Sunday, October 30, 2011

Benefits of machine vision inspection – porosity detection

Casting processes tend to produce porosity – tiny bubbles in the metal – and these are uncovered when the metal is machined. Porosity on sealing surfaces is a problem because the product may fail in service, so to prevent customer complaints, machined products are often inspected. As this is usually done by human inspectors it represents an opportunity for machine vision.

Most vision projects start out by trying to achieve the same level of defect detection as the human inspector, and that’s where things get interesting. If you look at the work instructions or quality standards you’ll see that they usually call out a minimum size for porosity of about 0.300mm.

Does this mean that the sealing surfaces don’t leak if the porosity is smaller? Not at all. The minimum size is based on what an inspector can see. 0.300mm is about the size of a speck of pepper, and since porosity looks black against a white surface, if it’s this size or bigger it’s usually detectable. Any smaller and there’s a high probability it will be missed.

But if the point of inspection is to ensure the customer gets only good product, shouldn’t the minimum size be dictated by the product and not the eyes of the inspector?

If larger porosity – up to 0.500mm is acceptable, then perhaps good product is being scrapped. After all, the inspector doesn’t measure the porosity, they just reject if any is seen. In this scenario machine vision will actually reduce waste.

Conversely, if the product still leaks at 0.300mm then perhaps the standard needs to be tighter. Machine vision can do that, so in this scenario it will lead to improved product performance.

Now let me throw a question at you. Most quality standards for porosity are defined by area, but pores have depth too. A pore with a 2D area of 0.250mm might still create a leak if it’s 1.000mm deep. What should we do about that?

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