Blade Inspection and Repair

Currently, it takes dedicated mechanical tooling jigs and fixtures, in conjunction with mechanical touch probes on coordinate measuring machines (CMMs), to accomplish blade repair inspection. However, jigs and tooling fixtures suffer from the following disadvantages:

• High initial cost
• Fast mechanical deterioration
• Lack of digital inspection records
• High dependency on the operator skills
• No flexibility for different blade models and sizes. In addition to these limitations, the mechanical probes on the CMMs can also suffer from a number of shortcomings:
• A mechanical probe needs to maintain continuous contact with the surface and, therefore, is limited in its scanning speed.
• The amount of useful data that can be collected using mechanical probes is limited.
• Since complicated freeform 3D geometry shapes characterize blades, the traditional CMM solution is very imited both in hardware and in measurement software.
• By nature, the dynamic range of a mechanical probe is zero or almost zero, which makes the scanning pattern, the alignment routine, and the programming of the scan path more complicated.
• MRO blades are significantly deviating in form and dimensions from the original CAD model, making the touch probe scanning problematic since basis of the programming is on CAD model freeform perpendicularity.
• Diameter of the stylus limit the resolution and size of the features that can be scanned, so features such as leading and trailing edges are not good candidates for any mechanical probe measurements.

One of the key challenges of a blade measurement is the blade alignment – finding the exact blade position in space relative to the CMM and moving the coordinate system from the CMM to the blade. It is a tricky trial and error approach unless one is using very high precision (and very expensive) mechanical jigs.

The need for an efficient, accurate, and fast non-contact laser based metrology system for fan, turbine, and compressor blades manufacturing and service has been an industry challenge for years. While various non-contact systems exist in the market for vision, calibration, and digitizing, none has overcome the technology barriers and provided the required performance and speed to bring efficient, fast, and top-accurate 3D sensor to the blade measurement world.

The traditional design of laser-based probes builds around the fundamental triangulation principle (right). It involves the manipulation of a laser beam projected back from the object through a dedicated set of optics in a fixed position and angle, and an optical detector (a position sensing device – PSD or a charge-coupled device) – CCD). These sensors are very sensitive to the optical parameters of the measured object, including color, material, glare and reflection, surface finish, and relative angle between the laser beam and the object. This sensitivity generates large deviations and unreliable output in the blade measurement results.

Non-contact vision systems started to offer various 3D applications leveraging on their 2D performance capabilities. However, when used for 3D applications, the vision systems provide fast 3D scanning but very limited accuracy.

A real breakthrough in this area is a new 3D laser-based scanning sensor developed by Nextec, combining advanced laser and vision technologies (right). It comprises an adaptable laser source, a sophisticated set of optics, 2D CCD sensor, and an advanced real time adaptive control. In addition, this sensor uses an extensive set of image processing algorithms to analyze the acquired 2D CCD image.
 

MRO Blade Industry Trend
With the growing demand for servicing and checking complex geometry blades, and a continuous search for higher speed and accuracy, the need for automatic blade alignment, fast data acquisition, and real time comparison of measured blade dimensions to its CAD design is a major requirement. MRO environments are trying to achieve the following initiatives:

• Achieve significantly higher throughput and yield
• Provide high accuracy inspection
• Achieve higher blade measurement speed
• Minimize the requirements from the operator
• Provide real time process control records.

Typical blade measurement applications include high precision software alignment to avoid expensive high precision jigs and fixtures; airfoil cross-sections inspection with emphasis on blade thickness profile, chord length deterioration and twist, and root features measurement.

Specialized Blade Solution
To address these challenges, development of this 3D laser-based scanning sensor, along with a state of the art motion controller and special application software, results in the WIZblade inspection solution – specifically for blade alignment, blade measurement, blade analysis, and reporting.

This solution provides a single point precision of 12µ and a feature precision of 2µ (100 points best fit), meeting the highest precision requirements for blade measurement. The small laser spot size enables to measure very fine geometry details, namely the leading edge and trailing edge actual profiles.

The combination of proprietary optics, unique adaptive control feature, and sophisticated image processing software allows this solution to handles all material and surface finish types, automatically performing self-calibration via real time closed loop adaptive control.

The combination of a large dynamic range and high accuracy makes it very simple to set up the alignment loop and the airfoil cross-section path and collect hundreds of points fast and accurate, with no noise.

Automating MRO Blade Repair Process
One of the typical applications for this WIZblade solution is the MRO service of fan blades. Typical fan blades suffer from service deterioration after thousands of flight hours.

This identification of deterioration is usually by:

• Blade edge deterioration resulting in chord length reduction at certain aerofoil regions
• Blade thickness reduction at certain high wear areas
• Blade twist distortion from an origi- nal new blade.

To restore blades to their original chord length and edge thickness, MRO companies use patch inserts. To help automate this process, Nextec engineers recently developed a close-loop solution for fan blade overhaul, which follows these steps:

First, the blade is positioned vertically on the WIZblade granite table. The WIZblade system then measures the entire blade in cross sections, with large blades typically having 25 cross-sections.

Next, the WIZblade software automatically calculates four different patch types as defined by the OEM (a large patch, a small patch, a diagonal patch on the leading edge side or a diagonal patch on the trailing edge side). If the chord length measures below a certain defined tolerance, the WIZblade software automatically generates a decision to cut and weld a patch. The operator has the data summary on the monitor screen with an option to select the patch type.

The laser spot will then trace the patch location and size to mark it on the blade airfoil. The recess is then cut on the blade by either CNC machining or by laser cutting. An insert with a thicker thickness is then welded in the recess by an electron beam welding (EBW) process to form the new patch.

The WIZblade software automatically generates the final airfoil sections, so that they blend with the original airfoil freeform shape, and sends the final patch cross-section profiles to the CAM software. The CAM software generates the relevant G-Code program for milling the individual patch on the individual blade. Since every individual fan blade is different in geometry and twist due to manufacturing tolerances and service effects, automatic machining of each blade patch is in accordance to this individual G-Code program.

The WIZblade solution also suits the production floor where it can inspect small and precise blades as well as large blades. The entire inspection process for a new blade type includes the blade CAD model input, cross sections selection definitions, automatic blade alignment, transformation of coordinate system into blade orientation (i.e., the stacking axis becomes the Z-axis in space), measurement of the relevant cross-sections, and actual deviations from the CAD model.

A typical automatic blade alignment Time is 50 seconds, and each full cross section scanning (double-sided) takes only eight seconds. During the scanning process, the system records the numerical data in real-time and displays measurement results on the screen. The data is automatically compared to the designed values and to the required tolerances. Thus, the measurement results and quality control reports are generated in a real time.

In instances where this approach has been implemented in blade production, the complete quality control procedure which lasted eight hours using a conventional CMM with a touch probe has been reduced to only 55 minutes. This allows manufacturers to double throughput, to overcome bottlenecks at the quality control stage, and achieve a faster delivery to the customer while testing a higher sample rate.

The WIZblade system can serve as a turnkey solution or a kit mounted on an existing CMM platform. Every single WIZblade system installed eliminates the need in purchasing four to five additional CMMs.
 

Nextec Laser Metrology
Eastlake, OH
www.nextec-laser.com