CAD/CAM Patternmaking of Large Volute Casings
A recent development exercise has been completed at Castings Technology International, in conjunction with Bryan Donkin company Ltd, Chesterfield, to manufacture a pattern for a large volute pump casing, which is used in the sewage treatment industry. The project, which was assisted by Sheffield Regional Technopole, involved the use of both CAD/CAM and conventional patternmaking approaches.
Howden Donkin Blowers of Chesterfield were launching a new range of high efficiency centrifugal compressors for the sewage treatment industry and The Bryan Donkin Foundry needed to cast a range of volute casings as part of this development. The casings were complex in design, thin walled with torroidal shaped hollow chambers and an unmachined internal profile which needed to accurately match the hydrodynamic design.
The requirement was to produce a generic range of castings of different sizes and this called for the benefits of the parametric features of CAD/CAM techniques which are available at Castings Technology International. These allow one set of data to be developed to cover the whole size range. The power of each compressor is a function of the impeller size resulting in a range of castings from 1-2 metres in diameter and from 500-1500kg in weight.
The first-off pattern and corebox was completed using the Rambaudi machining centre. This was one of the largest pump sizes which required a pattern base board of approximately 4m x 3m.
The project covered:
- working with compressor designers to adapt and later scale the existing 2D computer designs for the cast part.
- working with the Bryan Donkin Foundry moulders and methods engineers to optimise the pattern design to reduce moulding times
- commissioning rough block work from a conventional pattern shop to reduce time and cost.
- machining the core box and pattern shapes as single entities including the facing and joint surfaces. The patterns were delivered mounted and ready to use.
The first-off casting was dimensionally approved without any modification and three volute casings have now been cast from the new tooling. Conventionally, this type of casting, with a rising joint line, would have been made from a model pattern. The CNC machining techniques produced a split pattern with a complex, sloping joint line which facilitated much simpler moulding. For the batch of three castings the mould making time was reduced from 112 man hours (for a model pattern casting) to 48 man hours.
Howden Donkin are already receiving orders for other sizes of their compressor design. By computer scaling and machine cutting of the pattern, the technique is able to offer costs which are similar to conventional pattern making with considerably shorter lead times.
The technique has shown that significant timescale and accuracy benefits can be achieved from the use of CNC data when manufacturing the complex contours required by the volute pump patterns and coreboxes.
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