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Automation in manufacturing inserts. Image Credit: Iscar

Automation in manufacturing inserts. Image Credit: Iscar

The Right Grade Creates The Right Tool: Selecting Tool Materials

Cutting tools have different design configurations. Some of them are assembled comprising a body with replaceable cutting elements (indexable inserts, for example), another is wholly produced from solid material. Functionally, a cutting tool may be divided into a cutting part that is involved in cutting, and a mounting part, which is necessary for mounting the tool in a holder or a machine spindle.

A tool material is the material from which the cutting part of a tool is produced. This is the material that directly contacts a workpiece during cutting.

The tool material must be harder than the workpiece material so that it can cut the workpiece. In machining, the tool material needs to withstand mechanical and thermal loads, and oxidation. These factors cause gradual loss of the tool material or a change in its original shape: this is known as “tool wear”. When wear reaches a certain limit, the cutting part cannot work, and the tool fails. The machining time interval, within which the tool cuts normally from its original (new) state to a failure, is known as “tool life”. The tool must meet appropriate requirements of hardness, strength, and thermal and oxidation resistance to withstand wear and ensure an acceptable tool life.

Cutting tool manufacturers produce a variety of tools from different tool materials according to the desired tool application. “Which material is more suitable for my specific needs? Is the material of one producer better than another?” Customers often ask themselves these questions when selecting the tool or choosing their cutting tool supplier.

Industry utilizes the following tool material groups to produce cutting tools: high speed steel (HSS), cemented carbide (hard metal, HM), ceramics, cermet, and ultra-hard materials such as cubic boron nitride (CBN) and polycrystalline diamond (PCD). Each group features various types within the group; these are referred to as “tool material grades” or simply “grades”.

Classification

International standard ISO 513 classifies tool material based on their reasonable applicability with respect to the materials. ISCAR adopted this standard and uses the same approach in tool development. In accordance with the standard, the tool material grades are characterized by a class of engineering materials, to which a tool produced from the grade can be applied successfully. Each class has a specific identification letter and color:

A classification number follows the letter to show the hardness-toughness ratio of the grade according to a conventional scale. Higher numbers indicate an increase in grade toughness, while lower numbers indicate an increase in grade hardness.  Higher numbers represent increasing feed and lower numbers represent increasing speed.

Cemented Carbide

Cemented carbides are very hard materials and therefore they can cut most engineering materials, which are softer. Some carbide grades demonstrate better performance than others when applied to machining a specific class of materials.

A carbide grade is the result of combining cemented carbide, coating and post-coating treatment. Only one of these components – the carbide – is a essential component of the grade.  The integration of coating and post-coating elements depends on the main field of the grade application. Produced by powder metallurgy technology, cemented carbide is itself a composite material and comprises hard carbide particles “cemented” by a binding metal, which is principally cobalt. The term “cemented carbide” can refer both to the substrate of a coated grade and to an uncoated grade.

Main types of cemented carbide include tungsten (wolfram) carbide (WC), tungsten carbide and titanium carbide (TiC), and tungsten carbide, titanium carbide and tantalum carbide (TaC). Mixed binders, containing not only cobalt but additional elements such as ruthenium, may significantly improve grade performance.

Most cemented carbides used for producing cutting tools integrate wear-resistant coating and are known as “coated cemented carbides”. Applying a thin-layer coating to a cemented carbide considerably improves the carbide’s working characteristics. The coating may be one- or multi-layer depending on the number of coating materials. Materials used for coating cemented carbides include tungsten carbide and titanium carbide (TiC), alumina (aluminum oxide Al2O3), titanium carbo-nitride (TiCN), and titanium aluminum nitride (TiAlN). There are also various post-coating treatment processes that are applied to already coated cemented carbide, for example, to the rake surface of an indexable insert.

Insert structure_Substrate and multilayer coating. Image credit: Iscar

Coating Processes

Two methods may be utilized for coating: Chemical Vapor Deposition (CVD) and Physical Vapor Deposition (PVD). CVD coating is based on chemical reactions in a vaporized medium and PVD uses material sputtering. Technology development allows both methods – CVD and PVD – to be combined for coating cemented carbides as a means of controlling coating properties. For example, ISCAR’s carbide grade DT7150 features a tough substrate and a dual MT CVD (Medium Temperature CVD) and TiAlN PVD coating. The grade was originally developed to improve machining special-purpose hard cast iron.

Nano Layered PVD Coating

PVD coatings were introduced during the late 1980’s. Applying advanced nanotechnology, PVD coatings performed a gigantic step in overcoming complex problems that were impeding progress in the field. Developments in science and technology brought a new class of wear-resistant nano layered coatings. These coatings are a combination of layers having a thickness of up to 50 nm (nanometers) and demonstrate significant increases in the strength of the coating compared to conventional methods.

Applying SUMO TEC technology

SUMO TEC is a specific post-coating treatment process developed by ISCAR to improve both CVD and PVD coatings: CVD and PVD. In CVD coatings, the difference in thermal expansion coefficients between the substrate and the coating layers causes internal tensile stresses and micro cracks. PVD coatings produce surface droplets. These factors negatively affect the coating and shorten insert tool life. The SUMO TEC treatment has the effect of making coated surfaces even and uniform by reducing and even removing the defects –  minimizing inner stresses and droplets in the coating.

Classifying Grades

ISCAR, which produces a variety of cutting tools with cutting parts mainly fabricated from coated and uncoated cemented carbide, developed a tool material grade characterization system with designated letters indicating the material group and numbers representing as identity codes. The numbers also provide quick information on the grade type –  for example a two-digit number following “IC” in the designation of a cemented carbide grade means that it is an uncoated grade, while a three-digit number relates to a coated grade.

Occasionally, misconceptions occur concerning grade designation for a coating type.  IC300, for example, relates to the specific grade in its entirety – including both the grade substrate and coating. Wording such as “grade IC328 but with coating IC300” is inaccurate; the correct definition would be “substrate as in grade IC328 and coating as in grade IC300”.

Image Credit: Iscar

“The best grade is a grade you have now”

When a new insert (solid carbide tool or replaceable cutting head) is developed, it is necessary to decide from which grade it will be produced. The answer to this question depends on the insert’s designated application and this represents a starting point for tool designers in their work. Grade properties and their relative hardness-toughness ratio will be the main determinants to take into consideration. In some cases, stock availability and delivery terms may be the significant factors in the decision-making process.

As people engaged in production like to say, the best carbide grade is the grade that you have in your stock. This statement can be applied to the cutting tool as a whole; it is probably true if it relates to a production situation that requires an immediate decision. However, productive process planning –  or effective tool stock management –  requires a more in-depth applicative analysis of the pros and cons of the proposed carbide grades.

Selecting a grade is strongly connected with the cutting geometry of a tool and other factors. The cutting tool manufacturer should provide the customer appropriate information about grade properties to assist in their correct selection. While computerized grade selection systems are impressive and effective, often simple graphical figures, charts and tables can act as a good information “compass” to visualize a grade position in the field of application in accordance with standard ISO 513, and characterizing the grade properties compared with other grades.

ISCAR uses charts and tables to specify the cutting area of milling tools (Fig. 1, 2) and proposes suitable grades for replaceable inserts in indexable milling cutters, solid (mainly solid carbide) endmills, and replaceable solid milling heads with Multi-Master adaptation.

Fig 1 Grades for Indexable Milling. Image Credit: Iscar

ISCAR characterizes the grades as main and complementary. The main grades are more popular in machining specific engineering materials, but complementary grades can be effective as well in certain cases. When a main grade is not available for producing a certain product, a complementary grade provides an acceptable alternative.

Fig 2: Grades for Indexable Milling. Image Credit: Iscar

The tables provide summary data for grade applications and the charts show “an applicative map of grades”, in coordinates of classification numbers from standard ISO 513. The figures often prioritize the main grades by numbers in brackets below the designation of a grade. Prioritizing is general in character and is intended to assist in selecting the correct grade when there is insufficient information about the application.

The basic principle for selecting the grades is that when abrasive wear is dominant, a hard grade should be used, whereas a tough grade is needed for substantial mechanical loading during cutting. For example, for finish milling with typically small machining allowance (machining stock), high cutting speed and low feed, hard grades will be more efficient. However, tough grades will be required for heavy-duty roughing that removes significant volume of material and features considerable cutting load.

Using summarized tables and charts to represent performance attributes of various parameters is a well-known grade selection tool that is often preferred by tool manufacturers, even with the myriad of digital options available today. The simplicity and clear visuals offered by the traditional tables and charts provide important data in an easily interpretable and effective way, allowing optimal selection of the right tool materials for each application.

 

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