2 For The Road

High-speed grinders running CBN wheels with the ultrahigh-strength T2 bonding system increase production rates, reduce production costs, and improve workpiece-surface quality.

T2-platform CBN grinding wheels produce G-ratios as much as 10 higher than Saint-Gobain's previous bonds.

ULTRAHIGH-STRENGTH VITRIFIED BONDS
CBN-abrasive grains for precision grinding continually become available in grades with more controlled shape and toughness gradations. The challenge for grinding-wheel manufacturers is to develop and marry bond systems that capitalize on the performance potential of such grains.

Traditionally, CBN-wheel manufacturers employ several types of bonds, including electroplated (of nickel), metallic, resinoid, and vitrified. In high-speed, high-production, automated grinding, each bond type exhibits advantages and disadvantages, but the bond of choice for such applications is typically vitrified CBN. The major advantage of these wheels is that users can automatically dress them on a grinder. Also, the vitrified-bond structure and grade adjust for specific applications.

Grinding-wheel manufacturer Saint-Gobain Abrasives produces three such enhanced-strength vitrified-bond types that improve CBN abrasives. One produces G-ratios (the ratio of stock machined to wheel wear) 3 to 5 that of prior vitrified-bond-CBN wheels. The second improves upon those ratios by 40% to 67%, and the third ultrahigh-strength bond, called the T2 platform, produces G-ratios as much as 10 higher than previous bonds.

High-speed grinders running cubic-boron-nitride (CBN) wheels with a new ultrahigh-strength bonding system are the latest dynamic duo in auto-parts manufacturing. The combination increases production rates, reduces production costs, and improves workpiece-surface quality.

In peel-grinding tests, for instance, researchers teamed a high-speed grinder with 15.748-in. vitrified-CBN wheels sporting new T2 bond technology from Saint-Gobain of Worcester, Mass. They ground gearbox components, receiver gears, tripods, and motor-rotor shafts at wheel speeds from 4,724 to 5,512 in./sec and workpiece speeds from 700 to 7,000 rpm.

Additional tests with the T2 bonds were equally impressive. For example, in a gearbox output-shaft application with a finish requirement of <2.5 µ Rz with dressing, a standard CBN wheel required dressing every 25 workpieces. Incorporating a T2-bond wheel increased the dress interval to 75 workpieces and for a similar component from 40 to 120 pieces/dress interval.

During an actual high-speed, plunge-grind application, a North American manufacturer of steel camshafts upped grinding production from 100 parts/dress and 25,000 parts/wheel to 250 parts/dress and around 90,000 parts/wheel using T2-bond wheels.

What it takes to run T2
Vitrified-CBN wheels with T2 bonds run up to 630 in./sec, making them well-suited for high-speed grinding operations. But for optimum results, shops must address several important factors during use. These include wheel diameter, hub construction, dressing, and coolant selection.

Small wheel diameters allow for a more-compact grinder and are generally less expensive and easy to manufacture, balance, and handle. However, small wheels have inherent problems with maximum operating speed (mos).

Within the abrasive structure, wheel rotation induces stresses that are highest at the bore and decrease radially out to the periphery. For large wheels, replacing the core section with a high-strength material such as steel confines the vitrified layer to a thin outer rim where stresses are least.

Also, constructing the vitrified layer as segments with spaces between them further reduces stresses, which are hoop in nature. The spaces, essentially expansion joints, compensate for core expansion.

This approach works readily for mos up to 4,921 in./sec using wheels with thin CBN layers (<0.4 in.) and diameters of 11.8 in. or more. Further optimizing a wheel's core material and cross-section minimizes core expansion and reduces stress for speeds as high as 7,874 in./sec.

With wheel diameters under 11.8 in., centrifugal forces (mv ² /r) from higher required wheel rpm cause radial stress to dominate. In such conditions, bond mass (thickness) and strength controls burst speeds, and mos falls steadily with reduced diameter. With this in mind, users must calculate mos values on a case-by-case basis. However, high-strength bonds, such as the T2 system, raise the mos of wheels under 11.8 in. in diameter.

In addition to diameter, a wheel's hub construction, as it relates to proper mounting, is important for mos. Wheels must not shift position on the machine spindle, but what often happens is that the wheel expands radially, and its axial width contracts, thus reducing clamping pressure.

A common remedy is a bore with an interference fit (or close to it) and a bolt flange with as many bolts as possible before affecting hub strength. Should any slippage occur, the setup limits the wheel's freedom of movement.

Any slight wheel shift creates imbalance that leads to chatter. Shops must balance wheels individually and dynamically before mounting them on a high-speed machine. Then, they should dynamically balance the whole set of rotating elements on the machine at nominal speed. A two-plane, in-process dynamic balancer is recommended.

Balancing and overall machine condition are important to the service life of vitrified-CBN wheels with the T2 platform and workpiece quality. In tests conducted at the Oak Ridge National Laboratory, Oak Ridge, Tenn., technicians found that small perturbations of a few microns in the grinding process cut wheel life by half.

As with balancing, shops should run CBN wheels with vitrified binders at their nominal speeds when automatically dressing with rotary-diamond technology. This prevents changing the wheel shape (due to expansion) or center of rotation.

To conserve expensive abrasive while dressing, the dressing disk should in-feed at micron levels. However, this level of precision is difficult with large wheels that may require dressing once a shift or even once a week. So, it is essential that shops employ acoustic-emission-based sensors to detect the exact moment of contact between wheel and dressing disk.

Coolant is a must for high-speed grinding using vitrified-CBN wheels. It removes heat, flushes chips and abrasive particles, and lubricates. Shops should apply temperature-controlled and well-filtered coolant at the point of contact between a grinding wheel and the workpiece. A constant, laminar (air-free) flow and appropriate pressure to match wheel velocity are ideal.

However, necessary coolant pressure increases at the square of the wheel speed — 1,969 in./sec needs 15 bar, but 7,874 in./sec requires 240 bar. Such delivery requirements become increasingly impractical over 2,362 in./sec, except for extremely narrow wheels. Most successful applications incorporate a shoe-type nozzle that uses the wheel to accelerate coolant to the required speed.

Once in the grind zone, coolant creates hydrodynamic pressure. This, in turn, generates high normal force manifesting as profile or roundness errors at speeds as low as 2,362 to 3,150 in./sec, depending on part stiffness and support. Coolant also creates resistance for wheel rotation. The combined effects of inertia and coolant drag at 6,299 in./sec can be as much as 2 kW/0.0393 in. of wheel width. To get the benefits of high speed (high removal rates) with low grinding forces and less coolant effects, some shops peel grind using vitrified CBN wheels with T2 bonds.