In general, metal cutting refers to cutting steel and cast iron. However, in today’s workshop, we are facing other materials, such as stainless steel, iron-based and non-iron-based (such as titanium alloy) heat-resistant alloys and high-temperature alloys. There is a big difference between these materials and traditional steel parts, it is they that embodies the true meaning of “difficult to process”.
“Machinability” indicates the difficulty of processing a material at low cost. It is an average score obtained by comparing with a reference material. This value indicates potential problems in chip formation, cutting force, temperature, tool wear and workpiece quality.
We used to think of machinability as a material characteristic, which depends on all the factors involved in the metal cutting process. Five of the basic material characteristics have a great influence on machinability, namely adhesion, work hardening, Thermal conductivity, hardness and wear resistance.
When the material tends to adhere to a high level, it is necessary to use a tool material with good toughness and a specific plating layer. The cutting speed should also be increased. For materials prone to severe work hardening (deformation hardening), tools with sharp cutting edges are required. The cutting speed may vary, but the feed rate should be increased.
If the thermal conductivity of the material is low, the tool material should have a high red hardness. Both cutting speed and feed rate should be limited. Needless to say, hard workpiece materials require harder tool materials. In general, the feed rate and cutting depth need to be kept at a medium level. Wear-resistant workpiece materials require the use of highly wear-resistant tool materials. The cutting conditions must be adapted to the situation where the cutting utilization rate or cutting efficiency needs to be improved.
Based on the above, what conclusion can we draw?
By understanding the relationship between the above five basic material characteristics and cutting tools, we can greatly increase productivity and save costs. We only need to select cutting tools and cutting conditions according to the material characteristics of the workpiece to obtain reasonable expectations.
Is the “difficult to process” material really so difficult to handle? it’s not true. They are just different. How to deal with different materials?
For example: By comparing the five important characteristics of two typical workpiece materials 42CrMo4 (a typical alloy steel) and Inconel718 (a “difficult to machine” high temperature alloy), we can directly understand their differences. In view of the existing problems, the corresponding adjustment scheme can be easily proposed.
Question 1: What should I do if the adhesion tendency is too high?
Plan: Use the blade material grade with the correct coating and ensure that the chip formation is under control (the correct chipbreaker type). Also consider the importance of feed rate!
Question 2: What should I do if the work hardening is serious?
Plan: Use sharper cutting edges and cutting geometric angles.
Question 3: What should I do if the thermal conductivity is low?
Plan: Use blade material grade with high red hardness and proper cooling method. Of course, we must choose cutting conditions that are suitable for these characteristics. Compared with steel applications, Inconel 718’s cutting depth and feed rate may be at the same level (maybe the feed rate will be even higher), but a lower cutting speed (lower thermal conductivity) is required.