A particular challenge: Finishing of targeted surface areas on different hip stems
The production is to a large extent automated and allows tracing the entire manufacturing chain, from the raw material to the finished product. Until recently polishing of the neck area of the hip stems took place by a manual pre- and fine-grinding step, followed by an electropolishing process. This surface refinement operation was time-consuming, required a lot of manpower and was very costly. In addition, it was less than perfect with regard to consistency of the finishing results and general sustainability. Therefore, Thomas Müllner, general manager at MBN Präzisionstechnik GmbH, had been looking for an automated alternative for quite some time. He explains: ”Since only a precisely defined surface area must be polished, and a variety of different hip stem shafts must be processed, automating the surface finishing process turned out to be quite challenging. First processing trials were undertaken by a supplier competing with Rösler. However, the results did not meet our quality standards.”

An application for the drag finishing technology
Based on the recommendation of one of MBN’s customers further processing trials were conducted by the Rösler sales branch in Austria. This company is not only located nearby but also maintains a combined test and service center. The Rösler experts decided to run the processing trials in a drag finisher. This unique mass finishing system allows the precise and targeted surface finishing of high-value work pieces with complex shapes. Once the process parameters have been established, the process runs without any variations, thus ensuring absolutely repeatable finishing results. Thomas Müllner remembers: ”Rösler’s comprehensive know-how and experience from the job shop operation helped to quickly develop a two-stage process with the drag finishing technology. The process, consisting of a wet grinding / pre-polishing step, followed by a dry polishing stage, produces a consistently high quality in surprisingly short cycle times”. A major feature of the Rösler drag finishing technology is no doubt the vibratory motor mounted below the processing bowl. ”The vibration of the processing bowl ensures the optimal mixing of the processing media. This results in the absolutely homogeneous finishing of all relevant surface areas on the hip stems”, concludes the general manager.

Fully automatic drag finishing operation with two processing bowls
In the end, MBN Präzisionstechnik chose a compact drag finisher, model R 4/700 SF. This plug-and-play machine consists of a processing bowl with a diameter of 700 mm and a carousel with four rotating working spindles, each spindle allowing the mounting of three work pieces.  The carousel and rotating spindles are equipped with separate drive systems so that the carousel and spindle speeds can be set independently from each other.  The drag finishing machine is furnished with a second processing bowl. With a lift truck the two bowls can be easily and quickly exchanged.
Prior to the grinding operation the surface areas on the hip stems requiring no finish are masked. Then the implants are manually mounted to special work piece fixtures. ”For this purpose we developed a clamping device that can be used for all stem types and sizes”, explains Thomas Müllner. The work piece fixtures with attached hip stems are then attached to the working spindles.  
Once the respective processing program, stored in the programmable system controls, has started, the carousel is lowered so that the rotating spindles are immersed in the processing media. For the wet grinding process a mix of plastic media with different geometric shapes and a special compound are utilized. Carousel and spindles are rotating at the pre-defined speed in opposite rotational directions. After about half of the cycle time – amounting to less than one hour – the rotational direction is normally reversed. This ensures that the neck area of the hip stems receives an all-around perfect homogeneous finish. For cleaning of the process water from the wet grinding process, MBN Präzisionstechnik purchased a semi-automatic Rösler centrifugal process water recycling system, model Z 800. Recycling of the process water protects the environment and helps to significantly reduce the finishing costs.
For the polishing operation a plant-based dry polishing media is utilized. For this process stage, requiring a cycle time of well under 20 minutes, the processing bowl containing the finishing media is simply exchanged with the bowl filled with polishing media.  Thomas Müllner concludes: ”With the automatic drag finishing process we could significantly improve the quality and consistency of the finishing results. At the same time, compared to our previous finishing operation, the costs for grinding and polishing of the hip stem neck could be reduced by two thirds. Therefore, this investment will be amortized within a three year period”.