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.)
Example:
The next example more closely simulates the real intent of the dynamic profile modifier, where an already-dynamic control like cylindricity wouldn’t work:
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.
Dear colleagues,
Congratulations on the explanation of the dynamic profile.
Checking the standard, all examples used had basic dimensions. I didn’t understand how you came to the conclusion that it is not necessary to use basic dimensions. And the examples in standard this modifier is used as refinement of a profile. Could you clarify for me? Thanks in advance
The figure given above was oversimplified. So you are correct — the profile callout shown above should be a refinement of another profile tolerance (creating either a composite or a two-segment profile control), and then the diameter should be basic.
We were trying to give the general idea of the dynamic profile modifier by drawing an analogy to cylindricity. But we “dumbed it down” too much!
**Edit: The second figure above was added to help with Francisco’s question. The GD&T applies to an ellipse which is defined with basic dimensions (by way of the note).