Developments in Precision Grinding

The S41 universal grinder.

Grinding for automotive is a high-precision game, and the requirements are only getting higher, Kevin Chen, automotive accounts manager, United Grinding ( says. Take the example of continuously variable transmissions (CVTs), in which the key components all require high-precision grinding after machining. 

United Grinding responds with the Studer S22 and S41 models: high-volume production machines suitable for CVT key components grinding. The S41, a CNC universal cylindrical grinding machine, has high-precision axis drives with linear motors, fast direct-drive of the B-axis, and a large selection of wheelhead variants. Both the S22 and the S41, differentiated by their maximum center heights (125mm and 275 mm, respectively) and distance between centers (400 mm and 1600 mm) can accommodate up to four grinding wheels, so internal, external, and any other custom grinding requirements (forms, threads, contours, etc.) can be met in one clamping. A direct-drive turret wheelhead has 0.00005-in. resolution, and standard interfaces for loaders or other peripheral devices further accentuate automation aspects.

A recent addition to the S41 universal grinding machine is an optional tilting A-axis for grinding threads of various pitches. Useful for lead screws, ballscrews, and steering gear drives, A-axis availability on a universal machine eliminates added clampings and part handling on dedicated thread-grinding equipment. An S41 can even be equipped with two A-axis heads, meaning that rough grinding and thread grinding can be done in the same clamping, in addition to both internal and external thread grinding. Not only can they perform thread grinding, but they can also—depending on the grinding head variant (S41)—perform complete machining operations. 

For cylindrical grinding applications, Studer has developed the S151 which offers a 1300-mm maximum grinding length, programmable B-axis, automatic swiveling workpiece table, and in-process gauging and sensing. A PC is integrated into the CNC control system. According to the company, the possibility of fully integrating in-process gauging and sensor technology for process monitoring as well as contact detection and automatic balancing systems in the control enable standardized programming of the different systems.

“Dedicated machines are more and more being relegated to the past in favor of flexible and universal equipment, largely driven by both machine design and software,” says Hans Ueltschi, vice president of United Grinding’s cylindrical division. “Customers are increasingly seeing the value of being able to adjust the process more on their own.”

One of the test pieces set up for the S151 was grinding the internal diameter on a clutch retainer. The tolerance was held to 0.01 in. and cycle time was an impressive 30 seconds. 

Match Game
A new approach to match grinding—grinding so that there is a precise clearance between two parts, such as the external diameter of a piston to the internal diameter of a sleeve—has been developed by the company. Unlike match grinding of decades past, involving grinding individual components, checking the clearance, adjusting the equipment, and repeating, the latest equipment includes pre-gauge and process gauging monitors for handling tapers as well as clearance. Add a post-process feedback system all tied into the CNC and you’ve created an automated cell.

“We’ve engineered cells for grinding needle nozzles holding tolerances of 1.1 microns,” Ueltschi says. Depending on part-tolerance requirements, gauges can be added in one or multiple planes with automatic taper adjustment, if needed. Higher automation will increase setup time, and possibly higher cycle times as the gauging may take longer than the actual grinding, depending on the part, but this can reduce if not eliminate pre-machining of components, resulting in high workpiece quality and greatly reduced scrap.

Grinding with Diamonds
Because high-strength alloy materials such as Inconels are being used for applications like rotors, there is the use of more superabasive diamond wheels or CBN (cubic boron nitride) wheels in grinding equipment. This means a challenge for dressing the wheels. So Ueltschi says they’ve developed Studer WireDress technology, which essentially incorporates wire EDM technology into a grinding machine for profiling ultrahard wheels. Previously, ultrahard wheels had limited cutting life because they could be dressed only to a limited extent. Integrating wire EDM permits dressing at full wheel speed even with complex profiles, greatly extending complex grinding productivity. “This will really open the doors for automotive companies,” he says. “Harder wheels mean higher metal-removal rates and better surface finishes in hard materials.”

With metal-bonded grinding wheels dressed this way, it is possible to increase productivity by up to 30% in individual cases, compared to grinding with resin or ceramic bonded wheels, Ueltschi adds. In addition, such precise dressing process in conjunction with the high dimensional stability of the metal-bonded wheel enables workpieces with very challenging geometries to be produced repeatedly, previously not thought possible where cost-effectiveness was concerned.