Cryogenic assisted machining

Many parts used in the mechanical industry are obtained by machining. There are many families of materials, with a wide variety of associated characteristics.

There are, however, two trends:

• for materials whose machinability has been mastered, the quest for greater productivity gains leads to the imposition of stricter cutting conditions, in particular by increasing cutting speeds and feed rates;

• The aeronautics, aerospace, automotive and nuclear sectors are all demanding materials with ever-higher specifications, but which are more difficult to machine.

These developments both lead to higher temperatures at the contact interfaces between the tool, the chip and the workpiece, which can generate premature wear of the cutting tools, degradation of the workpiece's surface integrity and geometric dispersions in the final product.

To overcome these difficulties, it is possible to increase cutting tool performance by working on insert geometry or optimizing the choice of tool materials. The latter solution is used, for example, when machining nickel alloys with ceramic tools, or when turning hardened steel with cBN (cubic boron nitride) inserts. In this case, tool life is improved, but temperatures remain very high, which can lead to residual stresses, or even phase changes, harmful to the surface of the finished product.

Another approach is to cool the cutting zone down considerably, as conventional lubrication methods are often ineffective. Particularly when machining at high speeds, they can even have a significant impact on the working environment, producing toxic gases and fumes. The use of non-synthesized cryogenic fluids (liquid air, liquid argon, liquid nitrogen, etc.) during cutting is therefore an interesting alternative to dry machining. Cryogenic assistance during machining offers the following advantages:

• addition of a high-cooling fluid (temperature below – 100°C) to the cutting zone;

• non-stable liquid phase at room temperature, allowing chips to be evacuated in the first few moments before vaporizing;

• respect for the environment, property and people.

Cryogenic assistance is applied to several machining operations: abrasion, drilling, milling, turning... In the remainder of this article, the application of assistance to the turning process will be more specifically detailed. First, the principles and associated techniques will be presented. Criteria for choosing a cryogenic fluid will then be proposed. The gains achieved in terms of productivity and part quality will...