BOTTOM LINE

by Paul C Miller
Senior Editor

Superhard cutting tools: intuitively good

And some close contenders

Many improved cutting tools provide only marginal gains in productivity, and you need to keep careful records to see what, if any, benefits you actually get. Not so with superhard tools--diamond and CBN (cubic boron nitride). The longer tool life is dramatic, and production benefits are obvious.

The special tools are expensive, however, and the tool cost per workpiece produced may be about the same as for cheaper tools. Thus the benefits may not be so obvious to your purchasing agent, or accountant. But you as an engineer know that factors such as reduced toolchange time and longer cuts without tool adjustment can really boost productivity and product quality.

Intuitively, we know that hard tools are best, even though intuition is not always a scientific principle. For instance, the town of Brooklyn, OH, recently passed a law forbidding the use of cellular phones unless you keep booth hands on the wheel. Are laws forthcoming to prevent coffee drinking, eating, and newspaper reading while driving? These acts seem to be dangerous.

It doesn't stop there. Some reports say that conversation is unsafe, even with hands-free telephones. Next it'll be illegal to talk to people while driving, and thus kids will not allowed in cars at all. And what about radios or CD players?

But my intuition says that if we can't talk, drink, eat, or enjoy music while driving, we'll get bored and suffer severe fatigue. That's dangerous in the automobile as well as in the workplace.

Good luck charms

Many of the first diamonds were uncut and not worn. They served as talismans to ward off evil and bring good fortune. Golly, they can still do that if you're serious about productivity. Man-made diamonds and CBN cutting tools, in solid and crystalline form, can be well over 100 times longer lived in metalcutting operations. Intuition says that's good luck.

How hard is hard? Diamond hardness is about 8000 on the Vickers scale, and CBN ranges from 4000 to 5500. On the same scale, TiAlN (titanium aluminum nitride) ranges from 3000 to 3500, and TiCN (titanium carbonitride) is about 3000. TiN (titanium nitride) is about 2300, and plain cemented carbide is softer yet. Ceramic tools fall between carbide and CBN in hardness.

Diamond cuts about everything from glass to brass, but cannot handle the heat of ferrous materials. CBN, on the other hand, cuts cast iron, steel, hard alloys, and more. High-speed steel and carbide tools are often coated with harder substances using either the CVD (chemical vapor deposition) or PVD (physical vapor deposition) methods. CVD is a high-temperature process that ensures good bonding. PVD works at a lower temperature with less harm to certain substrates.

Diamond in the rough

Most industrial diamond is man-made. Sources include Norton Diamond Film, Northboro, MA. The firm offers many configurations including laser-cut and polished diamond with carbide backing for tool fabrication and wear parts. Complete diamond-tipped tools and tool-coating service are also available. The product can machine aluminum, silicon aluminum, carbon-reinforced plastics, carbon, graphite, fiberglass, layered composites, green ceramics, bronze alloys, rubber, and phenolics.

Norton says its CVD diamond is pure cubic polycrystalline diamond (unlike PCD, which consists of diamond grit sintered with metal). The CVD process avoids impurities that could compromise performance. In fact, the firm says its product approaches or exceeds the properties of Type IIa natural diamond, thus ensuring the benefits of high hardness, wear resistance, thermal conductivity, lubricity, and chemical stability. Circle 369

Recessing application

Boring and recessing tools from Bokum Tool Co, Madison Heights, MI, have been diamond-tipped to increase cutting-edge durability and improve surface finish. Although all applications cannot be run at sufficiently high speeds to warrant moving from carbide to diamond tooling, significant improvements in tool life and microfinish are possible if enough rigidity is provided. Generally, you should reduce speeds and feeds for typical OD turning operations.

In application, Bokum tools can spot-face to qualified depth from seal faces on aluminum transmission cases. Here, the tools provide superior finish and reduce tool-change interval to four to six months. Another part requires the generation of a tapered seat originating at a specific gage position. Once the machine and workpiece are qualified, the diamond tool holds the setup for many cuts. The diamond deteriorates much less rapidly than carbide, reducing downtime for recalibration. Circle 370

CBN inserts for turning

For hardness upgrades, WidiaValenite, Madison Heights, MI, offers solid CBN insert grades for difficult-to-machine materials such as Ni-HARD cast irons, chilled irons, and hardened tool steels. The inserts fit standard holders and include VC733 and VC724 grades created in specified sizes with tolerances of ±0.001". The PCBN inserts machine ferrous materials in the 40 to 70 RC range. The firm also provides PCD (diamond) tools to cut abrasive, low-tensile, nonferrous and nonmetallic workpieces at speeds up to 1500 sfm. Circle 371

Twice the turning

The FlipTip CBN hard-turning tool (see Cutting Tools cover story, pg 78) has a cutting edge that extends from top to bottom. It can be turned over to offer twice the performance of other disposable CBN tools. Diamond tools are also available. J & M Diamond Tools Inc, East Providence, RI. Circle 372

Superhard hones

Single-pass honing tools from Sidley Diamond Tool Co, Garden City, MI, can better the work of conventional hones and also perform light stock removal to replace ID grinding and reaming. The tools ensure maximum cylindricity, roundness, and straightness--eliminating taper problems and bell mouthing. The diamond hones can cut hardened steel, cast iron, and superalloys, because honing operations do not produce significant heat. However, the firm does provide PCBN hones as well as other diamond and CBN tools in many shapes and styles. Circle 373

Superhard alternative

Coated ceramics can be cost-effective substitutes for CBN tools in some situations. That's the report from Kennametal Inc, Latrobe, PA. For example, the firm's Kyon 4400 is a PVD TiN-coated ceramic engineered for turning hard steels and irons from 48 to 62 Rc. Its high wear resistance provides long tool life, and further gains accrue from multiple cutting edges. The tool is thus an effective alternative to CBN for hard turning. The new grade is available in both Kendex (clamp-on) and Kenloc (with-hole) styles. It also handles powder metals, nickel alloys, and nonferrous alloys in finishing and semifinish operations.
Circle 374

Tough threading carbide

For laydown-type threading, Carboloy Inc, Detroit, MI, has an improved version of its CP50 carbide grade. The product takes the form of a new titanium carbonitride PVD coating that improves cutting-edge resistance to cratering and flank wear. It combines relatively high cobalt content and micrograin tungsten carbide for threading steel, stainless, and cast iron. It also handles high-temperature alloys and austenitic stainless. CP50 has outperformed competitive grades in machining 316 stainless, and doubled the life of competitive grades on Nitralloy. Its toughness serves well for internal threading, where chips may be a problem. Circle 401

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