Tungsten carbide has been proven to be the most cost-effective and versatile tool knife material for cutting a wide range of materials found in the woodworking industry. Tungsten carbide was developed several decades ago for use in the metalworking industry. There are actually three carbides commonly used as cutting tool knife materials - tungsten carbide, titanium carbide, and boron carbide.
Tungsten carbide alloys used as cutting tool knife materials in the woodworking industry cover a wide range of compositions, types, and grades. The primary determining physical characteristics of the different alloys used are grain size and the percentages of the materials contained in each alloy type or grade. The grain size refers to the exact size of the individual particles in the mixture before it is pressed into the needed shape and sintered to produce a finished product.
Grain sizes are described as fine, medium, and coarse - meaning the individual particles will vary from less than one micron (one millionth of a meter) to as large as four microns. Grain size relates directly to mass (weight/density) and the strength of that particular alloy mixture. The sintering process does not vary greatly from any mixture to another. All are accomplished in a controlled atmosphere, without oxygen present, at a prescribed temperature.
Tungsten carbide is a mixture of tungsten and carbide bound together with a binder material of either cobalt or nickel. Nickel, though, is too costly for most practical uses.
The alloy content (or grade type) of tungsten carbide varies by the amount of cobalt present in the mixture - from a low of 1-1/2 percent to 2 percent for the very hardest grades to as much as 24 percent in the softest. All mixtures retain their ability to withstand high temperatures better than most other knife materials. Even temperatures as high as 1,400 degrees F won't affect the strength and hardness characteristics of these unusual metal compounds.
Tungsten carbide having less than 3 percent to 4 percent cobalt present in the mixture cannot be braze-attached to any other object. Brazing is a chemical bond that relies on cobalt to function at all. Cutting tools that rely on mechanically attached knives are usually made with very small amounts of cobalt, hence they are very hard.
There are at least 10 common compositions of tungsten carbide used in the woodworking industry. Specifying a particular alloy mixture is for the most part common sense. It should be based on a realistic and accurate description of the materials that will be cut or machined.
The most common misconception about knife materials is that harder alloys will provide a longer sharpening life and be more cost effective. Unfortunately, this is not accurate nor inclusive. Mixtures having less than 5 percent cobalt - the hardest of the brazed-on knives and all of insert knives - should be used to machine only in known homogenous materials that are free of foreign debris. Hardness does not relate to ability to withstand impact shock and subsequent fracture. Hardness is just one of several physical properties that we utilize to describe all materials. The other primary properties are elasticity, ductility and malleability, and toughness. Hardness also does not directly correlate to any of the four primary strength characteristics - tensile, compressive, fatigue, and yield. Very hard alloys are more costly to manufacture, both from the standpoint of the needed raw materials as well as the processes involved in their production.
Different alloy mixtures are appropriate for different applications, machining processes, and materials to be cut from the standpoints of cost and performance characteristics. The most significant determining factor is the material that will be machined, followed by the cutting tool type and the machine process involved.
General guidelines for selecting an alloy mixture of tungsten carbide should be based on the inherent advantages provided by the knife material.
• Tungsten carbide alloys with small amounts of cobalt binder - 3 percent to 5 percent - are very hard. They are capable of providing long life cutting even very abrasive materials such as high-pressure laminate coverings on man-made wood-based panels.
• Alloys with moderate cobalt content - 6 percent to 12 percent - provide a middle area where abrasive wear is not the primary consideration, but protecting the tool from impact fracture is a factor.
• For alloys with more than 12 percent cobalt, the concerns are extending tool wear while protecting the tool from impact damage and dealing with elevated tool tip temperatures.
• Tungsten carbide alloys having more than 20 percent cobalt present in the mixture have wear characteristics similar to high-speed steel, but can still withstand elevated operating temperatures without damage.
There are materials and machine processes that will require a look at all of the available tool knife materials. For instance, the patented cast alloys seem to be more appropriate for cutting solid wood materials with either a very low pH or high moisture content. High-speed steel is still the knife material of choice for cutting clear solid woods without glue lines in terms of overall operating costs and finish quality. A few of the newest laminating materials can be successfully machined only with diamond tooling.
Today, the woodworking industry continues to cut and shape an ever-widening collection of materials while requiring/demanding faster production speeds. We need to have a similar collection of cutting tools to accomplish our needed work. In the near future there will be more to choose from.
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