In order to efficiently manufacture more than 12,000 toothed parts with a wide range of dimensions, having lot sizes of one to 50, one needs to focus not only on the cutting parameters but also developing a strategy for handling the tremendous volume of setup time. The manufacturing engineers at Voith Turbo succeeded in this balancing act by selecting clamping devices for both individual part production and small-series production runs. In addition, the gear-cutting knowledge that was gained offers valuable insight for other machining processes, which unlocks considerable potential for significant future results.
“If we do not constantly improve ourselves, we lose,” according to Dieter Ruppe, workshop master at Voith Turbo in Heidenheim, which is located in the German state of Baden-Württemberg. His sentiment is also visible on the factory floor. The Drive Technology Division of the Voith Group not only produces movement, it is also dynamic in its own right. At the headquarters facility alone 3,500 employees ensure low friction losses and maximum drive. Over the last 140 years the core competence of Voith Turbo has developed into the production of hydrodynamic gear units for rail vehicles and commercial vehicles in the high-performance range. The basic conditions for production are difficult. Customer requests such as low price and high reliability, as well as availability of spare parts over a period of more than 20 years, must all be considered. These customer requests are implemented within the structures of a large company, however, and with lot sizes ranging from one to 50 there is an oversized job-shop feel to the enterprise as well.
Shorter Lead Times
An essential aspect of customer satisfaction is the delivery time of the transmissions. The company attempts to meet the requirement for shorter throughput times through advances in technology and lean manufacturing principles. Although a medium lot size of 15 quickly dispels thoughts of highly automated large-scale production. After all, the variety of toothed parts at Voith Turbo is enormous. In total there are 12,000 different types of toothed parts, ranging in diameter from approx. 50 mm to 2,000 mm. Johannes Wolfensberger, workshop director, cites the difficulties in this regard. “We move between pure single-part manufacturing and series production, and consequently we can only use current automation methods to a limited extent. In addition, we must guarantee availability of spare parts over a period of more than 20 years.” (Figure 1)
Competence and Reliability
However, Wolfensberger does not view an orientation to core competencies as an across the board reduction of manufacturing depth. “Today, as in the past, we have an extremely high level of manufacturing depth of over 50 percent,” he says. “Whether we increase or reduce this depth always depends on the question ‘Can we offer the machining or the process profitably and does the work step fit our profile?’ Although we manufacture practically all the parts ourselves, we differentiate very precisely. Individual machining steps are outsourced, others are reintegrated. Thus we are a specialist, but a specialist who nonetheless does not need to be able to do everything. Outsourcing means competent and reliable partners. This is why the circle of candidates is limited to a few. Moreover, the efforts associated with quality assurance and logistics are not insignificant.” (Figure 2)
The large variety of lot sizes and part sizes, as well as numerous tooth shapes and types, also require a significant effort relative to part-specific fixture construction. “The fact that we can manufacture all of the parts ourselves is a great benefit to us,” Wolfensberger says. “However, it is also our handicap. We must expend enormous effort on the fixture construction side. Part-specific fixtures must be designed, manufactured, and stored. In addition to an increase in productivity, the specification also cites a reduction in the scope of fixtures, along with faster availability. For simple spur gears this can be eight to 12 weeks, and for complex special variants it can be up to eight months.”
To improve order flow, the numerous gear variants are organized into parts families. The manufacturing orders are given to the machines as a package, while the fine scheduling of these work orders resides in the machine operator’s responsibility. Additionally, the number of work steps are consolidated on the appropriate machines with multitasking capability. Integration of so-called subordinate process steps such as parts cleaning, deburring, or marking is an essential means of shortening throughput times. Valuable time is lost for transport, storage, signing on and signing off these work sequences. “Handling these steps in a distributed manner at the machine significantly reduces our throughput times,” Wolfensberger explains. The organizational restructuring appears to be particularly worthwhile, he adds, because the machining time comprises just 5-15 percent of the actual throughput time. The rest of the time is comprised of standstill and non-productive machine times before the actual machining steps. (Figure 3)
The costs were adding up, and not only for design, manufacturing, and storage of the fixtures, but also the maintenance and transport costs combined to increase the pressure to change. Thus creation and optimization of the fixtures would negatively impact the throughput times of the gears. As Wolfensberger describes the problem, the customers no longer accepted the fact that they had to wait longer for the gear than the time it would take to manufacture the overall system.
Ultimately, new investment in a gear hobbing machine was the spark for an essential reduction of setup times and other machine idle times. Both the capacity of the machine, as well as the cutting data recommended by the tool manufacturers, promised a significant increase in productivity. However, to realize these productivity gains a new clamping approach had to be found. Good collaboration with the machine tool manufacturer was an advantage for Wolfensberger and his team, as the machine tool manufacturer was able to contribute helpful tips for the selection of the clamping device. Wolfensberger emphasized the significance of the clamping device by pointing out that in the discussions with the machine manufacturer’s fixture designers, selection of the optimal clamping device actually played a central role. The sought-for solution as a type of productivity multiplier had to satisfy the following requirements:
• Universal use for a broad spectrum of parts;
• Process reliability;
• Automation capability;
• User-friendliness and safety;
• Short delivery times;
• Good price/performance ratio.
Motivated by recommendations from the machine supplier, Voith engineers attended the HAINBUCH Technology Forum in Marbach. While reviewing the vast array of workholding technology that was available, the requirements list was further reinforced in favor of the MANOK stationary chuck. “I was really surprised by the versatility of the clamping solutions,” Ruppe says, “and also by how easy and quickly modern clamping devices can work today.” (Figure 4)
The MANOK stationary chuck not only satisfied the customer’s requirements, but as a supplemental benefit the clamping device could also provide each individual clamping head with a high clamping force of 70 kN and a broad clamping range of ± 0.3 mm. This means that exact fits for each workpiece are no longer required. Only roundness and concentricity are required on the clamping shoulder of the workpiece. The clamping device was able to fully satisfy the technical requirements, and it was also able to increase the machining parameters. “Earlier, we could often not achieve the recommendations of the tool manufacturers,” Wolfensberger recalls. “Frequently this was due to a clamping device that was too unstable. In addition, the vibrations in the machining process significantly lowered the service life of the tools. The recommendations can only be realized via an optimized total system consisting of a machine, cutting tool, and clamping device. In this specific case the machining time was reduced by 42 percent.”
In addition to considerable improvements in the milling process, the stationary chuck is also particularly impressive in part handling. “Setup times can be reduced by as much as 80 percent, plus there are numerous positive side effects,” Wolfensberger says, with Ruppe adding that “The clamping device functions simply, reliably, and quickly, and this has had a very positive effect, particularly in three-shift or night-shift operation. Uncomplicated handling reduced susceptibility to faults and increased process reliability.
“The tasks for the craftsman at the machine are constantly increasing, the craftsman has a lot of responsibility,” he says. “Consequently, simple and practical solutions that quickly reach the goal are in demand.” (Figure 5)
Significant savings also occur in the construction, manufacturing, and storage of the clamping devices. Wolfensberger thinks it would be possible to cut this cost pool in half if the new clamping is consistently implemented, and he and his colleagues will not rest on these isolated successes. “We are thinking of transferring this workholding to other machining steps, such as turning or grinding.”
Wolfensberger considers the balance between flexibility and standardization to be successful, which is positively demonstrated through the 20 percent reduction in throughput times for gear cutting. “Nevertheless, we are just getting started,” he says. “The overall potential is much greater. Whoever invests in a machine and stays with the old clamping methods does not get any tangible improvements, but rather squanders the potential that would have been possible through the active investment in machine, cutting tool, and workholding technologies.”