Fast machining of aluminium die castings
Fast machining of aluminium die castings
Speed is of the essence when machining automotive parts from aluminium die castings, as the industry is competitive and margins are tight. Spindle power is not the issue, as the depth of material removed is typically one millimetre. It is high axis feed rates that are essential to enable the cutter to get around the castings quickly.The largest aluminium die casters in the UK and a first-tier supplier to the automotive industry, JVM Castings, has found that for both large and small components, machining centres supplied by Whitehouse Machine Tools are ideal.
OKK meets the requirements for large castings To machine products at JVM’s Worcester factory from larger die casting machines rated up to 2,700 tonnes, two OKK HP400 horizontal machining centres with 4-axis interpolation, 630 x 620 x 710 mm working envelope and twin pallet changer (2APC) were selected back in 2005.
They were followed in 2007/8 by an identical model and another with an additional rotary 5th axis for positioning. A similarly sized, heavier duty 4-axis HM400 was installed in August 2009, bringing the total to five. Kevin Jones, engineering manager at JVM, said, “The beauty of the HP400 is that it accelerates in the linear axes at around 1g to 60 m/min rapid traverse in all axes, which is fast for a machine of this size with ballscrew drives.
“In addition, the BIG Plus 40-taper spindle is powerful (22 kW / 12,000 rpm) and rigid for a machine of this size and speed.”
He explained that the machine cuts at up to 40 m/min, exchanges tools in one second and completes a pallet change in five seconds. Table rotation through 90 degrees in half a second is effected while automatic tool change is in progress. In-cut productivity is therefore maximised and non-cutting time minimised. One of the OKK horizontal machining centres at Worcester is devoted to producing left- and right-hand body parts such as door frame pillars and transmission tunnel elements.
The other four machines manufacture three varieties of sump for the 5-litre V8 engine powering the Land Rover Discovery, Range Rover Sport and Jaguar XF saloon. Each machine is equipped with automatic, hydraulically actuating, flexible fixturing to enable them to produce any of the three sumps in two operations, one per pallet. Production rate has doubled in the past five years to 100,000 per annum. Despite the latest parts requiring additional operations, cycle times for machining the new, more complex sumps are just 8, 10 and 12 minutes respectively. For the previous, simpler models, however, cycle time was 7.5 minutes, hence the investment in further OKK plant.
The HP400 equipped with the 5th positioning axis has an additional duty, namely machining left- and right-hand cast aluminium shock towers for the Jaguar XF. According to JVM’s manufacturing engineer, Graham Jolliffe, sump length and the flatness of some milled faces have to be within 0.1 mm, ie as tight as the tolerances on the engine block to which the sump will be bolted. A feature of the OKK machines that supports production of high accuracy components is the comprehensive coolant management system, which uses seven individual pumps.
In addition to 20 bar through-tool coolant, there are coolant jets around the spindle, flume wash, bed wash, and a coolant curtain from the ceiling. Media-free filtration with refrigeration avoids temperature variation of the coolant, which is important when cutting aluminium owing to its high coefficient of thermal expansion. Temperature sensors are fitted to monitor the ambient temperature and that of the machine bed.
Accuracy is further promoted by the use of core-chilled, double-anchored ballscrews. Brother provides machining efficiency on smaller castings Following the award in 2008 of a contract to supply half a million machined aluminium brackets for Ford, JVM produced PSW (Part Submission Warrant) batch samples in October last year and will start full production in May 2010. The cast components are a fuel pump bracket and a vacuum pump mounting bracket for the new Ford Sigma diesel engine.
The parts are much smaller than the structural vehicle components produced at Worcester, so needed to be produced on a more compact machine. High speed was again imperative to keep down unit cost of manufacture. In the smaller size range, there are more 2APC machines available that might have fitted the bill, including turret-type drill / tappers and light-duty machining centres. So selection could have been a more difficult process than deciding on the OKK for the larger castings, as the OKK HP400 was clearly superior to other contenders.
However, after comparing axis speed and acceleration across a range of smaller plant, the machine that stood out was the TC-32BN QT from Brother, another Japanese machine tool builder also represented in the UK, coincidentally, by Whitehouse Machine Tools. Mr Jolliffe commented, “We have small, 30-taper vertical machining centres on site from other suppliers, but the Brother machine was notably faster than these and other models on the market that we considered. “Machine elements on the Brother are light so there is low inertia as the axes move, resulting in fast acceleration to 70 m/min rapids in all axes.
“Pallet change is 3.4 seconds and it takes just 2.1 seconds chip-to-chip, which includes time for the spindle to decelerate from a top speed of 16,000 rpm to zero for automatic tool change and accelerate again to maximum revs. Overall, idle times are considerably shorter than on other machines of similar capacity.”
The Brother is also quick when cutting due to the 16,000 rpm spindle, which is fast for this class of machine, and cutting feeds up to 20 m/min. 70 bar through-tool coolant increases drilling and tapping productivity, especially on deeper holes, and dramatically extends tool life.
“Rigid tapping on the TC-32BN is exceptional. We can tap at 8,000 rpm, which is over twice as quick as on other machines we looked at.”
On the subject of tooling, he also commented on Brother's cutter breakage monitoring system within the chain-type magazine, which is important for 24/7 operation where one operator looks after several machines. When the next tool is called up into the ATC position, a mechanical arm checks if the length of the tool matches the parameter in the control, or indeed is present. If not, the cycle stops automatically. As monitoring of the tool is carried out while the next cut is in progress, there is no loss of production.
Supplied as a turnkey package by Whitehouse Machine Tools, the 4-axis Brother cell at Worcester was delivered with proven programs, hydraulic automatic fixtures complete with air sensing, and tooling. Bespoke, trunnion-and-tailstock rotary indexing units are fitted on both pallets to accommodate the two component types. Services are fed through a rotary joint through the centre of the pallet for maximum efficiency.
On one pallet, two sets of two fuel pump brackets are clamped for ops 1 and 2 to be carried out in a 70-second cycle. After APC, op 1 castings are inverted manually into the op 2 positions and fresh castings are clamped for op 1. The indexing table is needed purely for gaining access to the components when machining, and also allows swarf to drop away. On the other pallet, the indexing table presents two sets of three components to the spindle, each of the fixtures being offset by 90 degrees for op 1 and op 2 to be carried out sequentially. Cycle time is 40 seconds. High accuracy is needed on both components, as they mount to the cylinder head and fuel pump, with some flatness tolerances down to 18 microns total.
Polycrystalline diamond tooling is used to achieve the required accuracy and speed. As Mr Jones points out, more and more aluminium is being used in modern vehicles to reduce weight and emissions. JVM is in a good position to increase its business, particularly as body and engine parts these days tend to have a structural function, dictating the use of machined castings rather than pressings. High-speed machining will always be necessary for profitability, however.