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.
|
