Here’s a question I received at the beginning of a recent GD&T training session. Often I’ll ask participants if there is anything in particular that they are looking to get out of the training. One gentleman didn’t hesitate:
“I’m here to learn why we shouldn’t use the concentricity symbol. No one has been able to tell me why I’m not supposed to use that symbol! Our drawings show a runout tolerance instead, but I’ve never heard a good answer why.”
First, what this person was told by others is indeed true: the concentricity symbol is often discouraged, and another choice such as position or circular runout is usually better. To answer the question, we must analyze how the terms are defined.
- A dictionary definition describes concentric as “having a common axis or center.” That is what we often require for parts such as a camshaft or guide pin holes, right? But the problem is in how we find that axis or center. There are different ways to derive an axis.
- The definition of concentricity in the GD&T standard (ASME Y14.5M-1994) is given in paragraph 5.12. “Concentricity is that condition where the median points of all diametrically opposed elements of a figure of revolution are congruent with the axis (or center point) of a datum feature.”
Now, place yourself in the shoes of an inspector. To check concentricity as defined in GD&T, we must perform a surface analysis at each cross section (or as many cross-sections as practical) to derive a series of median points. The result is an imaginary cloud of points that must all lie within the given tolerance zone. This obviously can be quite time consuming! If the tolerance symbol were simply changed to circular runout, the tolerance zone would be applied around the circumference of the part, which is something that can be physically gaged without getting involved in the tedious math required by concentricity. (The other alternative of position derives an axis from the mating envelope of the part, not each cross section.)
Concentricity is a legitimate choice for some applications where the main concern is equal mass distribution around the center. But for most mechanical parts, you should avoid concentricity. In fact, the automotive companies have explicitly instructed their designers and suppliers to avoid this symbol, along with symmetry, which involves the same concept for planar features.
Because of all this, GD&T folks sometimes use the more ambiguous term “coaxial” in discussion. This term conveys the general idea, without using the specific word concentric. But on a print, be very careful which symbol you choose!
Comments? Ideas for a future blog entry? Let me know!