Inspecting Finished Forgings with Resonant Inspection

July 1, 2005
A U.S. operation realizes more reliable defect detection and lower scrap rates and labor costs.

A forging operation in the United States recently installed two fully automated systems, supplied by Quasar International on a turnkey basis, to perform 100% inspections of hub-bearing component forgings. Management of the forge shop evaluated several NDT methods and chose the Quasar system for three reasons: more reliable defect detection, reduced scrap rate, and reduced labor cost.

The Quasar system has eliminated the false acceptance of defective parts due to human error and magnetic-particle equipment misapplication, or malfunction. Customers report that for the first time, they are receiving forgings with no significant defects. This forging operation — which supplies safety critical automotive components — was caught between customer demands for “zero defects,” or “six sigma,” and the generally accepted estimate that the typical magnetic particle operator detects only about 80% of the defective parts. Quasar offers a way to realistically meet customer demands.

The plant’s scrap rate is down compared to the previous magnetic-particle inspection, because parts with superficial mars, scratches, thin flaps or overlaps of metal that do not extend structurally into the forging, and other surface blemishes, are saved because these defects do not alter the structural integrity of the part.

Resonant inspection is based on structural discrimination and not visual indications. Typically, well over 50% of the forgings rejected by magnetic-particle inspection operators are acceptable when reviewed by plant managers. The percentage of false rejects exceeded 80% of the total rejects in several forging operations.

The most obvious effect, from a management perspective, is the reduction in the cost of inspection. The operation had been using an outside magnetic-particle inspection service at a cost of $0.32 to $0.42 per part. More recently, they moved the service in-house and reduced the cost to $0.18 per part. With the Quasar system, their cost is $0.04 per part, including labor and depreciation. This is a typical situation where the Quasar system pays for itself, in terms of labor cost, in less than a year.

In each of the two systems, the parts are manually loaded onto a turntable. A fully automated pick-and-place system moves the hub forgings from the turntable to the Quasar Production Nest. The nest is designed to provide a compatible interface with virtually any kind of material handling system. As long as the part is brought to within about 0.25 inch (6 mm) of the target drop location, the Production Nest is designed to “catch” the part and position it for testing. The Production Nest also provides shock and vibration isolation from the production machinery and senses the part temperature to compensate for variations.

The Quasar systems detect all structurally significant defects. Some of these include structural weakness caused by seams or inclusions in the steel feed stock; cracks from any cause, material-related problems, such as the wrong bar stock or out-of-spec steel; heat-treatment problems, which cause unacceptable or uneven hardness; and underfill, underweight, or overweight if it is significant.

Computer controlled sorting decisions
After the Production Nest receives the part, the automated computer controlled Quasar RI 2000 system takes about three seconds to measure a few key resonant frequencies. The computer then performs what amounts to an instantaneous structural analysis of that part, based on the resonances measured.

If the part falls within the acceptable limits for material, dimensions and freedom from structural defects, the part is accepted, and proceeds to packing and shipping. If the part is defective in any of the ways listed above the part is rejected, and the material handling system places the part in a reject bin. The entire test cycle takes about seven seconds.

The throughput is up to 500 parts per hour for each of the two systems. The throughput for a given application is a function of the part’s size and complexity and the relative defect size.

Quasar’s software makes it possible to inspect for defects that would be masked by the effect of normal product variations. The patented pattern recognition method detects localized structural differences that signal a crack or other defect. Material properties differences that are out of specification limits are also rejected. The responsible quality or production manager sets the acceptance limits, and the Quasar system enforces those standards. The same decision is made every time by the computer, with no chance for human judgment errors.

Quasar quantitatively measures the degree of defectiveness, so the greater the structural effect of the defect, the greater the rejection measurement. The forging plant management decides where to “draw the line” for acceptance and rejection for each product and for each customer. The engineer responsible for the Quasar systems uses the Quasar software tools to set up the sorting to those standards.