Which is better: dry or wet machining?

Throughout the sometimes-contentious world of machining, the debate rages about whether to apply flood coolant or cut dry. As in many areas of machining, the choice is not easy and requires careful and informed consideration. To further complicate the decision, minimum-quantity lubrication (MQL) can be a successful compromise that provides an efficient and effective answer to the troublesome question.

Wet Coolant

Cooling mixture, cutting lubricant, cutting fluid and coolant are common terms that refer to a liquid applied for both cooling and lubricating the tool/workpiece interface while machining.

All cutting generates unwelcome friction at that interface. The presence of coolant ensures that the friction between the two surfaces is reduced and lubrication significantly enhances the metal-removal process.

Flood coolant is not recommended for roughing steel with an extended-flute milling cutter. All images courtesy Iscar Metals.

During machining, the temperature in the cutting zone becomes extremely high, which depends on factors like the workpiece material, cutting data and cutting tool substrate. For example, the temperature for a tungsten-carbide tool at the point of contact between a cutting edge and the workpiece is estimated at 1,100° to 1,200° C (2,012° to 2,192° F).

The application of coolant lowers the cutting zone temperature and reduces the thermal load on the tool. In addition, coolant improves chip evacuation and also reduces the concentration of metal dust. Therefore, the coolant supply is directly connected to several important benefits. Specifically, coolant:

■ advances process performance by improving machining accuracy and surface finish;

■ increases profits by raising productivity and extending tool life; and

■ augments environmental control.

When interrupted milling, the cutting edge of the tool experiences a cyclic thermal load because the ambient temperature dramatically changes as the edge enters and then exits the cut. This exposes a tool’s cutting edge to severe heat stress—comparable to repeated thermal shock.

Cemented carbide is a sintered, powder metallurgy product and is sensitive to thermal shock loads, which destroy cutting edges. When using a carbide tool, coolant application may increase the “shock treatment” and unintentionally contribute to cutting edge failure. Extreme temperatures result in plastic deformation of the cutting edge, while the temperature difference leads to thermal cracks. This becomes even more exaggerated in situations that generate a high level of heat, such as when milling difficult-to-cut materials. Although wet machining provides multiple benefits, it certainly can be problematic when milling.

In many cases, however, having an efficient coolant supply is not only reasonable but absolutely necessary to ensure productive milling. For example, when machining titanium and heat-resistant superalloys, austenitic and duplex (austenitic-ferritic) stainless steels—or even special-purpose, hardened, alloyed cast iron—a considerable amount of friction and heat is generated. The flushing effect of coolant also significantly enhances chip evacuation and reduces recutting of chips, particularly when milling deep pockets or narrow slots.

Iscar’s Multi-Master ballnose milling head is intended for machining difficult-to-cut materials and has channels to supply coolant where it is needed.

Compared with traditional low-pressure coolant, normally delivered at around 20 bar (290 psi), high-pressure coolant (HPC) delivered at about 80 bar (1,160 psi) and ultrahigh-pressure coolant delivered at 350 bar (5,000 psi) are welcome developments.

When applying traditional low-pressure coolant, the high level of heat produces a vapor film in the cutting zone that intensifies heat transfer. However, a high-pressure jet that directly targets the cutting zone effectively penetrates this film and gets coolant into the cut. It also improves the cutting action by changing the shear-plane angle and creating thin, manageable chips. Taking advantage of HPC is only possible when using an appropriate machine tool or by retrofitting an existing machine.

Dry Machining, Other Options