Tool Matters by Brad Dullinger

Using coolant on cutting tools has been a matter of debate over the last few years, especially as coating technology has seen drastic improvements. Cutting fluids have three main functions: removing heat from the cutting tool, ejecting chips, and providing lubricity to the cutting tool.

Removing heat is an important function of cutting fluids, but as coating technology has improved, the amount of heat the tools can withstand has jumped drastically. Coatings like TiAlN or AlTiN actually need heat to be fully utilized. For that reason, we often do not want to use cutting fluid with these coatings because it can decrease tool life. This can change when machining a material which has a high tendency to work harden i.e. titanium, stainless steel and other materials with high nickel content. There still are some exceptions. For example, 316 stainless can be milled and turned successfully without coolant, but finding parameters can be challenging.

When you run dry you typically can increase the SFM by a minimum of 30%, due to the fact that you are removing the thermal shock which it had to endure when the cutting fluid was present. At the high SFM typically ran when machining dry, one drip of coolant on a part can cause the insert to fracture, causing a disaster. I have even seen situations where parts had a small amount of coolant residue from a previous operation cause the insert to fracture when it hit the residue. Granted this situation had extreme parameters, but the point is keep it wet or keep it dry. A common problem I see is a coolant line dripping on the part when dry turning, this can easily cause tool failure.

One of the biggest challenges when machining dry is clearing chips from the tool path. Recutting a chip will cause just about any tool to have instantaneous catastrophic failure. If you do not have the option to run an air blast or have some other means to keep the chips out of the way, it is typically better to run slower with coolant. This helps to ensure consistent tool life.

The third obstacle is the lubricity. When dry machining, your coating is the only place any lubricity can come from. Typically with milling and turning this doesn’t pose a problem. Where you will really notice the lubricity factor is missing, is when machining aluminum or other nonferrous materials, or when tapping. Both of these situations almost always require a cutting fluid of some sort.