Well, an updated version of the GD&T standard has been released! Although released in 2019, it carries the date of 2018 (big things move slowly, right?)
For those of you who use GD&T regularly, it’s probably worth the money to buy the new standard from ASME (either hard copy or PDF — same price). I’m still absorbing some of the changes, but for now I’ll mention a couple of significant things…
The most notable change is the deletion of concentricity and symmetry from the standard. You may already know that these two symbols have been confusing, because they were often misused or misinterpreted. (See our past blog post here for a glimpse into concentricity.)
So now the main choice for location will be position, but profile or runout symbols can also be used to control coaxial features.
…there are some other things in the new standard, but these are the main ones for now.
Another change to the standard was the addition of a new modifier called “dynamic profile.” It’s a little triangle symbol that can be placed within a profile’s feature control frame (after the tolerance number), and it means that the profile zone controls form (which profile usually does) but not the size of the feature.
For instance, suppose a cylindrical pin is to be controlled by profile of a surface. Without the dynamic profile modifier, the diameter of the pin must be basic, because profile shall be applied to a “true profile.” But with this new modifier, the pin’s diameter can have its own ± tolerance, but with a smaller, dynamic profile to control form. (The situation I’ve described would actually be identical to a cylindricity callout, so I’d only use dynamic profile on shapes where another GD&T symbol couldn’t apply.)
Again, there are some other things in the new standard, but these are the main ones for now. Also keep in mind that this new standard probably wouldn’t be used on real-world designs for quite a while; it takes time to digest the new information, update any CAD software, etc. So I don’t expect to see the 2018 standard imposed on a drawing until 2020 or beyond.