High-precision machining centers meet exacting tolerance requirements in manufacturing of pivot bearings for Porsche

System reliability leads to improvements in manufacturing quality as well as integration of a stable manufacturing process for the next five years

Schabmüller Automobiltechnik GmbH has been manufacturing the pivot bearings for the Porsche Macan since 2013. The machining is performed with BA W06-22 and BA one6 high-precision machining centers from Schwäbische Werkzeugmaschinen GmbH (SW), which are specially designed for challenging series production tasks. The high precision of the machines is especially essential in the automotive sector, where high standards of quality permit only very narrow tolerances. While the two-spindle BA W06-22 is used for the main bearing bore, spring strut connection and transverse link connection, the brake caliber connection is made in a second step on the one-spindle BA one6. In this step alone the company has achieved a significant increase in quality and integrated it into the manufacturing process.

 

The fact that the weekly output of 1700 sets of pivot bearings for Porsche is produced on two different machine types at Schabmüller is due to a conscious, strategic decision of the automotive supplier with headquarters in Großmehring, Germany: For manufacturing of the main bearing bores, the gooseneck connection (spring strut support) and transverse link, 90 percent of the machining takes place within the cycle time required by the end customer Porsche in a hemisphere access from above on the two-spindle BA W06-22. The size of the component and the annual number of units produced determine the number of spindles. Since machining of the brake caliper connection cannot be implemented in OP 10 due to the workpiece clamping and the proportion of machining is much smaller, this machining step is carried out on the one-spindle BA one6. "That makes it possible to run the machining steps of OP 10 and OP 20 in parallel," explains Helmut Häckl, Technical Director at Schabmüller. If the need arises, the BA one6 can also be used to compensate for production peaks.

 

To meet the automotive supplier's strict criteria – for example the position tolerances in each axle must not exceed two or three hundredths in some cases – the SW machines have been precisely tuned to this range of applications. This approach practically eliminates errors, as a pneumatic contact monitoring system checks whether the part is correctly clamped in the fixture – otherwise the process is not started. In this way deviations can be detected and eliminated immediately. The tool air sensing monitoring system also increases process reliability by checking the tool on the HSK interface of the spindle. Linear drive technology in all axes (in contrast to ball screw drive, linear drive exhibits extremely low wear) ensures that all cycle times required by the end customer are achieved and also provides consistent long-term workpiece quality. The thermal expansion of direct drives is also significantly less than for conventional drives, which makes it possible to guarantee high long-term accuracy. The in-house machines at Schabmüller also undergo a partial or general overhaul every three to five years to guarantee high-precision manufacturing for the long term as well.

 

One of the fastest machining centers: BA W06-22

The BA W06-22 that is used has a spindle distance of 600 mm and features minimum tool change times of 2.75 seconds, thus enabling high-precision machining of light metal workpiece in the shortest time possible. In particular, it requires a startup time of just 0.7 seconds to go from 0 to 17,500 rpms and achieves main drive acceleration rates of up to 2 G, making it one of the fastest machining centers on the market today. The working area for each spindle is 600 x 600 x 500 mm.

 

The machine is also designed as a monoblock construction, which minimizes deflection while ensuring optimum flow of forces between the machining unit and the workpiece carrier. In addition, it features a box-in-box 3-axis unit that was specially developed for highly dynamic machining operations in light metal applications. The 3-axis unit is designed with welded steel construction so that the masses in motion can be kept low with maximum system rigidity. "It is inserted into the monoblock in the rear. The horizontal arrangement of working spindle allows chips to fall freely," explains Matthias Maier, Sales Manager for Europe at SW. A double swivel carrier with integrated rotary axes (torque) allows for loading and unloading in parallel to machining time. The SW machines are also ideally designed for loading and unloading with a robot or from above via a portal.

 

BA one6 achieves position tolerances of < 0.005 mm

The one-spindle BA one6, which like the BA W06-22 is designed for machining non-magnetic materials such as aluminum and titanium alloys, offers a working area of 600 x 650 x 500 mm. "This model type focuses on machining with the highest requirements for accuracy," explains Maier. Linear and torque motors in the feed axes guarantee maximum precision here as well with a position tolerance of less than 0.005 mm, together with maximum dynamics and short auxiliary process times. The chip-to-chip time is just 2.5 seconds.

The BA one6 is also equipped with a double swivel carrier for loading and unloading workpieces in parallel to machining time. Since the layout of the machine is identical to that of the BA W06-22, it is ideally suited for combined interlinking: It makes a valuable addition to multi-spindle production lines for one-spindle premachining and rework of light metal workpieces, and can also compensate for peak production periods, for example if a two-spindle machine is not sufficient and a three or four-spindle machining center cannot be used due to the size of the part.

 

Savings expected due to quality enhancements and by integrating more reliability into the manufacturing process

Schabmüller uses SW machines to ensure high machining quality overall and a reproducible manufacturing process. This has a significant impact on personnel resources in quality assurance and maintenance. The machining time in the BA one6 is also reduced by optimizing the cutting data. To prevent machine stoppages, SW offers its PULSE services as well as remote maintenance and diagnostics. Also available is preventive maintenance with a specialist in the relevant machines who informs the customer about all the steps required. Maier sums us: "Generally speaking, it is always crucial to prevent a machine stoppage, because that can have far-reaching consequences for customers resulting in high expenses." Schabmüller and SW conduct service workshops together regularly. Suggestions for optimization are received in these workshops to be implemented by the machine manufacturer in terms of technical development. Maier describes the approach to work at SW: "We are especially responsive to our customers' problems and requirements and we also develop solutions especially for users." In this way technology is continually refined on the level of the individual customer.

"We made the decision for SW machines because the high quality gives us the peace of mind we need in a project," reports Häckl. "That means we save time and money too. We have also been working together with SW since 1994, so there is a feeling of mutual trust and the technology we need is already in-house." Another important factor is that with these machines we can expect problem-free manufacturing within the project runtimes of four and a half to five years. Häckl estimates: "When viewed over this time period, we have been able to save considerable costs that we would have had to invest in repairs or similar expenses over the same period if we had bought something cheap".

Schabmüller currently has a total of 21 SW machines in use. Some of them have been configured for automated use by bartsch GmbH, which has belonged to SW since 2015. By 2019 a second hall in Großmehring will progressively be fitted with SW machines: another 17 automated machining centers will be in use there. Up to 680,000 cylinder head covers will be manufactured there annually for the end customer Audi.