Technical Summary  
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Technical Summary

"Circular Geometry" inspection refers to the process of measuring parts that have cylindrical surfaces. In high performance machinery (Aircraft Engines, Gas Turbines, Automobiles, etc.), the geometry of these surfaces is critical, and it is often the case that very tight tolerances govern the surface-to-surface and part-to-part geometry.

Because of the extremely tight tolerances required in these situations, this type measurement is not well suited for coordinate measuring machines (CMMs), and in fact it is recommended that CMMs not be used. Instead, specialized metrology devices have been developed for this purpose where the part is rotated while data is collected using stationary LVDTs.

Sophisticated analysis of the inspection data is required to produce accurate measurements with respect to the datum surfaces of the part. AccuScan uses advanced fitting algorithms that meet or exceed those required by the governing specification for these type measurements, ANSI/ASME B89.3.1.

An Illustrated Example - Rotor Stacking
A specific example of the need for accurate circular geometry data is turbine metrology and the assembly of a multi-stage compressor or turbine rotor for an industrial gas turbine or aircraft engine, such as one of the compressor rotors produced by GE Gas Turbines.  With this rotor design, the individual pieces of the rotor are stacked like coins to produce a rotor assembly.  The stack is then held together by means of through bolts (see Figure 1).  This assembly process is called “rotor stacking”, and the goal of the stacking process is to produce the highest quality rotor, as determined by an in-process measurement (typically low rotor unbalance or runout). 


Figure 1 - Rotor Stacking Process

The difficulty with assembling a rotor of this type is that none of the individual parts have perfectly uniform thickness or perfectly concentric rabbets (adjoining male and female diameters), and these variations introduce geometric inconsistencies into the rotor, which can result in high runout measurements or high rotor unbalance (see Figure 2).


Figure 2 - Effect of Part Variability on Rotor Stack

Since the goal of the stacking process is to produce a rotor with these conditions minimized, it is important to try to optimize the assembly such that the individual part variations have a negligible effect on overall rotor quality.

In this case, a computer program is used to do predictive modeling of the assembly using the circular geometry data of the individual parts of the rotor. This circular geometry data can be calculated using an AccuScan inspection system.

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