Since the part or sample is altered or destroyed during testing, it cannot be retested.For example, in heat treating of steel tubes, a tensile test is often done to measure tensile strength.
Without the ability to retest, how can you estimate the Gage R&R? First, you have to be able 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. In the perfect world, if you took any sample from that batch, the test result would be the same. 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.Table 1: Nested Gage R&R Raw Data We will use analysis of variance (ANOVA) to analyze the results of our destructive Gage R&R study.This analysis method was described in detail on our three part series on ANOVA Gage R&R.Figure 1: Destructive (Nested) Gage R&RYou are involved in heat treating of parts and want to perform a Gage R&R analysis on the hardness tester. You are confident that the parts within a batch are homogeneous.To measure hardness, a piece of the product is cut, prepared and tested. You want to include three operators in the Gage R&R study.If you are using the measurement for process control or SPC, then you use the first or second method above.If you are using the measurement system for inspection only, you would use the specification approach. To use the process standard deviation, you need an estimate of that standard deviation.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.Figure 1 shows how a destructive Gage R&R is laid out.How you set up the experiment determines what sources of variation you can analyze.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.