Carbide cutters aren’t always what they seem

john_englishRouter bits, shaper cutters, molder/planer knives and saw blades are constantly being asked to cut, mill or shape new products in new ways. From plastics and resins to manufactured sheet cores, the tooling that we use is continually facing new challenges as material technology advances. Most of the tools we use in woodshops are carbide-tipped, carbide inserts or milled from solid carbide. Knowing something about the way that carbide is produced and graded might help a shop owner make more informed choices when buying bits and blades.

To begin with, tungsten carbide is generally described in terms of impact resistance, longevity and corrosion resistance.

If a table saw blade is traveling at 4,600 rpm, then each tooth will impact (crash into) the material being cut 4,600 times every minute. OK, the density of the material might slow it down a bit and the electric current can deviate a hair, but the bottom line is that each carbide tip smashes into resins and wood several thousand times during even a short cut on a table saw.

Take a look at a combination blade and it quickly becomes apparent that carbide tips are designed to abrade more than shear. That is, they force their way through the stock more than slicing it. The tips are hard and tough rather than razor sharp. And when the face of the cutter meets the wood, it usually does so at a slight angle, rather than a severe one. That is, the teeth lean slightly, but not very much. Because of that, the stress of impact can be spread over the entire front face of the tip, rather than concentrating along the top edge.

There is a direct tradeoff in carbide technology between hardness and sharpness. The harder the carbide, the better edge it will take. But that edge will not last long because hard carbide is very brittle. Upon impact, the fine edge will chip away very quickly. A tooth that is too soft, on the other hand, will not take a good edge to begin with. It can’t be properly sharpened. So, as with most things in life, the bits and blades we buy are a compromise. We give up a little sharpness so that the cutter can have relatively long intervals between sharpening. That concept has created a set of standards for tungsten carbide bits and blades where the inserts and tips don’t need to be formed to the shape of a knife edge, because they are not being asked to do the work of a knife. Rather than a knife or sword, the cutter works more like an ax where power replaces finesse.

What grades mean

Tom Walz is the president of Carbide Processors in Tacoma, Wash. The company has been supplying American industry with carbide solutions since 1981. Walz and his crew provide high-performance tooling for sawmills, major manufacturing plants and custom shops. Their first big customer was Weyerhaeuser Timber’s research and development team and the task back then was simple: make better saw blades for less money.

Asked about the different grades of carbide, Walz’s answer was surprising.

“There’s no comprehensive comparison of tungsten carbide between and among tungsten carbide suppliers,” he said. In other words, there is no industry standard. His website (www.carbideprocessors.com) explains: “A big part of the problem is the huge number of suppliers, grades and trade names. There are at least 5,000 different grades of tungsten carbide sold under more than 1,500 different trade names by more than 1,500 different companies. Tungsten carbide from two different manufacturers might have identical designation, but vary widely in almost every imaginable way including performance.”

So, when a woodworker orders a blade and looks on the packaging for a carbide grade, the C-3 or C-4 printed there is, well, subjective. Here’s the Carbide Processors website again, talking about the C grades that are commonly used in the United States:

“The original concept was to rate tungsten carbides according to the job that they had to do. If you had a particular job you would specify a ‘C’ grade of tungsten carbide and you could buy from anybody. This has lead to a situation where a C-7 tungsten carbide can be almost anything as long as it does C-7 style work. According to Machinery’s Handbook (an industry standards volume), it can range from 0 to 75 percent tungsten carbide, 8 to 80 percent titanium tungsten carbide, 0 to 10 percent cobalt and 0 to 15 percent nickel. The problem is that two C-7 tips from two manufacturers will almost certainly work very differently in two different applications.”

Most of us also assume that there is a straight-line progression from C-1 to C-14 and that each grade is a little harder than the previous one. That’s not exactly true. The grades are based on what the cutter can do rather than how hard it is (the grading curve is function rather than ingredients). Sharpeners often tell us that the higher grades have less cobalt in them and that makes each grade a little harder and more susceptible to shattering. (We’ve all seen the brittle corners knocked off a router bit when it hits an especially hard knot or drops on the floor.) But, again, it’s just not that simple.

“Following this line of thought,” Walz says on his website, “leads one to believe that the higher C number is harder and better for wear resistance. This is like classifying automobiles by size from a moped to an 18-wheel semi. This is clear and handy, but unfortunately it is not true.”

The secret recipe

The thing to understand about carbide bits and blades is that the tips and inserts are not solid tungsten carbide. The material is actually cooked up using several different ingredients. The core elements are tungsten (a metal) and carbon grains or powder, but these need to be held together in a matrix, a sort of molten soup that is usually either cobalt or nickel-based.

Atoms of tungsten arrange themselves into a pattern that looks like the weave of a chain-link fence, only in three dimensions. As heat and pressure are applied to the soup, the carbon atoms become lodged in the square holes of the fence. This means that the two materials are interwoven, rather than merely existing beside each other. This bonding is called carburizing and the result after cooling is tungsten carbide powder.

The powder is then blended with wax and cobalt in very specific quantities. The ratios will determine various properties that need to be incorporated in the final product such as longevity and impact resistance. The resulting product (the blended powder) is then pressed into a mold to shape it and heated (this process is called pre-sintering). The result is a putty-like substance reminiscent of soft chalk. This is then machined to its final shape and subjected to intense heat (about 2,600 degrees) and pressure (15 to 30 tons). The process actually shrinks the material up to 15 percent in any dimension, and up to 35 percent in volume, until it becomes exceedingly hard.

Sounds easy, right? Kind of thing one could do on a rainy Saturday in the garage.

According to Carbide Processors, the reason that cobalt is the most commonly used binder for tungsten is that “it has a high melting point (2,719 degrees), is strong at high temperatures, and forms a liquid phase with tungsten carbide grains at 1,275 degrees Celsius, which draws tungsten carbide in on itself by surface tension and helps eliminate voids and porosity.”

Because of the intricate manufacturing process, it’s easy to see why carbide inserts aren’t all alike. And because the grading system depends on the purpose of the product rather than the ingredients, it’s easy to see why all grades aren’t alike. That might sound daunting and confusing for somebody buying tooling, but there are some guidelines. For example, history is a good guide. If a shop has had success with a specific brand or product line, then sticking with that manufacturer is a comfortable decision. If the tungsten carbide is part of an over-the-counter product line that has been manufactured in the United States, one can call the manufacturer and discuss specific needs (for example, a shop might do a lot of work in solid-surface material or MDF). The company that sharpens a shop’s cutters can tell a lot about the quality of the tungsten carbide and its suitability to the type of work the woodshop does. 

For sources of carbide bits and blades, visit the Woodshop News online resource guide at www.woodshopnews.com.

This article originally appeared in the March 2014 issue.

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