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Since the dawn of the jet age, turbine blade design and

fabrication has been continually evolving. Airfl ow over the blade affects overall effi ciency of the turbine engine. Aerodynamic design of wings (air foils) and use of stronger, lighter-weight composite materials are evolving along with turbine engine de-sign. In recent years, engine de-sign has tried to balance the two confl icting demands of increased thrust and better fuel effi ciency. 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 en-gines. Airfl ow over and around the blades directly affects the amount of thrust produced, which propels the aircraft. Their complex curves have critical di-mensions that must be measured at numerous places across the blade. Typical measurements in-clude blade cross-sections at sev-eral 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

Quality in Aerospace – Turbine Blade MetrologyFrom Optical Gaging Products, Inc. (OGP®)

By William Verwys, Applications Engineering Manager, OGPEdited By Heather DeAngelis, Assistant Editor

By etching into glass, the glass became a permanent record of a particular blade.

The technology to measure turbine blades advanced alongside improve-ments in blade de-sign and fabrication. As manufacturing processes improve, tolerances get tighter. With visual inspec-tion of blade cross-sections as the pri-mary measurement, increasing the mag-nifi cation of inspec-tion devices allowed greater precision. Since optical com-parators were accept-ed tools for contour measurement in almost all in-dustries, 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 magnifi ed ten times. Quality control was verifi ed by comparing the mag-nifi ed etched contours to overlay charts with the master profi les on them.

Visual inspection of contours continued to be the inspection method of choice in years fol-

Illustration from 1946 edition of SAE Journal show-ing turbine blade

methods were required. One of the metrology prob-

lems with turbine blades from the very beginning is measuring the blade cross-section nonde-structively. This is because the cross-section has continually varying radii that twist in posi-tion 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 specifi c profi les at locations along the length of the blade, together with the magni-tude of the twist, were key goals of turbine blade metrology.

In the 1940s and ’50s, spe-cialized tracing devices (one-to-one pantographs) traced the cross-section 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 mea-sured directly. An evolution of this device manufactured by OGP in 1955 as the Panta Scrib-er Model 624, scribed the blade contours on glass. These scribed cross-sections were then staged on a special optical comparator for detailed inspection.

A unique characteristic of the PantaScriber 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.

lowing, 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 at positions along the length of the blade. The opening between op-posing sets of pins was projected on an optical comparator for in-spection as with shadowgraphs of the time.

In the 1960s, video cameras came into use in metrology. An optical device performed cross-

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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 ac-curately. Translating the blade while observ-

ing 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 cus-tom 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 al-loys 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 uni-form performance of the entire engine when operating at full thrust. Cooling holes in tur-bine blades are not a new concept. Since their implementation in the 1960s, cooling holes were typically measured by manually insert-ing pin gages into every hole in each blade.

Tighter tolerances and critical manufac-

OGP PantaScriber from 1955

Pin Form Blade Checker image

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turing demands have led to more thorough metrology. Cost and effi ciency 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 chal-

lenge. Mounting the blades in a compound rotary indexer overcomes this limitation and allows for automated metrology of both cool-ing 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 ap-plications. An attraction of video measure-ment is that it is non-contact. Today’s high performance cameras, quality optics, LED illuminators, servo driven stages, and high-speed computers are improving productiv-ity by measuring critical dimensions much faster and more accurately than past manual methods. Motorized zoom optics allow mea-surement of large areas at low magnifi cations and accurate measurement of each cooling hole at high magnifi cation without touch-ing the system. The programmability of these systems allows any operator to get repeatable performance for every part. From the manu-facturing process perspective, the measure-ment 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 fi xed position and moving it or the sys-tem optics only in the X, Y, and Z axes. Three axes of motion were not enough. Adding dual rotary indexers provides fi ve axes of motion. With such an arrangement, every cooling hole can be presented at an optimal orienta-tion to the imaging optics, no matter where it is located on the blade. Well-designed optical systems provide both the necessary magnifi ca-tion and suffi cient 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. Program-mable LED ringlights make optimal lighting easy. Selection of inner or outer rings of LEDs

Video Sectioner with close-up of blade and pro-jected illumination

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Fitting software takes the measurement data and does further mathematical processing to compare against CAD fi les. Two-dimensional fi tting can show profi le tolerance conditions for airfoil cross-sections and leading and trailing edge radii. Three-dimensional fi tting provides best fi t analysis of complex shapes for comparison with IGES design fi les. De-tailed data is available for every measured point, and interactive color models graphi-cally show tolerance conditions. Histograms show distribution of measured points while topographic maps show shape errors. Whis-ker plots show the location and magnitude of out-of-tolerance conditions.

Turbine blades used in aerospace jet en-gine applications are being designed for greater performance and fuel effi ciency. With blades being pushed to their limits, reliable, accurate metrology is required. The lat-est multisensor measurement systems, with their metrology and fi tting software applica-tions, are able to provide the necessary mea-surements at the throughput rates required to support development and production.

Turbine blade on dual rotary indexer on video measuring machine

affects the angle of the light striking the sur-face. Selection of segments of LEDs affects the direction of the light. And the intensity is easily varied for best signal-to-noise per-formance. Good illumination provides for consistent measurements, even of irregular or misshapen holes.

Metrology software provides an impor-tant cooling hole metrology function. A Centroid or blob analysis of each hole pres-ents 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 profi le the curves across a blade. By tight integration with the metrology software, scanning can take place as the rotary index-ers 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.

Cooling hole imaging with Centroid tool

Touch probe scanning turbine blade

Screen image of 3D best fi tting analysis of an airfoil