Quality in Aerospace Turbine Blade Metrology

From Optical Gaging Products, Inc. (OGP®)

Since the dawn of the jet age, turbine blade design and fabrication has been continually evolving. Airflow over the blade affects overall efficiency of the turbine engine. Aerodynamic design of wings (air foils) and use of stronger, lighter-weight composite materials are evolving along with turbine engine design. In recent years, engine design has tried to balance the two conflicting demands of increased thrust and better fuel efficiency. Competition for lucrative engine orders from the major airframe manufacturers drives innovation in designs. Of course, part of the development and testing process has been, and continues to be, thorough metrology.

Turbine blades are used in several stages of aircraft jet engines. Airflow over and around the blades directly affects the amount of thrust produced, which propels the aircraft. Their complex curves have critical dimensions that must be measured at numerous places across the blade. Typical measurements include blade cross-sections at several positions, leading edge radii, trailing edge radii, root forms, and cooling hole positions and sizes.

Developed during the 1940s, jet engines entered production after World War II. With designs so different from piston-driven radial engines, completely new manufacturing and metrology methods were required.

One of the metrology problems with turbine blades from the very beginning is measuring the blade cross-section nondestructively. This is because the crosssection has continually varying radii that twist in position from the root to the end of the blade. As a result, a typical optical comparator inspection with a projected shadow could not show these changes along the length of an entire blade. Extracting the specific profiles at locations along the length of the blade, together with the magnitude of the twist, were key goals of turbine blade metrology.

In the 1940s and '50s, specialized tracing devices (one-to-one pantographs) traced the crosssection on the chart gage while a probe was moved over the surface of the blade. That cross-section image was then compared to a model, or measured directly. An evolution of this device manu factured by OGP in 1955 as the Panta Scriber Model 624, scribed the blade contours on glass. These scribed cross-sections were then staged on a special optical com parator for detailed inspection.

A unique characteristic of the Panta Scriber was that multiple cross-sections could be etched into the same piece of glass. This allowed inspection and measurement of twist along the blade, and along the contours. By etching into glass, the glass became a permanent record of a particular blade.

The technology to measure turbine blades advanced alongside improvements in blade design and fabrication. As manufacturing processes improve, tolerances get tighter. With visual inspection of blade cross-sections as the primary measurement, increasing the magnification of inspection devices allowed greater precision. Since optical comparators were accepted tools for contour measurement in almost all industries, comparator technology was pushed as well. One example is the Model 60 Comparator from OGP in 1955. This device had a 60-in. diameter screen. By measuring the etched glass from the Panta Scriber on the Model 60, blades with cross sections as large as 6 in. were magnified ten times. Quality control was verified by comparing the magnified etched contours to overlay charts with the master profiles on them.

Visual inspection of contours continued to be the inspection method of choice in years following, but other techniques were developed for deriving the contours. One device was the Pin Form Blade Checker circa 1960. Similar to the "bed of nails" toy where the contour of something you push into the nails appears as a contour on the opposite side, this blade checker pushed pins along the cross-section atpositions along the length of the blade. The opening between opposing sets of pins was projected on an optical comparator for inspection.

In the 1960s, video cameras came into use in metrology. An optical device performed cross-sectioning with projected light. A slit of light was projected against the blade and imaged by a camera for projection on a TV monitor. With a reticle in the imaging system, radii could be measured quite accurately. Translating the blade while observing the projected image allowed continuous inspection along the entire length, versus the discrete steps required for previous methods. First produced by OGP in 1968, these video sectioners are still in production today.

This is by no means an exhaustive list of technologies used for measuring turbine blades since their inception. Numerous custom gages were used for these inspections. As noted earlier, the inspection technology has evolved alongside that of the blades and jet engines that use them. Today, there are larger and smaller turbines in use. Rolls-Royce's Trent engines used on Boeing 777 passenger jets each have 92 turbine blades. The 22 fan blades at the front of the GE90 engine used for this aircraft each weigh 46 pounds. High compression turbines towards the rear of the engine are smaller and closer together. With so much force and heat, the use of unique alloys has come into play. In addition, the use of cooling holes to dissipate heat from within each blade has become more common.

Today, turbine blade metrology includes more than the radii of blade cross-sections. The positions and sizes of cooling holes need to be consistent from blade to blade for uniform performance of the entire engine when operating at full thrust. Cooling holes in turbine blades are not a new concept. Since their implementation in the 1960s, cooling holes were typically measured by manually inserting pin gages into every hole in each blade.

Tighter tolerances and critical manufacturing demands have led to more thorough metrology. Cost and efficiency are driving the need for higher throughput, automated metrology. Go/no-go hole measurement with pin gages is not accurate enough when consistency in manufacturing processes is required. These 0.01-in. to 0.04-in. holes must be of a known size and in particular positions on the blades. Video measurement technology is good for this application, but automatically imaging every hole across the complex curves of these blades is a challenge. Mounting the blades in a compound rotary indexer overcomes this limitation and allows for automated metrology of both cooling holes and turbine blade critical radii and dimensions.

Video measuring systems are commonly used in manufacturing quality control for measuring dimensions of 2D and 3D parts made of many materials for a diversity of applications. An attraction of video measurement is that it is non-contact. Today's high performance cameras, quality optics, LED illuminators, servo driven stages, and high-speed computers are improving productivity by measuring critical dimensions much faster and more accurately than past manual methods. Motorized zoom optics allow measurement of large areas at low magnifications and accurate measurement of each cooling hole at high magnification without touching the system. The programmability of these systems allows any operator to get repeatable performance for every part. From the manufacturing process perspective, the measurement expertise resides in the system, not every individual operator. This removes one of the many sources of variability.

In the case of turbine blades with their complex curves it is impossible to measure all the important dimensions by placing the blade in a fixed position and moving it or the system optics only in the X, Y and Z axes. Three axes of motion were not enough. Adding dual rotary indexers provides five axes of motion. With such an arrangement, every cooling hole can be presented at an optimal orientation to the imaging optics, no matter where it is located on the blade. Well-designed optical systems provide both the necessary magnification and sufficient working distance to clear nearby parts of the blade.

Since the cooling holes are in surfaces of the blades that have a variety of convex and concave radii, optimal imaging of each hole requires different illumination. Programmable LED ringlights make optimal lighting easy. Selection of inner or outer rings of LEDs affects the angle of the light striking the surface. Selection of segments of LEDs affects the direction of the light. And the intensity is easily varied for best signal-to-noise performance. Good illumination provides for consistent measurements, even of irregular or misshapen holes.

Metrology software provides an important cooling hole metrology function. A Centroid or blob analysis of each hole presents the diameter and area. Through image processing the center location (true position) of each hole is provided - even if the hole is not symmetric in shape. Three dimensional metrology software retains the position of each cooling hole for viewing as a model and easy comparison to nominal design values.

As powerful as video metrology is for measuring cooling holes, some of the other critical blade dimensions are more easily measured with other sensor technologies. For example, scanning touch probes can profile the curves across a blade. By tight integration with the metrology software, scanning can take place as the rotary indexers move the blade. Alternatively, a laser can be scanned across the blade for non-contact measurement.

Acquiring the measurements is just one part of the analysis of turbine blade geometry. Fitting software takes the measurement data and does further mathematical processing to compare against CAD files. Two-dimensional fitting can show profile tolerance conditions for airfoil cross-sections and leading and trailing edge radii. Three-dimensional fitting provides best fit analysis of complex shapes for comparison with IGES design files. Detailed data is available for every measured point, and interactive color models graphically show tolerance conditions. Histograms show distribution of measured points while topographic maps show shape errors. Whisker plots show the location and magnitude of out-of-tolerance conditions.

Turbine blades used in aerospace jet engine applications are being designed for greater performance and fuel efficiency. With blades being pushed to their limits, reliable, accurate metrology is required. The latest multisensor measurement systems, with their metrology and fitting software applications, are able to provide the necessary measurements at the throughput rates required to support development and production.

For more information, please visit www.ogpnet.com.