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Keeping up with the casters

Andover die-caster MRT Castings, which specialises in high value sectors including medical and aerospace, has invested in new machining centres that combines a large working envelope with rapid metal processing

The company says that orders from these sectors have contributed to a 70% increase in turnover over the past 12 months, but the tonnage of parts produced has scarcely changed.

The increase in business is nearly all derived from machining the castings, which are becoming ever more complex, and from producing sub-assemblies.

A user of Brother machining centres since the early 1990s, MRT has invested in six further machines in the last year to cope with extra demand, bringing the total number of these Japanese-built, 30-taper machines on site to 18.

The latest two, designated S1000X1, are the first in the UK of a new model with a 1,000 mm X-axis – around one-third longer than that of the largest machine previously available.
Phil Rawnson, managing director of MRT commented, “Designers from Brother visited us in mid-2013 to ask what type of machine we would like them to develop next. Consulting with its customers is a good sign, as it means that the machine builder is listening to what the market wants.

“As we are milling, drilling and tapping near-net-shape aluminium castings in low to medium volumes and removing typically only a couple of millimetres of material, a 30-taper spindle is suitable for our needs. We told Brother that we wanted a larger machining envelope to give us more flexibility in the way that we fixture parts. Two years later, in June this year, the two S1000X1s were on our shop floor.”

As a 30-taper machining centre with a one-metre X-axis stroke did not previously exist, Mr Rawnson bought 40-taper machining centres to process MRT’s larger castings. There were disadvantages, however. 40-taper machines are of unnecessarily high power for skimming off 2 mm of aluminium and are consequently overly power-hungry. They also tend to be more expensive to buy and the rapid traverses and spindle accelerations are slower, so cycle times are longer and productivity is lower.

The S1000X1 design avoids these negatives by providing a highly productive machine with 50 m/min linear rapids (slightly higher in the 300 mm Z-axis). Tool change executed by the 21-station turret takes under one second, giving a chip-to-chip time of 1.4 seconds, and is performed at the same time as X and Y axis movements and rotary table indexing to reduce idle time further. Rapid cutter exchange is an important aspect, as a large mix of tooling is needed to machine MRT’s castings.

Machining speed is of the essence at the two machine shops on the Andover site. MRT uses four pressure die-casting cells, the fastest of which are capable of producing a casting every 40 seconds, and nearly all of them need to be machined in cycle times ranging from two to 20 minutes.

To keep up with the metal-cutting requirement over an extended day shift, the 18 Brother machines are of two configurations. Eleven have fixed tables, while the others are equipped with automatic twin-pallet changers utilising a total of 25 pallets set up with rotary trunnions and dedicated fixtures to speed changeover to the next batch. This occurs typically every one to two days per machine.

The twin-pallet Brothers are generally used if cycles are short, say less than five minutes, to minimise spindle idle time during sequential op 1 and op 2 machining on six sides of a casting. Fixed table machines are more economical if cycles are longer, as one operator is able to load and unload a pair of machines to complete the two operations in tandem.

Source: www.engineeringcapacity.com

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