Technical Articles:

Coating for Speed
February 1996/Volume 48/Number 1
By Wolfred Mielert

Recent trends in mold milling can be hazardous to the life of the cutting tool. But with newly developed coatings such as AlTiN, the risks can be reduced.

No one is more interested in advanced machining techniques than mold and die manufacturers. In the past few years, they have adopted a number of high-performance practices to transform what traditionally was slow and labor-intensive work into a fast and simple process. However, the heat and cutting forces generated by these modern machining methods pose a constant threat to the integrity of the cutting edge. This has led mold and die manufacturers to search for ever more sophisticated tool coatings to shield the cutting tool from these hazards.

Shops that practice state-of-the-art moldmaking have found ways to speed up, rearrange, or skip some of the steps that have traditionally been used to build a mold. In a conventional operation, a moldmaker starts with a model of the piece that the mold or die is to form. Using this piece as a guide, the manufacturer rough mills and then semifinish mills the mold out of a block of mold steel.

Once the mold is close to its final form, the mold pieces are hardened by heat treating. Treating the steel to attain a hardness of about Rc 45 ensures that the mold will be rugged enough to last through thousands or even millions of mold cycles without losing its dimensional accuracy. Accuracy and surface finish are critical in moldmaking. For a mold to create exact duplicates of its shape, mold parts must fit together precisely, and the surface of the mold generally must have a mirror finish or better. In traditional moldmaking, the builder tries to machine parts as close as possible to their final dimensions before heat treatment, because the steel is much easier to cut before it is hardened. But the treatment usually distorts the parts. Because this distortion is not predictable, the mold builder cannot compensate for it in the initial machining stages. As a result, he must machine the parts after heat treatment to achieve the molds' finish dimensions.

These final stages of a conventional moldmaking operation are the slowest and most labor-intensive. Because the mold parts are being machined in their hardened states, low speeds and feeds must be used to keep heat and abrasion to a minimum. Ultimately, to achieve the fit and finish required, the mold builder may have to spend many hours reworking, polishing, and fitting the pieces by hand.

Recent Changes
In recent years, competitive pressures have made these conventional methods impractical. Mold and die users, responding to their customers' demands for higher quality and lower cost, have begun to ask for faster delivery times and higher levels of accuracy and surface finish from their suppliers. However, they are not willing to pay a premium for these improvements. This has prompted mold and die makers to adopt practices such as high-speed milling to move parts out the door more quickly without raising their manufacturing costs.

With the introduction of high-performance machine tools, it has become possible for mold and die makers to increase the speed of their milling operations significantly. These new machines have ultra-high-speed processing and data-transfer capabilities that make it possible to mill at speeds up to 30,000 rpm and at feed rates up to 33 fpm. An operator using conventional moldmaking techniques might mill the part at a speed of 460 sfm. For high-speed milling, the speed may be increased up to 2,600 sfm. Moldmakers also are finding ways to machine parts in their hardened state. This replaces much of the time-consuming hand work they used to need to correct the part distortions caused by heat treatment. Modern tools and CNC machines are capable of cutting tempered hot-work steel. This is typically accomplished by machining the material at a constant, shallow depth. A hot-forging die that has been finish machined in its fully hardened state may need no other work, because forging does not require a die with an extremely smooth surface.

Mold builders machining hardened mold parts may not be able to generate the smoother surface finish needed for operations such as sheet-metal coining or blow molding, but they can come close. Hard-machined dies and molds for these operations may need only minimal stoning or polishing to be usable.

A third way moldmakers try to economize is to mill parts dry. This saves money on coolant and coolant-delivery equipment, and it eliminates the cost and bother of disposing of spent coolant and contaminated chips in an environmentally responsible way.

Taking Some Heat
Milling hardened mold steels at high speeds without coolant generates extreme heat and cutting forces. A fragile cutting tool that cannot perform at these high temperatures will limit the moldmaker's ability to adopt modern machining methods. High-speed milling, itself, is a response to the limitations of currently available cutting tools. The moldmaker's ultimate goal is to shorten his cycle times by increasing the feed rate (ipm). But increasing the feed rate alone would increase the feed per tooth (ipt) beyond the capacity of most cutting tools. To increase the feed rate while keeping the feed per tooth at a tolerable level, the moldmaker must increase the speed (sfm) as well.

To avoid problems caused by the heat generated during high-speed milling, some moldmakers choose to use brazed polycrystalline-cubic-boron-nitride (PCBN) tools. A PCBN-tipped tool running at high speed will last nearly five times longer than a carbide tool. But users also must consider the limitations on cutting conditions that will be imposed by the tool's cost and fragility. These expensive tools cannot tolerate even slight deviations from optimum cutting conditions. During cutting tests at one automotive plant, for example, vibrations caused by a stamping press on the same floor as the milling machine caused a PCBN tool to shatter. If optimum conditions cannot be ensured, then PCBN tools will not be an economical choice.

Protecting the Tool
A more economical alternative for shops that need to improve their cycle times is to use coated carbide inserts or solid tools. Research published by the Fraunhofer Institute for Production Technology in Germany shows that running coated carbide ballnose inserts at more than three times their normal speed does not significantly reduce their life, and it leads to less machining time, thereby reducing overall machining costs. In these tests a 5/8"-dia. ballnose endmill was used to machine 56NiCrMoV7 (nickel-chromium-molybdenum-vanadium steel with a hardness of RC 47 to 48). Machining cost was nearly the same when the tool was run at 1,640 sfm as when it was run at a conventional speed of 460 sfm. Increasing the speed improved the surface roughness by a factor of 2, and machining time by a factor of 4.

Coated carbide tools may not be as durable as PCBN tools, but they do not shatter as easily, and they are much less expensive. In fact, coated ballnose inserts cost only a little more than uncoated inserts. The price of a coated insert ranges from $12.50 to $35, while uncoated inserts cost between $12.20 and $28. By contrast, a brazed PCBN ballnose cutter costs between $750 and $1,200. When coated carbide tools were first introduced, some shops may have been reluctant to use them, because they found them unsuitable for high-speed hard machining. But newer coatings have the hardness, toughness, and adhesion integrity those first coatings lacked, and they are gaining wide acceptance. As a general rule of thumb, most users anticipate speed increases of 20% to 50% when they switch from uncoated to coated tools. This is an extremely conservative expectation based on the safe, conventional approach to machining many shops take. One recently developed coating that seems especially suited to the rigors of modern mold and die milling is aluminum titanium nitride (AlTiN).

Users of this coating have found that it can withstand the heat generated by a high-speed, hard-milling operation running without coolant. This heat resistance is a benefit of the coating's high hot oxidation threshold. The chemical decomposition of AlTiN starts at a much higher temperature than that of most coatings. AlTiN also resists the transfer of heat. This property protects the cutting edge by insulating the tool substrate from damaging high temperatures and directing the heat into the chip.

AlTiN is a proprietary material that is applied to the tool substrate with a special modified physical-vapor-deposition process. The coating was developed in Europe, and it will soon be available in the United States. The cost of AlTiN-coated tools is about 30% higher than the cost of other coated tools, but the use of the coating can increase tool life up to 400% beyond the life of other coated tools.

Choosing the Right Coating
Table 1 compares the thermal stability and surface hardness of different tool coatings and substrates. A substrate and coating should be chosen for their ability to remain harder than the material they are to cut, even at elevated temperatures. The coated tool's performance should be weighed against the tool's cost and the coating's compatibility with the workpiece material. In some cases, it may make more economic sense to use coolant with a coated tool that possesses lower thermal stability.

Users also may have to experiment with an application's machining parameters to gain the most economic benefit from a switch to coated tools. Some coatings, such as AlTiN, protect their substrates much better at higher temperatures. A tool with such a coating used at conventional speeds and feeds may not last significantly longer than a TiN-coated tool, because the operation will not generate sufficient heat. A user of coated tools will save money only when he uses them at aggressively high speeds and feeds. This will raise the temperature at the cutting edge enough to extend the life of the tool significantly, and it will lower the cost of the operation by reducing cycle times.

With coatings such as AlTiN, which actually perform better when the heat is turned up, mold and die makers can continue to pursue higher levels of performance and economy. As they dial up the speed to aggressively cut hardened materials, they don't have to worry about the effects of the heat they are generating on the cutting edge.

About the Author
Wolfried Mielert is CEO of Millstar LLC, Izar Tool LLC, and Galaxy Technologies LLC, Bloomfield, CT.

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