Unmanned cylindrical grinding for small series production

With a total of seven Studer grinding machines, MPS Micro Precision Systems AG (MPS) has a cylindrical grinding expertise second-to-none. Thanks to automation and process optimization, medium-sized series are also flexibly produced in unmanned processes 24/7 while future projects and prototypes can be brought to market faster thanks to a flexible production cell.

MPS employs more than 400 staff and has more than 140 turning, milling, grinding, polishing, and eroding machines.

“We have a very varied range of technology, if you want to reduce it to a common denominator, then it’s the manufacture and assembly of complex electromechanical assemblies within the tightest tolerances for high-precision applications in high-end segments,” says Manuel Nercide, MPS plant manager.

Consistent investments

“With seven Studer grinding machines, we currently have a high grinding capacity for producing high-precision components,” says Michael Bazzan, MPS production manager. “The grinding machines are matched to our varying lot size spectrum, which ranges from individual component production up to 25,000 pieces per year for some references.”

In accordance with the lot size spectrum, three of the seven Studer machines are currently equipped with an automatic loading system – the Studer S33 for production of recirculating ball screws and a Studer S21 in a flexible production cell (Flex Cell).

Flex Cell for prototype manufacturing

“The Flex Cell was recently integrated into cylindrical grinding production as a self-sufficient production unit to set up grinding processes for prototypes and new developments without having to intervene in production,” explains Didier Noirjean, MPS flex cell manager. “Thanks to this flexible production cell our process from development through to the finished component is much quicker. The decisive factor with the Studer S21 was it can be changed over quickly, and it can be flexibly configured for different components. Eccentric grinding processes are also possible with the S21, perfectly suited to our requirements.”

While the Flex Cell primarily involves prototypes, series parts are machined on the other six Studer machines, including recirculating ball screws with a high-precision design.

The smallest recirculating ball screws have a ball diameter of 0.8mm. All components are developed and manufactured in-house by MPS, including the balls. The larger series of screw spindles with gothic-arch thread are produced on a fully automated Studer S33.

“We are involved in research and development with these precision screw spindles,” Nercide explains. “The expertise developed by MPS Microsystems in the extremely gentle and precise positioning of lens groups enables the company to offer powerful alignment systems for optical components such as zoom and laser beam focusing systems.”

Customer requirements

MPS Microsystems miniature recirculating ball screws, or ball-type spindles, are made exclusively of stainless steel and are positioned with double nuts, enabling fine adjustment of the axial play. The often-customized spindles guarantee virtually smooth movement due to the high precision of their ground thread. There’s a wide range of customized options available for recirculating ball screws.

“Such applications require the highest precision and positioning accuracy – 100% backlash-free. And it’s precisely here that our spindle systems show their strengths,” Julien Grosjean, MPS screw grinding manager explains. “We grind a so-called gothic-arch thread profile within the tightest tolerance ranges, so we can reduce the radial clearance and backlash to zero.”

Together with Studer, a macro was developed for these gothic-arch thread forms, which are ground directly from the solid hardened blanks. The grinding parameters can be adapted very quickly to the respective screw spindles with this macro.

“The Studer S33 used here is an external cylindrical grinding machine with a turret wheelhead that has two external grinding wheels. One of the two wheels is used for thread profiling, while the second wheel is generally used for external cylindrical grinding,” explains Ulrich Weyermann, regional sales manager at Fritz Studer AG.

Handling unmanned production

To produce the medium-sized series cost-effectively, the Humard company developed a flexible handling system for automatic loading and unloading of workpieces in collaboration with MPS and Studer.

“A crucial factor in the design of the handling system is that it can be quickly adapted to different workpiece geometries, because our lot sizes are variable,” Bazzan notes.

Typical thread grinding process

The Studer S33 requires less than 15 minutes for the complete machining of a screw spindle 8mm in diameter and 120mm in length. The thread is roughed and finished with the same grinding wheel. The grinding wheel is dressed repeatedly in the micron range throughout the grinding process. More than 1,000 screw spindles of this size can be ground with one grinding wheel.

The grinding wheel dressing process is important, as dimension and form of the grinding wheel are calibrated and the cutting ability of the grinding wheel is defined.

“In the case of screw spindles, we typically need to dress repeatedly during each production process. Both a diamond and a dressing roll are used for dressing,” Grosjean explains. “You can define the roughness of the grinding wheel specifically with the dressing roll, controlling the grinding process and surface quality. We’ve carried out many tests in this regard, until finding the ideal dressing method for the roughing and finishing processes.”

“If you consider the complexity of a grinding process and the dimensions of the machines – keyword thermal growth – then producing within IT02 tolerances in unmanned operation is hardly a matter of course,” Weyermann notes. “All seven Studer machines deliver exactly the precision and process reliability demanded by MPS. The 24/7 production with the IT02 tolerance ranges that we require is only possible thanks to the extremely high repeatability precision of our Studer grinding machines.”

Investments reduce production costs

“Seven years ago we made between 2,000 and 5,000 spindles. We now produce 50,000 spindles a year and we have continually reduced our production costs thanks to investing in modern grinding machines,” Nercide says. “Seven years ago our production was four times as expensive. From this perspective, investments in modern production equipment are crucial for ensuring long-term viability. Ultimately, our production must satisfy the qualitative and quantitative requirements of our product range. We are perfectly positioned in the cylindrical grinding segment in this regard.”

United Grinding North America
https://www.grinding.com
IMTS 2022 Booth #236802

Fritz Studer AG
https://www.studer.com