Hermle 5-axis machining sets quality and productivity standards

Citation

(2002), "Hermle 5-axis machining sets quality and productivity standards", Aircraft Engineering and Aerospace Technology, Vol. 74 No. 4. https://doi.org/10.1108/aeat.2002.12774dab.002

Hermle 5-axis machining sets quality and productivity standards

Keywords: Quality, Productivity, Standards, Aerospace, Components

Taking pride of place as gate guardian at what is now the Hamble Structures factory at Hamble le Rice in Hampshire, UK, is a Folland Gnat jet trainer which was designed and built at the site and flew by the world famous Red Arrows acrobatic team (Plate 2).

Plate 2 Three Hermle 5-axis vertical machining centres with 12,000 revs/min spindle have reduced lead times at Hamble Structures from weeks to days

Gone are the days of complete aircraft build but still remaining under the Smiths Aerospace group umbrella is an enviable reputation of complex aerospace component supply. Indeed, over the last four years it has established a world-class track record of 99.8 per cent error-free delivery direct to the assembly lines of Boeing in Seattle.

As part of the company transformation to supply components to the likes of Airbus and Boeing is competitive pricing and a reputation for on-time deliveries. Forming a key role in the new business arena are three Hermle five-axis vertical machining centres supplied by Traub Heckert UK of Brackley.

We are informed that following their installation, Hamble Structures has been able to integrate guaranteed quality into its manufacturing with proven process capabilities which have enabled a massive cost-saving from jig-less assembly and the adoption of flowline techniques. Work in progress and stock levels have been reduced and the need for a store full of large assembly fixtures eliminated.

Initially, Hamble Structures installed two Hermle C800U simultaneous five-axis machining centres each with a two-axis trunnion unit and 12,000revs/min, 15kW spindles. These machines were able to combine jig-borer standards for accuracy and repeatability within a machining volume of 800mm by 600mm by 500mm. Such was the success of the machines in reducing lead times from weeks to just a few days, that a third Hermle, this time a larger capacity C1200U, was installed to add to the five-axis capability and machine Boeing 737 under-wing closing plates.

Hamble Structures initially bought the machines to perform a precision boring role and free up capacity on a flexible manufacturing system. This boring operation involved maintaining micron sized positional and dimensional tolerances on component 'K-holes' which are used for precise alignment during the jig-less assembly procedures on auxiliary track and main track ribs of the leading edges of Boeing 737–700 aircraft. "By using the Hermle machines, we were able to eliminate a bushing and reaming operation and the complete line boring operation, and due to the proven process capability of the machines, inspection frequency was also reduced," he says.

Part of the process of change has involved adoption of the Boeing-developed Advanced Quality System (AQS). This is aimed at right first time, production of components and sub assemblies in order to enhance productivity for Boeing and its suppliers. As a result of its conformity, Hamble Structures is able to supply on a straight-to-line, basis without any goods inwards inspection at the Seattle plant in America.

Derek Godsell explains: "One of the core principles of AQS is to make inspection a process. We therefore make use of the on-machine probing on the Hermle machines, supplemented by off-line use of electronic gauges by the machine operators. "As machining cycle times are between 10 and 20 minutes, this routine can be easily completed within the cutting cycles of the Hermles. Data captured from these inspection routines is then archived for SPC purposes and shared with the customer as a means of demonstrating that the process remains under total control.

While leading edge rib production remains a critical part of the workload for the Hermle C800Us, making full use of the machines, five-axis positioning capability, the same technology is now being expanded into the machining of other components. By contrast, the C1200U which was bought specifically for five-axis interpolative machining of the 737 under-wing closing plates which involve handed sets of eight components. These are required in multiple aircraft sets of 16 components per month.

The closing plates are about 500mm square by about 150mm deep, and are produced from a solid aluminium billet. While the exterior is a smooth contoured surface, the interior is similarly contoured but includes stiffening ribs and, as on the leading edge components, high precision assembly K-holes for jigless assembly.

"Given our experience with the C800U, the much larger capacity C1200U was the obvious machine for this application," said Derek Godsell. "These parts were produced on subcontract which meant the machine was relatively easy to justify on the grounds of cost savings and the just-in-time capabilities which then came under our control. Whereas we previously had lead times measured in weeks, the 1200U allows us to produce them almost on demand which means a massive reduction of expensive work in progress."

The components are machined from free machining 7050 grade aluminium, a relatively free machining aircraft grade material. However, this material does give rise to fine swarf particles which meant coolant filtration was critical. Although the parts have a fairly open surface, their complexity, together with the need to maintain a mirror-like surface finish, dictated the decision for five-axis techniques.

Components are machined using vacuum-type pull-down fixtures which have a quick-change capability to minimise non- cutting time. Most are completely machined using a spindle speed of 12,000 revs/min in a single set-up with the elapsed floor to floor time for each part being in the region of12 hours.

"The Hermle C1200U has proved very capable in this application," Derek Godsell confirmed. "It combines the capability for high rates of stock removal as we have to machine away over 75 per cent of the billet weight with very good positional and form accuracy. The basic design of the machine, with the A and B axes set below the center line of the trunnion mounting, is very good to maintain maximum rigidity and reduces linear travel and hence cycle time. This engineering also reflects in higher reliability and the repeatable accuracy performance that we have been able to achieve."

The overall benefit gained from the Hermle machines and AQS procedures has been significant with the move to jig-less assembly. "It has been a very successful project in terms of productivity and quality." Derek Godsell concluded: "Moreover, the efficiency savings have freed up factory space even though we have expanded production and is a very good illustration of the benefits of integrating quality into the manufacturing programme."

Details available from: Traub Heckert UK Ltd. Tel: +44 (0) 1280 705482; Website: www.traub-uk.com