The requirements for proper carbide tool selection have not changed over the years. The objective is still to achieve the best balance between tool hardness, toughness, abrasion resistance and corrosion resistance for both the tool manufacturing process and the intended application.
The application-related factors include the type of material that is being machined, the required cutting angles, the machining feeds and speeds, and the impact conditions. A product that contains defects such as loose knots and embedded foreign objects requires a tool material that has high-impact resistance. A product that contains few or no foreign objects or knots but is very abrasive requires a hard tool material with good abrasion resistance.
In some cases, excessive tool wear has been reported to be caused by carbide binder oxidation and/or corrosion. For these cases special carbide grades have been developed. There still does not exist a tool material that combines high hardness, toughness and abrasion resistance.
carbide particles. The products are then formed by pressing the mixture into a form under high pressure, and placed in a controlled atmosphere at temperatures ranging from 2,400 to
2,700 degrees F in a process called sintering. This process melts the binder material, which bonds with the carbide particles resulting in a carbide blank.
Though tungsten carbide was first developed in the late 1890s, it was not used as a tool material in metal cutting until the 1920s and was not commonly used as a tool material for the wood industry until the 1950s.
A disadvantage of traditional carbides is that they are brittle and cannot handle impacts as well as tool steels. Modern carbide formulations have improved the toughness without sacrificing hardness and can now operate in many of the areas that were normally reserved for high-speed steel. Submicron grades with relatively small amounts of binder can also be used successfully in highly abrasive applications such as machining MDF and particleboard.
There are dozens of grades of carbides with the grades being based primarily on the grain or particle size and the amount of binder used to hold the particles together. Carbides are normally separated into two groups or general
grades: those that can ma-
chine steel and those that
can be used for nonsteel ma-
L . terials. Nonsteel grades in-
clude those suitable for ma-
chining aluminum, copper,
and other nonferrous metals or nonmetals.
Grain size and binder content are the main factors that affect the properties of the cutting tool. As grain size is increased, hardness and abrasion resistance is decreased but toughness is increased. However, micro-grain grades are able to combine high hardness and toughness. High percentages of cobalt are typically in the range of 7 to 15 percent, though tools now exist with cobalt percentages as high as 30 percent. Cemented carbides with high cobalt percentages are typically used for roughing or interrupted cuts. Tooling with lower cobalt percentages ( 3 to 6 percent) are typically used for finishing cuts as well as machining highly abrasive materials such as MDF.
THE CUTTING EDGE
Carbide coatings such as titanium nitride have been used successfully in metal cutting. These coatings were developed around 1970 and include one or more layers of wear-resistant material bonded to the surface of the tooling. The coatings can include titanium carbide, titanium nitride, titanium aluminum nitride, aluminum oxide, and diamond vapor deposition for nonferrous materials.
Multilayer coatings were developed later to try to improve adhesion between the tool substrate and match the coefficient of thermal expansion. None of these types of coatings or other carbide “surface hardening” treatments have had much success in machining wood products. This is due in part to the sharper, nonra-diused tool edges required for wood cutting tools. This causes difficulty in providing uniform coating at the edge which results in premature coating failure. Another reason is that most wood machining operations are interrupted, which also promotes chipping of the coatings.
The term carbide or cemented carbides as applied to tooling refers to carbide particles (usually tungsten carbide) held together by a binder (usually cobalt).
The formation of carbide tool material begins with grinding the hard carbide particles to the desired size. The binder material is then mixed with the
Both the International Standards Organization and the American National Standards Institute have carbide grade classification schemes.
The ISO standard 513 scheme separates the machining grades into three separate color coded categories: straight tungsten carbide grades (color red, letter K) for cutting gray cast iron, nonferrous metals, and nonmetallics; highly alloyed grades (color blue, letter P) for machining steel; and less alloyed grades (color yellow, letter M), a multipurpose grade that can be used on steels and some cast irons. Each grade is also given a number that represents its relative position from maximum hardness to maximum toughness.
In the ANSI scheme the tungsten carbide grains are divided into two groups: the first group generally contains no titanium carbide (TiC) or tantalum carbide (TaC) and is used for machining nonsteel materials, while the steel cutting grades contain TiC and TaC.
A major advancement in carbide tooling has been in the reduction of grain size. Where carbide grain sizes in the range of 1 to 5 microns were common, submicron grains — down to 0.2 micron — are now available. These ultra-fine grains help make the tool very dense and resistant to highly abrasive materials. The fine grains can also be ground to a sharper edge, but these carbides can be difficult to braze due to reduced wetting and often require high-precision, well-cooled grinding procedures. Some manufacturers advertise “binderless” carbides, which typically have a submicron grain size and a low binder content in the range of 1 to 2 percent.
With so many carbide grades available it can be quite confusing to determine the correct grade for a particular job. Most large tooling manufacturers have guidelines, either online or in their catalogs, which provide grade suggestions based on the type of cutting required. These are just guidelines, since the actual grade required may also be affected by the tool manufacturing process (type of grinder available, whether brazing is required, etc.).
If little or no shock is expected on the tooling, such as from intermittent cutting, knots or foreign objects, then a medium to low binder content carbide should be considered for solid wood materials.
If shock resistance is needed, then a carbide grade with a relatively high binder content and micro grain particles should be considered for solid wood.
If high abrasion resistance is needed, such as for machining MDF and particleboard, then a low binder content and small to medium grain size is suggested.
For material with glue lines, a relatively high binder content with a relatively fine grain size should be considered.
A good starting point is the supplier’s recommendations followed by a series of trial and error machining tests.
Dr. Richard Lemaster is a research professor and director of the Wood Machining and Tooling Research Program at North Carolina State University. Tel: 919-515-1548. E-mail: richard_lemaster@ncsu.edu.
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