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Renishaw develops laser encoders for accurate machining of 40m composite parts

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article image HS10 laser encoder fires its beam down the air purged duct

A laser encoder from Renishaw is ensuring long axis accuracy during week-long machining cycles at Flow International Corporation's Composites Machining Center (CMC) as it hits the mark for single-setup processing of 40-metre long composite wing skins.

Measuring distance with the precision of a laser interferometer, Renishaw's HS10/HS20 laser encoder and its companion RCU10 real-time compensation unit, deliver part-per-million positioning accuracy on the CMC's X axis, eliminating variables that needed to be compensated for earlier when using a physical scale attached to the machine.

Flow's CMC is one of the new machine tools developed to meet the aerospace industry's need to produce large monolithic parts with minimal dimensional variation. 

Flow's Director of Aerospace, Mark Saberton explains that reduced variation in parts is an industry wide goal with the objective being to decrease assembly time, effort and tooling. The OEMs seek to eliminate assembly shimming, which adds weight to the product and time to the process; they also want parts to meet tolerances when they arrive on the assembly floor, rather than using fixtures and shims to bring parts into assembly tolerance. 

He added that less variance on a plane's parts ensured better control of the aircraft's final weight.

Flow's CMC has been created as a one-stop processing centre for composite parts that have been autoclaved. It can be configured as a mid-rail gantry or dual travelling column machine. The gantry design carries two rams on a single gantry with a 5-axis wrist on each ram, one for ultra high pressure waterjet cutting and the other for conventional high-speed routing. 

The CMC does all the finish machining on a wing skin including trimming, drilling, routing, surface milling, marking and inspection in one setup, with parts leaving the machine ready for assembly. The modular CMC is available in standard X axis lengths of 6-50m, as well as custom sizes. 

According to Todd Fuchs, Senior Electrical Project Engineer at Flow, using a physical scale 30-50m long attached to the machine introduces structural issues and makes thermal compensation much more difficult. He explains the temperature differences the machine sees over a 40m length during a 130-hour machining cycle can be relatively extreme, even in an air conditioned plant, with the variables becoming too complex between changes in the scale itself and the substrate of the machine. The HS20 laser encoder is the easiest way to obtain accurate feedback on such a long axis.

With a rack and pinion drive on each side of its gantry, the CMC uses split X axis feedback with a laser on each side, working as a master and slave for positioning. The lasers are located under the way cover bellows near the drivetrain, in a duct purged with clean, dry air, ensuring stable measurements and protection from airborne debris which might ‘break’ the laser beams.

Flow has used Renishaw's HS10 laser encoders and its successor HS20 on a total of 24 machine axes. Installations have been carried out in Germany, Spain, France and UK with support from Renishaw engineers. Both encoders use the same laser interferometry technology used for linear error measurement and compensation by machine calibration specialists, except the laser encoders are designed to be a permanent scale on the machine, replacing glass/tape fitment and resolvers.

Unlike a tape or glass scale, the laser encoder system has no short term errors which can ‘stack up’ on long axes, and its 1 m/sec measuring speed accommodates today's fast-moving machine tools. The HS20 is sealed to IP43 and hard anodised to provide all round protection in harsh machining environments. 

X axis positioning precision is also a factor during part probing, which is vital to the cutting process. The CMC uses a Renishaw RMP60 touch probe for locating the part during setup and confirming finished dimensions after machining. The touch probe's compact design allows it to access and measure any point on the part that the machine head can reach. Its frequency hopping spread spectrum (FHSS) radio signal transmission allows multiple probe systems and other industrial equipment to coexist in noisy industrial environments, particularly as wireless Wi-Fi and Bluetooth communications proliferate.

Having laser equipped machines in many European countries, Fuchs comments that the Renishaw HS20 laser encoder has proven easier to deploy than tape scales, and are reliable and robust as well. For long axes, where machines are cutting expensive and critical parts, customers appreciate X axis positioning that's as accurate as the gold standard of linear measurement, the laser interferometer. 

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