Suppose you have done that and your process standard deviation is 2.5. It can come from a control chart kept in production or from calculating the standard deviation from a large amount of production data (be wary of special causes). To use the process standard deviation, you need an estimate of that standard deviation. We will see the results for all three here. If you are using the measurement system for inspection only, you would use the specification approach. If you are using the measurement for process control or SPC, then you use the first or second method above. Which of these you use depends on how you use the measurement system. Total variation in the parts in the study.Remember there are three things you can compare the Gage R&R results to: The data were analyzed using the SPC for Excel software package. You can review these newsletters for more information on the calculations. This analysis method was described in detail on our three part series on ANOVA Gage R&R. We will use analysis of variance (ANOVA) to analyze the results of our destructive Gage R&R study. The resulting from the Gage R&R study are shown in Table 1. Operator 1 will measure the two parts for batches 1 to 5 operator 2 will measure 2 parts from batches 6 -10 and operator 3 will measure 2 parts from batches 11 - 15. You decide to use 15 batches and take 2 parts from each batch. But there are not always enough parts for each operator to test parts from each batch. You would like each operator to test two parts per batch. You want to include three operators in the Gage R&R study. You are confident that the parts within a batch are homogeneous. That piece is altered, so it cannot be retested. To measure hardness, a piece of the product is cut, prepared and tested. You are involved in heat treating of parts and want to perform a Gage R&R analysis on the hardness tester. Since each batch is unique to a single operator, this is called a nested Gage R&R. Operator 2 runs two parts from batch 3 and two parts from batch 4. Operator 1 runs two parts from batch 1 and two parts from batch 2. This is not enough for every operator to run parts from each batch since the part is destroyed during testing. In this example, there are only two parts from each batch. However, if each operator cannot measure parts from each batch (e.g., not enough parts from each batch to do this), then a nested Gage R&R must be used.įigure 1 shows how a destructive Gage R&R is laid out. If all the operators can measure parts from each batch, then you can use the traditional method of running a Gage R&R - the crossed design. In the perfect world, if you took any sample from that batch, the test result would be the same. This means that the batch is homogeneous. First, you have to be able to assume that a batch of material is so close to the same that you can reasonably assume that the parts in the batch are the “same” part. Without the ability to retest, how can you estimate the Gage R&R? There are two critical items here. The sample is destroyed during testing so you cannot have the same or different operators test that sample again. For example, in heat treating of steel tubes, a tensile test is often done to measure tensile strength. Since the part or sample is altered or destroyed during testing, it cannot be retested. We will start with a quick review of how a destructive Gage R&R experiment is set up. Contribution to Total Standard Deviation, Specifications and Process Standard Deviation.This month’s addition to our SPC knowledge base examines how you analyze a destructive Gage R&R study. With a destructive Gage R&R study, the part is destroyed or altered during testing and you cannot measure it multiple times. In the classical Gage R&R study, a part is not altered or destroyed - you can re-test the same part multiple times. Last month we took a look at the differences in how a classical Gage R&R study and a destructive Gage R&R study are set up. How you set up the experiment determines what sources of variation you can analyze. Our SPC knowledge database has additional articles on how to perform other types of Gage R&R studies.Īny Gage R&R study is really an experiment to determine the various sources of variation. Have you ever had to support your position that your measurement system is well suited for its intended purpose? This addition to our SPC knowledge database takes a look at how you can do this when a part is altered or destroyed during testing.
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