Silcoms — a niche supplier to the aerospace industry of nickel-alloy, titanium, stainless-steel and aluminium aero-engine ring components and assemblies (including seals, shrouds, segments and casings) — was awarded another contract last year to supply a complex rotating-fan seal for a large civil-aircraft engine programme.

Additional capacity was needed at Silcoms’ Bolton factory to cope with the increased workload; and while researching the most appropriate new equipment, managing director Jim Hill took the opportunity to “develop a new process route that would speed production of the rotational titanium part while maintaining the tight tolerances needed”.

The new method of manufacture is based on the use of a German-built Hermle C 50 UMT five-axis machining centre fitted with a torque table that allows in-cycle turning operations to be undertaken.

The machine — supplied by Gosport-based Kingsbury (www.kingsburyuk.com), the sole sales and service agent for Hermle in the UK, Ireland and Middle East — now performs three operations in two set-ups (the part previously required five operations on three different machines).

As a result, the floor-to-floor time is drastically reduced, while fewer set-ups mean that the risk of accumulative
tolerance error is minimised.

Machined from a titanium forging, the 1,015mm-diameter fan seal has to be turned to a final wall thickness of 3mm, while maintaining dimensional accuracy and avoiding distortion.

After a number of preparatory machining stages, the Hermle mill-turn centre completes the next five operations in two set-ups, taking 20hr.

Semi-finish and finish turning have been compressed into one operation on each side; while during the second clamping on the Hermle, turned and milled features are completed (these prismatic cycles were formerly undertaken on a different five-axis machining centre on site).

Tight tolerances

Critical dimensional features are held to ±20µm over the full diameter of the part, and ‘post operations’ remain the same; these include balance testing and machining to correct any imbalance, surface treatment and final turning to ‘restore’ the seal surface.

Mr Hill said: “Consolidating turning and milling on one machine has significant benefits for us.

Apart from a reduction in component handling and an improvement in accuracy, it shortens the lead time for converting a titanium forging into a finished seal — and reduces the total number of tools we need.

“Around 90% of machining on the Hermle is turning, so we wanted to be sure that the torque table on the C 50 UMT was up to the task, bearing in mind that titanium is a tough material to cut.

A demonstration at Hermle’s factory in Gosheim convinced us that the machine was ideal for the task, added to which the manufacturer has an excellent reputation for quality, reliability and service — as does Kingsbury.

“Moreover, it is possible to optimise the turning conditions by fully utilising the five-axis capability of the Hermle. As aerospace parts are being designed with ever-more-complex features, such capability and versatility are particularly important.”

The specification of the Hermle C 50 UMT includes: a 56kW 12,000 rev/min spindle (356Nm of torque); a trunnion swivel range of +100 to -130deg (this trunnion is driven from both sides to ensure the high level of rigidity needed for the ‘rigorous’ machining of titanium); a 500rev/min torque table; a work envelope of 1,000 x 1,100 x 750mm; and 6m/sec² acceleration in each axis to 60m/min rapid-traverse in X and Y, and 55m/min in Z.

Absolute measurement of all axis positions is fed back to the Siemens Sinumerik 840D control.