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What You May Not Know About Grinding-& Ought To

So far as metal removal operations go, grinding could be one that has greater applicability at operations at both OEMs and suppliers—with beneficial consequences.

So far as metal removal operations go, grinding could be one that has greater applicability at operations at both OEMs and suppliers—with beneficial consequences. To learn more, we put a few questions to Nelson Beaulieu, automotive accounts manager, United Grinding Technologies (Miamisburg, OH; www.grinding.com—with a URL like that, it is fairly evident that these folks know plenty about the operation).


AD&P What are the primary automotive parts ground right now?

BEAULIEU: The components can be broken down according to engine, drivetrain, chassis and auxiliary. Those parts typically ground are "class A" components. For the engine, there are camshafts, crankshafts, valves, connecting rods, and balance shafts. For the transmission, input, output, stator and reaction shafts, mainshafts, CVT components, gear faces—I.D.'s and O.D.'s—and bevel gears. On the chassis there are companion flanges, wheel hubs, rear axle casings, sector shaft OD's, steering rack OD's, universal joints, CV joints and shock absorber piston rods. Finally, in the auxiliary category, there are such things as turbocharger shafts, fuel injector needles and bodies, power steering metering shafts, electric motor armatures, hydraulic pump rotor slots, water pump shafts, inner and outer gerotors, and water pump outer rings.

AD&P What are other parts that ought to be ground?

BEAULIEU: Torque converter components, drivetrain yokes, steering rack tooth profiles, sector shaft gear tooth profiles, and snap ring grooves on drivetrain components.

AD&P Isn't grinding slow compared to other metal removal operations—and consequently more expensive?

BEAULIEU: Not necessarily. Compared with the traditional use of a grinding machine, the speed of specific operations may be faster using more traditional machining processes, but the throughput of completed components may be faster and less expensive using grinding machines. Additionally, machining operations that were once capable of maintaining quality requirements may no longer be capable as tolerances are reduced and materials are improved. When examining per-part manufacturing expense, previously hidden costs such as tooling inventories and tool maintenance costs were not part of the process equation, whereas today we now recognize the additional costs and apply them toward a better process solution using grinding machines.

AD&P How does grinding compete with processes like hard turning?

BEAULIEU: Grinding both competes and coexists with hard turning. In some cases, we combine both processes on the same platform, taking the best from both disciplines. Hard turning is particularly effective in fast metal removal for ID's and faces when both tolerance and surface finish permit. However, if the parts are thin walled, or there are interrupted surfaces, then grinding becomes more attractive in terms of process cost and manageability. For ID grinding, the ideal solution is to turn the heavy stock, and then leave a minimum of material for the finish grind. When the grinding wheel is smaller in diameter than the diameter it is grinding, wheel wear and removal rates are less than optimum.

In the case where a component is hardened in one area and transitions to an area that is not hardened, let's say on a wheel hub for instance, the tendency is for the tool to "jump" when either coming off or leading onto the different hardness area on the surfaces. This problem becomes much more evident as the machine wears and the slides start to react to screw wear and slideway position has to be counteracted by glass scales or encoders to overcome the sudden increase or decrease in tool pressure. As the industry transitions toward using the shaft OD section of the wheelhub as an inner bearing race, the once tolerable 0.0001" to 0.0003"difference in diameters resulting from tool jumping now plays a much greater role in long term processing and capability. Here high-speed grinding, or peel grinding, offers distinct advantages.

AD&P What about capital equipment costs—aren't grinding machines more expensive?

BEAULIEU: Grinding machines are recognized as typically more expensive than most commodity lathes and machining centers. And even among grinding machine builders, there are different levels of capital expenditure. When comparisons are made between grinding machine costs and other machine tools, the auxiliary costs of supporting traditional machining processes must be addressed, as well. The base price of machine tools may be a reasonable barometer when assessing commodity machine tools, but falls short when producers of high-production automotive components examine machine tools.

Accounting for tool inventory costs, post-process operations, such as deburring, straightening, polishing, as well as machine changeover costs close the gap between the cost of ownership of metalcutting machines and grinding machines.

For example, the cost of a profiled pull broach can run into the $20,000 to $30,000 range, and produce 2,000 to 3,000 parts before each regrind. Given the estimated life of the broach at 25,000 parts, the tool cost per part is easily at $1.00 per part. When the costs of regrind and tool change are considered, as well as the burden of an additional tool waiting in inventory, the cost of ownership over grinding machines begins to take on a new perspective. Grinding wheel costs for this example using vitrified CBN were in the $0.04 range, with the added advantage of simplified tool changes and much longer tool inventory costs. The process itself reduced compressive stresses being imparted by the broach, resulting in unmatched life cycles extending well beyond 250,000 cycles, where 125,000 cycles was the required target.

Where a line or cell would produce 5,000, 10,000 or 15,000 components per day, those processes which were used to produce that initial planned quantity of parts would be more formidable when compared to the then-existing grinding process. However, when those quantities dwindle, as they can today, the traditional process takes on a higher cost burden per part, and in many instances, the process now becomes more expensive than when it was first proposed. Reduced quantities in automotive production signal an opportunity to re-examine an existing process, and extract more profit out of the reduced volume using grinding machines as a processing platform.

AD&P Have there been technical developments that make grinding more applicable to automotive applications?

BEAULIEU: The gains made in abrasive products permit the use of ganged wheels mounted on common spindles, giving the operator the flexibility to completely process a component in or on one platform. Additionally, with turret-style indexing wheelheads and variable-speed high-frequency grinding spindles, grinding machines can perform a variety of operations with fewer grinding wheels. The common denominator of highest maintainable accuracy is a by-product of heavy metal removal using a grinding machine. Grinding wheel improvements allow engineers to estimate costs over time, reducing wheel changes, and applying a much wider range of performance parameters using the grinding machine to eliminate follow on operations.

Born out of toolroom environments, flexible software outperforms hard tooling changes with soft tooling changes, using common wheels at varying speeds and feeds to achieve a programmable desired result. With advanced software capability, set up time is greatly reduced, and as the demand for flexibility in high production operations is becoming a priority, the fast changeover of grinding machines and multiple operations which can now be performed on them results in a more economic approach to part processing.

Additionally, centerless grinding with and without centers is possible, as is peel grinding and plunge grinding on a common platform.

AD&P What's the one thing that process engineers in automotive ought to know about grinding but probably don't?

BEAULIEU: Grinding machines are metal removal machines, not simply finishing machines. Grinding machines offer a new dimension in profit processing components and not just finishing components. Look up and down the line for grinding opportunities, and not just at a "process problem that has to be ground."