Feature Carbide Cutters - Lylejamieson

Fe at u r eCarbideCuttersAre Here to StayLyle JamiesonJohn English wrote about themanufacturers that producecarbide cutters in the December2010 issue of American Woodturner(vol 25, no 6). Many woodturnersuse their own shopmade versions.This article will move the discussionof carbide cutters to another level todiscuss how we use themfor woodturning.During my lifetime of workingwith wood, I've acquired informationfrom others—reading books, attending demonstrations, and one-on-oneconversations—as well as from personal experience. I had a learningcurve to go through with carbidecutters, and this time much of myresearch came from the Internet, aswell as from my own experience.There are conflicting viewpointsexpressed on the Internet, but thereis consensus enough to supportmy conclusions.HistoryA few years ago, Mike Hunter askedme to consider using carbide cutterson my boring-bar system. In spiteof my initial comment, “No thanks,carbide does not get sharp enough forturning,” Mike began my education.He described the new technology ofwoodturner.orgnanograin carbide. “Okay,” I said,“prove it.” And he did—my assumption was wrong.Carbide cutters have been in usefor decades: metal machining, military, sports, plastics, wood-production duplicators, and in flat woodworking for tablesaw-blade tips. Theearliest mention I found for carbidewas in the 1860s, but the woodturning tool market did not acceptcarbide until recently.The first commercially produceduse of carbide for turning that I knowof was when Dennis Stewart put acarbide tip on his slicer tool, sometime in the early 1990s. It was theprecursor of the coring systems usedtoday. While it had wear resistancebetter than HSS or carbon steel, itwould not get as sharp. For the useDennis intended, however, it wasperfect. Then why didn’t other usesof carbide take off with Dennis’sexample? The answer is: That carbidedid not get as sharp as high-speed steel,and a diamond hone was required tosharpen it.MetallurgyThere are many different carbides andgrades of carbide. Carbide is not justcarbide. The quality of the manufacturing varies greatly, and the particlesthat make up the carbides are different sizes. A microscope is needed tosee the difference. Let’s break downcarbide cutters into two categories.First is tungsten carbide. It is formulated from a gray powder and theresult is three times stiffer than steel.The second category is the newnanograin tungsten carbide, sometimesreferred to as micro-grain carbide. Thenanograin, as you might guess, ismade of much smaller particulatethan for tungsten carbide—the difference in grain size is that of BBs tobeach balls. Nanograin carbide grainsare cemented with another metal,usually cobalt (Figure 1). Generally,there is 6% to 12% binder in thecarbide. With optimum grade selection, submicron-grain-size particles oftungsten carbide are manufactured tohave a razor edge. What does this haveto do with us in the turning world?21

Figure 1. Magnification of carbide: Illustration of (gray) carbide cemented with (red)cobalt under high magnification.Start with a sharp edgeIn woodturning, we begin with asharp edge on our tools, and theinstant we start turning, the sharpness of the edge degrades. Let’s dosome math. Take a 10"-diameterbowl and calculate the circumference: 10 3.14 30 ". Thirty inchesof wood are passing the cutter everyrotation. Let’s say we are turning at1,000 rpm. In one minute we havejust cut 30,000" of wood—almostone-half mile or 500" per second.Will any tool retain its sharp edgevery long?After the initial sharpened edgeis gone, the structure of the basematerial the cutter is made from isleft. This remaining cutting edge iscalled the land. With carbide, thebase material is very wear resistantand the tool will cut reasonably wellfor a long time on the land beforethe edge deteriorates enough tobecome unusable.With nanograin carbide, the finerparticulate will allow the edge tobe even sharper to begin with thanwith the old carbides—manufacturers are able to produce a razor-sharpedge. The land edgeleft after the initialnanograin razorsharp factory edgehas been used tocut wood will stillbe sharp becausethe fine grainstructure is resistantto wear.How can you tell ifyour tools are made fromthe old-style carbide orthe new nanograin carbide?The nanograin carbide is manufactured under high heat and highpressure. The surface ends up witha mirror or glossy finish. The oldcarbide will have a dull, flat-grayappearance. To confuse the distinction, some carbide manufacturersapply coatings to enhance sharpness. These coatings are usuallyyellow or gold in color and theyare intended to mask the dull gray.The coating wears off quickly andthe tool is now cutting with theland made of the same basemetal structure.nanograin carbide will begin withand hold a sharper edge than theold carbide and stay sharper longerthan HSS. This is demonstratedto me in daily use of the Hunternanograin carbide tools on myboring-bar system.With HSS tools, the sharpenededge will degrade rapidly, but wecan go quickly to the grinder andconstantly renew the sharp edgeto optimal performance. Thesharpened edge of HSS tools willbe sharper than the old-stylecarbide ever gets. You also canuse a burr on HSS tools that isnot present on carbide. Othersteels like stainless and composites have the same trade-off. Theknife industry has been strugglingwith this issue for centuries. Theold carbon-steel knives wouldget sharper and last reasonablywell for culinary needs, but theyrust and their appearance was aproblem. Flat woodworkers havebeen arguing forever about thebest tool steel for router bits andcarving tools.Grain sizes and sharpnessWhy use carbide?I spoke with Tom Walz, President ofCarbide Processors Inc. in Tacoma,WA, to compare nanograin carbideto HSS. He said, “Consider thatthe sharpness level of HSS is in therange of 1 to 20, with 1 being thesharpest possible for HSS starting out. Nanograin carbide, onthe other hand, starts out at 2 or3 sharpness. Turning with bothfor the same amount of time, HSSis dull and has reached a 20 whilenanograin carbide is still sharp at a4 or 5 in the 1 to 20 range.” My conclusion is that the finer structure ofNanograin carbide tools cannotbe resharpened to their originalrazor-sharp factory edge; they aredesigned to be disposable. Theyare, however, economical becausethey last so long. I believe theywill wear up to 100 times longerthan HSS.Nanograin carbide cutters leavea much better surface on thewood than the old carbide. Why?Because it begins and remainssharper, and we can use a slicingcut that leaves a cleaner surface onthe wood.22American Woodturner December 2011

Fe at u r eHow woodturners usecarbide toolsThere are two types of cuts we canmake while turning wood: a scraping cut or a slicing cut. It does notmatter whether the tool is HSS or acarbide. The rule for scraping is thecutting edge must touch the woodat a 90 angle or less. Cutting at thecenterline with the scraper heldflat on the toolrest, the 90 angleis achieved by having the handleslightly up from horizontal (nose ofthe tool pointing slightly down). Asthe wood passes by the cutting edge,it scrapes some wood off. If we touchthe wood with any sharp edge atmore than a 90 angle, handle downwith a scraper, the tool will dig in,starting a catch.The rule for a slicing cut is the bevelbehind the cutting edge must be supported against the wood. If you areslicing on an angle without bevelsupport, the cutting edge will grab,dig in, and skate across the wood’ssurface until you get a catch. This isa critical concept to understand andwhen you understand it, tool catcheswill become a thing of the past (seeFigure 2).The two different carbides areused for different cuts for differentreasons. The old carbide is used inthe scraping mode. Carbide cuttersare especially useful for beginners.Learning to use a scraper is easy, andthey can scrape for hours and theirtool will still be sharp enough.Old-style carbide cutters are alsogreat for what they were originallymade for—roughing out. The turnerjust presents the tool to the wood in ascraping mode and just pushes it intothe spinning wood and scrapes away,keeping the handle slightly up, never 12violating the 90 -angle rule. The oldcarbide cutters are wear resistant,so going through dirty bark andmiles and miles of waste wood, theywill hold their edge for a long time,longer than HSS scrapers. The tradeoff is that the old carbide cutters arenot as sharp as HSS scrapers or bowlgouges, which is okay—we are usingthem as roughing tools.The new nanograin carbide canbe used for either a scraping cut ora slicing cut. Professional turnerMike Jackofsky has set up thenanograin cutters to workonly in the scraping mode witha tip angle dedicated to scrapingacross the bottom, inside a hollowform. Mike Hunter developed hisnew Hercules tool to scrape inthis manner, which works betterthan the old carbide because itis sharper.39Negative-rakescraping1 (Above) Carbide cutter clock positionsfor a nanograin cutter on 3 16" (5 mm)square shaft to be used with swivelinghead boring bar.Bevel-supported cutWill not cutFigure 2. Nanograin carbide cutter in threepositions tilted to cut wood (or not).woodturner.org(Right) The carbide cutter is showncutting under the shoulder of a vessel.The cutting motion can be in bothdirections, as indicated by the arrow.Notice the swivel has positioned thecutter to allow scraping at the 9:00position. This means the carbide cutteris cutting at the 9:00 position and isdirected toward the tailstock whencutting under the rim of the vessel. Notethe obstacle created by leaving the wastewood behind the cutter, which couldeasily become a problem later.223