Flexible Manufacturing has transformed the traditional transfer line and machine providers are now offering cell-based, hybrid systems to meet the demands of cost saving and batch production. AMS looks at Heller’s Module Line System
The age of the traditional transfer line is over. The huge monolithic beasts that sat in lines at automotive OEMs are now a dying breed, consigned to extinction by the flexible demands of modern manufacturing. Conceived in an age when cost-per-part was king, with no other consideration involved, they ruled manufacturing shopfloors. It is now the turn of cell-based systems to answer industry’s demands for more flexibility in machining lower volumes of large components. Yet those experienced in transfer line concepts will recognise many of the old components in the newer variants, albeit in slimmed down and more flexible versions.
Need for flexibility
“The transfer line for us is really dead,” says Geoff Lloyd, Managing Director of Heller UK. “By that I mean that the dedicated machine, the one that cost millions, producing the same part day in and day out for anything up to 20 years. With engine parts today, that doesn’t really happen any longer. Engine parts have to change to meet market requirements, emissions being the big driver. Nobody today has an engine programme that lasts that long, so if you are going to invest capital to make engine parts, you need a fairly high level of flexibility.” The limitation of transfer lines lay in their rigidity and devotion to one, or a closely-knit family of components. To change from one variant to another meant emptying the line, manual tool changing and verification, and then gradually filling the line again – losing a shift or more of production time.
Whether the line is a transfer line, or a flexible transfer line, how the part is moved along the line between stations is crucial. “It’s very sequential and depending on the part, it could be an overhead gantry, or it could be a walking beam mechanism,” Lloyd says. “These were the old tried and tested means. Or it could be a pallet transfer, meaning that you load the parts onto a pallet and you have got multiple pallets going through the system. Then there is a pallet return, the parts unloaded at the end of the line and the empty pallets are brought back to be reloaded. Heller did quite a lot of pallet transfer systems.
“That was the transfer line and the workpiece would reach a station and, depending on what operation needed to be done, there would be a large milling spindle or a boring spindle, or even a multi-spindle head to drill lots of holes on the face of a cylinder head or a cylinder block. “If you had an hour and you had a three minute cycle time, you would know straight away that you needed 20 stations to be able to produce the part in that time.”
Transfer lines have been associated with the automotive industry since the thirties. In the days of huge buffer stock packing warehouses and the production line, the steady routine of the transfer line (TL) served the purpose. But then came the f-word: flexibility. The Flexible Manufacturing System (FMS) that was introduced to the automotive industry in the late sixties and early seventies changed everything. Machining centres, first conceived in the late fifties, became faster and more reliable. The first FMS installations grew among the machine tool builders and overflowed into medium/large, batch-produced components such as tractor transmission housings, water pumps, mining machinery components and large electric motors.
“The improved efficiency of the machine tool is a driver. The machine centre is quicker and more dynamic, and in some cases more durable and more capable,” says Lloyd. “We have a transfer line history and a new history, if you like, on making flexible modules, but it is important that the integrity of the machine is very much like it used to be. If it is going to work in an automotive plant, whether it be a transfer line or a machining centre, you have to expect it to work 24/7; it isn’t far off from that these days.” The automotive industry took only a passing interest in machine tooling, with volume production its main concern and transfer lines considered the best way of machining volume parts. Yet the 1980s saw OEM costs being driven down, with production inventory (work-in-progress) stocks getting a bad name. However, manufacturing engineers were reluctant to dispense with their highly-productive transfer lines, despite the fact that a networked and fully-integrated system of flexible machine tools, linked together in an almost-infinitely variable pattern, could return maximum versatility in production.
Flexible transfer lines
The car, once a status symbol of the wealthy and the adventurer, has become a virtually disposable fashion item. This shift has also played its part in rendering the transfer line technology effectively redundant. Carmakers now face a dilemma. They must pre-empt fashion trends, offer a variety of models and still meet new emissions regulations. Further, if they cannot bring a new engine to market at a low price and within a minimal timeframe, the company stands the chance of quite simply being dead in the water. To achieve these goals, manufacturers must have a production system that provides a minimum time to market, while allowing frequent changes in production volume, changes in component design, different process sequences and different part flow without incurring a part-cost penalty. The transfer line cannot achieve that: first developed as an inexpensive way to produce millions of identical components, its very success as a means of cheap volume production has brought about its demise. Yet highly successful in the context of lower volumes, machining centres alone are not the choice. The ideal manufacturing system, then, is one that offers high productivity, high volumes and high flexibility, all at low cost.
Matching industry demands
“Customers want to achieve minimal per piece costs,” says Bernd Zapf, Head of Development at Heller. “Two areas that require optimisation can be derived from this: firstly, the lifecycle costs consisting of investment costs and operating costs must be minimised; and secondly, the production volume should be maximised. On these we mirrored new and existing technologies and then assessed their contribution towards the respective objective. “In the past, development work was often not properly aligned to the actual requirements of the industry, and in this respect top performance with a view to certain technical data was plainly overvalued. True to the motto ‘a lot helps a lot’, revs, speeds and acceleration rates were improved but then seldom achieved in practice. Initially that may not necessarily be a disadvantage. If, however, technical progress adversely affects the life expectancy of the respective components, it is ultimately the customer who carries the burden.”
Heller has combined its understanding of transfer line and machining centre technology with its knowledge of the automotive industry and created a new production system which easily meets the demand for maximum versatility and minimum piece part cost – the Module Line System (MLS) from Heller meets all these requirements. “MLS is not a further development of the current level of technology but a completely new development,” Zapf continues. “The background to this strategic decision is the increased dynamics of production our customers have to cope with.
“The flexibility of a system is becoming increasingly important, but this must not be allowed to adversely affect productivity or per piece costs. Conventional systems are consistently set up so as to minimise per piece costs of an individual work piece. The drawback is, however, that they offer hardly any flexibility with respect to changes to the component, or changes in the production volume.”
MLS is a cleverly networked and fully integrated system composed of flexible machines, linked together in an almost infinitely variable pattern to give maximum versatility in production. It even allows the production facility to be built while the engine design is still fluid. Each machine forms one independent, but fully integrated, module, linked by a fully-automated parts and tool handling system, and a single integrated system controller. Heller offers its customers a diverse range of solutions best suited to the given application. These include overhead gantry or robots, floor-mounted robots, slide-mounted robots, or the option to manually load each module with scope for upgrading to automated methods later. This allows the greatest possible yield to be achieved at the least possible expense by increased machine running times, reduced idle times, flexible component flow, optimal machine utilisation, and high output, even in a reconfigured or only partially-utilised operation.
The MLS control combines the capability of each machine with the process sequence for each component to create the perfect match. Using adaptable machining modules, components are conveyed from station to station by centrally-automated, traction-less carriages. Components can skip stations as the machining sequence dictates. Tools are handled by the central control system and not necessarily tied to a specific machine. A component buffer is integrated into the MLS and intelligent component logistics ensure that each machine gets the next component while machining the current one.
The machine itself is then able to determine when to call up the next component. The optimal combination of tool change and tool magazine operation gives minimal idle time penalty: extremely fast chip-to-chip times of 2.4 seconds is combined with zero-tool-waiting time because the tool changes directly into the operating area. Flexible production modules machine the components on five sides in one clamping. In addition, the MLS can be reconfigured easily and inexpensively for batch components or extended to increase volumes. Floor space saving is more than a third compared with a machining centre system.
The success of the MLS lies in the approach Heller took to the concept of high volume, low cost production. Rather than continuing the over-worked notion of ‘faster is always better’, its engineers sought a completely new perspective, appreciating that ‘flat-out running’ can create at least as many problems as it solves. By optimising the performance of each element within the production process, tool handling for example, a significant decrease in overall cycle time is achieved.
“We wanted to tread a new path so we started by taking another careful look at the market conditions within the automotive industry,” Zapf notes. “The outcome: short innovation cycles, growing model variety and constant fluctuations in demand. This resulted in a very abstract, but very clear input for our work. The system needed to be tailored to the high-variety production of prismatic components, with due regard to current production conditions. It had to offer high-flexibility, coupled with lower costs per component. What’s more, there was also a very ambitious time plan: the prototype needed to be ready after just 24 months.
“Successful completion of a development task of this nature, within such a tight timeframe, can of course only be achieved by a highly-qualified project team. We therefore released 40 experts working in the respective departments from their day-to-day activities and assigned them to the project. Together we then began to derive concrete tasks from the abstract definition of the goal. The basis of all deliberations was the TCO principle (Total Cost of Ownership), by which the productivity is considered over the entire lifecycle of the system.
“At the beginning of the project we started an investigation into the effect of certain performance values on the overall result, during which we ascertained that an improvement of the axis dynamics, of jerk and rapid traverse, only equated to around 1 to 2 per cent higher productivity. On the other hand, an improvement in tool availability actually results in a 6 per cent increase. Taking potentially higher maintenance costs for high tech components into account, the pure dynamic advantage to the operator can ultimately also mean a disadvantage.
“Our main concern is not top performance but rather continuous performance, although our performance figures of 60m/min rapid traverse and acceleration rates of 8m/ s2 in all axes are absolutely respectable. With a view to lower operation costs, our focus with MLS lies on lowmaintenance or maintenance-free components, as well as on the optimisation of rapid traverse and acceleration in order to prevent excessive wear and tear. We also work, for instance, with linear direct drives in the automation system. The newly-developed linear motor with passive secondary part is insensitive to chips. The power is supplied via contact rails and the signal transmission is wireless. In this situation, high-maintenance trailing cables are a thing of the past. The fact that a transfer unit in threeshift operation can travel up to 10,000 km a year provides enormous savings potential that is often only recognised by the controlling department.”
Not all the technology in the system is new. It is an evolution of products while being a revolution of the process. “The machining modules are further developments of our current centres,” Zapf explains. “These include the single spindle MC10 and MC20, the MCT10 as a twinspindle module, as well as the MC200, MPC200 and MP200 as process modules for specialised tasks. The special aspect of the entire series is the use of a modular system. Individual functional units, such as the tool magazine, tool changer and work piece changer can be used across different machines and construction sizes. For the customer this also results in, amongst other things, a considerably more streamlined spare parts holding.
“The special design of the machining units is crucial to the huge productivity of the MLS. Our newly-developed rotating/swivel table enables five-sided machining of the work piece without having to reclamp it. It is equipped with high-torque direct drives in the A and B axes and travels on two guideways in the Z axis that lie outside the work area. This means that there are neither restrictions on tool length nor on work piece size. All machining modules feature a quick and integrated work piece changer that decouples the work area from the automation. With respect to the tool-totool time, the tool change system is also a completely new development. In order to minimise the machine width, we have moved the tool magazine as a chain magazine over the spindle. It operates extremely reliably in this position and offers exceptionally short tool-to-tool times. In concrete terms this means that there is no spindle waiting time in case of low tool operation time. Furthermore, all machines were consistently designed for dry machining, which for cost reasons has become a standard requirement in the automotive sector.
Flex flow extension
“Thanks to the linear material transport unit and the almost 30 per cent reduction in space requirement compared to a conventional system, all machines are linked to one another via very short paths. One automation system is sufficient for an entire installation. There is no rigid connection between the individual machining modules, just a flexible supply and distribution mechanism. “Thanks to the so-called ‘flex flow’ principle, the basic system remains open at all times for extensions, which are simply docked onto the existing installation. In individual cases, an extension of the linear unit, as well as the addition of a further machining module, is sufficient to expand the capacity. For cases of falling volumes, the system offers the possibility of utilising individual machining units for other purposes without disturbing the flow of material.”
New methods for machining
One example of a flexible cell in action can be found at Perkins Engines, a diesel engine manufacturer based in Peterborough, UK. The company’s most recent investment for prismatic parts, the cylinder head, was a Heller MLS line. “Now that is the future; that is where they are going,” Lloyd says. “They are actually parting company with their transfer lines. As new products come into being – new engines and new designs – they are going to employ more and more flexible methods.
“And within a given work envelope, if you have got a machining centre say, and on the pallet you can typically put a component that fits into a 630 cubic shape, or an 800 cubic shape, then that is the criteria. So they know that they can make today’s engine and, provided that the designer doesn’t go crazy and make it ridiculously big, then it will fit. So from engine changes they will always go on the same machines and that’s another very big driver. Floor space is also very important to these guys today; they don’t want enormous transfer lines that uses copious amounts of floor space.”
There are other options available to OEMs looking to replace transfer lines. At its Iowa plant, John Deere’s setup required queues of castings and the holding of large production inventories on its production line. Any tooling changeover took considerable time, let alone changing a line for a different casting. This is why the company invested approximately $100m to update its drivetrain component machining operations and, at the same time, to adopt a lean manufacturing strategy.
Three flexible manufacturing cells (FMC), each made up of four or more Mori Seiki MH-633 horizontal machining centres, with substantial tooling magazines and dedicated pallet transportation and storage systems, replaced the TLs. According to assembly line demand, the cells machine produces families of drivetrain parts for John Deere’s 7000, 8000 and 9000 series of tractors. Under normal working, Cell 1, with five HMCs, machines axle housings. Cell 2, with six HMCs, produces pump drive housings, range box housings and transmission manifolds. Cell 3 operates four HMCs to machine transmission covers and power take-off housings.
All-in-all, the FMCs have reduced machining cycle times from between 10 and 40 per cent, depending upon the complexity of the parts machined. Still, where continuous delivery of one, or a ‘like family’ of machined parts is required – at quantities over 250,000/year cast iron or 300,000/year aluminium alloy engine blocks – then, the flexible TL, or for larger call-offs the dedicated TL, remains the best answer.
Standards and specification
According to machining provider Cross Hüller, automotive OEMs tend to have similar production capabilities, not in terms of production volumes, but in terms of machining, control standards and reliability. Local influences, such as environmental concerns and labour skills, further influence system specification.
Production requirements are down and cycle times are up, so the tendency is to make efficient use of time through more flexible, multi-axis machining modules. TLs deal specifically with ‘tight’ families of parts that have a small range of product variation. For example, a cylinder block range with different heights of head face, cylinder bores, crank bores and bearing supports. Meanwhile, Heller has delivered hybrid transfer lines to two BMW operations machining V6 and V8 blocks. There are machining centres with 3-axis heads at the end of each line to cope with design modifications made to the blocks and cover variations in machining operations dictated by block specifications for several car models. The multispindle head changers in the TLs have a chip-to-chip time of 12 seconds.
Hybrid transfer lines
Generally speaking, where small to large batches of different variants in components, instead of a continuous volume, are required, then the FMC or agile systems take over. Such machining centres have become integrated into TLs to develop hybrid TLs, the highly dynamic machining centres replacing multi-spindle units to cope with variants. These machines feature a high-speed spindle positioning facility that gives a more reliable single-spindle machine with a toolchanger of sister tools that can replace a multi-spindle head module.
Overall, OEMs in developed countries still want TLs – though no longer the simple transfer line – while developing countries and low-labour-cost countries still specify the low-cost transfer line option.