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Coated-cobalt taps run long and hot: cobalt too steels, subtle geometry changes, and coatings add up to a tap that is greater than the sum of its parts.

Cobalt tool steels, subtle geometry changes, and coatings add up to a tap that is greater than the sum of the parts.

Forming internal threaded surfaces with taps requires a tool designed to handle tremendous chip loads and pressures. Because it is a confined operation, forming V-grooves in a hole, the tap must be tough and wear resistant, with good lubricity.

Tapping is a time-consuming and expensive operation, so there is a continual striving to improve tap life, and at the same time increase the speeds at which taps run. Improvements are often found by varying combinations of the basic tool steel used, the tool geometry, and in some cases the application of coatings.

The striving to improve, though, is complicated by the ever-increasing types of new materials that require threaded holes. Continual design refinements are necessary to efficiently tap these new materials.

A strong foundation:

Like any physical structure, a sound foundation is of utmost importance in taps. Although the type of tap required for a particular application depends on many variables, the material used to manufacture the tap also has a great impact on how effectively it works.

For most applications, tool steel is the material used because of its toughness and hardness. The type of tool steel, however, varies between tap manufacturers.

The most recent advancements in this area are tool steels with cobalt. The addition of a cobalt alloy results in a tool steel with high red hardness, or high heat hardness. This property allows taps to operate at higher speeds, imparting a degree of heat resistance to the tool. Other elements in tool steels that are desirable in tapping applications are carbide, vanadium, and tungsten. Carbide offers excellent lubricity, minimizing heat buildup that is a result of friction. Vanadium, while it may cause some brittleness, offers a high degree of hardness. And tungsten elements provide toughness.

How much is enough?

The answer to how much cobalt is enough is difficult to answer. Alan Shepherd, product manager at Emuge Corp, points out some of the economic considerations that could impact a tap manufacturer's decision. He explains that many times tap manufacturers also make other cutting tools, such as drills and end mills, and this may affect what tool steels they purchase.

"M42 is a premium tool steel with cobalt, excellent for drills and end mills. In the cobalt tool steels, it gets you the best value for your money, because, if you want anything else, anything different, you have to buy the whole heat," he states. "But, in our opinion, it (M42) does not necessarily make the best tap.

"The analogy that more is better doesn't always make sense," he continues. Because tapping puts such a large surface area of the tool in contact with the workpiece, Mr Shepherd believes too much cobalt can make the tap brittle and susceptible to chipping.

Emuge specifies its own tool steel, which is close to the American standard M35. Mr Shepherd designates the material a "designer steel for taps". It has a molybdenum base with about 5% cobalt, as well as some vanadium and tungsten. "We add the other elements to improve the integrity of the steel," Mr Shepherd claims.

Another leading supplier of taps, Vermont Tap & Die Co, uses an M3 type 2 tool steel that includes about 8 1/2% cobalt. "Our standard cobalt taps are all made out of the same grade of cobalt steel," reports Ted Henderer, chief engineer. "This grade has a relatively high level of vanadium and it also happens to be a powdered metal."

Mr Henderer stresses the importance of heat treatment in determining the brittleness of cobalt steels. Materials that contain cobalt have a high carbon content. The carbon and heat treatment are what cause brittleness."

Mr Henderer says that the company's cobalt tap was developed to achieve faster speeds and extend tool life, especially for machining heat-resistant alloys used in the aerospace industry. He points out that cobalt and carbon allow the tap to operate at high speeds; while the vanadium resists wear.

Greenfield Tap & Die, Greenfield, MA, will not reveal specifically what cobalt tool steel it uses for its premium taps. But John Emond, product specialist, mentioned that the industry standard for cobalt content is about 8%.

While these tap suppliers may differ on which is the best grade of tool steel for tapping applications, and what is the optimum amount of cobalt, they do agree on the importance of the substrate material when surface treatments are applied.

Coating-the next step:

"When we first got into coating, we had the expectation that you would be able to use a lesser-grade tool steel," says Ted Henderer about the company's initial coating applications in 1982. "But we found out immediately that the substrate material is extremely important."

Vermont Tap & Die uses Titanium Nitride (TiN) to coat general-purpose taps, heavy-duty taps, special taps for aerospace applications, and cobalt taps. The coating is applied using physical vapor deposition (PVD), a low-temperature process. The critical thing is not to soften the steels during coating. We are staying at a temperature where that doesn't happen," Mr Henderer explains. He reports the company offers other coatings, but typically at customer request.

Emuge offers TiN coatings but also offers other coatings as well. "Where TiN works well, it works extremely well," says Alan Shepherd. He points out, however, that with higher-alloy-content materials, he sees a drop-off in the effectiveness of TiN-coated taps.

Mr Shepherd also believes that it's not just the coating process that improves tap performance. Often, subtle geometry changes must be made in the tap design to realize the true benefits of coatings.

Design for coating:

"Adding a coating does improve tool life in some instances," Mr Shepherd explains. "Altering tool design specifically for adding a coating enhances it (tool life) even further."

As an example, Mr Shepherd talks about Emuge's focus on cast-iron applications. "We have three different styles of standard taps for cast iron.

"When we machine iron dry, we use a TiN iron tap," Mr Shepherd continues. "It looks like a tap for gray iron with the exception that we do change relief, back taper, and hook angles to make the tap perform better."

Ted Henderer agrees that coatings alone do not provide optimum performance, but that tap design must be addressed. "There are subtle design changes we made in our standard line of coated taps to enhance performance. Changes are primarily in relief geometry."

John Emond echoed the importance of design. "We did extensive, controlled testing of design features and coatings. When we were done, we felt that there were several changes along with the proper coating that gave the best performance for a given application and the money spent."

Mr Emond explains that when you get a skin hardness up around 80 Rc, as is typical with TiN, it may contribute to brittleness in the tool. "We don't think the real sharp angles found on an uncoated tap are the best way to go with TiN." The company reduces the angle on the cutting faces of its TiN-coated taps to make them penetrate more easily.

Tap synergy:

It is evident that tap performance is not directly attributable just to the tool steel, or the design, or the applied coating. All must be taken into account to achieve optimum performance and life.

A cobalt tool steel offers many benefits over a general-purpose steel, provided that it's used in the correct applications. But those benefits are not fully realized unless tap design is considered. "Just adding elements to a tool steel doesn't necessarily make a tool better," says Alan Shepherd. "You also have to cater the geometry along with it."

So what do you get with a redesigned, coated, cobalt tool steel tap? If used in the proper application, life expectancy can be anywhere from two or three times that of other taps or even up to 20 times.

Coated-cobalt taps are premium tools, and therefore more expensive. But in the right application, they deliver with lower cost per hole.
COPYRIGHT 1990 Nelson Publishing
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Copyright 1990 Gale, Cengage Learning. All rights reserved.

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Author:Arter, Richard K.
Publication:Tooling & Production
Date:Dec 1, 1990
Previous Article:Electronics in the toolroom - even this one.
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