Research on thread CNC milling processing technology

Table of Contents

Thread processing is a key process in mechanical manufacturing. Although traditional processing methods are mature, they have some limitations in accuracy and efficiency. With the rapid development of CNC technology, thread CNC milling technology has gradually become an advanced and efficient thread processing method. Compared with traditional methods, CNC milling has significant advantages in high precision, efficiency, and material adaptability. This article will discuss thread CNC milling technology in-depth and analyze its working principle, tool selection, machining accuracy, and CNC programming methods.

How are traditional threads processed? What are its disadvantages?

The traditional thread processing method mainly uses a turning tool to cut threads or uses taps and dies for manual tapping or threading. For large-diameter threads, single-tool boring methods are usually used on CNC milling machines, and processing is achieved by controlling the speed and pitch of the boring tool. The advantage of this method is that there is no need to purchase special tools and it is faster to implement. However, the quality of single-tool boring threads is not high and the tools wear out quickly, requiring frequent replacement, grinding, and realignment. In addition, the cutting force is large, multiple feeds are required, and the speed is very slow, resulting in low efficiency. Because the tool shank is long, vibration lines are easily produced on the thread surface, which affects the stability of the processing quality.

Advantages of the CNC Thread Milling Method

With the widespread application of CNC machine tools, thread processing technology has been further improved, and the CNC thread milling method has become an advanced technology in thread processing. Compared with traditional thread processing methods, CNC thread milling has significant advantages in accuracy and efficiency. In addition, it is not limited by thread structure and rotation direction. One thread milling cutter can process a variety of internal and external threads with different rotation directions, which greatly improves the flexibility of processing.

Threadmilling technology also offers other significant advantages. Thread milling cutters are usually made of carbide materials and can have a cutting speed of 80 to 200 meters per minute, while the cutting speed of traditional high-speed steel taps is only 10 to 30 meters per minute. This high cutting speed not only improves processing efficiency but also significantly improves the finish of the thread surface. In the thread processing of high-hardness materials and high-temperature alloy materials (such as titanium alloys and nickel-based alloys), carbide thread milling cutters show ideal performance and can effectively process materials with a hardness of HRC58~62, and have ultra-long service life. This kind of tool also shows excellent performance when processing high-temperature alloy materials, greatly extending the service life.

Compared with taps, when thread milling cutters deal with threaded holes with the same pitch and different diameters, they only need to adjust the CNC program to complete the processing without having to replace multiple tools. In addition, when the thread size is out of tolerance due to wear of the tap, the tool is often scrapped and cannot be used anymore. After the thread milling cutter is worn, the tool radius can be adjusted through the program to continue processing to maintain the thread size. For the processing of high-precision threaded holes, thread milling cutters use tool radius compensation, which is more convenient and effective than manufacturing high-precision taps.

In small-diameter thread processing, especially in the processing of high-hardness and high-temperature materials, the tap is easy to break, causing the thread hole to be blocked or even the part to be scrapped. In contrast, the thread milling cutter has a smaller diameter, so even if the cutter breaks, it will not block the hole, making it easy to remove and avoiding the risk of parts being scrapped. At the same time, the cutting force of thread milling is significantly reduced, especially when processing large-diameter threads, which reduces the load on the machine tool and solves the problem that the tap cannot work normally due to excessive load.

Thread CNC milling

Thread CNC milling is to realize thread processing through CNC machine tool movement. When working, the workpiece or the thread milling cutter rotates once, and the workpiece or the milling cutter moves one pitch along the axial direction to cut out all the threads.

1. Selection of thread CNC milling machine tools

When thread CNC milling is performed, it can be achieved as long as the machine tool is a three-axis linkage CNC milling machine.

2. Thread milling cutter selection

The thread CNC milling processing method uses a special tool-thread milling cutter. The thread milling cutter is a combination of several disc milling cutters set on a mandrel. Its appearance is very similar to the combination of a cylindrical end mill and a thread tap, as shown in Figure 1. But its thread-cutting edge is different from that of a tap, and there is no helical lift on the tool. There are currently two most commonly used thread milling cutters, namely carbide solid thread milling cutters and machine-clip thread milling cutters, as shown in Figures 1 and 2.

Figure 1 Carbide solid thread milling cutter

Figure 2 Machine-clamped thread milling cutter

Thread milling cutters only have 1 to 2 cutting edges in a circle. Therefore, the cutting amount of solid thread milling cutters and machine-type thread milling cutters is very small during operation.
Is different. When selecting a thread milling cutter, the pitch of the thread milling cutter (the distance between two points corresponding to two adjacent teeth on the thread milling cutter along the milling cutter axis) must be equal to the pitch of the thread being processed. For internal threads, except In addition to the above conditions, the outer diameter of the thread mill must be less than 0.8 times the diameter of the thread bottom hole to be processed.

3. CNC thread milling cutter position trajectory

The thread milling cutter position trajectory in CNC milling is an equal-pitch spiral, and the trajectory is shown in Figure 3. The mathematical model of the thread milling cutter position trajectory is shown in formula (1).

Figure 3 Milling cutter position trajectory

In the formula: αϵ [0,h*2π/ρ], h is the tread depth;
ρ is the thread pitch;
X_c, y_c, and z_c are thread position coordinates;

D is the milling cutter diameter;
d is the major diameter of the internal thread or the minor diameter of the external thread;
δ is the machining allowance. If the milling cutter is worn during finishing, it can be used as the milling cutter wear amount to compensate;
m is the control quantity of internal and external threads. When it is an internal thread, m=-1; when it is an external thread, m=+1;
n is the left-hand thread control quantity. When it is a left-hand thread, n=-1; when it is a right-hand thread, n=+1.
The cutting method is to cut in with a spiral line, as long as the cutting spiral line is above the thread to be processed. The retracting method is to exit in a straight line, toward the center for internal threads, and outward for external threads.

4. Thread milling step length

During thread milling, since the tool path is a helix, the helical interpolation instructions of the CNC system can be directly used. However, since the instruction format of each CNC system is different, you must be particularly familiar with the instructions when programming. To simplify programming, this article performs linear interpolation on the spiral, that is, the spiral is fitted into a line segment.

The determination of the tool path length is a basic and important issue in linear interpolation tool path generation. The small step length means that the density of tool position data on the tool path line is high, the part program expands, and the programming efficiency decreases. More importantly, in the general processing mode, the execution of the small-step part program will produce feed speed fluctuations. And the average speed decreases, thus affecting surface quality and processing efficiency. Excessive tool position step size means that the density of tool position data on the tool path line is small and the machining efficiency is high, but the contour approximation accuracy is reduced and the thread surface quality deteriorates. Therefore, the determination of a reasonable step size is very important.

The step length of thread milling is related to the variable a in formula (1). The greater the increment of a, the larger the step length and the greater the machining error, as shown in Figure 4. Therefore, controlling the increment of an in formula (1) can control the machining error and determine the step length of thread milling.

Figure 4 Determination of tool step length

When the increment Δ α of α is small, the curve between two adjacent tool points can be approximated as an arc with a radius r, as shown in Figure 4. Then the relationship between error e and allowable error E is:

Among them: r is the nominal diameter of the thread; E is the allowable error of thread processing.

Thread milling CNC programming system

According to formula (1), the author developed a thread milling CNC programming system, and the interface is shown in Figure 5. As long as you know the parameters of the thread, the position of the thread, the parameters of the milling cutter, the machining error, and the machining allowance, you can automatically output the thread CNC milling program.

Figure 5 Thread milling CNC programming system

Thread milling cutting parameter selection

Selecting reasonable cutting parameters is a key factor in improving processing efficiency, ensuring thread quality, and improving tool durability. If the parameters are improperly selected, the cutting will be unstable, the blade will chip, or the processing efficiency will be too low. In the most serious case, the quality of the thread will be affected.

1. Tool feed speed F

F=N*F_Z*Z              (2)
N is the spindle speed;
Z is the number of teeth per week of the thread milling cutter;
F_Z is the feed amount per tooth of the milling cutter.

After the feed per tooth of the milling cutter F_Z is determined, the key is to match the relationship between the cutting speed F and the spindle speed N according to equation (2). The feed per tooth of the milling cutter F_Z is provided by experience or the milling cutter manufacturer and is generally 0.1~0.2mm/Z.

2. Cutting depth αρ

αρ=m*(D_初-d)-δ            (3)
Among them, m, d,δ have the same meaning as formula (1), D_初 which are the initial direct diameter of the bottom hole of the internal thread and the initial cylindrical diameter of the external thread. The cutting depth αρ must be determined during programming and controlled by the machining allowance δ. The value of cutting depth αρ is 1~2mm.

Conclusion

Through practical application verification, thread CNC milling technology not only performs well in processing accuracy and efficiency but also greatly improves the service life and adaptability of the tool. With the help of the thread milling CNC programming system, the programming process is simpler, further improving processing efficiency and quality. The wide application of this technology not only optimizes the thread processing process but also provides more possibilities for high-precision thread manufacturing in the future. It is foreseeable that with the continuous advancement of CNC technology, thread CNC milling technology will play a more important role in the manufacturing industryα