Difficulties and Countermeasures of automatic lathe turning of stainless steel 420

CNC1 Analysis of problems and causes in the process of cutting stainless steel 420


In the part of trial production, we took a turning test of stainless steel 420 stainless steel according to the process of turning ordinary carbon steel. The tool wear is very serious, productivity is extremely low and the surface quality of parts is not up to the requirements.

Comparison of 420 stainless steel mechanical properties with 40# steel, 45# steel of carbon structural steel, stainless steel 420 steel strength is higher than 40# steel and 45# steel. It is a kind of high strength, good plasticity of medium carbon martensitic stainless steel. Because of serious work hardening, great cutting force, and high cutting temperature, all lead to serious tool wear, knife replacement increase, and downtime and machine adjustment time increase. Finally, productivity is reduced. At the same time, tools are easy to stick and generate steel scrap bugs which caused the change in size and surface roughness. And the chip is not easy curling and easy breaking which damages the machined surface and affects part quality.

Consequently, we can not use the process of cutting 45 steel, nor the general lathe machining method used. Because the general automatic lathe has fewer tools it demands the best to have one pass making the surface be machined to meet the requirements of the size and the surface roughness to ensure high productivity.

2 main technical measures

1) Change the hardness of the material by heat treatment

Martensitic stainless steel has different hardness after heat treatment which has a great impact on the turning processing. Table 1 is the turning result of stainless steel 420 steel with different hardness after heat treatment machined by tool YW2 materials. Obviously annealed martensitic stainless steel like state has poor turning performance even if the hardness is low. This is because the material has great plasticity and toughness, inhomogeneous microstructure, adhesion, strong weld property, and easy-to-produce scrap but which are not easy to get a good surface quality.

However, after quenching and tempering treatment stainless steel 420 material with hardness below HRC30 can have good machining performance and can be easy to reach better surface quality. Using material with hardness above HRC30 also can have good quality but the tool is easy to wear. So in the conditions allow the material hardness should be HRC25 HRC30 by quenching and tempering treatment before machining.

Table 1

2) Tool material selection

Commonly In the automatic lathe for turning stainless steel the hard alloy tool materials used are: YG6, YG8, YT15, YT30, YW1, YW2, and others.

High-speed steel tool materials commonly used are W18Cr4V, W6M05Cr4V2AL, and others. We had the turning test for tools of several materials with the same conditions of cutting parameters.

We can see from table 2: the cylindrical turning tool with TiC-TiCN-TiN composite coating blade has high durability, high cutting speed is desirable, high surface quality, and high productivity. However, we have no cutting blade for this material. Through the contrast test of table 2, the cutting performance of YW2 cemented carbide is good and it is used as a cutter blade.

Table 2 Different tool cutting contrast test

Material   The geometric angle of tool   Cutting consumption   Tool durability

Min   Surface roughnessμ

Rake angle γ  Back angle α    main angle Kr     edge angle λ

m/min   s


YG8 15 ° 8 ° 90 ° -10 45 ~ 550.172 ~ 82 Ra3.2

YT14 15 ° 8 ° 90 ° -10 45 ~ 550.180 ~ 95 Ra3.2

YW2 15 ° 8 ° 90 ° -10 45 ~ 550.190 ~ 110 Ra3.2


Composite coating blade 12 ° 8 ° 90 ° -4 45 ~ 550.1128 ~ 185 Ra6.3 (off)

Ra Ra1.6


3) Selection of the tool geometry and structure

For good tool materials, it is very important to choose a reasonable geometric angle.

1) Rake angle: For martensitic stainless steel the turning tool rake angle of 10 °20 °is suitable.

2): Back angle  5 ° 8 ° is more appropriate, but 10 degrees maximum.

3) Cutting edge inclination angle λ:  selecting  -10 ° 30 °.

4) The main deflection angle of Kr should be selected according to the shape of the parts, machining position, and tool condition.

5) The edge surface roughness should be no more than Ra0.4 Ra0.2.

In structure, our foreign circular tool uses an external oblique arc chip breaker groove, on the tooltip chip curling radius is big, and on the outer edge of the chip curling radius is small, the chip will be turned to the surface to be machined and broken. For the cutting knife, we will associate the angle control within 1o, this can improve chip removal conditions, and prolong the service life of the cutter.

1) Selection of the cutting

The cutting theory is that the cutting speed of V has the greatest impact on cutting temperature and tool wear. The second is cutting amount s and the third is cutting depth t. For the machining surface in the automatic lathe, the cutting depth of t is decided by parts size and blank size. Generally, it is 0 3mm. The cutting speed of hard processing material is much lower than ordinary steel cutting speed.

Cutting feed s has a low influence on the durability of the tool than the cutting speed. But it will affect the chip removal and breaking, pulling, or scratching the surface of the parts, affecting the surface quality. Generally believed that if the depth is large, the cutting feed should be reduced and the cutting feed is smaller than outside the circle. For the processed surface roughness requirements is not high the s can be 0.1mm/r 0.2mm/r.

In short, for difficult materials to machine general selection is of slower cutting speed and medium cutting feed.



2) The appropriate cooling and lubricating liquid

Cooling and lubricating liquid for cutting stainless steel should have the following characteristics:

A. The high cooling performance can take a large amount of cutting heat.

B. Stainless steel is tough, easy to produce the knife cutting tumor, and deterioration of machining surface. So it requires cooling and lubricating liquid has high performance of lubrication.

C. Good permeability which can play a better role in wedging, diffusion, and internal lubrication that is not easy to chip away.

The commonly used cooling and lubricating liquid for stainless steel are as follows: vulcanized oil, sulfide oil, acid kerosene oil or plant oil, carbon tetrachloride with mineral oil, emulsion, etc.

Considering the sulfur corrosion effects on the machine, vegetable oil (such as oil) easily attached to scabs in machine tools and deteriorates, we choose oil according to the weight ratio of 1:9. The permeability of carbon tetrachloride is good and oil has a good lubricating ability. The test shows that this kind of lubricant oil is suitable for the small surface roughness requirements of stainless steel parts of the semi-finish machining and finish machining process, particularly suitable for martensitic stainless steel parts.

3 Conclusion

Due to adopting a series of measures the stainless steel 420 machining process has been completed in the normal state. Sharpening frequency has been 1/3 of the trial production period. The production efficiency improved and the product quality fully meets the design requirements.

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