A deep coolant primer

Coolant has always been a component of successful machining processes. However, with more advanced machining techniques like deep-hole drilling and deep-pocket milling, it has become a critical element of the process.

When machining was characterized by big tools taking slow heavy cuts, coolant was applied primarily to absorb heat from the system. Keeping the tool and workpiece cool was the only concern. Shops were usually only concerned about maintaining the level in the tank so they didn’t starve the tool. In many cases concentration was monitored by the senses and maintenance was unscientific.

Coolant is far more critical in modern applications. Deep-pocket milling and deep-hole drilling are two good examples to illustrate this point.

What is deep-hole drilling? Generally, manufacturers define it is as drilling any hole that is eight times deeper than the diameter of the tool. So, drilling a 1″ diameter hole that is 8″ deep would be considered deep-hole drilling. The ratio of diameter to length is often referred to in shorthand as 8D, 10D, etc., and there is some variation in definition in the cutting tool market.

No matter the definition, when the drill gets deep into the part, the application of coolant is critical. Coolant has three primary functions in a deep-hole application. First, it absorbs heat energy released in the chip formation process. Second, it lubricates the tool chip interface as well as the flutes and the wall of the drilled hole. Finally, it is needed to assist the flutes in flushing the chips from the hole.

Lubricating the tool chip interface also aids chip formation. More importantly, it reduces heat from friction generated by the chip sliding across the tool while it also reduces built-up edge (BUE), which alters the geometry of the tool and can lead to a cycle of events that result in tool failure. As the BUE increases, the tool geometry is altered leading to more heat, which exacerbates the BUE process. The BUE will go through a cycle of buildup and break off, often taking small amounts of the tool edge with it. As the cycle continues it becomes exponentially worse until the cutting edge is no longer capable of forming a chip and the pressures exerted on the tool cause failure.

Cooling the tool and the workpiece enhances chip formation and encourages chip breakage. Every machinist knows broken chips are better than stringers and bird nests. However, broken chips are bad if they are not evacuated from the hole. Properly applied coolant flushes the broken chips from the hole preventing them from building up between the tool and the wall of the hole, which will ultimately lead to breakage.

Deep-pocket milling is another process where coolant is a critical part of success. Each machining process is different, and there is no common definition from one shop to the next. That said, deep-pocket milling has some common characteristics. The most common of which is depth of the work zone relative to the stick out length of the tool. Second, and the most significant, the work area is enclosed in a way that prevents chips from easily being evacuated from the cutting zone.