Machining System and Laser Encoders Team Up for Accurate Large Composite Parts

CMCPIC3 The Composites Machining Center handles composite parts up to 50 meters, or 131.2 feet, long. Credit: Flow International

As large composite structures become mainstays of aerospace design, part fabricators are turning to innovative machining systems to fine-tune the dimensions of these parts for assembly. One such system, from Flow International Corp., in Kent, Wash., is the Composites Machining Center, or CMC, a modular unit equipped with a 5-axis water jet and a 5-axis high-speed router that handles monolithic parts with X-axis lengths of 6 to 50 meters (19.6 to 164 ft).

The CMC typically finishes parts after autoclaving. It can be configured with either a mid-rail gantry or a dual traveling column. Each version mounts 5-axis wrists for the water jet, router, or other tools. In addition to trimming and routing parts, the CMC can be used for drilling, surface milling, marking, and inspection.

The machine's objective is to deliver parts for assembly with minimal dimensional variations -- i.e., parts that do not require fixtures and shims to meet assembly tolerances, and which as a result can be assembled faster and without additional labor. A tight tolerance can also reduce part weight.

These benefits are especially important to shops since many large composite parts can cost $1 million or more to fabricate and are intended for critical structural applications. Fitment issues will generate costly delays when production schedules falter as a result of unacceptable quality.

The stakes for part integrity are so high that Tier 1 aerospace suppliers worldwide, many of which have been designated manufacturing partners by aircraft OEM Airbus, use Flow International's machines for finishing composite parts.

A water jet is, of course, a better option for trimming composites than a conventional cutting tool, due to the high stiffness of reinforcing fibers and relatively weak matrices of the resins. A metalcutting tool would generate high heat during trimming, causing delamination and other damage to the composite structure.

It's not unusual for some parts, such as a 40-meter-long (131.2-ft-long) wing skin, to take 130 hours to machine on the CMC. Consistent accuracy over time is thus critical.

Two Renishaw laser encoders are mounted on both sides of the CMC. Their beams are directed down a special duct that is purged with clean air to preserve beam integrity. Credit: Flow International Two Renishaw laser encoders are mounted on both sides of the CMC. Their beams are directed down a special duct that is purged with clean air to preserve beam integrity. Credit: Flow International

One way that the CMC achieves the necessary precision is by installing either an HS10 or HS20 laser encoder from Renishaw Inc., of Hoffman Estates, Ill. The measuring device reportedly generates repeatability down to 0.0381 m (0.0015 in) over a 40-meter (131.2-ft) X-axis length. One Flow International engineer says the accuracy is "essentially equal to the laser interferometer used to calibrate the machine."

Renishaw, for its part, says the HS20 laser encoder is accurate in linear position feedback to 1 micrometer/meter (that is, 0.000039370 in/3.3 ft) on axes of 60 meters (196.8 ft).

The X-axis precision of the Renishaw laser encoders compensates for the deviation that occurs during long machining cycles, even in climate-controlled plants. Both the HS10 and HS20 are described by the company as more accurate than tape or glass scales and resolvers and do not generate short-term errors that accumulate over a long axis.

The laser encoders have measuring speeds of 1 m (3.3 ft)/min. This is fast enough to keep up with the latest water jets and routers.

In operation, two laser encoders are mounted on either side of the CMC. The system uses split X-axis feedback for positioning, with the lasers operating in a master-slave configuration. The lasers are located beneath the way-cover bellows near the drivetrain, in a special duct that is purged with clean, dry air to protect beam integrity and assure stable measurements.

In addition to the laser encoders, Renishaw supplies the RMP60 touch-probe for Flow International's machines. This locates a part during setup and confirms dimensions after machining. The probe accesses any point the machine head reaches and uses an FHSS (frequency-hopping spread spectrum) radio signal for transmission. The FHSS signal is designed for clarity in "noisy" industrial environments that have a lot of wireless devices in operation.


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