Precision engineering company is achieving ‘better-than-grinding’ tolerances  - sub 10 micron flatness and straightness -  on a machine with the capacity of components up to 18 tonnes, and is achieving 99.9% up-time. 

Brownhills-based Forth Engineering machines large and heavy components for a wide range of industries, including highly sophisticated scientific research establishments which need to achieve quite exceptional precision in order to - for instance - achieve the splitting of an atom. To achieve equally exceptional precision from its machine tools, Forth has an Asquith Butler Starmill 280 travelling gantry universal milling machine.

Says Mike Hedley, MD at Forth Engineering: ‘The Asquith Butler machine allows us to explore the full range of metal cutting and provides us with the competitive edge. The Starmill 280 has allowed us to achieve previously unattainable precision from a gantry milling machine even on very large, heavy components. In addition to sub-10 micron precision, the Starmill delivers 99.9% up time (as measured over fifteen months), a figure unheard of in almost any sphere of manufacturing.  Cycle times are lower than on equivalent machines because of the machine’s inherent rigidity, permitting higher feed rates for roughing operations, so we are experiencing a tremendous improvements in productivity as well as reducing costs by eliminating the grinding operation. The results from the Asquith Butler are very impressive.’

So what’s behind this installation? The devil’s in the detail, and the requirements for the machine were a high degree of accuracy, high reliability, minimal maintenance, minimal use of energy resources, and no infringement of health and safety considerations. The Starmill 280 achieves all of this, giving far greater accuracies in flatness and straightness than has previously been possible with travelling gantry machines. It is designed as a high performance, large capacity machine capable of both a high metal removal rate when rough milling as well as fast, accurate finishing operations. Modular construction enables each machine to be configured economically for an individual customer’s requirements. Provision of an accurate rotating servo controlled ram (C-axis) gives great versatility to the machine. Using an appropriate attachment, any of which have automatic load/unload facilities, five sided machining of a component in one set up is possible. This eliminates time-consuming re-setting and allows the best manufacturing practices to be employed. In addition, unusual machining operations are possible, for instance roughing of large bores by helical interpolation using a face mill in the right angle head. Milling of inaccessible faces is possible using contouring with a planetary milling attachment.

Based on the proven design of the Starcut machine, design engineers working at Asquith Butler’s manufacturing headquarters in Brighouse undertook a total re-evaluation of the Starcut performance to achieve the performance of the Starmill machine. Using FEA, the existing beam design was modelled using the known deflections. New beam designs were modelled using different types and arrangements of the ribbing and plate positions to enhance torsional stiffness, using the software to simulate the load. The new design of beam is 54% stiffer than the previous design, uses less material and additionally is easier to manufacture. The bed was completely re-designed, also using the FEA system, to be manufactured from steel rather than cast iron to eliminate the wave created by the travel of the gantry. Any deflection in the transverse bed would have a resultant effect on the surface flatness results achievable. The head slide was redesigned as was the ram bore which has the advantage of two quill clamps and an epoxy resin Moglice bearing liner. The attachment clamping force was increased to 12 tonnes from the existing 8 tonnes to give much enhanced rigidity. Diameter of the head is increased to 280mm from 250mm, giving greater bending and torsional stiffness; power is increased to 60kW from 45kW and maximum speed increased from 2000 to 4500revs/min.
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