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Why are there two terms, total indicator reading (TIR) and full indicator movement (FIM), for the same measurement?

Essentially, the total indicator reading (TIR) and the full indicator movement (FIM) measurements are different names for the same output. Both of these terms are assessing the degree of difference between the highest and the lowest point on the surface of a part. The subtle difference between them is that TIR relies on the readout of MIN and MAX from an indicator, whereas FIM relies on the zero cosine error and thus provides a slightly more accurate depiction of the actual movement of the indicator along the surface of the part. Both of these terms refer to the discrepancy along surface smoothness and shape, and can be used for similar purposes. The biggest reason that both terms exist is likely a delay in updating both professionals and materials. Most engineers today were educated using the term TIR and the majority of paperwork in the field still uses TIR terminology. FIM is a newer term and will take some time to become the standard in the field.
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