Automotive Engineering - December 2022 - 28

DIMENSIONAL METROLOGY
BEYOND THE CMM
A CT-scanned object compared with its respective
CAD data set. Color-coded analysis results and local
annotations show deviations. The software allows for
geometric comparison of voxel, mesh, and CAD data.
Computed Tomography
takes inspection beyond the
coordinate-measuring machine
in reducing machining scrap
and increasing throughput.
by Johannes Mann
I
n the world of high-volume automotive cylinder-head production,
the yields realized from casting processes count for a lot. Having
to reprocess faulty heads is inefficient enough. Sending them
through expensive post-cast machining processes, only to find
that they are not up to standard, is an equal or greater cost. Latestage
rejection of any machined product because of sub-spec quality
means that machining assets have been stranded, labor wasted,
throughput slowed and product-to-market-fulfillment time lost.
Porosity is the main culprit in rejection of cylinder heads, whether
aluminum or iron. All castings exhibit porosity. The real question is
" how much " and " where? " Porosity control is very important in
complex, thin-wall structures such as between water and oil carrying
passages, and potentially less critical in thicker, less-optimized
areas such as the engine block.
Excessive porosity can lead to structural fractures, breaks and
leakage in these areas and elsewhere. Downstream machining of
the deck and corresponding block surfaces is done, for example, to
level the mated surfaces. A near-perfect, non-porous surface between
these major components is required for proper gasket sealing
and lifespan, along with long-term engine performance.
Comprehensive data
Coordinate-measuring machines (CMMs) are an essential and standard
tool for dimensional metrology. Yet they don't quantify the
28 December 2022
actual, assembled clearance between mated surfaces
or test the fit and function of interior parts. CMMs complement
Computed Tomography (CT) analysis to cover
all aspects from ultra-precise surface measuring to interior
quality. CT analysis is a non-destructive testing
(NDT) method, as are the less-penetrating approaches
of magnetic particle and eddy current inspection.
Destructive slicing and testing generally are used on
raw castings before machining and heat treating and
before assembly. A shortcoming is that this approach
can eliminate good parts and decrease yields, can't be
economically done with regularity, and does not check
part fit and function. Additionally, destructive testing
only can expose porosity and fractures in predefined,
interior regions of the component, potentially not revealing
the true number and scale of the defects -
leading to false positive ('OK') decisions.
CT analysis, on the other hand, addresses this lack of
scope. It can be done on the raw casting, ahead of expensive
post-processing, in all regions of the cylinder
head. Once the cylinder-head features - outer envelope
of the head, bolt holes, under-valve cover features,
valve seats and such - are machined, the cast is complete
and ready for pre-assembly CT scanning and analysis
or final fitted-assembly testing. The process is
AUTOMOTIVE ENGINEERING
VOLUME GRAPHICS
Automotive Engineering - December 2022

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