Clamping of delicate workpieces without warping for precise processing

For the manufacture of valve housings for passenger car transmissions, a large German carmaker ordered a production line comprising four processing stations including the associated equipment from GROB Werke in Mindelheim. If conventional clamping with standard components had been applied, the workpieces would have been deformed; sufficiently accurate processing within close tolerances would not have been possible.

The solution were position-flexible clamping elements precisely adjusted to the application.
"Conventional tool clamping on three fixed rests plus an additional support perpendicularly to the surface to be processed does not work, since the autostability of the components is not sufficient", Key Account Manager Dietmar Wahl from ROEMHELD, the clamping technology specialist company with headquarters in Laubach, Hessen, sums up the starting situation in retrospect. The workpieces would be seriously deformed by the perpendicular action of the support only. An optional solution was additional lateral clamping of delicate workpiece areas by means of position-flexible clamping elements. Standard position-flexible clamping elements could already ensure a very slight shift of the workpiece during clamping, however the required process capability could not be ensured.

"This was why we had to develop an application-specific special solution."

In an automated production line, the aluminium castings consisting of a valve body and the associated cover are processed in four stations with different production applications. The valve body, incorporating numerous channels to control gear oil, has a length of 400 mm and a thickness of 21 mm; the cover dimensions are 350 mm and 24 mm. The first three stations are designed to scrub the component sides and drill holes in them.
The final step in the fourth station is precise finishing of the surfaces on a single-spindle special machine of the GROB processing centre G320 with four axes and a specially solid cross slide. The machine with its 20 kW spindle drive and processing distances in X/Y/Z of 615/655/630 mm is designed for the simultaneous processing of two workpieces. In total, around 480,000 of these challenging components are to be produced every year.

0.05 mm tolerance along the entire workpiece length

The bridge is automatically loaded and unloaded in a horizontal position. For the purpose of processing, it swivels vertically by 90 degrees. Subsequently, the surfaces of valve bodies and covers are machined off by a total of 0.2 mm in two cycles - 0.17 mm in the first and 0.03 mm in the finishing cycle. A special milling tool with a diameter of 440 mm is used; it completed both processing stages within about one minute. An integrated measuring sensor checks the specified reference points of the workpiece surface for processing accuracy.
"The evenness tolerance along the entire workpiece length is 0.05 mm; the specified flatness requirement on a length of 100 mm is 0.03 mm", Sebastian Dichmann, fixture designer with GROB in Mindelheim, describes the requirements regarding the component accuracy. These close tolerances represent a particular challenge. The basis for the tool was an existing clamping conception from a previous project. This conception had to be adjusted to the new components by the 28 year-old mechanical engineer, and modified from wet to dry processing.

Floating and warp-free clamping

After about two weeks or work, Dichmann hat completed the revised design in early 2014. The workpieces were to be clamped on three points specified by the Customer, each. The clamping elements used were swivel clamps with a forked head at the piston rod end. After the clamps is swivelled, the workpiece is clamped with a pre-defined clamping force of 2.25 kN per clamping point.
For reliable clamping without warping, another six position-flexible clamping elements are used - two on the valve body, four on the cover. These elements are small clamps with two mobile slides activated by a joint hydraulic connection. This way, the workpiece is clamped like by pliers and in a floating way, regardless of its position within the clamping range.
"Position-flexible clamping elements are primarily used for supporting, as well as for clamping delicate workpiece areas without the application of shear forces and hence without warping. For they adapt to the position of the clamping point without deforming it. At the same time, they have a vibration-reducing effect and absorb processing forces from all directions", describes Benjamin Nagel from ROEMHELD's product management department the purpose of the clamping element.

Close cooperation led to success

"Even with the original specification for the new fixture, position-flexible clamping elements, which should already have ensured precise processing, were used", says Dichmann. However, during commissioning of the adjusted fixture it showed that the required close tolerances were not always met reliably, the engineer reports: "The components shifted vertically upward by up to 0.03 mm at the clamping points of the position-flexible clamps. The process capability as required by the Customer could not be complied with."
In close cooperation, the clamping technology experts from ROEMHELD and Sebastian Dichmann searched for a solution - and quickly found one, the engineer says. "A position-flexible clamping element with specially adjusted slides should remedy the situation." This clamping element was manufactured in Laubach at high speed, for the acceptance procedure for the processing centre was imminent.
The challenge for the clamping experts was to provide the optimised slide with sufficient ease of operation and sensitivity despite the reduced backlash. Otherwise, the two dies could have moved unequally, which could have resulted in a stroke difference with one-sided shifting force and in warping during clamping. "We were able to eliminate this risk", explains Nagel. In order to take account of all contingencies, ROEMHELD's engineers also reviewed the Customer's clamping dies and made calculations, but did not identify any need for action. Today it is clear that the clamping elements meet all requirements, "we do not even have any trouble caused by the occurrence of chippings due to dry processing", says product manager Nagel.

Compliments for close cooperation

The engineers' efforts lead to success as desired: Due to the position-flexible clamping element for GROB, deformation of the clamped workpieces was reduced by two thirds to only 0.01 mm, which is significantly below the required tolerance limit. Upon successful acceptance by the Customer, everything was ready for the installation of the production line in late 2014.

Dichmann explicitly praises the cooperation with the Supplier. "We were under extreme pressure and very happy that our cooperation proceeded so quickly. The new, position-flexible clamping elements were rapidly manufactured and delivered by ROEMHELD. Communication, too, went excellent, and we as well as our Customer were very satisfied in the end." The German premium carmaker has ordered two more production lines for valve bodies and covers since the start of production.