Should You Try Dry?

Cutting without coolant or lubricant has always been the rule for certain applications. Now, the practice is expanding into new areas in U.S. machine shops. Here’s how and why dry metalcutting is evolving into a mainstream manufacturing trend.

Why try dry metalcutting? Although the practice has become more than just a passing trend in the American automotive industry and in foreign manufacturing plants, you’d still be hard-pressed to find a U.S. machine shop that isn’t using coolants and lubricants. It’s not hard to understand why a shop would be reluctant to give up its metalcutting fluids. After all, fluids lubricate and cool the point of cut, flush chips out of the cutting area, and control built-up edge (BUE) on the tool. With higher production rates, lower manufacturing costs, and all the other benefits associated with cutting fluids, why would a shop want to practice dry metalcutting?

First, there are environmental factors to consider. Dry metalcutting eliminates the worker health and safety hazards associated with coolant mist and wet shop floors. A study conducted by Harvard University for General Motors and the United Auto Workers union concluded that there are adverse health effects attributable to long-term exposure to straight-oil, soluble-oil, and synthetic metalcutting fluids. By cutting dry, you can circumvent these health issues, as well as environmental concerns related to cutting-fluid disposal. Rather than wet chips that require processing and filtration, you get clean, dry chips that are ready to be sold or recycled.

Economical factors also provide compelling reasons to try dry metalcutting. Once you take into account the costs for fluid procurement, maintenance, and disposal, along with any fines that may be incurred for violating governmental regulations, cutting fluid can amount to a sizable percentage of your total manufacturing cost. Some German companies have reported fluid costs as high as 16% of production costs, compared to tool costs of only 3% or 4%. U.S. shops are more reluctant to reveal their operating costs, but it’s known that cutting fluid is often the most costly of consumables in U.S. manufacturing. Fluid costs can be three to four times higher than tool costs for U.S. shops, according to Fred Teeter, marketing director at Balzers Tool Coating Inc., North Tonawanda, NY.

These environmental and economical benefits of dry metalcutting have already swayed many European manufacturers to give up their coolants and lubricants. Dry cutting is especially popular in Germany, where roughly half of the metalworking in Europe is done. Strict environmental regulations and rising costs for cutting-fluid disposal and recycling have encouraged many German shops to cut dry. Disposal costs alone make up 22% of total cutting-fluid-related costs in Germany, according to Dr. Tibor Cselle, director of R&D at Gottlieb Guhring KG, Albstadt, Germany. Balzers’ Fred Teeter says that environmental laws have driven up fluid-disposal costs in Germany to $1 per liter.

In the United States, even the shops that rely heavily on flood and high-pressure cooling systems may make the transition to dry metalcutting in the near future. Whether or not a shop adopts this process as a common practice depends on how the costs of doing without the benefits of cutting fluids compare to the costs of buying cutting fluids and maintaining and disposing of those fluids in compliance with environmental regulations. To calculate these costs, a shop must determine how well the workpiece, machine tool, and cutting tool can tolerate the heat and chips generated in a given dry-cutting operation.

The Workpiece

Without the cooling effects of fluid, a metalcutting process may produce excessive heat that subjects the workpiece material to high stress and the danger of thermal expansion. However, for many work materials, properly performed dry cutting can expel the heat with the chip to avoid these adverse thermal effects on the workpiece.

Cast iron is almost always cut dry. Some shops are even taking advantage of the heat generated in the dry cutting of cast iron to increase their productivity by following the recommendations of Makino Inc., Mason, OH. The machine tool manufacturer claims you can push metal-removal rates in cast iron to the maximum using its "red-crescent" technique. In this process for dry milling and drilling, polycrystalline-cubic-boron-nitride (PCBN) inserts are run at very high surface speeds and feed rates to push the heat ahead of the tool, creating a red glow. By reducing the metal’s yield strength, the concentrated heat allows you to increase your cutting speeds and feeds. According to Stan Weidmer, process development engineer at Makino, the red-crescent technique allows cast iron to be cut dry at speeds ranging from 6,000 to 10,000 sfm. Makino is working on alternative dry-cutting techniques to improve metal-removal rates in steel.

At the other end of the scale from cast iron and steel are some nickel-base alloys, titanium alloys, and exotic materials, which are extremely difficult to cut without using fluid. "The high-strength nickels and titaniums and exotics are hard enough to cut wet," says Bill Hughes, manager of metalcutting-systems development at Kennametal Inc., Latrobe, PA. "Cutting fluid is often required to meet the surface-finish requirements of parts made of these materials."

Cutting fluid is generally used to achieve mirror-like surface finishes in aluminum applications, particularly those involving high-volume production. However, machine tool builders and toolmakers are helping shops develop practical dry-cutting processes that can achieve high-quality surface finishes for some aluminum applications. "We are having success dry milling the 70 series aluminums in certain applications, but we cannot be nearly as aggressive," explains Weidmer. "Aluminum is extremely challenging, because it tends to be gummier and adhere more to the tool."

Hughes agrees that BUE is the main problem in the dry cutting of aluminum. "It’s not so much a problem with some other long-chipping materials," he says, "but aluminum is soft enough to weld to the tool." One way to avoid BUE when cutting aluminum dry is to cut at higher speeds. By transferring all the heat generated in the cutting process to the chip, higher speeds can help eliminate aluminum’s tendency to weld to the tool.

Turchan Technologies Group Inc., Dearborn, MI, has put this high-speed solution to the test. The company has installed high-speed turnkey systems for dry cutting aluminum components for the Big Three automakers, which are switching to aluminum from ferrous materials for many parts. Even at production rates of 600 parts per hour, Turchan claims its dry-metalcutting systems can generate surface finishes on aluminum parts that are as good or better than the finishes attainable using cutting fluid. A shop may be able to achieve similar success in other dry-metalcutting applications by using a machine specifically designed for the job.

The Machine Tool

The workpiece material dictates whether or not a shop needs to purchase a machine tool specifically designed for dry cutting. To achieve the higher speeds and feeds that are typically used for the dry cutting of aluminum, the complete structure of the machine has to be designed for rigidity to accommodate the higher spindle-speed requirements. Turchan’s president, Manuel Turchan, says that machine rigidity is necessary for dry endmilling and drilling of aluminum with small-diameter tools at spindle speeds greater than 60,000 rpm. Cast iron and steel, on the other hand, can be dry milled or drilled with the machines already installed in most shops, according to Makino’s Stan Weidmer. Unlike aluminum, the dry cutting of cast iron or steel doesn’t necessarily require higher spindle horsepower or greater machine rigidity. Since chiploads are about the same cutting dry as they are cutting wet, torque requirements and tool forces are also about the same. “In the cutting of cast iron and steel, a machine that’s performing very well wet should also perform very well dry,” says Weidmer. “It’s just a matter of getting rid of the heat.” Although it isn’t necessary to increase cutting parameters for cast iron or steel, higher spindle speeds and feeds may allow the chips to be ejected from the cutting zone before the heat can penetrate the workpiece or tool.

Makino’s technique for expelling chips from the cutting area is to run air through a through-coolant spindle on an existing machine. "Standard shop air is rated at 80 psi," explains Weidmer. "That generally isn’t enough to get out the chips, and you end up recutting chips and sacrificing tool life." He recommends running an additional air line to the machine that supplies compressed air rated at 150 to 200 psi for dry cutting. "A separate compressor makes a big difference in chip evacuation," says Weidmer.

Compressed-air or vacuum systems can blow or suck ferrous chips out of the cutting zone, but their effectiveness depends on controlled chip formation at the cutting tool. "You have to be able to make very reliable chips to ensure that the chip-handling equipment can take them away," says Hughes. "Otherwise, the equipment may become overloaded by long, stringy chips that it’s not designed to handle."

The configuration of the machine must enable proper chip collection and evacuation to prevent dry chips from accumulating and building up heat, which may cause thermal growth of the machine. Chip removal may be aided by a machine’s chip-auger system. "The way covers that protect the guideways usually are slanted so that the cast iron or steel chips will slide down into those augers," explains Weidmer. "If you’re cutting correctly, most of the heat should be going out in the chips. As long as you don’t have large piles of chips sitting in any area of the machine, you shouldn’t have a problem with thermal displacement."

The chip augers take care of the heavy pArticles, but small, airborne particulates must be vacuumed out. For cutting cast iron without fluid in high-volume applications, Makino recommends the use of an air-filtration unit to prevent dust from escaping into the atmosphere. In extreme cases, shop vents may have to be relocated to provide adequate ventilation. Additional precautions may or may not be necessary, depending on how well the cutting process lends itself to being performed dry.

The Operation

There are limitations to the types of operations that can be done completely dry. In fact, some industry experts, such as Clyde Sluhan, founder of Master Chemical Corp., Perrysburg, OH, insist that dry-metalcutting technology is suitable only for milling and turning operations. "Dry-cutting technology cannot match the production and manufacturing efficiencies provided by standard cutting fluids," Sluhan contends. The general consensus seems to be that deep-hole drilling, reaming, broaching, and grinding are very difficult to do without any cutting fluid at all.

Dry drilling is complicated because there is constant heat in the cutting zone due to the continuous engagement of the tool in the cut. The drill’s constant contact with the workpiece may cause some thermal growth, which can create tolerance problems for the hole. In drilling, the greater the hole’s depth-to-diameter ratio is, the more problems there are with heat and chip flushing. "When you’re deep down in the hole, it’s very tough to get the chips out without some type of coolant or lubricant," says Magnus Ekback, product manager at Sandvik Coromant Co., Fair Lawn, NJ.

When drilling a shallow hole, heat and chip evacuation aren’t as significant. However, drilling often requires some type of coolant or lubricant to produce exacting finishes and rigid tolerances at high production rates. In fact, some dry-drilling applications are performed with minimal lubrication; they are technically "dry" because the small amount of fluid used can be completely dissipated through heat.

It is often favorable to eliminate cutting fluid from milling operations. Although machinists like to use cutting fluid to facilitate chip evacuation, particularly during endmilling, fluid tends to intensify the temperature variations that occur during interrupted cutting, leading to thermal shock and thermal cracking of the cutting edge. "During dry milling, the average temperature will be higher," says Sandvik’s Magnus Ekback, "but you won’t run the risk of temperature variations that are greater than the tool can handle." Even for operations well-suited to dry metalcutting, heat that isn’t expelled with the chips can cause tool fracture. You can support the removal of heat by the chips by selecting the proper tool and using suitable cutting parameters.

The Cutting Tool

You may be able to cut dry using a tool with a standard geometry, but you probably won’t be able to run it as aggressively as you could if you were using cutting fluid. Although many European toolmakers have developed tool geometries specifically for dry metalcutting, their U.S. counterparts typically recommend tools in their existing product lines that have suitable geometries for such applications. Tool manufacturers offer chipbreaker geometries for better control and evacuation of chips, and they are working on proprietary tool designs to control BUE and prevent wear at the cutting edges in dry operations. The tool material you select for a dry-metalcutting application is just as important as the tool geometry you choose. Not all tool materials have the properties required for dry cutting with the same parameters used for wet cutting. Due to the complex thermal- and mechanical-load conditions, the cutting tool’s hot hardness and toughness are crucial in dry cutting.

Cutting dry with ceramic and cermet tools typically isn’t a problem. In fact, tool manufacturers often recommend cutting dry when using ceramics and cermets due to the danger of thermal shock when using cutting fluid. Improper fluid application can result in irregular distribution of fluid in the cut, which creates an unstable heat zone for the cutter. These temperature variations can cause premature tool failure. Standard HSS and carbide, however, generally aren’t suitable tool materials for dry milling, drilling, or turning. HSS has poor deformation resistance, and carbide lacks the necessary toughness. According to Guhring’s Tibor Cselle, ultrafine-grain carbide has the heat resistance needed to prevent tool failure during dry milling and drilling. This carbide doesn’t lose its hardness, and it allows the tool to have very sharp edges that generate less heat than the honed edges on a standard-carbide tool. With the proper coating, however, a standard HSS or carbide tool may be suitable for the dry cutting of cast iron, steel, or aluminum. In a dry operation, a coated carbide tool generally performs better in terms of tool wear than an uncoated carbide tool.

Coatings serve a similar function to cutting fluids by providing a protective layer that isolates the tool from the chip’s heat. "If you can insulate the tool with a coating that will not transfer the heat, you can maintain a sharper, harder edge much longer," says Raymond May, program manager at Multi-Arc Inc., Rockaway, NJ.

A coating not only optimizes flank wear and cutting time, but it also protects the substrate from catastrophic thermal and chemical effects. "You want a very thermally stable coating that won’t break down at high temperatures," explains May. "You also want a very chemically stable coating. In almost all high-speed, dry-metalcutting applications, high temperatures are a catalyst for chemical reactions." A coating with a smooth surface finish also helps minimize the catalyst effect of heat by reducing friction. "Most titanium-aluminum-nitride (TiAlN) coatings have those characteristics," says May. "TiAlN coatings with a higher aluminum content are harder, but they all have about the same oxidation temperature." Because TiAlN has a higher oxidation temperature than titanium nitride (TiN) or titanium carbonitride (TiCN), it is preferred for many dry-metalcutting applications. It can withstand higher temperatures without breaking down and causing degradation of the tool substrate.

TiAlN can be combined with a soft coating based on molybdenum disulfide (MoS₂) to form a multilayer coating with high wear resistance and a low coefficient of friction. The MoS₂-base coating can also be used on its own to reduce BUE and speed chip evacuation. Tungsten-carbide/carbon and diamond-like carbon (DLC) coatings also show very good potential for replicating the effects of cutting fluid.

In the dry cutting of aluminum and nonferrous metals, diamond coatings perform much like DLC or MoS₂-base coatings, with the additional benefit of high hardness. Diamond composites and CBN are under development for coating tools designed to cut cast iron, hypereutectic aluminum alloys, and various steels and titanium alloys without cutting fluid.

Coating suppliers are investigating other coating properties, such as good adhesion and layer thickness, that may be important for wear resistance of tools that are cutting dry. More research and development of coatings is essential for dry metalcutting to continue to grow in U.S. shops.

The Future

While developers of dry-cutting technology believe it will become an absolute necessity for U.S. shops to go dry, cutting-fluid manufacturers and suppliers aren’t too concerned about dry metalcutting putting them out of business. "Dry cutting has been available as an alternative process for 30 years," says Rick Chambers, Valcool products manager at Valenite Inc., Madison Heights, MI. "It’s being practiced in some U.S. shops, but only for limited applications." Although fluid manufacturers and suppliers concede that dry metalcutting may be suited to a particular application, such as cutting cast iron, they don’t see it becoming a common practice in U.S. shops. "For the most part, all machine shops will always have to rely upon traditional cutting fluids," opines Master Chemical’s Clyde Sluhan.

Fluid manufacturers and suppliers dispute the idea that dry metalcutting is more economical. They justify the costs of maintaining and disposing of cutting fluids by the benefits you get in terms of tool life and part quality. Cutting fluids have added value if they’re used and maintained properly. “In addition to maintaining your fluid, you have to maintain your machine tool and use the proper tools at the right speeds and feeds," says Rick Chambers. "If you can get longer life out of the fluid, that minimizes the amount you must pay for disposal." Fluid manufacturers are researching and formulating products that are less expensive for shops to maintain, easier to recycle, and safer for the environment.

Those who are working to find the right components for a successful dry-cutting operation may disagree that the practice offers limited benefits. But all may agree that there are cases when high temperatures in the cutting zone and high friction forces at the point of contact between the tool and the workpiece may lead to unacceptable results if metalcutting is performed without cutting fluid. Fluid may be essential for removing chips during some endmilling and drilling applications or for collecting and washing away cast-iron dust. It may be needed to moderate temperatures in the workpiece for maintaining accuracy or improving surface texture. And you simply cannot reap all the benefits of dry cutting unless fluid can be eliminated from all the processes in the production of a part.

Ultimately, the decision to cut metal dry or wet may not be up to the individual shop. Eventually, governmental regulations may exact such a heavy toll on shops using fluid that they’ll have to give it up. "We think that what happened in Europe is going to happen here," says Makino’s Stan Weidmer. "Costs for cutting-fluid disposal are going up. As OSHA [Occupational Safety and Health Administration] and EPA [Environmental Protection Agency] regulations get tighter and tighter, there will be a lot of disadvantages to keeping cutting fluid up and flowing."

Kennametal’s Bill Hughes agrees that stricter laws will provide the incentive for shops to switch to dry cutting. He cites OSHA’s proposal to tighten up the limit for suspended pArticles in the atmosphere from 5.0mg/m³ to 0.5mg/m³. "That will significantly change what is happening in wet processing, requiring better machine enclosures and mist-collection systems," says Hughes. "Rather than going through complex system-related changes, some U.S. shops may decide simply to avoid cutting fluid by going dry."

Because most existing machinery cannot be retrofitted for certain dry-cutting applications (i.e. aluminum), going dry may require a shop that has invested in a machine with a flood or high-pressure cooling system to purchase a totally new machining system. "But for the investment in new dry-metalcutting equipment, the economics are extremely favorable," argues Manuel Turchan. "In most cases, a dry-metalcutting system will repay its entire capital cost in a period of 18 months to 32 months." In addition to eliminating costs for cutting-fluid purchase, maintenance, and disposal, dry cutting yields pristine chips that can be immediately sold or recycled, rather than wet chips that require processing. Turchan adds that you can eliminate the costs for cleaning machined workpieces, since chip-removal systems used in dry metalcutting leave parts clean and dry.

"Many shops want to go dry," says Makino’s Stan Weidmer. "But it’s going to take the technology to show them it’s practical and cost-effective to cut without fluid." He says that some shops are resistant to going dry because they’re afraid it will mean sacrificing tool life and cutting performance. But as machining and tooling technology improves, U.S. shops may find dry metalcutting to be economically justifiable in many instances.