Tech Talk
What is "Tungsten Carbide (WC)"?
Carbide is a manufactured alloy that was developed over 70 years ago and has been the material of choice for machining and sawing wood for over 30 years. There are as many different types and grades of carbide as there are applications for it. Carbide is used to cut steel, rock, recycle pavement, and even as a wear surface. Some of these applications require a very high level of wearability, while others require a great deal of tensile strength, or shock resistance.
Tungsten Carbide (WC) in its various forms is the type most suitable for woodworking. In its raw state it is a powder-like material that is made into useful shapes by compressing it at high temperatures, with a binding agent adhesive of Cobalt (Co), to form different shapes and sizes of bar and preformed pieces. As you increase the Tungsten Carbide content relative to the Cobalt binder, the wear resistance increases but the resistance to shock decreases. In other words, it will last longer, but is more prone to crack or break if impacted by inclusions in the material. For this reason, the carbide recommended for cutting solid wood has more binder in it than the carbide that should be used for cutting MDF on automated equipment, where the cutting resistance is constant, without the shock load of varying grain structures and knots. To establish a sharp cutting edge these cast pieces of carbide are shaped and ground with fine grit diamond grinding stones. The structure of carbide will not grind to an edge quite as sharp as steel but it will, however, last a great deal longer.In the last few years there have been many developments in Tungsten Carbide structure. The newer grades have a grain size that has been reduced to sub-micron levels. Wear in carbide is largely due to the corrosion of the Cobalt binder. As it wears away the fine grains of carbide break away. The smaller grain size of the newer grades decreases the exposure of the binder at the cutting edge, thereby increasing the life of the edge. These new grades also grind to edge. It's difficult to comprehend, but the temperatures at the cutting edge of a carbide blade cutting MDF have been estimated to reach over 500°C. This is higher than the temperature created when cutting metal, primarily because a coolant is used in metal cutting, and the metal itself conducts and dissipates the heat generated - whereas MDF is a very poor conductor. These extremely high temperatures contribute greatly to the corrosion or breakdown of the binding agent. Once the binding agent is worn away, the fine carbide grains also break away, blunting the cutting edge. As the tool gets dull the resistance and pressure at the edge increases, and so does the surface temperature. This causes the cutting edge to break down exponentially. The Cobalt lets go faster and also corrodes faster. We often find that tools that have been left on too long, even as little as 20% too long, have three times the wear of a tool taken off at the correct time. In woodworking there are two designs of carbide tooling - brazed tools and insert tools. The ability to braze carbide to steel cutter bodies is reduced as the amount of binder is reduced in the carbide bar or tooth. Therefore, the newer carbide grades with their extremely low levels of cobalt are only found in the hardest grade of micro-grain insert knives - and there is more than one grade of carbide insert.The hardest (lowest binder content) knives are not always suitable, as they may break too easily in some applications. Constant density material machined on CNC equipment is about the only place the hardest grades will perform efficiently. It is also important that the equipment and the knife holder be well maintained. A worn router collet, for example, could force you to use a grade of insert that is less productive simply because the worn collet is causing too much knife breakage.The carbide industry is in a constant state of development and recently has been introducing new binding agents to replace cobalt. They are harder and more resistant to corrosion, having the effect of increasing the life of a sharp cutting edge. The carbide will have more binder content without sacrificing tool life, and have the tensile strength of the softer grades. I am told that one of the leading European tool manufacturers will be introducing a new grade of carbide at Ligna which has a very hard binding agent, which they expect will expand the uses of carbide and increase its durability. The changes are coming as fast as they are in almost every sector of industry, so don't hang on too long to old opinions about methods of machining wood. More than likely, what didn't perform in the past has been improved.On the left, standard grain carbide. Note the larger gaps that are filled with binder (cobalt). Micro-Grain carbide, on the right, has more carbide and less binder at the cutting surface. by: Doug Reid, President, B.C. Saw
