How To Measure Concentricity

concentricity

As evidenced by the title, today we’re going to take a look at how to measure concentricity, what it is, and other related areas. First of all, let’s start off with a description.

What Is Concentricity?

It is sometimes referred to as coaxially. It is a tolerance that controls, to a datum axis, the central axis of a referenced feature. The referenced feature and axes for datum are derived from the feature or median points of a part. It’s exceedingly complicated, this concentricity. Why? Because it is reliant upon derived axis as opposed tangible feature or surface measurements.

When referring to GD&T symmetry, concentricity is considered the circular form. The measurement of a derived midpoint axis to a datum axis is referred to as concentricity, while symmetry applies to a feature’s true midpoint plane to a datum axis or plane. In both cases, it is extremely troublesome to calculate an exact measurement. You may hear the term “runout” referred to. This is a combination of circularity and concentricity.

By fixing all datum points, runout can be measured by rotating the part along the rotational axis. A spindle of some sort or set of V blocks is commonly used to constrain the part while still making sure that it can rotate.

Tolerance Zone – GD&T

GD&T (or geometric dimensioning and tolerancing) is a system for communicating and defining tolerances in engineering. It uses computer-generated 3D solid models and engineering drawings’ symbolic language that describe, explicitly, nominal geometry and its allowable variation.

A three-dimensional cylindrical tolerance zone can be referred to as concentricity. That is, as long as it is defined by a datum access into which all of the referenced circular features derived median points fall.

GD&T is essential to the comprehension of detailed engineering drawings. It is frequently put into use by manufacturing, commercial design, electronics, aerospace, automotive industries and more. When designing almost anything, tolerance is a critical issue. It must be consistent throughout the manufacturing process of a product.

Measurement and Gaging

Due to the complexity in establishing a feature’s midpoints, one of the most difficult GD&T symbols for which to measure is concentricity. The initial step is establishing a measurable datum axis. Following this, a series of cross-sections must be measured. The next step is to obtain the exact plot of the surface and determine the cross section’s median points. To see whether or not they fall into the cylindrical tolerances zone, the points must be plotted. A computer measurement device such as a CMM is the only way that this can be done. And it takes a lot of time.

By now, surely, you realize the complex nature of concentricity. It is for this reason that it is reserved for parts which, in order to function properly, need a high degree of precision. One example would be transmission gears. To ensure that all the axes line up correctly, and to avoid oscillations and wear, they always need to be coaxial. Easier to use, on the other hand, and sometimes capable of being a replacement for the need for concentricity, is runout – which was mentioned previously.

Willrich Precision Instruments carries a full line of concentricity gages in various series and models. Talk to one of our metrology experts today to see which model is appropriate for your needs. We also carry every measurement instrument imaginable when it comes to the gaging and metrology industry.

by George Chitos

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