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Metal Castings, one of the leading aluminium pressure die casters in Britain and a first-tier supplier to the automotive industry, has identified what it believes to be the best large-capacity, light-duty machine for such applications – the HP400 horizontal machining centre from OKK, Japan, represented in the UK by Whitehouse Machine Tools.

As 80 per cent of Metal Castings' output goes to the highly competitive automotive sector, the ability to maximise in-cut productivity and minimise non-cutting time is crucial to winning business and maintaining margins. Despite having a 630 x 620 x 710 mm working envelope and a pallet swing diameter of 700 mm, the HP400 accelerates at around 1g to 60 m/min rapid traverse in all axes, cuts at up to 40 m/min, exchanges tools in one second and completes a pallet change in five seconds.

Features contributing further to short cycle times are simultaneous interpolation in X, Y, Z and B, and table rotation through 90 degrees in half a second while automatic tool change is taking place.

Two OKK HP400 2APC machines have been installed at Metal Castings' Worcester plant together with a component washing machine and a leak tester to form a production cell, which runs 24 hours a day with one operator per shift. Before the cell was installed, castings machined in-house mainly fell within a 300 mm cube, machining of larger parts being subcontracted out as necessary.

As Metal Castings now has die casting machines up to 2,700 tonnes capacity, the number of large parts such as door frames and gearbox cases has started to increase. A strong in-house machining capability was considered desirable to maintain control over customer deliveries and at the same time gain extra revenue from adding value to the larger castings.

Commented Manufacturing Engineer, Graham Jolliffe, "Pressure die castings used to be produced in very high volumes running into hundreds of thousands over the lifetime of a product. Now runs are generally much smaller due to shorter product life cycles and more frequent design changes, not only in the automotive sector, but also in white goods and other industries.

"More and more of our customers want their castings supplied finish-machined these days. For most of our recent projects we tend not to use special purpose machines, which are difficult to change in production. Capacity changes, component modification through development, or retooling for next-generation parts all dictate investment in standard CNC machines that can be reconfigured easily.

"For our larger castings, we calculated that two horizontal-spindle, APC (automatic pallet change) machines would be more productive than three single-pallet, vertical machining centres. However, most machines with a big working envelope generally have a powerful spindle motor with, for example, a BT50 or HSK taper and perhaps twin ballscrews in some linear axes, but we did not need such a high specification for cutting aluminium. Big machines also tend to have slower axis movements than their smaller, more nimble counterparts.

"The OKKs with their 40-taper BIG Plus spindle delivering 7.5 kW of power and 12,000 rpm are ideal for our applications, as they have faster axis movements than the other machines we considered, were less expensive and are also covered by five years' warranty."

Castings put onto the HP400s at Worcester are generally more complex than components machined on the drill-tappers. Whilst one minute might be a typical cycle time on the latter, six minutes is more usual on the 4-axis OKK machines and a gearbox casing can take more than 15 minutes. Only 10 per cent of castings produced at Worcester are too large to be processed on these machines.

One of the first jobs produced in the OKK cell, which justified the investment, was the production of the sump for a Jaguar engine in two operations, one per pallet. Current production level is 35,000 per year. Some tolerances are fairly tight, such as a cast dowel hole that needs to be opened out to within 11 microns total tolerance; and a milled face that has to be held within a 0.1 mm profile to ensure accurate assembly of the sump to the engine.

A feature of the machine 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, a coolant curtain from the ceiling and a coolant gun. Media-free filtration is standard and refrigeration is included to avoid temperature variation of the coolant, which is important when cutting aluminium owing to its high coefficient of thermal expansion. Temperature sensors are also 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.

Metal Castings uses CAD models from customers to develop the casting processes, but programming of the HP400s is carried out manually at the Fanuc 160i MB control, as the machining cycles are fairly straightforward. The electronic model of the sump was, however, used by Hyfore to assist in making the automatic hydraulic fixtures, which incorporate sensors to check that the component has been clamped correctly. Whitehouse fitted rotary couplings on the pallets to supply constant power to the fixtures.

Another long-running job introduced to the OKK cell is the production of a remote gear change housing and another related housing for a Land Rover engine.