What’s the Difference Between TIR and FIM?
This blog post may seem like splitting hairs to some of you, but it’s a question that came up in my class. And you know the saying: if someone has a question, chances are that others are thinking the same question.
When inspecting parts for runout (and a few other characteristics), you may know that the classical method involves holding a dial indicator to the surface and then watching for the highest and lowest reading. This difference is then compared to the specification allowed by the drawing. Here’s a visual of runout being checked on an end face:
The absolute value of highest to lowest gage reading is often called “TIR,” or “total indicator reading.” In the past it was quite common to specify runout by adding a note to the drawing such as “.040 max. TIR.” (Nowadays, it is more proper to use GD&T to control this, especially because the former method is ambiguous when it comes to identifying the datum.)
Well, somewhere along the line another acronym crept into the vocabulary: “FIM,” which is “full indicator movement.” It essentially means the same thing — the total variation from highest to lowest gage point. But the Y14.5 standard uses FIM exclusively in its explanations for runout. Is there a reason?
Yes, and here’s where the “splitting hairs” part comes in: the way TIR is phrased emphasizes the reading shown on the dial face. But there may be a small error inherent in the dial indicator: after all, it’s made of springs and other mechanisms that might display a number that varies from the actual distance travelled by the gage tip. So the term FIM implies that we want to know the distance that the tip actually moves.
It may be semantics, but the world of technical standards is permeated by legalisms, and this was a small change in terminology to avoid the discussion of inherent error in the reading shown on the dial face. Aren’t you glad someone asked…. :)