Slashing process times with single lift and set-up

Citation

(2003), "Slashing process times with single lift and set-up", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 5. https://doi.org/10.1108/aeat.2003.12775eab.002

Slashing process times with single lift and set-up

Slashing process times with single lift and set-up

Keywords: Machining, Grinding, Cycle times

In sub-contract machining and manufacturing environments, the production mantra is single lift and set-up; the ability to reduce process chains by performing all operations on one machine. This saves production time, reduces errors and improves precision, quality and efficiency. Unfortunately, in many sectors of industry, conventional thinking militates against achieving this objective. Convention dictates that milling or turning operations are the norm where high metal removal rates are required, with grinding reserved for high accuracy surface finishing. Work undertaken by Professor David Stephenson at Cranfield University turns this conventional thinking upon its head. The work, undertaken on a Holroyd Edgetek superabrasive machine (Plate 2), is said to show clearly that using the latest in high efficiency deep grinding (HEDG) grinding techniques, high removal rates – up to 30 mm depth of cut – and excellent surface finish – to optical quality – can be achieved on the same machine, slashing previously accepted process cycle times for machining operations from hours to minutes in many cases (Plate 3).

Plate 2 The Holroyd Edgetek superabrasive machine

Plate 3 HEDG turns conventional machining practice on its head, slashing process times with single lift and set-up

The problem with conventional machine tools is that high metal removal rates can only be achieved if components are relatively soft. Once the required depth of cut is achieved, the component is removed for hardening followed by surface finishing. This expensive, multi-operation process can reportedly be reduced to just one using HEDG techniques, even if the material is the hardest nickel-based alloys used in aerospace applications or the powdered metals used for timing gears and sprockets in car engines. In both the cases, extremely high metal removal rates, at wheel speeds of up to 200 m/s will be accompanied by excellent surface finish and high dimensional accuracy.

HEDG is a relatively new machining technology, operating under conditions far beyond the range of conventional grinding processes – even those of creep feed grinding. Compared to the latter, HEDG claims much higher specific removal rates, 50-2000 m³/mm/s compared to 0.1-1 mm³/mm/s. In addition, HEDG claims improved burn threshold qualities, which result in a lower finished surface temperature of the workpiece, and the overall improvement in the quality of the finish itself.

These benefits are said to accrue due to the use of CBN superabrasives, which claims high thermal conductivity to remove heat from the grinding process, and also to the unique quality with HEDG that specific grinding energy decreases as grinding chip thickness increases. This results in specific energies that are similar to conventional cutting processes such as high-speed milling.

The superior burn threshold qualities of HEDG were determined in extensive machining trials conducted by Professor Stephenson's team at Cranfield University. The trials were conducted on a Holroyd Edgetek five-axis superabrasive grinding machine. The machine is equipped with a 27 kW spindle, which can operate at speeds up to 14,000 rpm, enabling wheel speeds of 200 m/s to be attained together with large depths of cut. The Edgetek machine is fitted with a very stiff grinding spindle, which uses hybrid ceramic bearings that facilitate the very large cuts possible with HEDG. In addition, the granite polymer composite base offers excellent damping properties, virtually neutralising resonant frequencies within the machine that could impair its accuracy.

Commenting upon the machining trials, Professor Stephenson said: "Although our results are naturally very detailed, the general trend shows a reduction in specific grinding energy as specific stock removal rates increase. The thermal modelling results correlate well with the experimental observations of grinding burn. Modelling indicates the potential benefits associated with HEDG, using high work speeds and large depth of cut to minimise the finished surface temperature. Thermal modelling also highlighted the significant contribution that CBN abrasives can make to heat removal."

"These results speak for themselves" said Dr Tony Bannan, Engineering Director at Holroyd. "The large depths of cut and accompanying lower temperature workpiece characteristics that can be achieved with our Edgetek machines – even across difficult to work materials, are the key to achieving improved productivity, reduced process times and improved quality in highly competitive global markets. There is no doubt that Edgetek technology is the future for machining, with the added benefit that is available now."

Following the success of its burn trials, Cranfield have been grinding glass cathode ray tubes on the Edgetek machine, achieving optical surface finishes down to 30 nm Ra on what are complex 3D contoured surfaces. In another project, even greater levels of optical finish, 11 nm Ra, have been achieved, when grinding crankshafts. Importantly, the mirror finish on the crankshaft is achieved by using the same super abrasive grinding set-up on the Edgetek, without additional – and expensive – lapping or polishing operations.

Both these projects have been undertaken on behalf of industrial customers. Much of Cranfield's work is collaborative research undertaken on behalf of specific companies, or with groups of companies, which associate with the university and provide funds to purchase equipment for longer term projects. In addition, Cranfield is also at the centre of joint industry – government initiatives, as evidenced by the 50 per cent funding provided by the Joint Research Equipment Initiative for the original Edgetek machine, Rolls Royce and SKF being the other partners. The EPSRC, through the Innovative Manufacturing Research Centre, is also helping Cranfield to become the centre of excellence for superabrasive machining worldwide, by part funding with the industry purchase of another Holroyd Edgetek machine, a superabrasive turning system. David Stephenson said: "The facilities provided by this new machine will enhance even further our possibilities for collaborative research with industry, primarily because we will be able to broaden the scope and nature of our superabrasive activities."

Another current Cranfield project that is using HEDG techniques is "Ultraflex", a European automotive industry-funded project, for the development of innovative manufacturing technologies for reducing process chains. The specific objectives of the project Ultraflex are to develop and demonstrate the following two processes:

• •

  cost-effective and high productivity machining process of high-alloy steel before hardening operations and processes;

• •

  pultra-precision and environmentally friendly machining process of high-alloy steel after hardening operations.

Holroyd's Edgetek machine is playing a pivotal role in developing these processes. Two performance targets for the project are:

1. 1.

   working towards optical quality, machined surface finish levels of Ra 0.01 μm; and

2. 2.

   the ability to rough finish a high-alloy steel CVT gearshaft within a minute.

"The results achieved by Cranfield overturn conventional thinking on machining strategy and support the view that superabrasive machining has massive implications for the future of a wide range of machining operations" said Tony Bannan. "Moreover, the dramatic reduction in cycle times and tooling costs which result from a switch to one of our Edgetek superabrasive machine tools can give companies that essential productivity 'edge' which enables them to remain competitive in the extremely fast moving global economy."

Details available from: Holroyd. Tel: +44 (0) 1706 526590; Fax: +44 (0) 1706 353350; E-mail: phannah@holroyd.renold.com; Web site: www.holroyd.com