Breakthrough in aerospace machining times, accuracy and surface finish

Citation

(2003), "Breakthrough in aerospace machining times, accuracy and surface finish", Aircraft Engineering and Aerospace Technology, Vol. 75 No. 2. https://doi.org/10.1108/aeat.2003.12775baf.011

Breakthrough in aerospace machining times, accuracy and surface finish

Breakthrough in aerospace machining times, accuracy and surface finish

Keywords: Aerospace, Machining, Surface finishing

A research project undertaken by a team at Cranfield University's School of Industrial and Manufacturing Science has employed Holroyd's superabrasive Edgetek machines to achieve significant breakthroughs when machining Nickel-based superalloys; materials which are notoriously difficult to work (Plate 2). The breakthroughs, in terms of reduced cycle times, better surface finish and improved accuracy, have been achieved using the latest high efficiency deep grinding (HEDG) techniques. They are likely to have a profound influence on the way that superalloys are machined in the future within the aerospace industry.

Plate 2 Cranfield team achieve breakthroughs in aerospace machining time, accuracy and surface finish using Holroyd Edgetek machines

The Cranfield team, led by Professor David Stephenson, carried out lengthy "burn threshold studies" on Inconel 718 – a superalloy commonly employed for gas turbine components, using Holroyd's "Edgetek" Superabrasive Machining process. Manufactured at Holroyd's UK factory in Rochdale, the multi-axis, CNC controlled Edgetek machines deploy proven HEDG techniques, using cubic boron nitride (CBN) wheels at surface speeds of up to 200 mls. This enables them to achieve high depths of cut and optimised metal removal rates far exceeding those of more conventional machines, such as CNC lathes, milling and machining centres and grinders.

An Edgetek machine (a five-axis unit) was specified for the Cranfield research project due to its ability to machine nickel-based superalloys such as Iconel 718, which are extremely difficult to work. The ability of these alloys to retain much of their strength at elevated temperatures means that wear rate on conventional tooling is rapid, even when cutting at low speeds. In addition, problems such as work-piece burn and poor surface integrity are common, due to the low thermal conductivity of the alloys. Over the last 30 years developments in creep feed grinding techniques have addressed some of these problems, notably in achieving higher rates of metal removal, but economic pressures to reduce manufacturing costs further, and to improve productivity, still remain.

As a direct result of these pressures, a growing interest in HEDG has developed, which can be considered as a combination of creep feed grinding and high speed grinding, using high wheel speeds often in excess of 200 m/s. A main area of research has been to attempt to prove a commonly held HEDG theory that very high stock removal rates are associated with low work-piece temperatures. Key to this, were the grinding trials carried out at Cranfield University.

The trials were undertaken on blocks of Inconel 718 measuring 40 x 40 x 40 mm, using a "down grinding" mode and grinding parameters were varied over the following range: depth of cut: 0.05-1.5 mm, work speed: 0.05-55 mm/s and wheel speed: 50-150 m/s. For each test a series of 15 mm wide cuts was taken and resultant forces were measured using a three-axis dynamometer.

Although the results of the trials are naturally very detailed, the general trend shows a reduction in specific grinding energy as specific stock removal rates increase. Commenting upon this, Professor Stephenson said "The thermal modelling results correlate well with 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, highlighting the large depths of cut and accompanying lower temperature work-piece characteristic of HEDG, have significant practical relevance in terms of aerospace manufacturing processes, in that the two promote dramatically improved productivity, less surface deformation, improved accuracy and better surface finish.

"We are delighted, but not surprised by these results", said Holroyd Sales Director, Paul Hannah. "The massive productivity gains and cost savings experienced by those who have made the switch to Edgetek superabrasive machining speak for themselves and prove that the technological developments pioneered by our machines represent a highly significant breakthrough for the future of machining."

In addition to its key role in the Burn Threshold Studies, Holroyd's Edgetek machine is also central to another Cranfield project, "Ultraflex", which is running simultaneously. "Ultraflex" is a European/automotive industry-funded project, for the development of innovative manufacturing technologies for reducing process chains. Its specific objectives are to develop and to demonstrate the following two processes:

• •

  cost-effective and high productivity machining process of high alloy steel before hardening operation and hardening process,

• •

  ultra-precision and environmentally friendly machining process of high alloy steel after hardening operation.

The Edgetek machine is playing a key role in developing these processes and in achieving the two performance targets for the project:

1. 1.

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

2. 2.

   the ability to rough finish a high alloy steel CVT gearshaft in less than 2 min.

The groundbreaking research carried out by Cranfield University on both burn threshold studies and project Ultraflex and the positive results obtained, proves conclusively the effectiveness of Superabrasive machining on difficult to work materials, not only within its "traditional" sector of high volume-production associated with milling machine accuracies, but also on very close tolerance work with high levels of surface finish. However, despite these results, neither Cranfield University nor Holroyd are resting upon their laurels. Holroyd is conducting ongoing technical developments at its factory in Rochdale which complement the innovative research carried out by Cranfield University, developing further the potential that exists in this rapidly growing area of machine tool technology.

Stressing the latter point, Holroyd's Paul Hannah said, "Feedback from customers and the results of detailed research into superabrasive machining both support the view that this exciting technological breakthrough has massive implications for the future of a wide range of machining operations. The dramatic reduction in cycle times and tooling costs which result from a switch to Holroyd's Edgetek' superabrasive machine tools contribute to significant improvements in overall productivity for every company using them."

For futher details contact: Holroyd. Tel: +44 (0)1706 526590; Fax: +44 (0)1706 353350; E-mail: phannah@holroyd.renold.com; Web site: www.holroyd.com