Centrax, producer of gas turbines and turbine components, has reduced the time required to inspect turbine blades 97% by switching from a coordinate measuring machine (CMM) to the Maxos optical scanning system.
"We originally planned to purchase several touch probe CMMs to keep pace with production during a ramp-up of our blisk (blade-integrated disk) manufacturing program," said Kevin Vickers of Centrax. "But we discovered that the Maxos system is so much faster than the other machines we had considered that a single Maxos scanner handles our planned output."
Centrax's Turbine Components Division produces compressor and turbine aerofoils, discs, shafts, casings, and associated hardware including subassemblies and engine modules. Blisks consist of rotor disks with integral blades or vanes for gas turbines. A major advantage of the blisk over the conventional disk/blade arrangement is potential weight saving through elimination of the fixings that secure the blade root to the disk. One of the greatest challenges of producing blisks is inspecting them to ensure that they meet the very close required tolerances.
The conventional approach, with touch probe CMMs that are moved from point to point, is very slow because the touch probe must make actual physical contact with each point that it measures. Speed of the measurements is limited by how quickly the probe can be moved around the blade. Faster noncontact scanners have recently been offered for this market, but their weakness is that they can only scan freshly machined or highly polished surfaces if the surfaces are sprayed with a coating. This need for a coating creates problems because it takes time, raises the risk of contaminating the blades, and introduces dimensional inaccuracy.
The Maxos scanner uses a proprietary noncontact probe consisting of a point of white light that allows the collection of individual points at a rate of 70 per second. Like a touch probe CMM, it collects individual points, but unlike a conventional CMM, it continues on its path at high speed and without pausing.
Because the Maxos scanner measures with a single white light point, the cause of inaccuracy and approximation inherent in 3D measurement with a ball probe is eliminated. Consequently, Maxos has an exceptionally high accuracy of ± 2 µm on matte surfaces and ± 10 µm on polished metal. It can achieve a point spacing resolution of 0.2 µm without pausing. Additionally, since it has no ball probe and measures a single point at a time, it is not limited by ball-offset geometry and can inspect radii of less than 0.2 mm. This feature is key to the measurement of leading and trailing edges on turbine blades..