Can you predict the functional fitness of a component based on dimensional accuracy?

by | Aug 13, 2018 | MetrologyEd | 0 comments

A commenter has raised an important aspect of dimensional metrology in this thread. The point is a very important and fundamental one connecting dimensional measurement and functioning of a manufactured component. The core question that we should ask when we do a dimensional measurement is this – how does this measurement help me in ensuring that the component will function properly?

Let us rephrase the question a little – can a dimensional measurement guarantee proper functionality? Well, the short answer is No. As a manufacturer of a dimensional measurement equipment, I would like to escape responsibility!! The nuanced answer is a long-winded one. Have you seen a situation where all dimensions of an actual manufactured component are well within the tolerance of the given design drawing, but the component does not function properly? The situation may sound familiar to many – QC rejects the whole batch and blames Manufacturing Team, which in turn blames supplier or the Design Team, who say they did exactly as told by customer and the drawings were approved by them. In the end, customer is upset because things don’t work, and no one is happy!!

The real issue is the extension of geometric dimensioning and tolerancing analysis from being a necessary condition to a sufficiency criterion during evaluation a manufactured component. In many situations this may be true, but in some cases, it leads to wrong conclusions and heartburn (and money-burn). To understand why, allow me to use a simple analogy in everyday life. Consider you are on a holiday on the banks of a river. Water is flowing constantly, and you can see fishes swimming around. You take out your shiny new camera phone and take a picture. From this picture, can you estimate the flow rate in the river? Our situation is similar. Let me explain why.

Coordinate-based measuring systems fundamentally pick points on the edge/surface of the object of measurement, and construct ideal geometries (like straight lines, circles, etc), thereby generating a geometric model of the component. The system proceeds to measure this model, NOT the component!  But in reality, a perfect line or circle is as achievable as a frictionless surface in physics! Hence, measuring systems give the nearest ideal geometry to what is actually there in the component. This is computed as a “best-fit” for the given set of points (the algorithms for calculating this may vary from machine to machine). If you ask the system to generate a circle, it will always give you a circle, although your component may be oval in shape. Hence, the actual measurement numbers that are output from your measuring system may NOT indicate what is actually happening in the component. While the best-fit answer may show that the component is accurate, the component may fail ( for e.g., if you are looking at a connector in a flow piping system, you may see a leak). The remedy to this kind of a problem is to assess the geometry more thoroughly and hence newer GD&T techniques and standards have been developed (this kind of situation is likely the mother of “Profile of Shape” concept in the ASME Y14.5-2009 GD&T standard).

The lesson to take from this discussion is that we should not blindly accept a component simply because it matches ALL dimensions within specified tolerances, and/or reject all components because some of them may not. You need to take care of which dimensions are critical for function, and have a clear understanding and methodology in place to assess the fitness of a component for functionality.

Awaiting your comments and response !