In last month’s post we looked at the 6 common tolerance zones that are used in what is the majority of tolerancing in today’s engineering drawings. While these 6 makeup the vast majority of tolerance zones in the world of GD&T there is one other type that occasionally finds its way into a tolerance scheme, and that is the spherical tolerance zone.
The ASME Y14.5-2009 standard, in paragraph 7.4.6, defines the use of location tolerancing with spherical diameters directing both the feature and the positional tolerance value to be preceded with a spherical diameter symbol (shown in fig. 7-35 on page 127 of the standard.)
Spherical diameter features have their centers identified with a point, which can be used as a datum or simply the location of a part feature. The illustration below, posted online from the University of Florida, shows the 2” diameter spherical feature called out as a spherical diameter with a positional tolerance imposed to datums D and P.
The example below pictures a part with a spherical feature with its location controlled by the feature control frame to a .2 mm spherical diameter tolerance zone at least material condition. Additionally this example shows the feature itself identified in what may be a basic dimension box and the dimension locating the spherical radius of the feature is shown as 40mm with an applied distance tolerance, which may interfere with the geometric position frame.
This example needs to be given some latitude in that I don’t have the engineer that made it to talk to. I see many things, in my research, that cause me to question the choices made in many tolerancing schemes but the more I study the more I learn how many different ways people can try to make these conventions fit their requirements. And while I’m not exactly a humble individual I am learning that humility is often a fitting reaction to discovering someone as lost as yourself.
The Y 14.5 standard also explains that a sphere’s location can be controlled with other than a spherical diameter zone if the need is for the spherical feature to be located in a bidirectional zone (shown in Fig. 7-28 on page 124 of the standard.)
I’m including an interesting side note here about the solution for spherical position examples. We were studying position solutions this week in a class I was teaching and one of the students asked if the same type of math was used to solve position involving spheres.
I had to admit I didn’t know and told the group I’d see what I could find, which includes the material covered here so far. In my research I found several solutions listed for specific examples that after experimenting with them I found that they fit the same formula as used for regular feature location as with holes and bosses, only using 3 deviations to sum the squares of instead of 2, such as in this formula: 2+B2+C2).
The man that runs Dimensional Consulting, David Slopsema, has been helpful in the past, so I wrote and asked if he knew how the calculation for a spherical diameter is performed and he sent the following, which confirms the other calculations I found.
“You just need to find the true distance between the actual position of the feature and the true position. That distance must be smaller than the radius of the spherical tolerance zone.”
You may notice the formula he shows is missing the 2X multiplier but his comment about the radius of the spherical tolerance zone explains its interpretation to be half of the diameter zone.
As a further side note readers may want to check online for Dimensional Consulting’s free online GD&T textbook. Mr. Slopsema wrote this entertaining book using his passion for all things canine to help people learn and use GD&T. It is very easy to understand and I recommend it to my students and now to you readers. Thank You David Slopsema.
Any comments from our readers are appreciated. I look forward to hearing your commentary and you can look forward to future posts, detailing material on all things GD&T.