Why dry machining

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. 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. 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:. 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. Cemented carbide is a sintered, powder metallurgy product and is sensitive to thermal shock loads, which destroy cutting edges.

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. Cutting fluids prevent the problem by lubricating the edge, flushing the chips away and cooling the workpiece. To ensure that the cutting fluid performs these functions well, titanium alloys prefer cutting fluids delivered at high pressure, usually in the range of 4, to 7, psi. On occasion, powdered metals also need a cutting fluid to generate a thin coating of oil as a rust inhibitor.

While some shops have learned the value of dry machining by accident, many others have failed to see the benefits even when they have purposefully attempted it. The reason is that success at dry machining requires more than just eliminating coolant—it requires a methodical approach to controlling heat in the overall process. The most important way that the tool affects heat transfer is by creating good chips. Chips can carry away 85 percent of the heat generated from the cutting action and allow only 5 percent to enter the workpiece while 10 percent flows into the tool and elsewhere.

Modern chip grooves pressed into the surface of tools are a great help in breaking chips into manageable shapes and sizes. Because the chips are hotter and therefore more ductile than their counterparts in wet machining, they are more difficult to break and more likely to produce dangerous chip tangles resulting in poor surface finishes.

Using a chip groove designed to shear stringy materials will help to solve this problem. Although such edges tend to have more positive rakes, they are not as fragile and susceptible to breakage as they would be in wet applications. The high cutting temperatures inherent in dry machining usually soften the carbide slightly, which increases its toughness, reducing the likelihood of chipping and improving the reliability and longevity of the tool.

For the same reason, switching to a slightly harder tool upon going dry rarely reduces tool life or degrades the consistency of the cut.

In fact, the opposite is true. The harder substrate ensures that the edge retains its integrity at high cutting temperatures, yet the slight softening prevents it from being too brittle.

Consequently, users can specify a harder grade of carbide to resist both the deformation and cratering chemical dissolution of the tool edge that would otherwise shorten tool life unacceptably in dry applications.

Because the tools designed for dry machining can be sharper and tend to be freer cutting than their counterparts for wet machining, they actually generate less friction and help to control heat. Studies in drilling have shown that reducing the edge hone to create a sharper drill can reduce the cutting temperature by 40 percent.

Not only do sharp edges keep the temperature low, but they also reduce runout and improve surface finish. Another way to assist chip breakage and evacuation from the cut is to replace a liquid cutting fluid with a gaseous one, shop air being the most common. Although it is not efficient at cooling, a blast of shop air is sometimes enough to blow chips from the cut to prevent them from being re-cut and from transferring unwanted heat into the work and machine.

When lubrication is necessary, users can apply a high-efficiency lubricant as a mist that is consumed in the cutting process. The most effective method is a relatively new technique sometimes referred to as minimum quantity lubrication MQL , which injects minute amounts of coolant through the tool. Tool coatings also play an important role in guarding the cutting edge during dry machining. Some of the most effective cutting tool inserts for dry machining combine a specially engineered coating system with a cobalt-enriched zone substrate offering a hard interior and a tough surface.

An exceptionally thick, micron multi-layer coating is produced using a combination of conventional and medium-temperature chemical vapor deposition processes. The first titanium carbonitride layer produces the necessary adhesion to the substrate as well as edge toughness. Next, a layer of fine-grained aluminum oxide provides the effective thermal barrier needed for dry machining and high cutting speeds. A second sandwich layer of abrasion-resistant titanium carbonitride helps control flank and crater wear, while the top layer of titanium nitride provides built-up edge resistance and makes it easier to determine the wear on the insert.

Lubricious coatings reduce heat generation by decreasing friction. Coatings such as molybdenum disulfide and tungsten carbide-carbon have low coefficients of friction and can lubricate the cutting action.

Unfortunately, these coatings are soft and have relatively poor tool life. To compensate for this limitation, these coatings are often used with hard underlayers, such as titanium carbide, titanium aluminum nitride, aluminum oxide or some combination.

Cutting temperature remains very high during machining. Cutting temperature remains significantly low because of reduced rate of heat generation and continuous removal of heat by cutting fluid. High cutting temperature sometimes limits the level of process parameters. Thus high cutting velocity, feed rate and depth of cut cannot be utilized, which leads to low MRR and low productivity.

Because of low cutting temperature, high values of speed, feed and depth of cut can be utilized without much problem. Thus high material removal rate MRR and improved productivity can be achieved.

High cutting temperature accelerates the tool wear rate and thus reduces tool life. It also tends to deform the cutting edges plastically.

For the same work-tool material combination and process parameters, tool under wet machining exhibits prolonged life due to degraded wear rate. Tendency of plastic deformation is also meagre. Sometimes finished surface oxidizes or burns due to excessive cutting heat.

It changes the surface properties and appearance leading to rejected item. Cutting fluid can protect the finished surface from oxidation by maintaining the cutting temperature minimum.

Due to excessive cutting heat, chip colour changes undesirably. Chip colour usually remains same to that of work material. 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. There are perhaps four main drivers of dry machining: removing the thermal shock cycle means extended tool life; the cost of coolant, where some subcontractors say the price is rising; waste coolant requires professional disposal, adding to cost; and operator health.

At the speeds that spindles run at today, often the coolant just atomises. The operator is exposed to the atomised coolant on the tool changeover. Machine speeds are making dry machining more practicable. A generation ago a typical mm diameter face mill could be left running for more than an hour on a 10mm depth of cut, churning through at rpm.

Although the heat produced was limited by low surface speeds, coolant was useful to keep the component cool while the tool was engaged with the piece for hours on end. So modern milling cutters tend to run with light depth of cut but very high speeds and feeds.