METHODOLOGY FOR ASSESSING MEASUREMENT ERROR FOR CASTING SURFACE INSPECTION
G. Daricilar and F. Peters Iowa State University, Ames, IA
Copyright © 2011 American Foundry Society Abstract
Visual assessment of objects is critical to many fields including metalcasting. While the output for such a task is often a simple attribute, the problem studied here is for inspection tasks requiring an output that defines shape, size and location of anomalous areas, which are random and are not defined a priori. This paper defines a methodology to quantify the amount of repeatability and reproducibility variation. The application of the methodology for the visual surface assessment of steel castings reveals
Introduction
This paper develops a methodology for determining the measurement error for the visual assessment process used to determine the presence and size of surface anomalies. Specifically, the method was developed for the inspection of steel casting surfaces, but could be extended to many other visual assessment tasks. During the processing of the castings, the surface is typically inspected several times to determine if it meets customer requirements. The inspec- tions identify unacceptable casting surface defects such as inclusions, porosity, burnt-on sand, and flash. The parts that are marked for surface defects are then taken through a series of cleaning operations, where the marked defects are mitigated by welding and/or grinding.
The most common method for communicating the sur- face requirements and determining acceptable surfaces is to use surface comparator plates or photographs. These methods are only qualitative and based on human judg- ment as there is not a method of specifically classify- ing casting surface conditions. For instance, employing a surface prolifometer as is done for machined surfaces cannot capture the longer range of the surface indications found on a casting. One commonly used standard, ASTM A802, relies on the comparison of the subject surface with comparator plates from the Steel Castings Research and Trade Association.1,2
Photographs of some sample plates
are shown in Figure 1. Another standard, MSS SP-55, re- lies on photographs of casting surfaces, such as shown in Figure 2, for the inspector to make comparisons with the subject surface.3
International Journal of Metalcasting/Summer 2011 The result is that the description of
significant repeatability and reproducibility error. The work presented here is the impetus for current efforts that are defining the capabilities of the visual inspection process and ways to improve it through the selection, training and retraining of operators and through better control of the process.
Keywords: metalcasting, subjective evaluations, surface inspection
the casting surfaces is uncertain at best and impossible at worst. The lack of consistent surface evaluations poten- tially results in unnecessary costs. Undetected surface de- fects result in unacceptable quality standards and returns from the customer; marking minor surface imperfections as defects will result in excessive processing.
NOTE: The comparators for definition of casting surface quality, referred to in this paper can be purchased from Cast- ings Technology International (England).
As with all assessment procedures, measurement error will inherently exist. Measurement is more complicated when concerned with freeform areas; likewise, the assess- ment of measurement error in these cases is also more difficult. The objective of this study is to develop a meth- odology to quantify the amount of variation in terms of repeatability (variation within the same operator) and re- producibility (variation between different operators) for visual assessment tasks.
There are many literature sources on measurement error for a variety of measurement systems. Much of this work is for measurements in which the output is a continuous variable such as a length, viscosity, or temperature, which does not apply to the problem at hand with visual inspec- tions. The Automotive Industry Action Group (AIAG) published a reference manual for analysis of such mea- surement systems.4
A significant group of literature sources include those re- garding attribute measurements in which the output is bi-
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