For the production of valve housings for passenger car transmissions, a major German automobile manufacturer ordered a production line consisting of four machining centres including the corresponding fixtures from GROB Werke in Mindelheim.

The workpieces would have been deformed if they had been clamped using conventional clamping with standard components; sufficiently precise machining within the narrow tolerances would not have been possible. The solution was provided by position-flexible clamping elements, which were exactly adapted to the application.

“The classical workpiece clamping on three fixed supports with an additional support perpendicular to the surface to be machined does not work because the intrinsic stability of the components is too low”, Key Account Manager Dietmar Wahl from the clamping technology specialist ROEMHELD in Laubach, Hesse, summarises the initial situation in retrospect.

The workpieces would already deform too much due to the vertically acting contact force of the support. The alternative solution was an additional lateral clamping of unstable workpiece areas with position-flexible clamping elements. With the position-flexible clamping elements from the catalogue, it was already possible to achieve a minimal displacement of the workpiece during clamping, but not yet the required process capability. “For this reason, we had to develop an application-specific special solution.”

The bridge is automatically loaded and unloaded in a horizontal position. For machining, it swivels by 90 degrees into the vertical position. Then the surfaces of the valve housing and cover are removed by a total of 0.2 mm in two operations, first by 0.17 mm and then by 0.03 mm during finishing. A special milling tool with a diameter of 440 mm is used, which completes both machining steps in about one minute. An integrated measuring probe checks the accuracy of the machining process at predetermined reference points on the workpiece surface.

“The flatness tolerance over the entire workpiece length is 0.05 mm, the specification for straightness over a length of 100 mm is 0.03 mm”, describes Sebastian Dichmann, fixture designer at GROB in Mindelheim, the specifications regarding the accuracy of the component. These tight tolerances represent a special challenge. An already existing clamping concept from a previous project served as the basis for the fixture. This had to be adapted to the new components by the 28-year-old mechanical engineer and converted from wet machining to dry machining.

After about two weeks of design, Dichmann had completed the revision in early 2014. The workpieces were to be clamped at three clamping points specified by the customer. Swing clamps with fork head at the end of the piston rod were used as clamping elements. As soon as the clamping arm is swivelled in, the workpiece is clamped with a defined clamping force of 2.25 kN per clamping point.

For reliable, distortion-free clamping, six additional position-flexible clamping elements are applied: two on the valve housing and four on the cover. These elements are small vices with two movable slides which are operated by a common hydraulic connection. Thus, the workpiece will be clamped nippers-like and floating, independently of its position within the clamping area.

“Position-flexible clamping elements are used primarily for supporting and clamping unstable workpiece areas without side load and distortion. They adapt themselves to the position of the clamping point without deforming it. At the same time they have a vibration-damping effect and absorb machining forces from all directions”, Benjamin Nagel, responsible for product management at ROEMHELD, describes the purpose of the clamping device.

The engineers' efforts led to the desired success: Thanks to the new position-flexible clamping element for GROB, the deformation of the clamped workpieces was reduced by two thirds to only 0.01 mm, well below the required tolerance limit. After successful acceptance by the customer, nothing more stood in the way of installing the production line at the end of 2014.

Dichmann expressly praises the cooperation with the supplier: “We were under enormous time pressure and therefore I was particularly pleased that the cooperation proceeded extremely quickly. The new position-flexible clamping elements were manufactured and delivered by ROEMHELD very quickly. Also, the communication was excellent and in the end we were pleased, as was our customer.” Since the start of production, the German premium car manufacturer has ordered two additional production lines for valve housings and covers.