Dry and Near-dry Machining
Eliminating cutting fluid is a good ecological solution, but it has limitations
December 1, 2011
Eliminating cutting fluids can lower production costs.
Metalworking machining fluids have undergone intense regulatory scrutiny throughout the last two decades, led by the United Auto Workers (UAW) in the U.S. who petitioned the Occupational Safety and Health Administration (OSHA). The UAW's aim is to lower the permissible exposure limit for metalworking fluids from 5.0 mg/m3 to 0.5 mg/m3.
Consequently, OSHA established the Metalworking Fluid Standards Advisory Committee (MWFSAC) in 1997, which developed specific standards and guidelines related to metalworking fluids. In its final report in 1999, MWFSAC recommended that the exposure limit be 0.5 mg/m3 and that medical surveillance, exposure monitoring, system management, workplace monitoring, and employee training are necessary to monitor worker exposure to metalworking fluids.
In the current competitive manufacturing environment, users of metalworking fluids are looking to reduce costs and improve productivity. The costs of maintaining and eventually disposing of metalworking fluids, combined with the previously mentioned health and safety concerns, have led to a heightened interest in either eliminating metalworking fluids altogether or limiting the amount of metalworking fluid applied.
The former process is known as dry machining, while the latter is referred to as near-dry machining or minimum-quantity lubrication (MQL).
Dry machining is machining without any fluids, while MQL is the use of a minute amount of fluid that is applied directly to the cutting edge (either internally or externally). In near-dry machining, fluid vaporizes during the process, leaving dry chips. Numerous case studies have proven that much faster cutting conditions can be successfully achieved with dry or near-dry machining than what was possible several years ago.
Fluid Usage
During machining, 70 percent of the heat generated originates from plastic deformation of the workpiece. The remaining 30 percent arises from friction at the chip/tool and tool/workpiece interfaces. Without metalworking fluid, excessive tool wear and inferior surface finish may occur during machining. Both of these factors significantly increase manufacturing costs and reduce productivity.
The use of fluids during machining serves several purposes, such as lubrication and cooling at the tool/workpiece interface, and flushing of the chips. Failing to evacuate the chips efficiently can result in subsequent recutting, which tends to compress the removed material against the freshly machined surface and to some extent weld the chip fragments to the surface. This can also reduce the quality of the surface finish.
Lubricating fluids also are susceptible to bacterial propagation and odor generation, which could pose serious health hazards. Another drawback associated with these fluids is the cost of procurement, disposal, maintenance, and labor. Stains on the part or contamination can also be problematic.
Holemaking operations require lubrication at the drill's contact point with the workpiece material to eject the removed material from the hole. Lack of fluids will cause the chips to adhere to the hole walls, and average roughness of the machined surface might be twice as much when compared to a wet operation. Lubricating the interface between the drill head and the walls of the hole can lead to a significant reduction of torque during the operation.
In milling and turning, transfer of the heat from the cutting zone toward the removed chips is an indicator of positive machining characteristics. Good chip design enables a deflection of 85 percent of the generated heat away from the cutting zone, while the remaining heat flows into the workpiece or dissipates into the tool. This heat generation phenomenon has a detrimental effect on the cutting tool's life.
During a milling operation, for example, the cutting edges tend to heat and cool as they enter and exit the piece being machined. These temperature fluctuations create a sequence of expansions and contractions that lead to fatigue stress and thermal cracks. The introduction of lubricating fluid often makes the situation even worse.
A long-lasting debate among research engineers revolves around the question whether cutting fluid actually reaches the zone between the bottom side of the chip and the cutting tool. If it does, the fluid's effect is limited, assuming that it cools only the shearing vicinity. This hot/cold interaction only intensifies the temperature gradients and increases the thermal stress.
With regards to economic considerations, 20 years ago the purchase, management, and disposal of lubricating fluids accounted for less than 3 percent of production costs. Today the same operations constitute 16 percent of the cost of the average job. In line with this significant trend, manufacturers are willing to accept slightly shorter tool life as they look to reduce the cost of buying and maintaining lubricating fluids.
Consequently, a growing number of manufacturers are turning to dry or near-dry machining with the intention of benefiting from coolant cost savings or improved tool life.
Substrate and Coatings
Dry and near-dry machining supports the adoption of green machining techniques, with the added benefit of cost reduction. Thus, manufacturers of cutting tools have developed the latest generation of cutting tool materials, such as advanced coated carbide, ceramics, cermets, cubic boron nitride (CBN), and polycrystalline diamond (PCD).
The submicron grain structure of the solid carbide substrate retains cutting edge integrity, even at high cutting temperatures. It also accommodates a sufficient softening effect to combat deformation and resist cratering. The high cutting temperatures that occur during dry machining tend to soften the carbide structure slightly, which in turn increases its toughness, prevents chipping, and prolongs tool life.
The latest coating technologies are based on titanium nitride (TiN), titanium carbon nitride (TiCN), and titanium aluminum nitride (TiAlN), which were developed to withstand more severe operating conditions.
TiAlN, in particular, has emerged as an outstanding coating that exhibits thermal stability up to 900 degrees C. The coating displays a very low friction coefficient, enabling its use under almost all dry machining conditions.
Dry Machining Limitations
While the technology to carry out dry machining has improved, metalworking fluids are still needed to ensure that the highest speeds and feeds can be used and that the surface finish of workpieces meets expectations.
Certain dry machining trials have indicated that new tool coatings have been helpful, but the problem of dry machining at a rate needed to achieve high productivity remains.
Industry has not seen any increase in dry machining beyond its current use for cast iron. Dry machining, presumably, cannot overcome the benefits metalworking fluids provide. The tradeoff in using metalworking fluids is normally a compromise between fluids disposal costs and productivity.
Near-dry Machining
Near-dry machining is the application-of small amounts of lubricant to the tool/workpiece interface. The key to the proc– ess is atomizing the lubricant, with air as the carrier, into a fine aerosol.
In near-dry machining, the lubricant is applied directly through a nozzle pierced through the cutting tool; the flooding method cannot be used in near-dry machining. That brought about the development of MQL technology in which air and oil are mixed together as closely as possible to the cutting tool.
MQL trials using this system have indicated that near-dry machining demonstrates the best performance on cast aluminum alloys and steel alloys.
Dry Machining's Future
While dry and near-dry machining will not replace wet machining in the near future, these two techniques will provide cost-effective alternatives in niche applications.
Despite the growing popularity of dry machining and MQL methods, certain materials still require cutting fluids to be machined. Aluminum, for example, has to be machined with high-pressure coolant to prevent the built-up edge (BUE) phenomenon. In addition, machining high-temperature and nickel-based alloys also requires copious amounts of coolant to prevent strain hardening of the subsurface layers.
Deficiencies such as chipbreaking, exhibited when machining stainless steel, can be solved with the use of a coolant stream.
When to Choose High-pressure Coolant
High-pressure coolant is the latest technology, used particularly for exotic and stainless steel materials. High pressure creates a localized stream of pressure coolant, which eliminates the formation of vapor and assists chipbreaking into small segments.
The combination of coolant and high pressure (70 to 140 bar) precisely targeting the cutting zone provides advantages conventional flood coolants can't. The coolant is delivered with enough force to reach the cutting zone as a liquid, not a vapor. In the liquid phase, it actually lubricates the cutting process as it quenches the molten chip, shattering it into smaller, more manageable pieces.
Also, because of its internal flow path, high-pressure coolant keeps the spindle,tool, insert, and workpiece cooler, leading to much longer tool and equipment life. Another benefit is that delivering coolant close to the secondary shear zone improves machinability of the material under high-speed conditions.
Coolant pressure can be as high as 400 bar, but most applications are in the 70- to 100-bar range. At the extremely high cutting rates in today's high-pressure turning applications, high-pressure coolant flow is an integral part of the cutting process. Besides preventing overheating and thermal shock in the cutting zone,the coolant also enhances cutting action.
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