Dimensional Metrology Beyond the CMM

Computed Tomography takes inspection beyond the coordinate-measuring machine in reducing machining scrap and increasing throughput.

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.

In 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. Late-stage 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.

Wall thickness analysis is especially critical for cast parts. Mold and part design, along with material and process variations, can lead a component to fall short of its specification. CT software analysis provides deep insights into all stages of development, helping ensure final quality and higher yields.

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 seal- ing 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 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.

Layout of a typical inline inspection station featuring Computed Tomography.

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 straightforward and can be fully automated. The data generated encompasses the product in its entirety.

Improving yields, reducing cost

What is an optimal way to use CT scanning software for improving the total efficiency of casting a variety of automotive metal components? Several leading OEMs and suppliers combine inline and at-line inspection during production. Raw castings and finished, machined castings are candidates for either, mostly in random or predetermined lots.

Examining raw castings prevents process faults and mold design errors from carrying over into production. Examination of post-cast, cleaned, machined and treated heads allows teams to understand metallurgical changes derived from the finishing steps. It’s important to realize that CT inspection at both raw and finished product development stages lends huge insights into CAD design, mold design and the casting process itself, as well as the final variations that can occur in the “as-manufactured” product.

Conducting a software analysis of the scan in the beginning and later periods essentially permits continuous process control. Companies can determine, based on the complexity of the analysis, whether parts should be taken out of the running production or inspected as they are made.

Are the inspection costs worth the savings? Again, post-processing a sub-spec raw cast part that gets rejected downstream is as costly or more so than creating the initial casting. Consider, too, that poor-quality parts can make their way to the consumer, causing recalls and damage to the company’s brand. There are countless examples of warped heads and poorly sealed gaskets that led to warranty claims and recalls.

From 6% to 2% scrap rate

One large automotive OEM using CT analysis software during cylinder-head production discovered immediate gains and rapid return-on-investment. Using select, at-line inspection methods for aluminum DOHC heads, the OEM went from a disappointing 6% scrap rate (at the raw casting stage) to just a 2% scrap-and-reprocessing rate. For gravity casting, this is an excellent production yield. Moreover, the resulting statistical information taught a great deal about the process drift and interdependency of each juncture of head design and manufacturing.

Since its implementation of CT software analysis, this manufacturer has better-utilized machining and post-processing resources. Bottlenecks have been removed. The company has saved energy, labor and all costs associated with upstream design and downstream manufacturing. CT analysis contributes to total cost reduction and quality improvement.

Are the CT equipment and software acquisition costs worth it? Automated inspection is the way to free valuable employees and their knowledge to solve future growth and innovation challenges. Capture productivity and quality – and let automation handle the repetitive inspection tasks. Every group in product development can benefit from the information gained.

Johannes Mann is director, global industry processes, at Hexagon’s Manufacturing Intelligence division.