A Marwin 40m long Alumax CNC twin-gantry milling machine, used for ‘prepping’ wing stringers, has been retrofitted with a user-friendly contouring CNC capable of controlling up to 13 axes

The 40m long by 3m wide bed of the Marwin Alumax CNC twin-gantry milling machine located in Hangar 14 on the 800 acre Cambridge site of Marshall Aerospace enables ‘prepping’ operations to be carried out on aircraft stringers up to 22m in length. Purchased initially as two separate machines, which were then dismantled, moved and re-installed as one machine by Electron Engineering Services, a Heidenhain-approved TNC retrofitter, this refurbished machine is playing a key role in the supply of 11,700 stringers destined for the Airbus range of aircraft.

This is under the terms of an initial two-year contract with CAV Aerospace that requires the Manufacturing 3rd Party Department of Marshall Aerospace to hold buffer stocks of ‘prepped’ stringers ready for supply to the Tier One supplier.

According to Colin Flack, business development manager (Manufacturing), it became a ‘race against’ time to complete both the extensive hangar refurbishment and the commissioning of the re-manufactured gantry mill.

In fact, the entire project was completed within the 24 week deadline, with the official opening taking place as planned in October 2006.

Since then, Hanger 14 has become home to several thousand ‘prepped’ stringers as well as to a sizeable quantity of free issue material and, says Colin Flack, the expectation is that the sizeable order from CAV Aerospace will be the precursor to further support work in the future.

Refurbishment of the gantry mill by Halifax-based EES involved the replacement of an obsolete Allen Bradley control with a Heidenhain iTNC 530 contouring control capable of controlling up to 13 axes.
This has been fitted into a new cabinet on the first travelling gantry, which is equipped with two spindle heads.

A new, heavy-duty, 40kW spindle complete with chiller unit carries a large indexable-insert face mill that machines the upper and lower faces of stringers, with a 3.4mm depth of cut on the first X-axis pass over the work piece and a 0.8mm depth of cut on the return pass completing the milling of each face.

A second Gamfior high speed spindle, which can also swivel (A axis) through +/-20 deg, traverses the Y-axis to machine profiles on the stringer, as and when required.

EES is one of Europe’s leading machine tool CNC retrofit and refurbishment specialists, and its recommendation of the Heidenhain iTNC 530 CNC was accepted after in-depth discussions with Marshall Aerospace.

This was on the basis that future ‘prepping’ work involving more complex shapes and profiles will require a significant five-axis capability.

The intention is to equip the second travelling gantry, which has still to be refurbished, with two five-axis spindle heads that will be under the control of a second iTNC 530 CNC.

Operators report that the new control is extremely user-friendly, although Colin Flack says current machining operations are not especially complex.

Longer term, however, the decision to install the Heidenhain CNCs is regarded by Marshall Aerospace as essential to the cost-effective production of highly accurate, complex workpiece contours machined in the shortest possible time.

The iTNC’s short block processing time of 0.5ms for a 3D line segment without tool compensation permits fast traversing speeds, even on complex contours, and with its optimised tool path control, pre-calculation of the contour, and algorithms for ‘jerk limitation’, it ensures that the end result is a perfect mirror surface finish.

According to Heidenhain, this reflects the fact that for more than 20 years TNC contouring controls have been proving themselves in daily use on milling, drilling and boring machines and machining centres.

The iTNC 530 is the latest version and features a more powerful processor architecture.

However, like its predecessors, the iTNC 530 is both easy to use and flexible as it can be programmed remotely on a CAD/CAM system or on the shopfloor, with complex programs verified quickly and simply by way of the control’s optimised graphics build-up.
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