High-tolerance machining cell

Citation

(2001), "High-tolerance machining cell", Aircraft Engineering and Aerospace Technology, Vol. 73 No. 1. https://doi.org/10.1108/aeat.2001.12773aab.016

High-tolerance machining cell

High-tolerance machining cell

Keywords: NCMT, BAe Systems, Machining, Component, Manufacture

A new cell for producing high accuracy missile components at the Basildon, UK factory of BAe Systems, started full production at the end of last year. The cell (Plate 3) is based on a Makino machining centre fed by an in-line pallet pool and an Okuma lathe, all supplied by NCMT at a cost of in excess of £1 million.

Plate 3 The Makino A66 machining centre is served by a 12-station linear pallet pool

A Zeiss co-ordinate measuring machine plus other gauging equipment is included in the cell, which is housed in a temperature controlled room consistent with routinely achieving machined tolerances down to ±5 microns.

An unusual feature of the prismatic machining cycles is the inclusion of grinding operations using a wheel that is exchanged automatically, like metal cutting tools, from the magazines of the machining centre. Owing to the relatively small size of the wheels, of the order of 50mm diameter, the technique employed is a simplified version of the so-called Viper grinding process developed for Rolls-Royce by Makino, NCMT and Tyrolit, but without the two-axis NC coolant nozzle assembly, forced lubrication and double skin guards.

Noteworthy also is the ability to perform five-axis machining on the four-axis (X, Y, Z, B) machining centre. A pallet is sent to one of the two work-set stations for fitting of a rotary A-axis, after which it is transferred to the Makino and exchanged automatically. The services are plugged manually into the pallet at Basildon, although there is also a system for automatically engaging them, which Makino has developed particularly with the aerospace industry in mind.

Two years ago, the production management team at the then GEC Marconi company in Basildon identified a requirement for high tolerance machining of stainless steel and titanium castings as well as solid aluminium. The option of subcontracting the work was dismissed, as external suppliers could not guarantee a total tolerance of ten microns when dealing with production quantities without resorting to hand finishing, which considerably raised the manufacturing cost.

Other Makino and Okuma machines were being successfully used on site, so NCMT was approached to turnkey part of a new cell capable of producing six major missile components. They are a stainless steel chassis, a titanium rear tube, a prism housing and a mirror carrier both machined from stainless steel bar, an aluminium nose housing, and a yoke comprising aluminium and stainless steel parts which are machined again after assembly (Plate 4).

All the above components except the nose and prism housings need to be put on to the Okuma LU-15M twin-turret lathe in addition to the Makino A66 machining centre. Other components are produced entirely on the lathe, including optical housings machined from 50mm diameter bar, whose drawings stipulate roundness of 10 microns, ± 5 microns fit for the lens mountings and ± 8 microns on external dimensions.

Plate 4 An original stainless steel casting (left) next to a finish machined missile chassis, produced in six separate clampings on the Makino machining centre and Okuma lathe at BAe Systems

Extensive use is made of the driven stations in both of the turrets but pick-off for back end operations is avoided in order that perfect concentricity can be maintained. In one optical housing, a square aperture is milled in the reverse of a component before part-off.

A good example of a component machined in successive operations on both the machining centre and the lathe is the stainless steel chassis, produced from a casting approximately 230mm long x 175mm OD. It is fixtured on a pallet, locating off three faces on the casting. The inside face and end face are rough milled, material is roughed from the main bore and two tooling holes are drilled. Locating on the lathe off the same three faces, two rim diameters at 90 degrees to each other are rough turned to create face A and edge B. Back on the machining centre, face A and the two tooling holes take over as location points. Another face and the prism housing are roughed, as are the top and bottom gimbal arms.

Fixtured again on the lathe using the original location points, rim diameters A and B are finish-turned, the former to 5 microns flatness and to a parallelism of 10 microns to other machined features. On the machining centre and reverting to face A and the tooling holes as fixturing points, various lower tolerance flat faces are finish-machined, drilling, tapping and slowing operations are completed, and the bores are finished using Mapal tools to ±5 microns.

It is here that the in-process grinding operation comes into play. In the same set-up the profiled aluminium oxide wheel mounted on an extension arbour grinds two faces for locating the rear optical tube and parallelism and flatness to datum face A. The through-spindle coolant facility is used to supplement the external nozzles, directing copious quantities of coolant to the point of grinding. A diamond dresser is mounted on the fixture, which is also used to profile the straight-sided wheels during the pre-production phase, avoiding wheel collision with the complicated machined casting.

One fined refixturing for machining slots and holes which are inaccessible at other clampings completes the machining sequence.

Provision has been made in the cell for a second Makino A66 which will be served by the same pallet pool to cope with an anticipated increase in demand for missiles. The additional A66 will provide the extra prismatic machining capacity needed. Furthermore, the existence of two 128- position tool magazines will provide plenty of capacity for what by then will be a 180-tool library, plus sister tooling.

The face-and-taper tool-holding system BIG Plus, also supplied by NCMT, is used to provide the necessary rigidity for attaining the very high tolerances. To achieve contact between the back of the tool flange and the front of the spindle nose as well as contact along the length of the taper, both spindle and toolholder are precision-ground and a drawbar pressure of 800kg is used, deforming the taper slightly to allow simultaneous face contact. The advantages claimed are well documented; one benefit is said to be that rigidity and hence machined finish are improved due to the increased diameter where the tool meets the spindle.

Furthermore face contact is thought to have the additional benefit of squaring the tool in the spindle reliably after every tool change, resulting in a high level of repeatability. Tests are reported to show a positional variance of less than one micron over a large number of automatic tool changes.

Details available from: NCMT Ltd. Tel: +44 (0) 20 8398 4277; Fax: +44 (0) 20 8398 3631.