MODERN TOOL STEELS and GRINDERS Tom Wirsing

MODERN TOOL STEELS AND GRINDERS Tom Wirsing

Grinding a CPM 10V particle-metal gouge on a CBN grinding wheel. Microscopic views of the grinds reveal that CBN wheels are capable of sharpening particle-metal tools to a very keen edge.

here have been many changes A11 (Crucible Industries’ version of which carbon, forming what are called car- in both the steels used in wood- is called CPM 10V) have become widely bides. Carbides contribute positively to T turning tools and in the grinding available and are gaining in popularity. abrasion resistance, heat resistance, and wheels we use to sharpen them. These What are the differences between toughness and are much harder than changes have created confusion among conventional high-speed steels such as the surrounding steel. But the higher woodturners. We wonder why we some- M2 and M42, and particle metals such the percentage of carbide-producing times don’t get the results we expect from as M4 and CPM 10V (A11)? Which are alloys, the larger the carbides become. expensive new tools. Following is a sum- the better choices? It helps to under- Excessively large carbides create two mary of how tool steels have changed stand how they are made (Photo 1). problems: they are too hard to be and what it means for woodturners. In manufacturing conventional ground (sharpened) with most conven- HSS, iron and carbon are melted in a tional grinding wheels and they rob the Evolving tool steels furnace, and alloys are added to the surrounding steel of their benefits. For many years, M2 has been the most molten steel. The molten steel is then What we’d like is steel with very small, widely used and successful conventional poured into molds and allowed to cool. very hard, abrasion-resistant carbides high-speed steel (HSS) used for wood- The alloys make the steel more resis- that are abundant and well-distributed. turning tools. M2 revolutionized wood- tant to abrasion (wear) and to damage The carbides of vanadium are particu- turning by providing tools that were by excessive heat, as well as tougher larly hard and contribute very positively much more resistant to both wear and and more resistant to breakage. Since to abrasion resistance. That’s why steels heat than previous tools. More recently, alloys in steel make it better, logically high in vanadium stay sharp longer. we have seen growth in the popularity we would like to add more. But there The carbides of molybdenum contribute of M42, another conventional HSS. We is a limit to the percentage of alloys to wear-resistance, and molybdenum have also seen the introduction of parti- we can add to conventional HSS. As contributes to toughness. Carbides of cle-metal tools (also called powder-metal the steel cools, atoms of some of the chromium and tungsten contribute tools). Tools with steels such as M4 and alloys chemically bond with atoms of positively, too. How can we add all the alloy we would like and at the same time avoid excessively large carbides? Enter When it’s time to buy new tools, take advantage particle-metal technology. In the manufacture of particle metals, of developments in modern tool steels. iron and carbon are melted in a furnace, and the alloys are added. But instead

38 American Woodturner June 2018 FEATURE

of pouring the molten steel into molds There are other particle metals, but for The CBN abrasive is typically bonded and allowing excessively large carbides purposes of brevity, we will limit our dis- to the surface of a steel or aluminum to form as the molten steel cools, the cussion to these two excellent examples. grinding wheel, so the abrasive sits molten steel is blown through a nozzle, proud of the wheel. When grinding creating microscopic droplets of steel. Grinding wheels with a CBN wheel, just the abrasive The droplets are on the order of 6 to 10 While particle metals perform excep- touches the tool, cutting faster and microns in diameter, and the carbides tionally well, the carbides within them cooler. The abrasive in a conventional, within the droplets are 2 to 3 microns have created new challenges. Most vitrified cast wheel, on the other hand, in size. One micron is one-millionth conventional grinding wheels are is mixed in with a bonding matrix of a meter. The steel powder is then unable to sharpen particle-metal tools (called the bond) that holds the wheel put into a metal canister and subjected to a keen edge. That’s why conven- together. Since both the bond and the to extremely high pressure, at forging tional wisdom says we cannot get par- abrasive rub on the tool as it is being temperatures, in a process called hot ticle-metal tools as sharp as HSS tools. ground, conventional wheels don’t cut isostatic pressing (HIP), transforming But that’s no longer true—enter cubic as quickly, and they get the tool hotter the powder into solid steel. The particles boron nitride (CBN) grinding wheels. than do CBN wheels (Photos 2, 3). bond together so thoroughly that the CBN abrasive is almost as hard as Since the abrasive in most con- resulting steel is very dense, with almost natural diamond and is highly resis- ventional wheels isn’t hard or sharp no porosity. We now have solid particle tant to abrasion; it maintains its sharp enough to cut through the carbides, it metal with very small, very hard, well- edges so it cuts quickly and efficiently. either shatters the carbides or knocks distributed carbides. Particle metals have revolutionized woodturning tools. M2 vs. CPM 10V steel A closer look From left to right: M2 HSS gouge and Figure 1 shows the chemical composi- scraper, and a CPM 10V particle-metal tion of M2 and M42 conventional gouge and scraper. All have been sharpened with CBN grinding wheels. HSSs, as defined by the American Iron While conventional wisdom says we and Steel Institute (AISI). M2 contains cannot get particle-metal tools as sharp as conventional HSS, with CBN wheels this is vanadium, molybdenum, tungsten, no longer true. and chromium, all of which contribute to the formation of carbides, making 1 M2 quite abrasion-resistant and tough. M42 also contains molybdenum and chromium, and smaller percentages Chemical composition of M2 and M42 of vanadium and tungsten. To com- pensate for its lower percentage of Conventional HSS Fe% C% Si% Cr% V% W% Mo% Co% S% Mn% vanadium, whose carbides are very M2 81.43 1 0.33 4.13 1.98 6.13 5 0 0 0 abrasion-resistant, M42 has 8% cobalt, M42 74.47 1.08 0.45 3.9 1.2 1.5 9.4 8.0 0 0 which contributes to hardness. What Figure 1. AISI definitions allow for tolerances, so exact percentages of alloys may vary. M42 lacks in vanadium carbides, it Symbols: Fe=Iron, C=Carbon, Si=Silicon, Cr=Chromium, V=Vanadium, W=Tungsten, compensates for with greater hardness. Mo=Molybdenum, Co=Cobalt, S=Sulfur, Mn=Manganese. M2 is very widely used in woodturning tools. M42 is less widely used, but its popularity is growing. Figure 2 shows the chemical compo- Chemical composition of M4 and CPM 10V sition of M4 and CPM 10V (AISI A11) Particle-metal Steels Fe% C% Si% Cr% V% W% Mo% Co% S% Mn% particle metals. M4 is an excellent M4 79.53 1.42 0 4.0 4.0 5.5 5.25 0 0 0.3 particle metal but has been adopted by A11 (CPM 10V) 79.78 2.45 0.9 5.25 9.75 0 1.3 0 0.07 0.5 a relatively small number of tool manu- Figure 2. The higher vanadium content of both M4 and 10V provides greater abrasion facturers. CPM 10V (AISI A11) has been resistance, so they stay sharp longer. 10V, with almost 10% vanadium, is exceptionally adopted by more tool manufacturers, abrasion-resistant. and its market acceptance is growing. woodturner.org 39 them out of the cutting edge, leaving Conventional vs. CBN grinding wheels a microscopically ragged cutting edge. It simply isn’t possible to get a particle- metal tool as sharp with most conventional wheels as it is with CBN. And CBN wheels work wonderfully on M2 and M42 as well as on particle-metal tools, getting both steels much sharper. CBN wheels have three other great advantages. First, they are much better balanced than conventional wheels, so 2 3 they run more smoothly. Even though It is true that CBN wheels (right) are generally more expensive than conventional wheels (left), but the price difference is narrowing. CBN wheels have a much longer service life conventional wheels can be balanced, than conventional wheels, so in the long run, the cost of ownership of CBN wheels is as soon as they are dressed, the balance lower. Plus, they offer distinct advantages over conventional wheels. changes. It is almost impossible to keep conventional wheels balanced, and

Comparing microscopic views M2 ground two ways (4) M2 tool ground on an 80-grit conventional wheel. Note the “teeth” along the cutting edge. The parallel lines are the scratch pattern on the tool’s bevel.

(5) M2 tool ground on a 600-grit CBN wheel, which produces smaller, sharper, and more uniform teeth.

4 5

CPM 10V ground two ways (6) CPM 10V tool ground on an 80-grit conventional wheel. Compare this with the view of the same tool ground on a CBN wheel. CBN gets the tool remarkably sharper.

(7) CPM 10V tool ground on a 600-grit CBN wheel. As with the M2 tool, the 600-grit wheel produces a sharper cutting edge, with far more teeth 6 7 along the cutting edge.

40 American Woodturner June 2018 FEATURE

Conclusion A Note About Heat Treatment of Steel If you have M2 tools, continue to use them proudly. M2 has been the most All woodturning tools must be heat-treated in order to fully develop the microstructure of the successful steel for woodturning tools steel to yield its desired characteristics of hardness, toughness, and resistance to wear. During heat treatment, tools are heated in a furnace to a precise temperature, then quenched to rapidly for many years. But when it’s time to lower the temperature of the steel to make it hard. Cryogenic treatment is sometimes employed buy new tools, take advantage of devel- in the quenching process. Tools are then tempered to reduce stresses in the hardened steel, en- opments in modern tool steels. And hancing toughness and resistance to shock. No matter how good the steel, if tools are not heat- with CBN grinding wheels, you can get treated using accurately controlled processes, tools will not perform effectively. A full description particle-metal tools every bit as sharp as of heat treatment processes is quite involved and too lengthy for inclusion in this article. conventional HSS tools, and they stay sharp longer. Invest in CBN grinding wheels as soon as your budget allows. the vibration makes it more difficult to teeth across the cutting edge, the tool And consider getting at least one very grind accurately. Smooth-running CBN sharpened on a very fine CBN wheel fine-grit CBN wheel to grind gouges wheels facilitate better grinds. Second, will not only be sharper, it will dull used for finishing cuts. A dull tool CBN wheels never need to be dressed, more slowly. That gives a fine-grit CBN will always produce tearout, always! so their diameter never changes. For wheel a big advantage over an 80-grit The sharper the tool, the better it will those who use jigs when grinding, the conventional wheel. perform. Fine-grit CBN wheels produce cutting angles on tools change every Now consider that with particle- superior grinds. Modern woodturners time a conventional wheel is dressed metal tools, each tooth has hundreds should take advantage of evolving tech- because the diameter of the wheel is of carbides within it that are finer, nologies in tool steels and grinders. reduced. Unless the jig is readjusted harder, and more evenly distributed. frequently, cutting angles are chang- Conventional HSS tools also have Tom Wirsing is a physicist who has ing. With conventional wheels, your many carbides within each tooth, had a lifelong interest in metallurgy jig’s grinds may not be nearly as repeat- but the carbides are larger and not and woodturning. He enjoys teaching able and consistent as you think. With as well distributed within the metal. woodturning and has taught at regional and national symposia, as well as at CBN wheels, the angles don’t change. Therefore, even though there is an numerous AAW chapter events across the Third, CBN wheels produce far less dust advantage in grinding both conven- U.S. and Canada. Tom is a past president of than conventional wheels. Breathing tional and particle-metal tools on fine the AAW. For more, visit thomaswirsing.com. the dust produced by dressing conven- grit CBN wheels, the particle-metal tional wheels and by grinding tools on tool is the winner. them can be a health hazard. What grits do I use in my work- JOURNAL ARCHIVE Is there really high-speed steel in your new turning tools? One other difference with CBN shop? I have two 8" (200mm), 1725- CONNECTION Putting wheels is that they are readily available rpm grinders with two CBN wheels Use AAW’s online Explore! Steel the or several thousand years, a to the Test piece of steel (or at least a steel- By Alan Lacer Fedged tool) came between spiraling shipping costs and steel the woodturner and the wood. prices, how can these tools be sold as Although the skill of the turner is a high-speed steel (sometimes stated as huge consideration in woodturning, recording of false, fictitious, or fraudu M2), some at prices below the price of the properties of the turning tool lent statements or entries on the cer in much finer grits than conventional on each: an 80- and 180-grit on one a handle on the English tools? influence such things as longevity of tificate may be punishable as a felony - the cutting edge (how long it holds I wanted to see if at least the steel under federal law.” - was the same. The traditional way of an edge), whether it tends to break or determining whether a tool is high- Stork’s process involved the cutting bend easily, and how the tool reacts up of each tool and subjecting it to carbon or HSS was with a spark test at to heat (whether in use on wood or a chemical analysis by optical emis the grinder. This test turned out not to while grinding). sions. This process analyzes the Since the 1980s, the transition from spectra from an arcing area of the - wheels, including 80, 180, 350, 600, grinder, and a 350- and 600-grit high-carbon steel woodturning tools sample. In addition, the Rockwell (0.5 to just over 2 percent carbon) to Hardness C Scale (HRC) was mea high-speed-steel tools (iron, carbon, sured at three points on the sample to and additional alloys) is now nearly arrive at an average hardness. - complete in the sale of new tools. Costs for each test ranged from $50 Few high-carbon steel tools are now to $150 per turning tool, a fee that available for sale in the USA (usually most turners would never consider tool to find more only from sources of used items and for a tool costing as little as $6 in some and 1000 grits. When a tool is ground on the other. I use the 80-grit for estate sales). High-speed steel (HSS) instances. The lab’s sampling of steel was developed for the metal trades effectively destroyed the tool. and has been around for more than According to Dr. Jeryl Wright 100 years, but is a relatively new steel Sections of lathe tools embedded in of Crucible Materials Corporation, for the woodturner. composite materials and prepared for there are no legal definitions of HSS. emission (spark) test. Although steel manufactured in However, there are American and Sheffield, England, dominated the tool international standards and defini resources—articles be foolproof, as some tools sparked as market for decades, there has been a tions. The common understanding of HSS, but lacked sufficient quantities of on an 80-grit conventional wheel, rough-shaping tools. It cuts fast and flood of HSS turning tools in recent HSS is steel that will resist softening- those materials that produce the ben years coming into the market from the at higher temperatures (usually can efits of the genuine article. Far East (primarily from China). withstand a dull red heat, around - 1,000˚ This influx of inexpensive im Tests at certified lab F) and excellent wear resistance. ported tools, often at a fraction of the The tests were conducted at Stork More specifically: price of the English turning tools,- “High-speed steels are high-alloy, tung Material Technology (storksmt.com), really caught my attention. But with sten, molybdenum, vanadium, and and videos—on a certified laboratory in Huntington Refining cobaltthe bearing steels designed to cutEdge - Beach, California. The warning at the the scratch pattern on the bevel pro- cool. I use the 180-grit primarily for other materials efficiently at high speeds, bottom of each Stork test stated: “The and must stand up to the extreme heat generated at the tool’s cutting edge. This Skews andheat can reach 1,000˚FGouges and more de 52 Improve Turned Surfaces with Simple Sharpening- Techniques tool steels and American Woodturner Summer 2008 duced by the abrasive in the wheel general sharpening and to sharpen Alan Lacer with Jeryl Wright I have heard many discussions and fielded a lot of interesting questions while traveling amongst woodturners. A common assertion is that some steels don’t sharpening. Two get as sharp as others or that high carbon steel (HCS) tools get much sharper creates a series of “teeth” along the my negative-rake scrapers. The 180- than high speed steel (HSS) tools. Another common view is that honing is a waste of time: the burr will “strop off” in the wood; honing takes so much time it’s inefficient; or woodturners don’t need a really sharp tool. Often, such views about steel and honing are spoken as fact, not just opinion. My experiences examples: Alan Lacer’s run counter to these viewpoints, so I knew something more was needed to cutting edge of the tool. These teeth grit wheel produces a burr that is just test these “facts” in an objective, scientific manner.

Methods of honing

To place the bevel of a Summer 2008 article, skew chisel correctly Polishing flutes on a diamond hone, are clearly visible under a microscope right for negative-rake scraping. I use hold the handle securely and use an up-and-down To polish or refine the motion, starting at flute of a gouge, use a the back of the bevel. wheel made of MDF, Move the hone to turned to a convex 1 simultaneously touch shape that fits the the area just below the profile of the flute. Coat cutting edge and the the wheel with an emery “Putting the Steel to back of the bevel. stick. Work only the last 1" or so of the flute. The (Photos 4–7). If the same tool is ground my 350-grit wheel to sharpen gouges The flute of a gouge wheel should spin away must also be honed. 3 from you. The curved radius of a slipstone or a tapered diamond cone is held flat in the flute and 2 (4A) Gouge flute with worked with a back- milling marks (100X). and-forth motion. (4B) Gouge flute the Test” (vol 23, no after polishing (100X). on a 600-grit CBN wheel, the teeth used for rough-shaping platters and 4A 4B 22 are sharper and more uniform, and bowls, and I use the 600-grit wheel for 2, page 52), and Alan American Woodturner Summer 2009 there are many more teeth across the sharpening gouges used for finishing Lacer and Jeryl Wright’s Summer cutting edge. The tips of the teeth do cuts, where a smooth surface with no 2009 article, “Refining the Edge: Skews and the cutting and therefore wear down tearout is essential. If I had only one Gouges” (vol 24, no 2, page 22). Log on at as the tool cuts wood and gradu- grinder, I would choose 180- and 600- woodturner.org to access Explore!. ally becomes dull. With many more grit CBN wheels. woodturner.org 41