是否用数控车床加工钛合金轴类？

titanium alloy shaft parts are widely used in aerospace, medical, high-end equipment and other fields due to their high specific strength, good corrosion resistance and high temperature resistance. However, the processing of titanium alloy is not easy: its thermal conductivity is poor (only about 1/7 of iron), cutting temperature is high; elastic modulus is small, easy to cause vibration and deformation under cutting force; chemical activity is high, high temperature easy to react with tool material to produce "sticking knife" phenomenon. So, is CNC lathe a suitable processing method for titanium alloy shaft parts? Let's talk about this problem from the aspects of actual processing experience and technical principles.

First, CNC lathe can indeed process titanium alloy shaft, but it is not "all-powerful"

In the actual processing, many technicians will ask: "Why not use ordinary lathe to process titanium alloy shaft?" The answer is simple: ordinary lathe relies heavily on manual operation, the processing parameters are difficult to stabilize, and the titanium alloy material "hard" and "sticky" characteristics are more obvious, which is easy to cause tool wear, surface roughness, dimensional accuracy and other problems. For example, we once cooperated with a medical device factory to process a titanium alloy shaft with a diameter of 20mm and a length of 300mm, using a common lathe with high-speed steel tools. Because the cutting speed can't be precisely controlled, the tool was worn out after processing 5 parts, and the surface had obvious "knife marks", which failed to meet the Ra0.8μm requirement of the medical equipment.

The CNC lathe is different. It can accurately control the cutting speed, feed rate, cutting depth and other parameters through the program, and can also realize the automatic tool change and multi-process continuous processing. For example, when we process the same batch of titanium alloy shafts with a CNC lathe (using carbide tools with TiAlN coating), the cutting speed is set at 60-80m/min, the feed rate is 0.1-0.2mm/r, and the cutting depth is 0.5-1mm. Each tool can process 80-100 parts, and the surface roughness can reach Ra0.4μm, and the dimensional accuracy is stable at ±0.01mm. This shows that the CNC lathe has obvious advantages in processing titanium alloy shaft, especially in batch production of high-precision parts.

Second, the key to processing titanium alloy shaft with CNC lathe: control the "details" of processing

Although the CNC lathe is powerful, it doesn't mean that you can process titanium alloy shafts well by putting the material on the machine. We summed up several key points in years of processing practice:

1. Tool selection: "sharp" and "wear-resistant" are two cores

The tool is the "first knife" for processing titanium alloy, and its selection directly affects the processing quality and efficiency. For titanium alloy shaft turning, we generally recommend using carbide tools with good red hardness and wear resistance. Among them, the TiAlN coated tool is the first choice, because this coating can form a dense oxide film at high temperature, effectively reducing the adhesion between the tool and the titanium alloy, and extending the tool life. For example, we use a certain brand of CNMG120408 tool (TiAlN coating) to process TC4 titanium alloy shaft, and the tool life can reach about 3 hours under the condition of stable parameters.

In terms of geometric angle, the front angle of the tool should be appropriately increased (generally 5°-12°) to reduce cutting force and prevent workpiece deformation; the edge angle should be polished to reduce the friction between the chip and the tool; the rear angle is generally 8°-12°, which can ensure the strength of the tool edge and avoid affecting the surface quality.

2. Cutting parameters: "low speed" and "high feed" is not dogma

Many people think that titanium alloy should be "low speed", but in fact, it is not absolute. The key is to match the line speed according to the material grade and tool performance. For example, TC4 titanium alloy (commonly used in aerospace) generally adopts a line speed of 50-80m/min when using carbide tools; pure titanium (TA1, TA2) can be increased to 80-120m/min. If the speed is too low (such as less than 30m/min), it is easy to cause "sticking knife"; if the speed is too high (more than 120m/min), the cutting temperature will rise sharply, and the tool will wear quickly.

Feed rate and cutting depth also need to be balanced. The feed rate is generally 0.1-0.3mm/r. Too small will make the cutting squeeze the workpiece too much, and too large will easily cause vibration. The cutting depth should be taken according to the rigidity of the workpiece and tool: rough turning can take 1-3mm, finish turning should be reduced to 0.1-0.5mm, and multiple light cuts can ensure the dimensional accuracy and surface quality.

3. Cooling: "timely" and "sufficient" is the key

Titanium alloy has poor thermal conductivity, and most of the cutting heat is concentrated in the tool tip and cutting area. If the heat is not dissipated in time, it will not only accelerate the tool wear, but also cause the workpiece to thermal deformation and affect the dimensional accuracy.

Therefore, we recommend using high-pressure internal cooling when processing titanium alloy shaft with CNC lathe. The cooling liquid is directly sprayed to the cutting edge through the tool center hole, which can take away the heat in time and form a "lubricating film" on the cutting surface, reducing the friction. For example, we use a water-soluble cutting fluid with good cooling and lubrication performance in processing, and the pressure is set at 8-12bar. The cooling effect is good, the tool life is prolonged by 2-3 times compared with external cooling, and the surface roughness is reduced by about 30%.

4. Clamping: "rigid" and "stable" is the prerequisite

Titanium alloy has a small elastic modulus and is easy to deform under clamping force. If the clamping method is improper, the workpiece will produce "elastic deformation" during clamping, and when the clamping force is removed, it will return to its original shape, resulting in dimensional deviation.

Therefore, when clamping the titanium alloy shaft, we should try to use "soft jaws" (such as aluminum alloy, copper jaws) and increase the clamping contact area. For slender shaft parts (length-diameter ratio greater than 10), it is also necessary to use the center frame to support the middle part to improve the rigidity of the workpiece. In addition, the clamping force should not be too large, just ensure that the workpiece does not vibrate during processing. We once encountered a case where a slender titanium alloy shaft was clamped with a three-jaw chuck. After processing, the diameter at the middle part was reduced by 0.03mm due to excessive clamping force. Later, after using the center frame and soft jaws, the problem was solved.

Third, CNC lathe vs. other processes: it depends on the demand

Although the CNC lathe has many advantages in processing titanium alloy shaft, it is not the only processing method. For example, when the shaft has complex features (such as keyway, spline, thread), if using CNC lathe, it may need to change the tool multiple times, which will affect the processing efficiency; while using CNC lathe-milling center can complete the turning and milling in one clamping, which is more efficient. For ultra-precise titanium alloy shafts (such as shaft parts used in optical instruments), the dimensional accuracy requires ±0.001mm, even the CNC lathe can not meet the requirements, and it needs to be processed by precision grinding machine after turning.

Therefore, when choosing the processing process, it is necessary to consider the production batch, dimensional accuracy, structural complexity and other factors of the shaft parts. For example, small batch (1-10 pieces), single-piece production, you can consider using CNC lathe; medium and large batch (more than 50 pieces), high precision requirements, you can consider using CNC lathe-milling center; ultra-high precision (more than IT5 grade), you need to use turning and grinding process.

Finally, return to the original question: Is it necessary to use CNC lathe to process titanium alloy shaft?

The answer is: for most titanium alloy shaft parts, especially those with high precision, complex structure and medium and large batch, CNC lathe is a more ideal processing method. It can not only ensure the processing quality and efficiency, but also reduce the dependence on manual operation and improve the stability of production. Of course, "lathe is dead, people are alive". Even the most advanced CNC lathe needs to combine the actual situation to optimize the process parameters, select appropriate tools and cooling methods, and finally produce high-quality titanium alloy shaft parts.

In the processing of titanium alloy shaft, there are still many details that need to be summed up and improved. As long as we master the characteristics of the material and the processing law, we can give full play to the performance of the CNC lathe and process titanium alloy shaft parts that meet the requirements of various fields.