the following points.


(1) The shank or workpiece is fine. The shank extension is longer, and the shank or workpiece is finer. The rigidity of the shank or the workpiece is poor. If the cutting amount is too large, vibration will occur during cutting, so that the surface of the machined thread will have vibrating knife marks. The surface quality is very poor. . When the thread is cut at a high speed, the cutting thickness is too small or the chips are discharged in an oblique direction, and the flank surface is roughened, resulting in a large surface roughness of the thread. Therefore, the section of the shank should be increased as much as possible to reduce the shank extension length. Choosing the right cutting amount has a great influence on the surface quality.


(2) There is a problem with the angle of turning of the turning tool, and the radial rake angle is too large.


If the radial rake angle is large, or if there is a large gap in the center slide screw nut, it is easy to generate a knives to generate a shock knife pattern. The solution is to reduce the radial rake of the turning tool. When high-speed steel cuts the thread, the chip thickness of zui after the knife is generally larger than 0.1mm, and the chip is discharged along the vertical axis direction without destroying the quality of the processed surface.


(3) The edge of the thread turning tool will generate built-up edge. BUE is continuously generated, grown up, and shed as processing proceeds. At the same time, because some of the built-up edge debris is embedded on the surface of the workpiece and forms hard spots on the surface of the workpiece, these conditions will seriously affect the thread surface roughness. The common method to avoid the buildup of BUE is to increase or decrease the cutting speed. When the tool is sharpened, the backlash angle and the cutting edge angle should be increased, and the correct cutting fluid should be selected according to the material.


7. Conclusion


The causes of product defects in the processing of threads are various, and in addition to the above-mentioned factors such as machine tools, equipment, tools, and operators, there are other comprehensive factors. Therefore, troubleshooting failures should be based on specific analysis of specific circumstances, through a variety of testing and diagnostic tools, supplemented by work experience, identify specific factors, and take reasonable and effective solutions.

Contact Us
Shandong Jining Qingfeng Machinery Hardware Co.,Ltd

Address: NO.47 Jinyu Road, Jining, Shandong

Tel: +86-537-3281595

Fax: +86-537-3281597

E-mail: info@qfmh.com

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