Coatings coupled with speeds and feeds increase productivity and tool life.

by Michael Plankey

Ask an engineer or machine operator what he wants from his cutting tools, and the answer is universal: increased productivity and greater tool life. Several factors affect productivity and tool life. Two of the most important are machining parameters (speeds and feeds) in combination with various coatings.

Speeds are determined by surface feet per minute (SFM), which is defined as the distance in linear feet the cutting tool travels in one minute. Feed, or inches per revolution, is the distance the tool cuts into the workpiece for each revolution of the cutting tool.

Speeds and feeds vary depending on the type of material being machined, the substrate of the cutting tool, whether the tool is running with or without coolant, and whether the tool is coated or uncoated.

Coatings were designed to increase cutting tool performance and overall productivity.

When a cutting tool enters the workpiece, it rubs, causing friction. The friction turns to heat, and if it becomes hot enough, the tool can approach the temperature at which it was hardened. When the tool cools down, the cutting edge tends to be drawn back, or annealed, which results in reduced hardness of the cutting edges.

This reduced hardness causes the cutting tool to wear prematurely when used again, and shortens the tool’s wear life. Coatings help solve this problem by keeping the heat off of the cutting edge, and in the chip. Coated cutting tools have a freer cutting action and therefore less contact with the chip while they are being formed, which keeps the temperature of the cutting tool lips and rake cooler.

Coated is cooler
Coated tools run better at higher feeds and speeds and also produce a better finished hole, thread or slot because of freer cutting action and lower co-efficient of friction. Coated cutting tools also cut closer to their own size. All cutting tools generally cut holes larger than the diameter of the tool itself, primarily because of built-up edge (BUE), which is material build up on the cutting tool’s edge.

The built-up edge occasionally breaks off the cutting edges and embeds itself either in the chips being produced, or in the surface of the hole, thread or slot being produced, causing the surface finish of the part to be rough. Coated tools have a lower co-efficient of friction, which gives them freer cutting action, dramatically decreases built-up edge, and produces superior part finish and more accurately sized holes, threads or slots.

Just as machining with coated tools at higher speeds and feeds improves productivity, not following the suggested higher machining parameters decreases productivity.

Coated cutting tools run at higher speeds and feeds, which dramatically increase productivity and increase tool life. For example, if a machine operator were required to drill in a material such as carbon steel with an uncoated half-inch jobber drill, the suggested speed would be 100 SFM. If he used a titanium nitride-coated drill instead, the suggested speed would be increased by 25 percent and the feed would be 10 percent higher than the machining parameters for the uncoated drill. This would mean increasing from 100 to 125 surface feet per minute and increasing speed from .008/revolution to .009/revolution. By upgrading to the TiN-coated tool, the user would realize a production increase of 366 inches drilled per hour to 515 inches drilled per hour, a 40 percent productivity increase per hour. The machine operator would have increased tool life as an added benefit as well.

Three common coatings
Cutting tool manufacturers base suggested feeds and speeds for coated tools on performance tests. Three of the most requested coatings are titanium nitride (TiN), titanium carbonitride (TiCN) and titanium aluminum nitride (TiAlN).

TiN coating is gold in color and is intended for aggressive machining of steels. TiN-coated tools should generally be machined at 20 percent faster speeds and 10 percent higher feeds than uncoated tools.

TiCN coating is blue-gray in color, is extremely hard and wear-resistant, and is intended for very aggressive machining of stainless steel and non-ferrous materials. TiCN-coated tools should generally be machined at 35 percent faster speeds and 50 percent higher feeds than uncoated tools.

TiAlN coating is violet blue-gray in color, is extremely hard and wear-resistant, and is intended for aggressive machining of stainless steels, high-alloy carbon steels, nickel-based high-temperature alloys and titanium alloys. TiAlN-coated tools should generally be machined at 50 percent higher speeds and 25 percent higher feeds than uncoated tools.

Coated tools will improve productivity when machined at higher speeds and feeds. Guidelines vary depending on the type of tool being machined, and all coatings do not deliver equal performance on all materials. Follow the cutting tool manufacturer’s guidelines about which coatings work best with specific tools and materials.

Michael Plankey is a technical specialist for Kennametal Industrial Products Group. He can be reached at (706) 650-4220 or michael.plankey@gfii.com.

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