MARCH 1997 WESTEC ‘97 Preview Society of Manufacturing Engineers 1 AreAre TheyThey forfor Experts differ on GrindingGrinding oror Milling?Milling? what to call a process that uses a CBN grinding wheel that removes metal at milling speeds and produces tiny milling chips Free-cutting, single-layer, plat- hen a process uses a CBN with increased wheel speed it is possible to plated grinding wheel yet increase removal rate (productivity).” ed CBN form wheels, like these produces a chip that looks from Abrasive Technology, have W like a miniature milling tight tolerances and cover a wide chip under a microscope we ask: is it Call it Superabrasive grinding or milling? It uses high wheel range of forms. rotation speeds and traverse rates and rel- Machining atively deep depths of cut to remove hard Wes Lee president of Edgetek Machine The basic Edgetek machine has a 35- aerospace alloy materials at speeds Corp (Meriden CT) which has sold over 55 hp (26-kW) spindle and runs at speeds to approximately equal to the speeds that CNC machine tools using plated single- 14,000 rpm. Its extremely rigid cast-poly- milling cuts soft steels. It is definitely not layer CBN form wheels formerly referred mer base and column absorbs vibration grinding in the usual way we think of to the process as HEDG grinding. The from heavy cutting forces more effectively grinding. Nor is it creep-feed grinding company’s first two machines were than cast iron. At the top spindle speed of since the machine table does not creep but shipped in 1991; today Lee insists that the 14,000 rpm. a 6" (152 mm) diameter wheel traverses in the 30-100 ipm (760-2540 mm process is not grinding at all. He calls it runs at 22,000 sfm (6706 m/min) and an 8" min) range. This process has been gradual- “superabrasive machining.” The word (203 mm) wheel at about 30,000 sfm (9144 ly catching on in American industry in the ‘grinding” does not appear in company lit- m/min). These speeds and rigidity let the last two years. erature. One brochure says: “The super- machine remove metal fast especially in University professors call this process abrasive machining process has been slotting or form-grinding operations. The high-efficiency deep grinding (HEDG) but compared to grinding but it is more accu- motor’s power and the wheel diameters that term has not caught on generally. rately described as a milling process. The and widths are typically smaller than on Milton C. Shaw professor emeritus of engi- preformed steel core wheels are electro- the big creep-feed grinders, so the forms neering at Arizona State University plated with CBN and mounted on the and grooves suited to it are not as wide as (Tempe AZ) described HEDG grinding in spindle. The wheels act more like a milling those tackled by the creep-feed grinders. his 1996 book Principles of Abrasive cutter than a grinding wheel. At high rpms. The price, however, is about a third to Processing this way: “When CBN is used at and relatively high feed rates the CBN a half the cost of the big creep-feed high wheel speed and at an increased wheels provide surface finishes equal to grinders. The Edgetek machine has three, removal rate in creep-feed grinding with a grinding.” four, and five-axis versions, and all come copious supply of fluid a new grinding with a large coolant tank and powerful concept called high-efficiency deep grind- FREDERICK MASON high-pressure coolant system. Oil is the ing (HEDG) emerges. When CBN is used SENIOR EDITOR, SPECIAL PROJECTS coolant used most frequently; nozzle REPRINTED FROM MARCH 1997-MANUFACTURING ENGINEERING 2 and wheel form closely so that, when plat- ed with a given grit size, the wheel will conform precisely to the form to be machined. About 40-50% of the CBN crys- tal is exposed above the bond, providing ample room for chip formation. The wheels act like a very sharp wheel and, so, cut aggressively and with less power than aluminum oxide wheels. Plated CBN form wheels have several advantages over bonded conventional or superabrasive wheels, according to Joe Eramo of Abrasive Technology Inc. (Lewis Center, OH). “Plated wheels have higher material removal rate capabilities, but are freer cutting, which reduces thermal met- allurgical damage to the work’s surface. Plated wheels grind virtually the same pro- file from the first cut to the last, grinding hundreds or thousands of workpieces. The times associated with set ups, wheel changing, and wheel dressing are signifi- cantly reduced or eliminated.” “We can turn the wheel’s steel core to precision complex forms.” According to Eramo, “Plated technology has improved to the point that profile tolerances of ± 0.0006" [0.015 mml may be held today, an improvement over the ± 0.002" [0.05 mm] of five years ago. The steel core provides for safety at high operating speeds [>20,000 sfm or 6100 m/min]. The wheels may be stripped of worn CBN and replated several times, resulting in a greatly reduced average wheel price.” design and placement need optimization. Corp.” He called the Edgetek “a true HEDG “Although plated wheels cost less than This process can eliminate manufac- machine [with] huge Q'w [cubic inches per vitrified and metal-bonded wheels, it is turing steps. It can machine hardened minute of metal removal per inch of width] not the cost that is driving the trend parts–those formerly milled in their soft values.” He reported high metal removal toward plated CBN but the productivity,” condition, then heat-treated, and finally rates at high wheel speed on both materi- reports Eramo. Smaller production runs ground–in their hardened condition, elim- als, without burning with a clean wheel and more setups, he says, means that inating milling. Other benefits from face. Working with two 8" (203 mm) diam- many companies no longer have the luxu- machining with plated CBN include high eter wheels and with water-soluble oil at a ry to amortize the cost of diamond rolls productivity, good surface integrity and 22% concentration, he ground Inconel 718 and the long nonproductive setup times size control, consistent finish, and usually at speeds from 10,480 fpm to 31,440 fpm associated with bonded wheels, over long a burr-free part. Less thermal stress, (3195 to 9583 m/ min) without leaving production runs. Plated technology fills enhances part quality. Surface quality on burn marks. “The huge Q'w values on the need for high metal removal rates with groove walls is close to that of convention- Inconel 718 [reaching about Q'w = 7] are short setup times and eliminates the need al grinding, about a 32 µin. AA finish. quite impressive” compared with metal for dressing.” Groove or slot bottoms look more like a removal rates achieved in the better- American Pfauter offers a similar view, finely machined finish, around 90 µin. AA known process of creep feed grinding with reporting that CBN is free cutting. “It does finish. The CBN grains do not wear, so aluminum-oxide wheels. not cause burning because the heat goes there is no burnishing effect. Radii on into the chip. The thermal conductivity of forms can be reliably held to about 0.020" Why Plated CBN Wheels? CBN, compared to aluminum-oxide, is 46 (0.51 mm), although wheels have been to 1. Using CBN, about 4% of the heat gen- plated with radii down to 0.005" (0.13 mm). From Edgetek’s general-purpose erated goes into the work. With aluminum Well-known grinding expert Dr. Superabrasive machining technology, to oxide wheels, about 63% of the heat gener- Richard P. Lindsay, Lindsay Consulting, American Pfauter’s CBN micromachining ated goes into the work.” (Marlborough, MA) tested the Edgetek technology for parallel-axis gears, (See machine for two days with plated-CBN sidebar) the process commonly relies on Vitrified CBN is an wheels in late 1995 and issued a report nondressable but replatable single-layer Alternative entitled “Grinding Chrome Vanadium plated CBN wheels. Wheel suppliers con- Steel and Inco 718 at Edgetek Machine trol CBN crystal sizes, the plating process, Another company, Campbell Grinder REPRINTED FROM MARCH 1997-MANUFACTURING ENGINEERING 3 (Muskegon Heights, Ml), entered the mar- ket for aerospace grinding with CBN a few Gear Cutting Machine Builder Calls it years ago, when they shipped three machines equipped with plated wheels to ingle-layer plated-CBN Pratt & Whitney. General Electric is also S machining with formed buying a number of these machines. Their wheels, developed in the 1980s, is basic CBN machine, which, incidentally, probably the most significant they call a HEDG grinder, grinds vanes, development in gear manufactur- blades, shrouds, and shafts, one at a time. ing in recent decades. It has The machine is similar in size and power replaced, in some applications to Edgetek’s. Bruce Hammond, mechani- cal design engineer at Campbell, says, within the gear cutting industry, “Our machines are built extremely stiff. pregrind gear hobbers and They move through the part like milling. In shapers that cut gears from the one straight test cut we took a 0.400" [10 solid. Users change wheels to suit mm] depth of cut, 4" [100 mm] wide at a the gear size and form. Typically, feed of 3 ipm [76 mm/min] The widest root but depending on stock form we have ground is a 3" wide form for allowances, two-double passes, a land-based generator. We have found each double pass consisting of a that in some cases our process can grind at down stroke and an up stroke as CBN-plated dual wheel set grinding gear a dramatically lower cost-per-part than on a milling machine, perform the with roughing and finishing wheel on Pfauter P400G vertical gear and form creep-feed grinding.” The company is rough cut and the finish cut on shipping about one machine per month at grinder.