600 Centre cell turns the key for Weston Aerospace

Citation

(2001), "600 Centre cell turns the key for Weston Aerospace", Aircraft Engineering and Aerospace Technology, Vol. 73 No. 4. https://doi.org/10.1108/aeat.2001.12773dab.003

600 Centre cell turns the key for Weston Aerospace

600 Centre cell turns the key for Weston AerospaceKeywords: 600 Centre, Weston Aerospace, Machining

During the early part of 1999 Weston Aerospace, Farnborough, UK, embarked on a programme of process development intended to bring manufacture of a key thermocouple body component in-house.

An important part of the manufacturing process was a joint cell development between Weston Aerospace and 600 Centre of Shepshed, which supplied three of the four machines – an Okamoto OGM 360 CNC cylindrical grinder, a 600 Centre CNC vertical EDM machine and a Fanuc Alpha 0C AWF Robocut wire EDM machine.

From the placement of order, the three machines were delivered, installed and the first parts run-off inside 12 weeks. This time scale covered the design and manufacture of complex tooling, application engineering and process prove-out against stringent quality requirements and acceptance trials.

As a result, the new method is reported to have reduced costs by some 50 per cent of the out-sourced price, generates virtually no scrap components and provides predictable delivery of high quality components ready for assembly. Taking into account the production cost saving, Chris Rooke, manufacturing manager-machining at Weston Aerospace, estimates payback time on the investment as being less than nine months.

The exhaust temperature of a gas turbine engine is a primary indicator of how efficiently the system is working. To gauge this parameter, many of the world's leading aero engine manufacturers utilise multiple thermocouple arrays based on sensors developed and manufactured by Weston Aerospace. Thermocouple assemblies, like most aero engine components, must satisfy rigid quality criteria while the manufacturing process laid down for their production is subject to third party approval which must be strictly adhered to.

Says Chris Rooke: "The introduction of kaizen some four years ago marked a watershed for us, helping to transform our business from a manufacturer relying heavily on subcontractors into a highly efficient, profitable operation. As a result, we have been able to draw a significant proportion of high value added production in house. And, to that end, our parent company, The Roxboro Group plc, has funded considerable investment in equipment and training initiatives which have helped to achieve a rapid return."

A key part of this regeneration has been a move to cell-based manufacturing and team-based administration on the shopfloor. Regular application of kaizen principles to improving defined production procedures has seen typical component lead times fall from 15 days to 3½ days.

The confidence generated by the success of previous initiatives enabled Weston to approach manufacture of the thermocouple bodies with a certain level of self-assurance. This was despite the fact that the component is deceptively complicated and manufactured from a directionally solidified nickel alloy casting which is extremely difficult to machine yet surprisingly delicate and susceptible to damage.

"Our previous subcontract supplier was having major problems in meeting the specification", Chris Rooke explains, "and while the as-supplied cost was high, we actually doubt whether it was really profitable for them".

As part of the change to manufacture in-house, Weston Aerospace had to satisfy its customers and a number of statutory certification bodies as to the integrity of its production route. And, whereas the original supplier had the benefit of certain process concessions, the route defined by the Farnborough team and 600 Centre had to be demonstrably capable of meeting the specification.

The finished cast component bears a resemblance to a thin metal ball-point pen body but includes a number of transition blends along the length with critical position tolerances only checkable using a shadowgraph. It also includes a boss, threequarters of the length from the point, a face which is grooved and must also be produced out of parallel with the other by 1.5 degrees of arc so that, when seated, the finished thermocouple will clear the rotating turbine blades in the assembled engine.

As there are two versions of the body, a variable number of slots are required between the boss and the point. The linear and angular position must also be precisely referenced within 0.025mm to the "high point", of the angled face on the boss. In addition, the tapered end of the component is cut across at a precise angle and its start point is again aligned to the high point. The central bore on one variant also has to be precisely finished machined on the Fanuc wire EDM using a 0.5mm wire.

"It is a very complicated component", Chris Rooke insists, "but it is required in reasonable volume of 6,500 per year. As it is a high value-added part, we were able to justify developing a dedicated cell and the associated process controls. From our experience of machining nickel alloys, we were optimistic that introducing cylindrical grinding could overcome some of the problems but, we had no production experience of EDM, were on a tight schedule and needed a machine supplier that could provide a high degree of application engineering expertise".

He continued: "Ideally I would have liked a full turnkey solution but finding a single supplier for a CNC lathe, CNC grinder, EDM wire and EDM sink was a tall order. 600 Centre had this capability for EDM and grinding, and were willing to participate despite the short lead time and we felt comfortable working with them. Ultimately, we elected to source our own CNC lathe which with hindsight, probably gave us a better insight into the overall process as we became active, rather than passive partners in the activities".

The basic production route comprises rough grinding of the cast component on the Okamoto cylindrical grinder to provide initial process datums. The casting is then held on its long end for the short end to be turned, then reversed for turning the taper, groove in the boss and a counterbore. The part then returns to the Okamoto for finish stem and taper grinding. A separate grinding operation creates the high point and the part then has the slots produced on the EDM sinking machine.

EDM operations involve up to ten components at a time, using a high repeatability location system to align the custom-built fixture. Slotting is carried out using an optional C-axis on the 600 Centre vertical EDM to position the side-mounted copper electrode. On one of the variants there are two sizes of slot and the machine is equipped with a tool changer to accommodate this requirement. Following slotting, the part is located on the Fanuc Alpha wire machine for cut-off at either end. If the component needs the bore to be machined, a separate set-up on the Fanuc machine is carried out.

The key to the production process lay in developing a repeatable method for producing the high point and maintaining it as the reference for subsequent processes. The cell machining method not only allows generation of the high point but allows it, or any feature that is referenced to it, to be produced at random. It comprises a high precision D-shaped screw to clamp a split collet which holds the component on the short end side of the boss.

The method dictates that once the part is clamped in the collet, then all machining alignments can be referenced to the flat of the D. To produce the high point, the "D-bit", locates in a mating shaped driver on the Okamoto which aligns the part at precisely 1.5° to the horizontal enabling a repeatable grind of the high point.

"The first time we used the D-bit it looked very odd but we've become used to it now", John Gibbs the cell leader remarked. "By also using the D-bit to locate on the EDM machines we can preserve alignment relative to the high point, or alternatively, we can align the high point relative to those same features. This gives us considerable flexibility in the operational sequence. In addition, on the EDM machines, the multiple part fixtures, automatic toolchange and auto wire re-threading while submerged also allow us to operate under lights-out conditions between shifts."

An on-going capability study commenced with checking all dimensions on the first three batches produced within the cell. Regular checks include shadowgraph assessment of external dimensions plus X-ray of all components to determine wall thickness and parallelism of the central bore.

Chris Rooke reports that: "Production is now around 150/week and our maximum capacity is about 180, based on two people per shift working a two-shift system".

He concluded: "The cell is performing very well and has certainly justified the decision to bring the components in-house. Within Weston Aerospace the commitment from the team who now operate the cell has been extremely high which is another factor attributable to kaizen. Equally, 600 Centre has made a major contribution to making the cell work in respect of responding to a very challenging target for machine delivery and process development".

Further details are available from 600 Centre. Tel: +44 (0)1509 600600; Fax: +44 (0) 1509 600159; E-mail: cntools@600centre.co.uk; Web site: http://www.600centre.co.uk