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Reducing friction lets a new coating score in three ways for dry machining.

Reducing friction lets a new coating score in three ways for dry machining.

Milling was performed dry, at a cutting speed of 300 m/min and a feed of 0.15 mm/rev.

A surface roughness test shows the difference in roughness between a diamond coating (top) and a DLC coating on a carbide substrate.

An optical micrograph depicts the rake face of an uncoated and a DLC-coated insert after dry milling tests on aluminum alloys.

Sumitomo's new DL1000 inserts feature a DLC coating.

Friction is bad for cutting tools. It causes chips to weld to cutting edges and heat to build up in both tools and workpieces. These conditions, in turn, often lead to high cutting forces that can cause part deflection. However, a new cutting tool coating not only promises to overcome these problems, but its goal is to actually increase productivity, extend tool life, and deliver excellent surface finishes. The coating reduces friction so well that it even makes rubber slippery.

To show how well the coating performs, an engineer at Sumitomo Electric Carbide Inc., Mount Prospect, Ill., threw a piece of rubber onto a table in front of Thomas Jensby, the company's engineering manager. According to Jensby, the rubber "puck" promptly stopped where it landed. The engineer then threw a second chunk of rubber on the table. This one skated across the surface without slowing down, Jensby recalls.

The second piece was coated with a diamond-like-carbon (DLC) coating that, claims Sumitomo, is the first of its kind for dry machining of aluminum alloys. Debuted at IMTS on the DL1000 carbide milling insert, the DLC coating is exceptionally smooth and has an extremely low friction coefficient, 0.05 to 0.2 µ, for aluminum alloys. The coating's lubricity greatly improves chip flow versus uncoated inserts, reports the company.

"The DL1000 produces a thinner and shorter chip than an uncoated-carbide insert," Jensby explains. "There's less adhesion because chips don't stick to the rake face as long." Eliminating chip weld maintains the tool's edge sharpness and lets shops dry machine aluminum without marring or scratching finishes.

The coating also combats heat, which is one of the primary failures for cutting tools. "Heat is what generally ends up eroding tool life," remarks Jensby. The DLC coating reduces the cutting temperature approximately 25%. Sumitomo reports that its coated tools last about twice as long as their uncoated counterparts.

In addition, the DL1000 cuts with less pressure than an uncoated tool, reducing axial cutting force more than 50%.

The process behind the coating
DLC coatings are based on the same carbon chemistry as diamond and graphite and feature an amorphous structure that provides both high hardness and good lubricity. These coatings have protected wear parts in the automotive and other industries for a number of years. However, they posed a major drawback when applied to cutting tools: Basically, they peeled off when exposed to the heat and force generated in machining applications.

"In the automotive-process coating," explains Jensby, "coating thicknesses vary from 1 to 30 µ. The hardness of these coatings ranges from 1,000 to 3,000 Vickers, with the hardest coatings being somewhat difficult to achieve. Unfortunately, when these coatings get thick, they have a tendency to peel off. That's the problem with the wear-part coatings on cutting tools — they probably could work to some degree, but they were not developed for cutting tools."

Sumitomo overcame this problem using a low-temperature coating process. The ensuing coating is extremely thin and resists chipping and peeling.

The low-temperature process also provides another benefit, says Jensby. "Diamond coatings are generally deposited through a high-temperature CVD process that can cause physical changes to the carbide substrate of the tool. The DLC coating is done at low temperatures — actually not much higher than room temperature — so there is no change in the tooling geometry from the coating process. The temperature is so low, our engineers coated rubber without melting it."

Game plan for the future
Sumitomo is currently examining new applications and tool materials for its DLC coating, which it has tested at cutting speeds up to 10,000 sfm. The coatings work best in low-silicon aluminum and in finish/semifinishing milling applications. However, future applications should involve other nonferrous materials as well as turning and boring tools.

The company is also examining tooling materials beyond carbide. For instance, it has coated tools tipped with polycrystalline diamond (PCD). "Now, one might wonder why anyone would put a diamond-like film over a PCD tool," comments Jensby. He says that the smoothness and the lubricity of the coating provides better surface finishes and permits users to run their PCD tooling without coolant.

"DLC coatings do not compete with PCD products, which are mainly roughing tools," says Jensby. Instead, the coatings might someday coat such PCD-tipped tools as reamers, drills, and counterbores.

Put to the test

A paper by Keiichi Tsuda, Haruyo Fukui, and Yoshikatsu Mori, three researchers from Sumitomo Electric in Japan, describes various tests performed on the company's new DLC coating. Among the results detailed is the shape of the chips generated during dry cutting tests.

The researchers compared a DLC-coated insert to an uncoated tool in both A50502 and ADC12 aluminum. In both tests, the chips from the DLC-coated insert were about 1.5 the length of those from the uncoated insert.

As the photos show, test cuts on the A5052 material yielded two markedly different types of chips. The chips from the uncoated tool are whitened and have a large curl diameter. In contrast, the chips produced by the DLC-coated tool have a metallic shine and are small and curled into a spiral.

Although not pictured, researchers also ran tests machining with coolant and the uncoated tool. The resulting chips still did not show the spiral curl formed using DLC coatings.

As for the ADC12 material, chips made by the uncoated tool have no metallic shine and are gray. Some chips are saw-toothed along the edges, others broken midway. There is no uniform shape to any of the chips. Researchers believe the white or gray color of the chips is caused by an increase in cutting temperature.

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