Software speeds jet baffle production

Alphacam helps reduce the noise signature of modern jet engines in a project that streamlines the production of acoustic liners using its advanced CNC machining software.

CAD/CAM software and services specialist Alphacam is helping to reduce the noise signature of modern jet engines, thanks to a recently completed project that streamlines the production of acoustic liners using its advanced CNC machining software.

The liners are manufactured from composite materials and achieve their noise suppression characteristics through a matrix of tens of thousands of drilled holes, which are precisely positioned across their surface.

The Alphacam project was undertaken for a leading aerospace component manufacturer and focused on the machining of 2mm diameter holes at 4mm nominal pitch centres around the circumference of liners fitted to turbofan engines.

Following successful proving trials earlier in the year, batch production is now underway at Alphacam's customer using a bespoke drilling machine, with the composite liner mounted on a rotary indexing table.

The angles of the machine's drill heads are determined using machined templates which, in this application, enable the process parts to be divided into three separate drilling zones.

Typically, components are mounted "bell mouth" down and indexed in approximately 0.5 degree increments, with the comb of drills producing a series of holes in rings around the part's circumference.

The drill heads are then stepped in the vertical axis and readjusted to account for the taper of the components, before the process is repeated.

According to Alphacam's Project Manager Mark Aldous, the accurate positioning and orientation of the drilled holes are complicated significantly by the nonuniform geometry of the job.

"The liners are not true cones".

"They are not circular in cross section and their profiles are not completely straight, resulting in some complex compound geometry, which the programming system has to take account of", he notes.

As a result, incremental movements of the comb of drill heads in the vertical direction - as the machine moves to the next level to process another row of holes - produces a nonuniform variation in the spacing of each drill head above the component surface at the new height.

Likewise, variations in the part's radial cross section mean that the start distances of each drill head from the surface can vary by as much as 16mm, as the liner is indexed around its circumference.

"With a drilling feed depth range of less than 4mm on each head, individual drills are either in danger of failing to reach the surface at all, or clashing with it as the component is indexed, depending on the start point of that vertical row", adds Aldous.

"In addition, the variation in surface geometry poses interrelated variations in the slope angle of the liner's surface at any given point".

"It is therefore necessary to determine the local variables to be encountered during each drilling pass and compare the theoretical hole positions and angles against actual values produced by the programme, just to ensure that the machining is always within production tolerances".

The achievement of this goal with the minimum of in-process adjustment is one of the keys to the success of the project, as it plays a fundamental role in enabling cycle times to be minimised.

With a maximum angular tolerance of +7 degrees on the normality of each hole to the local surface, it proved necessary for the final programming solution to calculate the maximum, average and minimum curvature and slope at each location - and compensate for their variations accordingly.

In operation, Alphacam's customer initially imports Catia geometry into the programming application in IGES format.

Following specification of the hole parameters, the Alphacam system automatically determines the drilling sequence and generates fully specified machining code.

"Typically, it takes around 25 minutes for the system to create all the geometry for the hole matrices, with a further 35 minutes required to complete the final programming", says Aldous.

As part of the programming cycle, the Alphacam system runs a complete machining simulation and a comparison check against the theoretical drilling pattern prior to manufacture.

Similarly, with a typical drilling cycle run time of around 9 hours, the finished programme includes automatic stop points to enable drill bits to be replaced after the production of a prespecified number of holes.

The programming project's ability to streamline the definition, specification and drilling of the hole matrices represents a massive saving in time over previously used methods for Alphacam's customer.

The Alphacam system also incorporates the flexibility to be applied equally effectively to the fast, accurate and cost-effective drilling of acoustic liners for a wide range of other aircraft engine types through the use of environmentally friendly technology.

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