Diamond coatings sparkle.

Proprietary process coats round and complex tools and wear components fast and could hasten adoption of dry machining techniques.

A breakthrough in diamond coating technology could take us a little closer to the day when dry machining is the rule rather than the exception, at least for aluminum and nonferrous metals. Further developments in tooling materials, coating technology, and machine design, however, are still needed to make inroads into dry machining ferrous metals, such as steel and cast iron.

Looming in the background is the specter of government regulatory pressure brought to bear on ridding the workplace of potentially dangerous exposure to coolant in the form of mists that are inhaled or fluids that are touched. Europeans, particularly the Germans, have already taken the lead in adopting more stringent requirements for machining with and without coolant.

One thing is certain: the mindset of metalworkers to accept coolant as the only way to lubricate, remove heat, and evacuate chips in metalcutting operations is being challenged to change. Other options to current coolant usage including new formulations by the coolant manufacturers, misting systems, and substitution with cooling gases are all currently being studied.

Just how serious industry is about the problem will be seen in a conference to be held in mid November in Dearborn, MI, on the environmental and epidemiological effects of coolant systems in the plant environment. Sponsorship of the conference reads like a Who's Who of organized labor, trade associations, government agencies, and health care organizations. Their concern: the impact of coolant handling and delivery on the work-force. What will come of their findings remains to be seen, but the fact that so many are talking about coolant management signals its growing importance to metalworking manufacturing.

The diamond connection

The introduction by QQC Inc, a member of the Turchan Technologies Group, Dearborn, MI, of a new diamond coating technology marks the commercial introduction of a process that has already proven itself in high-speed dry machining of automotive powertrain components at GM's Power Train Div, Bay City, MI. Buoyed by the success of the large diamond-coated milling tools, Turchan engineers turned to developing a coating process for small diameter tools that could be adapted to other machining operations.

QQC Inc was formed expressly for this purpose, explains company president Manual Turchan. The solution seemed to lie in developing a way to apply diamond coatings to round and/or shaped tools for which conventional PVD and CVD coating technologies are somewhat limited in their ability to apply uniform diamond coatings to meet the demands of production machining, he explains.

QQC's "laser" based process is claimed to be about 1000 times faster than competitive CVD and PVD processes, reducing to minutes the time that it normally takes (hours) to apply coatings to tooling.

Turchan's experience with diamond coated-milling cutters included machining automatic transmission components such as channel plates, valve bodies, and chain covers, where one or both sides of the relatively flimsy parts are rough milled using diamond insert face mills at rates up to 600 parts/hr. Some systems have also been built for the machining of cylinder heads, achieving overall flatness of 0.0001[inches] on a production basis at rates of 230 parts/hr.

Key to dry machining at the Bay City, MI, GM plant is total integration of chip removal processes into its machining system design. Turchan's vacuum chip disposal system permits the systems to operate totally coolant free. Chips are recovered in a dry state. High-speed machining makes it possible to eliminate coolants from the process. At the high cutting speeds used on these systems (about 15,000 sfm), adiabatic conditions are achieved and virtually all the heat generated goes into the chips, so coolant is not required to maintain thermal stability of the system.

The QQC coatings can be applied to tools including cutting tool inserts, round tools such as drills and reamers, curved and irregular surfaces, and large fiat areas in thicknesses up to 1mm and to any grade of tungsten carbide, ceramics, cast iron, stainless steels, and a variety of other ferrous and nonferrous metals and non-metallics. The process produces a true metallurgical bond between the diamond coating and the substrate material which gives the coating a very high bond strength to resist spalling, cracking, and stress-related failures.

Currently under investigation by Turchan are applications of its high-speed dry machining technology to drilling, which is typically the slowest machining operation on a transfer line and where chip removal through redesign of flutes is mandatory.

Tooling may only be the tip of an iceberg of potential applications for the diamond coating process, however. QQC says it has supplied a variety of diamond-coated tools for the can making industry, diamond-coated extrusion dies for aluminum, and balls, cages, and races for ceramic bearings. Other applications in the medical industry including the steel or ceramic balls in hip replacements and in the electronics industry also look promising.

Dry quest goes on

The goal of achieving dry machining continues to be one of those much sought-after and talked about machining objectives. Coolants serve many purposes in metal removal. With their help, the workpiece and tool are cooled, wear is reduced, and chips are flushed out of the machine. But there are essential disadvantages. Machines cost more because of the need to process and circulate the coolant.

Other costs include the cost of protecting the environment from oil mist, the costs of purchasing and disposing of the coolant, the cost of disposing of the contaminated chips, and the strain on workers and the environment caused by coolant in all its forms.

Expenses normally related to the use of coolant include not only the cost of the coolant and its disposal, but coolant filtration, mist filtration, cleaning of parts, and the cost of separation of coolant from chips.

Elimination of coolant, however, has far-reaching effects on the machine, the tooling, and the process. The tool must withstand the thermal and mechanical stresses of the process and still be economical. In a high production environment it takes on the added burden of meeting consistent workpiece quality in large batch sizes.

One area of development is in materials and coatings for tooling. Today, coated carbides and ceramics allow cutting speeds of about 2000 m/min (6500 fpm). At the same time, the improved cutting materials have increased tool life, so that in the final analysis these new tools offer a substantial economic advantage in spite of their higher initial cost.

Cermets have been used much more extensively in Japan than in the US and Western Europe, particularly for semifinishing and finishing operations. One distinct advantage: price is a fraction above that for uncoated tungsten carbide tools and only a fraction of polycrystalline diamond tooling as the accompanying cost comparison graph from R Edwards' book published by The Institute of Materials, Cutting Tools, illustrates.

Cermets have been very successful in Japan for a variety of reasons. Metalworkers there have benefited from the use of stiffer, more powerful machines and have wanted to be less dependent on tungsten, a strategic material in the 1950s and 1960s, according to Mr Edwards. Today, pricing of tungsten can fluctuate widely with interruptions in supply; 70% of the world's supply comes from China.

Cermets (CERamic with METallic binder) are a titanium carboni-tride (TiCN) composite consisting primarily of titanium carbide (TiC) and titanium nitrite (TiN) with a metallic binder such as nickel and cobalt. This unique combination of particles is the reason cermets show such high wear resistance even though many are not coated with a hard metal material, according to Mitsubishi Materials USA Corp, Garden Grove, CA.

Mitsubishi NX series cermets are used as a major cutting tool on finishing of steel for turning operations, ductile cast iron, and roughing of mild steels at a fixed depth. The company has also developed a cermet insert which is a multicomponent carbonitride based called NX530 Grade which is the toughest cermet with the best flank wear. One coated variant, UP35N is a coated TiN (PVD) cermet that delivers a high luster finish along with longer tool life on interrupted cuts in both wet and dry turning applications. For milling operations, the company has developed an especially tough multicomponent carbonitride insert, the NX530, which exhibits excellent flank wear. The company expects the shift from cemented and coated carbides to cermets to continue.

Mr Edwards attributes more versatility to cermets than is often thought: "Although their hardness is similar to that of hardmetals, they have a lower susceptibility to diffusion, favorable frictional behavior, and low susceptibility to wear. These factors make it possible to cut clean metal at higher speeds than would normally be used for hardmetal. Their major area of application is in finish turning and semi-finish turning of steels. Excellent results can also be obtained with light milling cuts on difficult-to-machine steels such as stainless steel."

New machine design

Rethinking machine design will play a significant role in just how well and how fast dry machining techniques can be implemented. According to Bll Hughes, manager metalcutting systems development, Kennametal Inc, Latrobe, PA, removing coolants from the machining paradigm would require total rethinking of the process. Everything from chip management to what happens inside the machine would be fair game, he believes, as the role of coolant both as coolant and lubricant becomes better understood.

"There is a strong sense that standards need to be formulated," says Mr Hughes, who points out that too often coolant delivery systems are an afterthought in machine design. "Before we can be comfortable with the tradeoffs that would be made between tool life and machining without coolant, it would be necessary to get a good handle on the total costs and the options that metalworkers have."

Kennametal, which has surveyed its customers to find out how far along toward adopting dry machining techniques they are, is conducting machining tests to establish cutting results and conditions without coolant.

Newly designed machines can make a difference. American Pfauter's P100 CNC Hobbing Machine, introduced at 11.EMO, is designed for dry machining or wet machining of automotive sun gears and pinions and high-production power gear tools. It was shown cutting a gear in 6-sec cycle using a solid carbide hob. Actual cutting time was 4 sec for the P100, which features a 4000 rpm spindle, a 3000 rpm table speed, and automated loading. Run slower, the P100 can produce a finish on the gear that would eliminate the need for a finishing shave operation.

The horizontal spindle, direct drive P100 hobbing machine features a work area completely enclosed in a stainless steel shroud, which helps direct "hot" chips downward and completely away from the work area to maintain thermal stability. This standalone machine features a compact footprint of only 2.3 cu meters and a 2-sec parts autoload feature. It can be operated wet or dry with carbide or HSS tooling for hobbing, grinding, polishing and even hard finishing.

Liebherr's prototypical LC 82 hobbing machine, developed for high-speed machining of manual auto transmissions, has reportedly reduced machining time in tests to 17 sec using TiN coated carbide tools and cutting without coolant at 350 m/min. This compares favorably with 28 sec total machining time using TiN-coating HSS hobs and cutting at 120 m/min with coolant and a feedrate of 3 mm/rev.

Using a cermet hob without coolant further reduced machining time to 13 sec. In addition to reducing piece costs by 15% to 25% before savings due to the elimination of coolant, reduction of the overall machining time to 13 sec using the cermet tool would in effect double capacity of the machine, the company claims. Quality of the work-piece machined dry with the TiN-coated cermet hob was found to be comparable to that of a conventionally cut gear. The process is compatible with subsequent heat treatment and hard-gear finishing (honing) or with an alternative finishing process such as shaving and hardening.

Emuge Corp, Northborough, MA, has developed and tested taps which allow lubricant-free thread-cutting in a broad range of materials. First it was necessary to find a geometry with low friction for cutting true-to-gage and clean threads, a flute form that allows the chip to roll off and leave the bore without hindrance, and select hard surface coatings that would protect the taps from high friction strain. Thread-cutting tests with various of Emuge's Oko series taps produced acceptable results in unalloyed and low carbon construction steels, unalloyed tool steels, heat treatable steels, cast materials, and corrosion- and acid-proof steels.

As one observer put it, dry machining almost has to come, but dry doesn't make sense unless machining is fast. And the faster you go, the better it works.