Functional and Free

Functional and Free
George Schuetz, Mahr Federal Inc.

When gaging with mechanical or electronic transducers, you can sometimes get more than you bargained for in terms of the results from the gage.

"Functional checks" involve using contact gages to simulate measuring a form or surface characteristic on a part. A form machine or an optical system is ultimately going to be the best way to perform roundness, flatness, straightness, or similar checks. The reason is simply that they do the check correctly per the ANSI spec for Dimensioning and Tolerancing.

In a perfect world, every critical dimension on a part would be tested with the best and most reliable equipment available. Unfortunately, we do not live in a perfect world. There is usually a long laundry list of things that get in the way—like the cost of the best equipment, the skill of the required operator, and the time required to perform the measurement. In a production environment, parts have to be measured quickly and affordably, often by the operator running the production equipment.

The best way to do form checks is, of course, with a form machine. Between the accuracy of the mechanics in these gages and their software capabilities, a lot of analysis can take place. But form machines come with a high price and using them takes time and training.

When a form machine is set up to make circularity (roundness) check, a number of steps are taken before the actual measurement is made. At least two traces are made of the part to establish an axis, and once this is established, additional traces are made at different planes to inspect for this form feature. A single circularity trace could take two or three minutes for each roundness check required.

The same is true with other form checks, including flatness, straightness, and parallelism, to mention a few. All these checks can be performed on a form machine—with high accuracy, but also high cost and time.

But many of these checks are also possible on a bench or fixture gage at a fraction of the cost and much more quickly. That's what we mean by "functional checks." However, as with every other decision, the risks and rewards have to be assessed. The functional check will be quick and easy, but you have to decide how critical the check is and whether the risk is too great to measure the part this way.

If the part has very, very tight tolerances and is part of a life and death application, it may be a requirement to spend the time and money doing the best possible job of inspecting the part. However, if the tolerances are not that tight and there is a lot of pressure to get as many parts produced in the shortest amount of time possible, a functional check may be the answer.

Usually, such parts will be qualified first with the best method of inspection to understand the manufacturing process and the stability of the process. If it can be determined that the process is under control and will continue to produce the same level of quality, then the decision to use a functional check may make sense.

Form checks, such as circularity, parallelism, etc., are frequently used to refine the size requirements for the part. As such, they are always less than the size tolerance. For instance, a part may have a certain length tolerance. It may also have a parallelism tolerance on the length. The parallelism tolerance will almost certainly be smaller than the length tolerance. Thus, the parallelism check can be a free functional check as part of the length check.

Let's take a simple application where we need to measure the thickness, flatness and parallelism of a part. Typically, thickness would be measured using two opposing probes and a flat reference surface. A form machine could then be used to measure the flatness of each side and the parallelism of the surfaces. But that would be costly and take a lot of time. A functional check of the part could be performed instead by looking at the probe measurements in different combinations—i.e., measuring dynamically—to make the required form measurements. As can be seen in the figure, you can use the probes simultaneously to get your required measurements.

This is where free comes in: the probes are already there. By using them in different combinations—and in a dynamic mode—more results can be obtained for the same measurement "cost."

"Functional Checks" involve using contact gages to simulate measuring a form or surface characteristic on a part.