Machining of stainless steel

▇ With the vigorous development of aviation, aerospace, petroleum, chemical, metallurgical and food industries, stainless steel materials have been widely used.

However, stainless steel materials have high toughness, high thermal strength, low thermal conductivity, large plastic deformation during machining, severe work hardening, high cutting heat, and difficult heat dissipation. The generation of chippings not only exacerbates the wear of the tool, but also affects the surface roughness of the machining.
In addition, because the chips are not easy to curl and break, it will also damage the processed surface and affect the quality of the workpiece.
In order to improve machining efficiency and workpiece quality, it is important to choose the right tool material, milling geometry and cutting amount.

1. Selection of tool material 

The right choice of cutting tool material is the decisive factor for ensuring the efficient processing of stainless steel. According to the cutting characteristics of stainless steel, the tool material should have sufficient strength, toughness, high hardness and high wear resistance, and the adhesion to stainless steel should be small. Commonly used cutting tool materials are two types of carbide and high-speed steel.

Because the cutting speed of high speed steel when milling stainless steel can’t be too high, it affects the improvement of production efficiency. For end mills, the tool material should be hard alloy with high strength and good thermal conductivity, because of its hardness and wear resistance, it is better than high-speed steel.

Commonly used cemented carbide materials are: tungsten cobalt, tungsten cobalt titanium, general purpose. Tungsten-cobalt-based cemented carbides have good toughness and thermal conductivity, and are not easy to bond with chips, so they are suitable for rough milling of stainless steel; while tungsten-cobalt-titanium cemented carbides have hardness, wear resistance, heat resistance and oxidation resistance, and good toughness, suitable for precision milling of stainless steel.

When processing 1Cr18Ni9Ti austenitic stainless steel, it is not suitable to use tungsten-cobalt-titanium cemented carbide. Because Ti in stainless steel and Ti in tungsten-cobalt-titanium cemented carbide have affinity, the chips easily take away the Ti in the alloy, which promotes the tool. Worn out.

2. Selection of end mills geometry angle 

The geometric angle of the tool cutting part has a great impact on the productivity of stainless steel cutting, the durability of the tool, the roughness of the surface being processed, the cutting force, and work hardening. Reasonable selection and improvement of the geometric parameters of the tool are to ensure the processing quality, an effective way to improve efficiency and reduce costs.

(1) Selection of the first clearance angle of the end mills (Radius Angle)

The size of the rake angle determines the sharpness and strength of the cutting flute. Increasing the rake angle can reduce the deformation of the chip, thereby reducing the cutting force and cutting rate, reducing the cutting temperature, and improving the durability of the tool. However, increasing the rake angle will reduce the wedge angle, reduce the strength of the end mills, cause chipping, and reduce the durability of the tool.

When milling stainless steel, the rake angle should be appropriately larger without reducing the strength of the tool. When the rake angle of the tool is large, the deformation is small, the cutting force and cutting heat are reduced, the work hardening tendency is reduced, and the tool durability is improved. Generally, the rake angle of the tool should be 12 ° -20 °.

(2) Selection of the second clearance angle α0 of the milling cutter (Second Angle)

During the cutting process, the rake angle can reduce the friction between the flank and the cutting surface. If the clearance angle is too large, the wedge angle will decrease, which will worsen the heat dissipation conditions, reduce the cutting edge strength, and reduce the tool durability. Temperature increases tool wear.

Generally, the backward angle doesn’t change much, but it must have a reasonable value to help improve the durability of the tool. When milling stainless steel, because the elasticity and plasticity of stainless steel are greater than ordinary carbon steel, if end mills backward angle are too small will increase the contact area between the cutting surface and the cutter back angle, and the high temperature area generated by friction is concentrated on the cutter back angle, speed ​​up end mills wear and reduce the surface finish of the machining.

Therefore, the backward angle of the end mills when milling stainless steel is slightly larger than that of ordinary carbon steel, but too large the back angle will reduce the strength of the end mills and directly affect the durability of the end mills. Therefore, in general, the first clearance angle of the end mills should be taken 20 ° ~ 30 °.

(3) Choice of end mills helix angle

When the cutting depth ap and the feed amount f are constant, reducing the helix angle Kr can improve the heat dissipation conditions, reduce tool damage, and make the end mills cutting smoothly. However, reducing the helix angle will increase the radial force and easily cause vibration during cutting. Milling stainless steel has a strong tendency to harden and is prone to vibration, which in turn will make work hardening serious.

Therefore, the helix angle is generally 45 ° ~ 55 °. The specific angle should be selected according to the rigidity and cutting amount of the machine tool, part, and tool system.

(4) Selection rake angle of end mills

The end mills rake angle inclination can control the chip flow direction. When the end mills inclination angle λs is negative, the chips flow to the milling surface; when the end mills inclination angle λs is positive, the chips flow to the surface to be milling. In order to prevent chips from scratching the machining surface, the value of the end mills inclination angle λs is positive during finishing.
When λs is positive, the cutting edge strength is weak and the workpiece is first contacted, which is easy to be damaged. When λs is negative, the cutting edge strength is high and impact resistance can prevent the cutting edge from being broken, and smooth the cutting processed. Generally, the inclination angle of the cutting edge of the tool should be 0 ° ~ 20 °.

3. Selection of cutting rate 

The cutting rate has a great impact on production efficiency and processing quality, so after determining the geometric parameters of the tool, a reasonable cutting rate must be selected. When choosing the cutting rate, you should pay attention to the following factors:

The first is to choose the cutting rate according to the hardness of stainless steel and various types of semi-finished workpiece;
The second is to choose the cutting rate according to the tool material, welding quality and sharpening conditions of the turning tool;
The third is to choose the cutting rate according to the diameter of the part, machining allowance and milling machine accuracy.
Meanwhile, in order to suppress the chippings while cutting and improve the surface quality, when using carbide tools for cutting, the cutting rate should be slightly lower than that of general carbon steel workpieces, especially the cutting speed should not higher than (vc = 50 ~ 80m / min); the cutting depth ap should not be too small to avoid the cutting edge and the cutting edge penetrating the hardened layer, ap = 0.4 ~ 4mm; therefore, the feed rate f has less impact on the tool durability than the cutting speed, but it will affect chip breaking, straining and abrading the surface of the workpiece, affecting the surface quality of the processing, the feed amount is generally f = 0.1 ~ 0.5mm / r.
Stainless steel, especially austenitic stainless steel, has good flexibility. During cutting, the chips generated are difficult to break, which increases the friction between the chips and the rake face of the tool and increases the cutting force. At the same time, the hardness and strength of the material being cut will increase due to work hardening, which will also increase the cutting force.
For this reason, on the basis of reasonable selection of the tool material, the geometric angle of the tool and the cutting rate, a comparative test of cutting forces was performed on stainless steel and 45 steel. The test results show that with the same cutting rate, the cutting force is only 8.5% higher when machining stainless steel than when machining 45 steel.

4. Conclusion 

Reasonable selection of tool material, tool geometry angle and cutting amount can completely achieve the improvement of stainless steel cutting production efficiency and quality of processed workpieces.