Case Studies: Optical Scanning Solution
January 2, 2008
Centrax (Devon, England), a 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 from Southlake, TX-based NVision Inc. “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,” says Kevin Vickers, research and development engineer for 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 sub-assemblies and engine modules. Blisks consist of rotor disks with integral blades or vanes for gas turbines. One of a blisk’s major advantages over the conventional disk and blade arrangement is the potential weight savings through the 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 to inspecting blisks is with touch-probe CMMs that are moved from point to point. But this technique is very slow because the touch probe must make physical contact with each point that it measures. The speed of the measurements is limited by how quickly the probe can be moved around the blade. Noncontact scanners that are faster than touch-probe CMMs have recently been offered to this market, but their weakness lies in that they cannot scan freshly machined or highly polished surfaces unless 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 3-D measurement with a ball probe is eliminated. Consequently, Maxos has an exceptionally high accuracy of ±2 micrometers on matte surfaces and ±10 micrometers on polished metal. It can achieve a point spacing resolution of 0.2 micrometer without pausing. Additionally, because 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 millimeter. This feature is key to the measurement of leading and trailing edges on turbine blades.
- NVision Inc.