Cemented carbide scrap is classified into two main categories: solid hard scrap and powdery soft scrap. The former includes used or defective cutting tips, drills and dies/molds, while the latter refers to powder sludge generated during grinding and polishing at manufacturers of cemented carbide materials and cemented carbide tools. The sorting and collection of such scrap are crucial processes in the completion of a recycling system. When Sumitomo Electric started to recycle scrap in the 1980s, it also started to collect scrap. Recycling collection boxes are made available to customers, which use cemented carbide tools. The boxes, which are used to sort cemented carbide tools and other materials, are collected periodically. The company also accepts scrap from competitors’ products. It has also started collection by using a home delivery service. All the cemented carbide scrap collected is transported to the Igetalloy Scrap Center for sorting. The entire collection process is undertaken by Sumitomo Electric Tool Net, Inc., which sells cemented carbide tools. Shinji Nakao, General Manager of the Environment and Resources Sales Dept. of Sumitomo Electric Tool Net, has been engaged in the collection from the outset.

Doiron added that it’s not just the cutting tools themselves that an operator needs to consider. Having a rigid setup with the cutting tool fully inserted into the toolholder to the maximum depth is an optimal scenario. If an operator can keep the tool engaged in the holder to the correct gauge line, tool wear can be reduced, which is already a significant challenge even without overextending the tool.

"Enhancing the efficiency of machining cobalt chrome can be accomplished by keeping a close eye on the tool life, for all the tools in the process, and frequently changing tools to ensure that you are cutting the material and not rubbing the material with dull tools," said Foschaar. "Most times cutting parameters are drastically reduced in this material to avoid poor tool life, but there is a fine line or small window to machine within for cobalt chrome."

Beyond substrate, geometry, and edge prep, adding a coating can be a big asset. Cetkovic explained that a coating can have a huge influence on chip evacuation, which is extremely important in this heat-resistant material.

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Manufacturers in the orthopedic and dental implant sectors are familiar with difficult-to-machine materials like cobalt chrome. These superalloys are known for their biocompatibility, making them a great choice for medical device manufacturing. Even the aerospace industry has taken note of the unique characteristics and high degree of hardness that cobalt chrome offers. As this material becomes more mainstream, machine shops have to adapt to meet the challenges when milling this hard-to-cut material, particularly because the cost of cobalt chrome is quite high.

"This in turn causes the next challenge, which is work hardening of the material," said Foschaar. "These challenges can be minimized by selecting the proper cutting tools for the application."

Emuge’s TiNox-Cut product range consists of submicron grain carbide end mills for difficult-to-cut materials, particularly aerospace materials. Photo courtesy of Emuge.

The Japanese government has set the procurement of rare metals as a national project due to changes in the environment surrounding rare metals. From the latter half of the 20th century to date, the framework of the global economy has changed significantly due to the rise and development of emerging countries, including China. Global production and consumption have accelerated, resulting in the expansion of consumption of rare metals. Against this backdrop, there has been a rising tide of resource nationalism (the belief that the resources available in a country should be managed and developed by that country) among producing countries. Tungsten is no exception. Previously, Japan imported much of its tungsten from China, which accounted for more than 80% of the global production volume. Today, tungsten is a mineral subject to export control in China. The production volume in other countries, including Vietnam and Russia, is limited. It is evident that tungsten will be depleted in the near future. Under these circumstances, the Sumitomo Electric Group has embarked on the recycling of tungsten. This is a challenging project to free the company from dependence on imports and achieve a stable supply by recycling tungsten. The Administrative Dept. of the Advanced Materials Business Unit is responsible for procuring metallic materials for cutting tools, including tungsten. General Manager Yoshimitsu Okamori is one of the members who have led the efforts to recycle tungsten.

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There are several challenges that operators should be aware of when milling cobalt chrome. The material tends to be extremely abrasive, which causes the cutting tools to dull prematurely and reduces the life of the tool.

Using the appropriate cutting tools is essential to optimizing cutting conditions for cobalt chrome. According to Cetkovic, one of the biggest cutting tool considerations should be strength of the tool design. He recommends starting with a reinforced shank diameter. Choosing a milling tool geometry with a positive break angle will reduce cutting forces and deflection, which tend to be problematic with this material.

Big Kaiser’s end mills are designed to increase productivity and meet some of the most challenging application requirements. These micro cutting tools are produced using solid carbide with a reinforced shank to maximize stability. Photo courtesy of Big Kaiser.

"I think everyone who cuts this material, or at least has been in the industry, understands how difficult it is," said Doiron.

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He explained that for operators or shops that haven’t come across it or are now just starting to get into it, they may find it very disappointing because of the SFM rating and tool life expectancy. However, once a shop has worked out the correct machining parameters and knows how to approach the material, it can successfully machine cobalt chrome and get the best tool performance.

Luminoso has a bachelor of arts from Carleton University, a bachelor of education from Ottawa University, and a graduate certificate in book, magazine, and digital publishing from Centennial College.

Lindsay Luminoso, sr. editor/digital editor, contributes to both Canadian Metalworking and Canadian Fabricating & Welding. She worked as an associate editor/web editor, at Canadian Metalworking from 2014-2016 and was most recently an associate editor at Design Engineering.

"These materials also can be polished at an extremely high level, which eliminates surface areas where bacteria can form in medical devices," said Dan Doiron, milling product manager, Emuge Corp., West Boylston, Mass.

"Cobalt chrome is considered a DTC [difficult-to-cut] material due to its high hardness, corrosion resistance, high melting points, and incredible strength at high temperatures," said Chris Foschaar, application and sales engineer, Horn USA Inc., Franklin, Tenn. Foschaar added that there are many variations of the material, with an average hardness of 40-50 HRC. Depending on the type of cobalt chrome, the material also can carry hard spots throughout upwards of 58-60 HRC.

"Tools designed for nickel-based alloys or tools for hardened materials are also suitable for this material," said Foschaar. "Nose end mills or square end mills with corner radii are usually ideal for this material. It’s also important that the tools are sharp. Dead sharp tools should be avoided due to the rapid decline of the cutting edge, which will cause rubbing, and a work-hardening situation will occur."

Japan depends on imports for most of its energy and mineral resources. At present, there are no mines in Japan that produce rare metals, including tungsten. For this reason, ensuring the stable supply of energy and mineral resources is one of most important national issues. Against this backdrop, the Ministry of Economy, Trade and Industry formulated the New National Energy Strategy in 2006. It mentioned the need to step up efforts to promote recycling of metallic mineral resources. The Guidelines for Securing Resources, which were established in 2008, set out a policy to support the acquisition of important resources, including rare metals. In the following year, the Strategy to Secure Rare Metals was published. In 2012, five minerals— neodymium, dysprosium, tantalum, cobalt and tungsten—were selected as priority minerals to be recycled. A national project is under way to secure these rare metals. Why are rare metals so important? Although they are consumed in small quantities, they are indispensable materials in the manufacture of LCD TVs, mobile phones and vehicles. In fact, they are essential in maintaining and strengthening the international competitiveness of Japan’s manufacturing industry. Notably, global demand for rare metals is expected to increase in the fields of nextgeneration vehicles, motors and storage batteries, which are expected to come into widespread use in the building of a low-carbon society. In terms of the use of tungsten, cemented carbide accounts for about 61% of the global market on average. In Japan, the percentage of cemented carbide is particularly high (about 76%).* Derived by mixing WC with cobalt (Co), cemented carbide achieves high hardness, high wear resistance, and high heat resistance. Cemented carbide tools, such as drills, and cemented carbide inserts for cutting tools have achieved high-speed machining and revolutionized the operation of manufacturing and machining sites. In Japan, the Sumitomo Electric Group has become one of the pioneering companies of this technology. Cemented carbide tools are crucial for many manufacturers. It is safe to say that these tools are the lifeline of manufacturing and machining. Tungsten is an essential material in the production of these tools.

Proper cooling at the cutting edge, reducing tool projections as much as possible, and using a very strong, rigid fixture setup on the machine will help deal with the challenges of milling cobalt chrome. With the high price of the material, ensuring process reliability through proper tool selection and cutting parameters is essential.

Cetkovic added that operators need to be aware of the high work-hardening rate of this material. He suggests that a slow feed rate, although better for cutting tool life, could also result in this work hardening, so it’s important to discuss with cutting tool manufacturers the range of cutting speeds (SFM) that the tool can handle in the material. To extend tool life, he also recommends a low depth of cut.

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This is generally because of the low thermal conductivity of the material. Cetkovic explained that when cutting, the tool will absorb a lot of the heat. Adding coolant as the tool exits the cut can cause it too cool down quickly, which can cause the tool to fracture because of thermal shock. Machining cobalt chrome dry can help operators avoid situations like this.

"We are able to take a lot of our more common tool geometries and apply a special coating or put a radius on the edge to help them perform in some of these more difficult-to-machine materials," he added.

“Recycling of cemented carbide scrap is important partly because it is highly effective in terms of environmental conservation. The percentage of tungsten contained in ore is less than 1%. Meanwhile, the percentage of tungsten contained in cemented carbide tools is about 85%. From the viewpoint of refining efficiency, it is far more environmentally friendly to extract tungsten from the scrap of cemented carbide tools. It should also be noted that global tungsten reserves are estimated at about 3.2 million tons. Thus, the resource is likely to be depleted in the near future. Global competition for tungsten will intensify. We must further expand the scope of recycling to protect Japan’s manufacturing industry. While such efforts partly reflect our business strategy to encourage customers to use Sumitomo Electric products, we will publicize the importance of recycling through activities to foster motivation and raise awareness with the SDGs in mind,” said Nakao.

Another significant consideration is that cobalt chrome is very heat-resistant with low thermal conductivity. During cutting of standard steels, heat generally is evacuated through the chip. However, with cobalt chrome, heat does not generate into the chips.

Because of this, the heat concentrates at the cutting edge of the tool," said Doiron. "This is where the geometry of the tool plays a big role in its ability to absorb that heat and chip away the material. Also, when working with cobalt chrome, establishing proper cutting parameters is vital to prevent tool failure."

Foschaar added that solid-carbide tools are preferred for machining cobalt chrome materials. Carbide cutting tools generally have thermal shock-resistant properties while at the same time have a high degree of bending strength and fracture toughness, making them suited for the characteristics of this material.

“One of the tasks of our department is to strengthen the material flow from ore and scrap to the manufacture of cemented carbide tools, which are the final products. Our solution was to establish a recycling technology and a recycling center in the U.S. [discussed below]. The Sumitomo Electric Group started recycling tungsten in the 1980s. At that time, a recycling process called the “zinc process” was used to recycle scrap into raw material powder while retaining its constituents. In 2011, Sumitomo Electric developed a new chemical process to recycle scrap into tungsten trioxide (WO3), making great strides in recycling. Today, the recycled volume is equivalent to the weight of the cemented carbide tools sold by the Sumitomo Electric Group in Japan. However, this does not necessarily mean that Sumitomo Electric products accounted for 100% of the collected scrap. Part of the scrap was exported as valuables or melted with steel scrap. Of the total scrap generated in Japan, an estimated 30% is returned to our group. As the first step, we hope to increase the percentage to about 50%. To this end, we must expand both the collection capacity and the processing capacity. In terms of achieving stable supply, we are studying the possibility of investing to acquire interests in mines in addition to promoting recycling,” said Okamori.

Beyond these characteristics, cobalt chrome is heat-treatable, not magnetic, offers good hot and cold formability, and has high coefficient of thermal expansion and very high resistance to acid, making these alloys a great fit for dental equipment, medical tech, the energy sector, chemical industries, and aerospace.

Adding to the fact that this material is difficult to mill, the hard spots also can lead to inconsistent machining results. According to Cory Cetkovic, product manager, Big Kaiser, Hoffman Estates, Ill., it is important to take note of these hard spots in the material and determine the most appropriate tool to handle a range of hardnesses within the material.

"We also tend to see some variability when it comes to coolant delivery," said Cetkovic. "We’ve also seen a large number of cases where customers are dry machining these materials."

Horn’s Torus end mill is carbide grade TSCC for machining of cobalt chrome alloys. It comes with a 2- to 5-fluted design with a 30-degree helix angle. Photo courtesy of Horn.

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