At the interface between a cutting tool edge and a metallic workpiece, the temperature can vary from 200°C to over 1,300°C. At such temperatures softer metals such as aluminium melt and the cut surface of the component can be damaged. However, with modern coatings and tool technology the expensive tooling is not badly affected by heat alone. It is thermal shock, or rapid temperature variations that weakens tools and this is far more likely to reduce tool life than simply exposing tools to high temperatures.
As Le Coz and Dudzinski at the LEM3 Research Centre at the Université de Lorraine explain, in a 2014 paper, “a moderate level of cutting-edge temperature and a low thermal shock reduces the tool-wear phenomena, and a low temperature gradient in the machined sub-layer reduces the risk of high-tensile residual stresses”.
In the majority of metal-machining operations, liquid coolant is still used to reduce temperature of both workpiece and tool and to evacuate chips from the cutting area. This amounts to hundreds of thousands of gallons of coolant fluid per year, and from £4 and more per litre, a lot of money.
The problem with using coolant for milling specifically is thermal shock. As the cutting edge engages with the workpiece and cuts the material, it generates friction and gets very hot during the cut. But as soon as the cutting edge is removed, coolant is poured on and instantly cools it, then the edge is immediately returned to the cutting operation and heats up.
This is true, whether it is a solid tool or indexable insert. “You are getting hot-cold-hot-cold constantly at the cutting edge, which does not do the tool any good,” said Adrian Fitts, business development manager at WNT (UK). “This produces thermal shock and breaks down the cutting edge far quicker than allowing it to run hot full time.”
Dry machining is becoming more prevalent, in milling especially. In drilling, coolant is required because the tool has prolonged exposure to the material and fluid is essential to evacuate the chips. And dry machining in turning is rare as the cutting edge is constantly in contact with the workpiece, so without some cooling, the cutting edge will eventually fail. Milling is the main beneficiary.
“In milling applications, for example, cutting is always intermittent and the risk of thermal cracking caused by the coolant is even higher,” said Paul Campbell, product specialist at Sandvik Coromant. “Dry machining is therefore the primary choice to increase tool life, especially when cutting with steels, cast iron and some stainless materials.”
Read more: The benefits of dry machining