The viability of dry cutting depends on several factors. The characteristics of some workpieces may make dry-cutting impractical: machining materials that are poorly heat-dissipating, work hardening or viscous workpieces may still require coolant. The characteristics of the metal cutting process itself also play a crucial role. For example, in milling and turning, the formation of chips is unimpeded, and in the drilling process, the cutting tools, chips and the heat of cutting are deeply hidden inside the workpiece, making chip removal and heat dissipation difficult.

Bob Hellinger, domestic sales manager at tool manufacturer Guhring, believes that in some turning operations, dry cutting can be beneficial because most of the cutting heat can be carried away by the chips. However, in the drilling process, the cutting heat is more transmitted into the tool, which may cause the workpiece material to be welded to the bit and cause the bit to fail.

Although dry drilling is challenging, Hellinger says, the process and its closest minimum amount of lubrication (MQL) drilling are becoming increasingly popular in the processing of aluminum and cast iron materials. “The automotive industry uses a lot of these materials and has the benefit of reducing the use of coolant through dry drilling and MQL drilling.” However, he adds that tool coatings are essential when performing dry drilling.

According to Dr. Dennis T. Quinto, a surface engineering consultant and senior tool industry expert, one of the main developments in dry drilling tool coatings is the addition of a relatively soft coating with self-lubricating properties. Such coatings typically include a tungsten carbide/carbon (WC/C) coating, a diamond-like amorphous carbon (DLC) coating, or a molybdenum based coating.

"Soft coating" is a relative concept. Conventional hard coats (such as TiCN or TiAlN coatings) have a hardness of approximately HV2000-3000, while self-lubricating coatings have a hardness of approximately HV500-1200. Dr. Quinto says that a softer coating is usually applied over the hard coat. “Apply an optimized PVD coating on the tool substrate and then apply WC/C, DLC or molybdenum-based materials to it.”

The lubricating coating applied over the hard coat serves primarily two purposes. First, reduce the friction of the cutting edge of the tool and reduce the "run-in" phenomenon. At the first few seconds or minutes after the start of the cut, the tool usually has a phase of rapid wear up (ie, the run-in phase), after which the tool wear gradually decreases into a relatively minor, gradual normal wear phase. Dr. Quinto said, “In the initial wear phase, the soft coating can reduce the cutting force. Then, when the soft coating is worn out, it is mainly played by the underlying hard coating.”

The second function of the lubricious coating is to accelerate chip evacuation, which is critical for dry drilling. Dr. Quinto explained, “When the chips are formed, they must be quickly drained to avoid chip clogging.” A soft coating with lubrication applied to the flutes behind the actual cutting edge of the drill can promote faster chip flow. The ground exits the hole through the flute, because no serious abrasive wear occurs in the flute.

Guhring's Hellinger said the company began working with the German government about 10 years ago to develop technologies that reduce the use of coolant in machining. “The first true dry cutting tool coating we developed consists of a multi-layer TiAlN and TiN coating with a molybdenum disulfide coating on the top layer.” This composite coating is coated with a PVD process. cover. The coating sequence is: a multi-layer hard coating at the bottom (called a Firex coating by Guhring), first coated with a TiN coating, coated with a TiAlN coating, and then alternately coated with the two coatings. Floor. Hellinger explains, “These hard coatings are resistant to wear and heat because the material begins to oxidize at 800 ° C.” He believes that the top molybdenum disulfide coating (called Moly-Glide coating) Layer) Much like Teflon for non-stick surfaces. Although it does not have the wear-resistant function of the hard coat, "When it is applied to the top of any coating or substrate, it can prevent other materials from sticking to it. This can eliminate the built-up edge and make the chips Smooth flow, it acts like a lubricant."

Regarding the comparison of coating properties, Hellinger cited an example of machining a hole with a hole diameter of 18 mm and a hole depth of 50 mm on a cast iron material. A TiAlN coated drill bit was used for dry drilling at a cutting speed of 100 sfm and a feed rate of 14.7 ipm, and 400 holes could be machined before the drill had to be reground. When using Guhring's drills of the same diameter but using the Firex/MolyGlide multilayer coating to process the same cutting parameters, 1200 holes can be machined before regrinding the drill. Although the cost of the Firex/MolyGlide coated drill bit is about 30% higher than the original TiAlN coated drill bit, the cost of regrind and downtime due to the shorter life of the original drill bit is three times higher than the replacement drill bit.

Guhring recently introduced a nano-multilayer grade Firex coating consisting of approximately 80 nanometer-scale coatings with a total thickness of 1.5-5 μm. Hellinger explains, “The thickness of each coating is thinner, helping to prevent the crack from spreading and preventing the crack from extending down to the base material and causing a greater impact on the tool.”

Dr. Quinto believes that the PVD process has the advantage of coating this multilayer coating because it provides a “mixing and matching coating capability. You can design the thickness of each layer in the multilayer coating. Very thin, usually as small as 20-30nm. You can adjust the composition and structure of the coating at any time, and you can increase the toughness of the coating by design."

The optimal adjustment of the thickness of each layer of the coating is the focus of a diamond-like carbon (DLC) coating recently upgraded by tool manufacturer Sumitomo Electric Hardmetals. Rich Maton, the company's assistant engineering manager, said the coating was originally a single-use coating (not coated on a conventional hard coating as it is now) for cutting aluminum and others. Non-ferrous materials. “When the tool cuts the aluminum part, the chips continually slide over the blade face. The aluminum is extremely abrasive and generates a lot of cutting heat.” The DLC coating reduces the coefficient of friction between the workpiece and the tool.

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