Product category:
Flexible machining cells and systems (FMS)
News Release from: Mike Page - editor's feature articles | Subject: Transfer lines
Edited by the Manufacturingtalk Editorial
Team on 04 June 2004
Transfer lines are still the high volume
producers
Machining cell based systems are answering the automotive industry's demands for more flexibility in machining component batches but transfer lines are still the high volume producers.
Machining cell based systems are answering the automotive industry's demands for more flexibility in machining lower volumes of large component variants The transfer line concept has been reconfigured to bring in more flexibility and still remains the most effective solution for large volume, continuous production of limited variants, reports Mike Page
This article was originally published on Manufacturingtalk on 29 Oct 2001 at 8.00am (UK)
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To many mechanically minded manufacturing engineers, the 'classical' transfer line has always had an appeal.
One could admire the 'classic' transfer line's steady, progressive, efficient and relentless production of cylinder heads, cylinder blocks, crank cases and crankshafts.
Even the smaller rotary systems that machined water pumps and hydraulic system parts presented that solid routine of converting raw aluminium or iron castings into precisely machined, ready-to-assemble parts.
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Limitations - the limitation of transfer lines lay in their rigidity and devotion to one, or a closely-knit, small, like 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 - loosing a shift or more of production time.
Transfer lines have been associated with the automotive industry since the 1930s.
As long as 'bean-counters' (accountants) did not worry about production inventory in 'buffer' stocks by the line, and in warehouses serving the assembly lines, then the steady routine of the transfer line (TL) served the purpose.
Adding flexibility - flexibility, as we know it today, crept in with the 'flexible manufacturing system' (FMS) ideas of the late 1960s and through the 1970s.
Machining centres - first conceived in the late 1950s - became faster and more reliable.
The first FMS installations grew among the machine tool builders themselves and overflowed into medium/large batch produced components such as tractor transmission housings, water pumps, mining machinery components, large electric motors and so on.
The automotive industry took only a passing interest as volume production was its main concern, and the TL was seen as the best way of machining lots of parts.
In the 1980s, OEM costs began to be driven down, and production inventory (work-in-progress) stocks got a bad name.
However, manufacturing engineers were reluctant to let go their TLs.
For example, an UK tractor plant, in 1987, operated a 30-plus year old Cincinnati TL, which had undergone maybe ten refurbishments and retoolings as tractor engines changed.
To some, the classical TL had become a dinosaur.
Machining centres replace transfer lines - the Waterloo (Iowa), USA, plant of John Deere operated TLs - some up to 30 years old - and reckoned that they took up too much floor space.
Their operation demanded queues of castings and the holding of large production inventories.
Any tooling changeover took a long time, let alone changing a line for a different casting.
These were the reasons why John Deere invested some $100 million 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 (World: www.moriseiki.co.jp USA: www.moriseiki.com UK: www.pollardfredk.com ), with substantial tooling magazines and dedicated pallet transportation and storage systems replaced the TLs.
The cells machine, according to assembly line demand, 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 cells run on three shifts, generally on a five-day week.
That the cells save space is illustrated by Cell 1.
It occupies 8000ft2 and has replaced three TLs occupying 64,000ft2.
The TLs worked on the bases of 40 days' inventory compared with just two days on the FMCs.
All-in-all, the FMCs have reduced machining cycle times from 10 to 40%, depending upon the complexity of the parts machined.
It could be said that John Deere's manufacturing engineers took a 'quantum leap' from TLs to FMCs.
Apart from the wish to go 'lean' and machine on demand, parts variety had increased.
For example, 24 new tractor models were produced in one year, 2001, compared with a handful when the TLs were first installed.
Not going out of fashion - the transition one has seen at John Deere, from TLs to FMCs, does not mean that TLs are 'going out of fashion'.
Where continuous delivery of one, or a 'like family' of machined parts is required - say over 250,000/year cast iron (CI) or 300,000/year aluminium alloy engine blocks - then, the 'flexible' TL, or for larger call-offs, the dedicated TL remains the best answer.
Two former competitors, in the USA and Germany - The Cross Company and Huller Hille - are now members of the German Thyssen group (www.thyssenkrupp-metalcutting.com/ ).
The UK company of Cross Huller said that automotive OEMs tend to have similar production capabilities - not in terms of production volumes - but similarities in terms of machining and control standards, reliability, and so on.
Then local influences, such as environmental concerns and labour skills, influence detail machining system specification.
The world has changed as regards high volume machining, said Cross Huller.
Production is dispersed more.
The OEM requirement might be for 1 million engine blocks/year, but may be divided among four plants, at 250,000 blocks/year each.
Therefore a TL may not be cost-effective.
So what do you do with reference to flexibility? Today's TL is not the same 'kit' as, say, 30 years ago.
Production requirements are down and cycle times are up, so the tendency is to make more 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.
In the modern TL, said Cross Huller, automatic multi-spindle head-changing modules cope with different hole patterns.
Gebruder Heller in Nurtingen, Germany (www.heller-machinetools.com) builds TLs and machining centres.
'Flexible' or 'agile' systems based on machining centres cater for production volumes up to 250,000-300,000 parts/year, with variations according to whether parts a re cast iron or aluminium.
A lot of lines supplied by Heller are mixed TL and machining centres, or TLs with automatic multi-spindle head-changing modules.
The most dramatic changes seen in TL design have been aimed at achieving reconfiguration.
Wide use is made of 1-3 axis machining modules, sometimes a fourth axis on the machine module bed, while achieving a commonality in design in terms services and so on.
Heller has always considered itself to be proficient in milling, as demonstrated by a TL designed for the machining of sawn and faced square section steel billets, with hardness to 64 HRC.
The product is common rail pump housing mounted four or eight per fixture.
Guhring (www.guehring.de) and Mapal (www.mapal.com) tooling is used to rough out and finish bores under flood coolant conditions.
While the TL is carrying out these operations, a system based on 26 machining centres is employed in machining castings for common rail pump drive housings.
Examples of 'hybrid' TLs delivered by Heller include two lines for BMW to machine 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 multi-spindle head changers in the TLs have a chip-to-chip time of 12 sec.
Interestingly; some OEMs are trying to eliminate deep-hole drilling in engine blocks by using steel tube inserts cast in during the moulding process.
Connecting rods - the TL is still the main tool for producing con-rods.
Alfing Kessler Sondermaschinen (AKS) builds TLs and machining centre systems (www.alfing.de).
For con-rods, today's TL combines rough boring, honing, notching, fracturing, re-assembly and fine boring.
Latest systems use laser notching in conjunction with Lasag.
Laser notching is now standard for steel conrods, generally forged steel grade C70.
Sintered steel conrods also fracture satisfyingly; AKS has supplied several such lines to machine, fracture and finish machine powder metallurgy (PM) con rods to US OEMs.
AKS commented that it is getting into discussions with OEMs at early design stages and is able to suggest design changes to bring down machining costs.
Most lines accept palletised castings for automatic loading into the TL, coping typically with outputs of 500 conrods/h.
TLs also feature in crankshaft and camshaft production for the drilling and deep hole drilling of oil holes, drilling and tapping and end-machining and balancing.
Etxe-Tar (www.etxe-tar.com), Spain, has supplied many such lines, interfaced with Schenck balancing systems (www.schenck.net) to most European OEMs.
OEMs are asking for more flexibility, for which Etxe-Tar has developed 1-3 axis CNC modules, say to cope with different lengths of one family of crankshaft.
The company has developed a robot-served rotary transfer machine to machine flywheels.
Another trend is that automotive foundries are requesting 'cubing' lines and Etxe-Tar has delivered such lines to the Spanish Victorio Luzuriaga foundry (Fagor group), Renault's Valladolid engine plant and to DaimlerChrysler and Ford foundries in the USA.
Control systems - coming back to the John Deere plant and the observation that 8000ft2 of 2003 technology FMCs had replaced 64,000ft of 30-year-old TLs and associated production inventory space, the 2003 TL is a very different set-up compared with 30 years ago.
Cross Huller said that the biggest change in TLs is the architecture.
The TL of 35 years ago was hard-wired, with electro-mechanical relays and rows of large electrical cabinets by the line or up in a gallery.
Now, all control is decentralised.
It copes with machining variations at each station.
There is more control complexity and to prevent control costs from spiralling companies standardise.
Clearly defined modules are used for each axis situation.
For example, a 3-axis unit has three standard axes, not a 'special' 3-axis unit.
Also system designers have gone from the 'bespoke' design - where all units are different and customised - to 'platforms'.
The platforms offer limited choices, but there is much more scope for mixing.
In the same vein, Rockwell Automation (www.rockwellautomation.com) noted that the reduction in control cabinet space gives better visibility.
Control cabinets have halved in size over the last 20-25 years while PLCs are a quarter of the size compared with 20 years ago.
The specifier has also the choice of asking for 'on machine' solutions using IP67 devices and I/O modules all 'bolted on' to the machine are networked, not cabled, and systems are quicker to install.
Depending upon the TL supplier, networking systems can be based on Profibus (generally Siemens), DeviceNet or Interbus.
Safety systems are often based on Pilz or ASI Safe systems.
There is the question of standards too.
OEMs want systems in their plants to 'look and feel' the same for all machining systems and develop software and man-machine-interface standards.
The more 'switched on' users will work with control system hardware/software suppliers to define these standards, said Rockwell.
Rockwell uses automatically generated software techniques producing process flow chart, process flow, generating codes and so get all screens to 'feel' the same in terms of diagnostics, tool management.
Every automotive OEM has different ideas and a favourite network.
But the key thing is to link up these systems to the OEMs' IT systems.
Summing up - the development of machining centres, and latterly 'highly dynamic', high speed ballscrew or linear motor powered axis machining centres, has seen FMCs and 'agile' systems (particularly the robot or gantry loading of workpieces directly to 'intelligent' fixtures in machining centres) replace traditional TLs.
Generally speaking, where small to large batches of different variants in components, instead of a continuous volume, are required then the FMC or agile system takes over.
Such machining centres have become integrated into TLs to develop 'hybrid' TLs, the 'highly dynamic' machining centres replacing multi-spindle units or coping with variants.
Such machining centres include, for example, Cross Huller's 'Specht', PCI's 'Tripteor 8', Cincinnati Lamb's 'Bobcat' or Fatronik's 'Ulysses'.
These machines feature a high-speed spindle positioning facility such that a more reliable single-spindle machine with toolchanger of 'sister' tools can replace a multi-spindle head module.
A relatively new competitor to the TL is the Opick-up spindle1 vertical turning centre capable of machining prismatic components.
Emag, Germany (www.emag.com), has demonstrated this in the machining of crankshafts and transmission housings.
Overall, OEMs in the 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.
Mike Page.
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