Blade Measurement: Software's Leading Edge

Dan Jeanloz | January February 2011

Shown is a blade Aerofoil Program being executed in PC-DMIS measurement software.There is nothing new about the use of blades to create air movement. We have been doing it for centuries. Even so, the pace of development for new blade designs has never been faster, particularly in the jet engine industry. For vendors of measurement systems and software serving this industry, the pace of development has spurred the development of a new generation of powerful, flexible, innovative products.

Blade manufacturers are using various combinations of machining processes, including forging, casting, ECM (electro chemical machining), and multi-axis CNC machining. All of these require specialized measurement software to verify the artifacts they produce. For example, in a process like casting, it is very important that the software support high measurement throughputs to verify blades produced in large batches quickly. On the other hand, in high precision manufacturing operations, where the final blade shapes are produced, throughput takes a backseat to high measurement precision and advanced analytical capabilities. The same is true for R&D.

The ability to scan and accurately reverse engineer blades is very important to third party blade manufacturers who must secure regulatory approval for their products. Sampling a large number of blades, they use their software’s analytical capabilities to superimpose the models generated by measurements coming from a large number of blades over one another to produce a composite model demonstrating that average geometries meet critical specifications.

A primary objective of all types of blade manufacturers is to refine blade materials and geometries to produce lightweight blades that pull air in, compress it, and deliver it into combustion chambers more efficiently – inturn creating greater power with lower fuel consumption.

OEMs have undertaken substantial R&D efforts to make their blades lighter by using composites of traditional materials combined with carbon fiber composites or aluminum. To preserve strength at the most critical points, they mate these with leading blade edges of titanium.

As for geometries, blades have evolved from almost flat 2D fans into deeply sculptured 3D air-scoop surfaces. There are many air compression stages and different combinations of blade assemblies that work together to compress air and deliver it efficiently to the combustion section of the engine. Because of the wide ranges of blade geometries and sizes in these sections, there continues to be opportunities for refining their arrangement and positions relative to each other to deliver higher efficiencies with less material and weight.

The requirement to enhance engine efficiency driving these changes is balanced by an unwavering commitment to preserve strength-to-weight ratios and structural integrity for the sake of safety. As a result, inspection equipment and software used for blade R&D, and for monitoring critical variables once a design is in production, has never been more important. While the engine OEMs have been hard at work devising better blade designs, measurement software vendors have been making comparable improvements in their products to match the pace of their customers’ developmental and manufacturing innovations.

Systems and Throughputs
There are more than 200 seats of blade measurement software in North America either in production or R&D environments. The users work for major jet engine OEMs, their suppliers, and third party blade manufacturers. The equipment they use can sometimes be very large – for example, gantry type CMMs that measure large blades for 10ft diameter engines. These blades can be up to 4ft long and weigh more than 60 lb. There are a large numbers of small tabletop CMMs, and some white light measurement systems that can measure blades and partial blades small enough to hold in the palm of your hand. These systems range in cost from about $100,000 to $750,000.

The majority of systems in use today are equipped with hard probes with scanning capabilities. The wide spread adoption of scanning probes in the past several years has dramatically improved measurement throughput in the industry. Even so, users are continually asking for systems that will measure faster. An answer to this requirement might prove to be white light scanning, which can theoretically provide measurement throughputs that are significantly higher and with greater accuracy.

Another major benefit of this technology is the zero probe diameter of the white light beam. This eliminates all probe compensation error. Hard probing systems minimize probe compensation error by using ultra-small (e.g. 1mm probe tips) along with advanced probe compensation algorithms. However, it is impossible to eliminate all probe compensation error from a hard probe system.

The output of white light measurement is readily comparable to the CAD model, making it a very good tool for R&D. Unfortunately; in production manufacturing multiple steps require verification. At $200,000 to $300,000 per system, equipping a plant with several white light systems might be cost prohibitive.

There are also a number of problems with white light technology that must be overcome before they are ready for large-scale use. One of most perplexing is figuring out how to deliver an unobstructed beam of light to all areas on a deeply sculptured blade.

Recent Software Advances
Graphic reporting capabilities of PC-DMIS measurement software make it easy to communicate blade measurement results by superimposing data and analyses on a CAD model of the blade segment.Until vendors overcome the cost and directional measurements problems of white light sensing, blade measurement will continue to be dominated by hard probing systems. While this is an older technology, software development efforts for these systems have been aggressive. Here are some important software enhancements introduced in the past several years to improve blade measurement capabilities.

Scanning: The incorporation of scanning capabilities within blade measurement software has significantly increased the throughput of blade measurement systems. It is now possible to scan a 1" cord section of a blade in about 20 seconds, and a 4" cord section in about 30 seconds. This represents a four-fold improvement over what is achievable with conventional touch trigger probing systems. Additionally, special software modules make it possible to develop scanning routines off-line while CMMs are productively measuring.

Blisk Measurement: Blisk machining relies on multi-axis CNC equipment to produce a single piece blade and disk assembly all in one operation. This results in higher component integrity and greater manufacturing efficiency. New measurement software functionality allows 5-axis CMMs to efficiently measure a complete blisk in one operation, and then analyze critical features relying on specialized blisk measurement capabilities. Where it used to take about 12 hours to measure a blisk with a touch trigger probe on a CMM, it is now possible to do the same job in about two hours using a CMM with a scanning probe and rotary table.

Variable Tolerancing: R&D tests in wind tunnels show that there are certain areas along the leading edge of the blade where tolerances are super-critical. Other tolerances along the same edge have been shown to be far less critical. By relaxing these tolerances, it is possible to maintain the close critical tolerances and stay within the allotted error budget for measurements.

In the past, this sort of variable tolerancing was only possible using incremental steps. New software algorithms (in PC-DMIS Blade 2010) make it possible to apply sophisticated tolerance variations along a curve, making it possible to maintain tighter surveillance of critical leading edge specifications without increasing the number of parts that are falsely rejected due to inflexible tolerancing protocols.

Partial and Skewed Sections: To develop more air pulling capacity within the tight confines of the blade assembly, designers have resorted to adding short partial and skewed blade sections at the root of the blade. In the past, blade measurement software did not allow for simple programming and analysis of these idiosyncratic sections. Now these capabilities are available in some software, making it possible to completely analyze or verify the complete blade geometry, automatically, with a CMM.

Robust DBs and Advanced Data Sharing: Today’s more complex blade geometries make it necessary to collect greater volumes of data for blade analysis and verification. Scanning probes and related software makes this easier. The increased volume of data has made it essential to collect information into massive, robust databases with industry standard access protocols (e.g. SQL Server) for fast access and easy sharing.

These new data architectures have in turn opened up new opportunities for data analysis and sharing. For example, web-based reporting software available for PC-DMIS measurement software allows data to be viewed remotely by those with access privileges, and configured into a wide variety of reports that can be distributed and read using standard web browsers putting everyone who is involved with the blade - from design to second and third tier suppliers - literally on the same page.

More To Come
Vendors of measurement software to the blade industry are still hard at work developing yet another generation of software for this demanding industry. Expect to see support for new geometries, additional analytical capabilities, and better accommodation of zero probe tip diameter white light scanning.

In the meantime, users have an array of tools for improving their measurement throughput, and analysis and reporting capabilities, using the generation of software that has just arrived on their doorstep.