Coated round and indexable tools continue to be sales leaders for general-purpose cutting. New geometries, when coated, also help tools last longer and cut quicker, according to cutting tool experts.

Indexable tools are sold more often with titanium aluminum, or TiALN, than any other coating, according to T.J. Nemic, president of Nemic Machinery in Grand Rapids, Mich. The most successful products have multi-layered coatings that include TiALN, which is used for machining harder steels or high-temperature materials because higher-than-normal heat is generated.

According to Nemic, the number of coating types has increased dramatically in recent years.

"Everyone has their own name for coatings, multi-layer and otherwise, so it's hard to say which coating type has the best performance," he said.

Polycrystalline diamond and cubic boron nitride are popular if the price is right, he said.

"They are very good in graphite and companies are trying new applications with diamond in steels," Nemic said.

Milling operations have become more demanding of tools because manufacturers want more productivity through faster speeds and feeds, and longer-lasting cutting edges. Advanced coatings and micro-grain carbides help realize those goals.

Another tool coating, Laser-Cut 964, is a proprietary single layer coating, 65 millionths of an inch thick. It's also known as "rainbow coating" for its gleaming, multicolored appearance. Reports indicate it provides up to 10 times the average edge life of other coated tools.

"Laser 964 is very new and quite expensive for applications in my area," said Harold Brooks, vice president of sales for Cumberland Die Supply in Nashville, Tenn. "However, for exotic metals in applications where the cost is justified, such as defense work, the coating doesn't wear like TiALN or some others."

Recommending coatings on cutting tools all comes down to wear, Brooks said. For example, coating D2 steel with tungsten carbide is adding expense for little value because the edge breaks down under heat, regardless of the coating.

A typical application for carbide coating is cutting nickel alloys, which are abrasive and difficult, particularly in interrupted cuts, he said. M4 and M42 steel tools coated with tungsten carbide reduce chatter and don't wear as easily.

Polycrystalline diamond and CBN-coated tools are expensive but the cost is justified for tungsten carbide toolmakers, he added.

"When you're machining carbide, diamond is a starting point and as tools get better at holding their edge, the costs become more justified," he said.