Milling operations face the challenge of high volume and quality requirements in the automotive industry
Supplying machined parts to the automotive industry brings a unique set of challenges, due largely to the combination of high volumes and meticulous quality demands. The problem is exacerbated in milling operations by the prismatic nature of machining. Here, only carefully considered machines, fixtures and tooling can provide the recipe for the successful production of automotive parts.
A case in point can be seen at the Welshpool, UK, plant of aluminium diecaster CastAlum, which started adding value to its products by machining castings. The development was triggered by the award of two major, long-term automotive contracts for the supply of machined steering-gear housings and transmission cases.
The projected production volumes from the two new contracts, which this year will see CastAlum machine 700,000 castings, warranted consideration of a transfer line. However, the company says it would have been difficult to adapt if the automotive components were to change in future. A transfer line would also have meant a substantial investment before the volumes ramped up.
Progressive installation of HMCs
The sequenced purchase of twin-pallet, four-axis horizontal machining centres (HMCs) was deemed to be the answer, as they could be installed progressively to suit rising throughput and are easy to reconfigure for almost any component. CastAlum opted for Heller H2000 models in combination with tooling supplied by Mapal, which audits CastAlum’s tooling cabinets twice weekly to ensure continuous availability.
The production hall at CastAlum has ten Heller H2000 HMCs
Castings arrive from the foundry mounted on carriers that travel on a long, U-shaped chain conveyor running past all of the H2000s, creating a lean manufacturing environment in a tight footprint. Each carrier has a chip that identifies the type of casting and the intended destination machine, automatically diverting the castings into buffer areas on both sides of the line, which enables operator access.
A raw casting is loaded manually into a fixture on a second machine pallet, ready for the first operation once a previous, fully machined part has been unloaded and a part-machined component has been relocated into another fixture for the second operation. Both operations are performed on two different components per pallet every time it visits the spindle. So after a pallet index, a finish-machined casting comes off and is sent via the conveyor to the inspection department, and on to dispatch.
“The Heller machining centres have an average uptime of 95%, which is very high for manually loaded, twin-pallet machines,” says Keith Brown, CastAlum’s managing director. “It’s due to the efficient presentation of material to the second pallets and from there to the spindles, which minimises changeover times. The machines work around the clock, six days a week, holding tolerances down to 10µm with a process capability of 1.67Cpk.”
Reducing throughput time
Another UK subcontract manufacturer, BCW Engineering of Burnley, also supports a seemingly growing preference for horizontal-spindle machining centres. The company recently invested in an Okuma MB5000H HMC with a ten-pallet pool to add capacity for producing precision automotive components.
“The reduction we are seeing in throughput times is partly due to being able to present a component on its pallet to the spindle more quickly as it comes out of the pallet pool already in its fixture, so set-up does not delay the start of the next cycle,” explains Tony Kilfoyle, group engineering director. “Changeover is therefore limited to the automatic pallet change time, which is no more than 35 seconds. Once the part is being machined, the cycle is also quicker because idle times are short due to the 60metre/min rapids in the linear axes and 2.5 seconds tool change. Additionally, the in-cut elements of the cycle are shorter, as metal removal is fast with the 15,000rpm spindle.”
"The level of quality and degree of innovation is extremely high in this industry, so we must keep pace from a manufacturing perspective in order to remain competitive" – Thomas Kaiser, Kaiser Werkzeugbau
Large reductions in machining times are evident across many automotive parts produced in the cell at BCW for customers such as Jaguar Land Rover, Ford and Aston Martin. The subcontractor is a first-tier supplier to many of them, developing and supplying prototypes, as well as delivering production components to tight schedules.
The machine was supplied by UK agent NCMT, with the ten-pallet cell featuring a number of options including a full NC rotary table, a 218-position tool magazine and high-pressure coolant delivery at 70 bar. Provision of two extra pallets was included in the purchase, enabling 12 different fixtures to be set up at any one time. A Renishaw OMP60 touch probe for workpiece datuming and gauging has also been fitted, as well as a TS34 table-mounted probe for tool breakage detection and automatic feedback of tool-length offsets. A swarf management system and mist extraction unit have been installed at the rear of the machine.
A modular approach
The alternative to HMCs is to take a more modular approach using multiple spindles simultaneously. Modular machining centres such as the Mikron Multistep XT-200 produce prismatic parts in a single clamping with short chip-to-chip times, typically less than one second. Furthermore, the system can be retooled in just a few minutes, with up to four machining spindles in use at the same time. The upshot is lower cost per part, by circa 35% in the case of a German manufacturer that produces 500,000 turbocharger housings in five different variants every year.
Batch sizes of the turbocharger housings vary between 500 and 10,000. As a result, machine operators have to retool at frequent intervals. With the previous machines, this used to take 80 minutes, but today, with the Multistep XT-200, 20 minutes is sufficient. In addition, there were previously 12 employees operating four machines in a 300 sq.m floor area. The machines were multi-spindle machining centres and automatic lathes, and workpieces had to be clamped twice. Now, the German company produces the turbocharger casings with just six employees and two machines in half the floorspace. All of these factors have helped to lower cost per part by 35%.
The Multistep XT-200 machines turbocharger housings on five-and-a-half sides in one set-up. Depending on the variant, between 30 and 40 machining steps, such as turning, milling and drilling, are necessary, with tolerances of ±25µm and a process capability of 1.67Cpk. According to Mikron, other successful applications of this technology include ABS systems, turbocharger impellers, nozzle holders for common-rail injection systems, hydraulic-pump housings and heatsinks.