Table of Contents
16.2.2 Damaging Effects from Finishing
In the following, damaging effects from finishing are understood to be effects that have an unallowable influence on the specified characteristics of the part that are necessary for operating behavior. The term “finishing” refers to all work and testing that is done following raw part production. These problems may already occur and be recognized during the finishing process. In this case, the primary damage will be in the form of additional costs and time due to scrapping or repair. In addition, there may be penalties for failing to meet supply contracts, as well as the incalculable costs of the resulting loss of prestige.
Above and beyond this, undetected finishing problems and damages in parts that are installed in engines are highly relevant to flight safety. This means that the consequences may be far more serious than in cases in which the damages were discovered during the production process.
A prerequisite for damage limitation is identifying and limiting the damage mechanism, causal influences, and affected parts.
Experience has shown that damage mechanisms can be attributed to typical damage symptoms/effects, part-specific problems, and finishing processes. Depending on the point in the finishing process or operation in which the damage occurs or is recognized, a few guidelines can help in identifying the mechanism.
If the problem is detected during the production process, it will often not be immediately attributable to a specific process (e.g. in the case of cracking). In order to solve the problem as quickly as possible, the damage symptoms/effects should be used to identify the causal process. For this reason, these effects are collected in this chapter.
If a specific process is suspected to be the cause, process-specific problems should be consulted to determine the damage mechanism. These are found in Chapter 16.2.1.
In the case of operating damages, in addition to researching effects, part-specific problems should especially be researched in order to determine the damage mechanism. An aid for important engine components is found on pages 16.2-6 to 16.2-12.
The large number of possible damaging effects from finishing requires that they be limited to those which experience has shown to be important (safety) or frequent (costs and probability). Technicians will not always immediately know which seemingly harmless finishing problems lead to extensive, cost-intensive actions such as scrapping or extensive reworking. Because of this, it is important that damage prevention is given sufficient emphasis before (work preparation) and during the finishing process. Therefore, this text explains causal relationships that are often not apparent at first glance. For this, it is important to know and understand part-specific functions and operating influences.
If one is aware that characteristics such as burrs or cracks can occur during many different processes, process combinations, and influences, then it is clear that flaws are not necessarily caused by the process during which they are discovered. This makes the understanding of finishing damages and the development of solutions especially demanding (also see Chapter 17.1).
The increasing demands on the load levels and operating characteristics of parts, combined with a high stage of maturity, will promote the trend towards improving the operating behavior with a combination of small steps using synergy. Turbine engines are currently in a stage of development that is similar to that of Formula 1 cars. Success can only be attained through the complete optimization of very minor influences. This becomes clear when one considers that an average speed advantage of one thousandth represents a significant technical advantage. Finishing consists of a chain of many separate processes that act upon one another according to the sequence in which they are implemented. Therefore, details that were disregarded in the past must now be focused upon. The topography of functional surfaces such as blades, blade root contact surfaces, and rubbing surfaces are examples in which even minor improvements must be utilized. Design and construction of the parts must be incorporated into the optimization process along with finishing. This convincingly reinforces the idea of teamwork in a whole process that considers all relevant influences.