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User-friendly CNB grinding.

User-friendly CBN grinding

If you're still using aluminum oxide (Al2O3) wheels for production grinding of hardened steels. hard cast irons, and difficult-to-grind superalloys, you could be losing the battle to keep your company competitive. Engineers throughout the world are improving productivity and quality, and reducing grinding cost per piece, by replacing Al2O3 wheels with CBN (cubic boron nitride) wheels made with porous vitrified bonds.

Because of the high hardness and wear resistance of CBN--a superabrasive--porous vitrified-bond CBN wheels wear slowly, even when grinding at high material-removal rates. They remain sharp and free-cutting when grinding under conditions that rapidly dull Al2O3 wheels. Typically, grinding cylce times is reduced 10 to 30 percent. Wheel life is high; G ratios (stock removed/wheel wear) of over 53,000 have been achieved.

The CBN material also offers the benefit of fewer stops for wheel maintenance. To hold correct part geometry, Al2O3 wheels often must be dressed after every grinding cycle. But the only attention porous vitrified-bond CBN wheels normally require is an occasional light truing a process similar to dressing an Al2O3 wheels.

Test at Litton Industial Automation, Waynesboro, PA, show an additional benefit of CBN over Al2O3: There's less risk of metallurgical damage when grinding cam profiles with CBN wheels. The superbrasive can prevent burning, cracking, grinding stress, and problems with wringing.

Litton's evaluation included the cost of changing both grinding wheels and truing tools. Unlike conventional CBN wheels made with nonporous resins or metal bonds, porous vitrified-bond CBN wheels are easy to true. Furthermore, they are not dulled by truing and thus don't require conditioning to open up the wheel. The bottom line is economic. Cam-lobe-grinding costs per camshaft were reduced up to 50 percent.

Parts testify

Workpieces ground by CBN abrasives give better performance in service than identical parts ground with Al2O3 wheels. Because Al2O3 wheels dull rapidly, they grind hot. Worse, Al2O3 is a thermal insulator, so much of the grinding heat transfers to the workpiece. Cooling the over-heated material sets up residual tensile stresses, which may cause early fatigue failure of the part.

CBN wheels, on the other hand, usually grind cooler than Al2O3 wheels because the abrasive stays sharp under high grinding forces. Nevertheless, they still generate a lot of heat. But CBN is a thermal superconductor, and the CBN crystals transfer heat away from the workpiece, reducing the posibility of thermal damage.

Finally, CBN crystals in the wheel actualy peen the workpiece surface as they grind it, setting up residual compresive stresses that oppose fatigue failure. This action can boost fatigue life 30 percent of more, as reported by firms that have converted from Al2O3 to CBN wheels.

Cam application

Porous vitrified-bond CBN wheels serve in most types of grinding, and are especially recommended for form and creep-feed operations. In one example, a European automaker converted from Al2O3 wheels to CBN for grinding can profiles on camshafts. He more than doubled productivity.

The camshaff is cast steel, ground with a 20"-dia, 1"-wide wheel. The Al2O3 wheel had to be dressed after grinding each lobe. In contrast, the CBN wheel produces dozens of camshafts per truing. Grinding time per camshaft dropped from 5 min to 2 min, 10 sec.

Despite less-frequent maintenance, the CBN wheel produces more accurate and more consistent cam profiles than the Al2O3 wheel, resulting in better engine performance. Also, it consistently holds surface finishes in the desired 15 to 20 microinch Ra range.

US manufacturers of passenger-car and trunk engines (including diesels) see cam-lobe grinding with the new CBN wheels as a big step forward in their drive to increase productivity, achieve better quality, and reduce costs.

A bearing manufacturer , too gained the CBN benefits when grinding bores of ball-bearing rings. The ring material is a high-chromium-steel bearing alloy, hardened to 62 Rc.

Using 0.64"-dia, 0.52"-wide CBN bore-grind wheels, engineers developed a 12-sec grinding cycle to remove 0.012" stock on the bore diameter. When Al2O3 wheels were used for this operation, they had to be dressed after each grinding cycle, losing about 0.0001" of material on the diameter.

The vitrified CBN wheel, on the other hand, delivers 100 parts per truing. Only 0.0001" of material is removed on the wheel diameter during truing, which is done with a rotary diamond tool operating in the downcut mode. In this case, grinding cycle time has been reduced by 25 percent.

Application guidelines

Although the new wheels can grind some soft ferrous materials effectively, the greatest productivity improvements occur with difficult-to-grind steels, cast irons, and high-temperature superalloys. There's no mystery about using CBN wheels. Procedures generally follow those of AI2O3 wheels. The most important factors are use of high wheel speeds, proper truing methods, and plenty of the right coolant.

When installing a CBN wheel on an existing machine, make sure the machine is rigid enough to contain high grinding forces without deflection. Older machines probably will cause problems with vibrating frames, worn feed slides, and inadequate spindle bearings. All machines should have hydrostatic spindle bearings to provide high stiffness.

Also, check the machine's wheel-dressing system for rigidity. The forces generated when truing a CBN wheel are higher than those generated when dressing an AI2O3 wheel, because CBN is so hard.

Spindle horsepower should be sufficient to avoid wheel slow-down under severe grinding conditions. Ideally, even for AI2O3 wheels, spindle rpm should be high enough to permit grinding at wheel speeds in the 6500 to 8500 sfm range, and higher speeds are desirable with CBN. Material-removal rates, surface finishes, and wheel life all improve as wheel speeds are increased. However, don't exceed the manufacturer's maximum recommendation.

CBN wheels are recommended for wet grinding only. Consequently, another machine requirement is a system for applying large volumes of filtered coolant. Position coolant nozzles so they direct substantial volumes of fluid directly to the wheel-workpiece interface.

To minimize frictional heating and promote good chip flow, use a coolant with high lubricity. A 5 to 10 percent concentration of soluble-oil/water emulsion fortified by EP additives provides efficient material removal and long wheel life in most cases. However, severe grinding operations may require a straight oil to maximize wheel life.

Wheel design

Most vitrified-bond CBN wheels have a porosity in the 20 to 40 percent range. The porous structure of the wheel has two beneficial effects:

First, the pores trap coolant and carry it into the grinding zone, resulting in more effective cooling and lubrication. This reduces frictional heating and promotes longer wheel life.

Second, the pores provide clearance for chips. With ample chip clearance, wheels cut more freely, hence more efficiently and with reduced wheel wear.

Wheels with porosity at the low end of the 20 to 40 percent range tend to grind hard; i.e., both free-cutting action and wear rate are reduced. As porosity increases, wheels have softer grinding action and wear faster. Hard wheels are a good choice for close control of form because of the low wear rates. Soft wheels are better for fast stock removal.

The vitrified-bond wheels are available with CBN abrasive in a wide range of mesh sizes, from 60/80 (very coarse) to 325/400 (very fine). Coarse abrasive provides high stock-removal rates, serving where relatively rough surface finishes are acceptable. Finer CBN abrasive produces smoother surface finishes.

Because of the sharpness of the abrasive, CBN wheels made with a given mesh size produce rougher finishes than AI2O3 wheels made with the same-size abrasive. However, the better wear resistance of CBN ensures more consistent finishes. And, even when smaller abrasive mesh sizes are used, CBN wheels can remove stock faster than the AI2O3 wheels they replace.

The term abrasive concentration refers to the percentage of CBN abrasive in the bond matrix material by volume. The figures range from 18 percent (75 concentration) to 50 percent (200 concentration). As these numbers increase, the wheel grinds harder and surface finishes are smoother. Most of the new CBN wheels are in the 100 to 150 range.

Truing hints

Try not to use single-point diamond-dressing tools on CBN wheels, because they develop wear flats, become overheated, and thus have short life. Instead, consider impregnated diamond nibs or form blocks, diamond truing wheels, diamond form wheels, and diamond rotary profile dressers. Hardened-steel crushing rolls also work well, if the machine has crush-dressing capability.

Truing is a severe operation, so apply large volumes of coolant to prevent overheating of both truing tool and CBN wheel.

Normally, only a very small amount of wheel material is removed when truing porous vitrified-bond CBN wheels. Removing more material than necessary shortens wheel life without improving straightness or wheel form. Generally, it's best to true the CBN wheels while ground parts are still well within tolerances. Only a light touch-up will be required, minimizing consumption of wheel material.

For more information on porous vitrified-bond CBN wheels, contact Dunnington Div, Wendt Grinding Corp, Rt 100, Chester Springs, PA 19425. Circle 487.

PHOTO : Structure of CBN grinding wheel made with porous-vitrified-bond matrix, shown at 103X enlargement. Porosity level is 20 percent. Photo courtesy GE Superabrasives, Worthington, OH.

PHOTO : This CBN wheel is at work grinding a camshaft. Dark area on rim is CBN material. Although the superabrasive layer is relatively thin, the wheel has long life.

PHOTO : Adcole machine checks profile of automotive camshaft ground with CBN wheel on Landis masterless camgrinding machine.

PHOTO : Parts ground with CBN wheels include (top to bottom) milling cutters, threading tools, sawblade segments, molds, and pump components.
COPYRIGHT 1989 Nelson Publishing
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Copyright 1989 Gale, Cengage Learning. All rights reserved.

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Title Annotation:cubic boron nitride
Author:Cassidy, William J.
Publication:Tooling & Production
Date:May 1, 1989
Previous Article:Managing job-shop data.
Next Article:Deburring basics.

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