1. ## Feature of size

Folks I'm diving into the GD&T magical world and now I'm learning about feature of size although something is getting me confused. When I talk about feature of size the first thing comes in my mind is that a feature of size is a feature... Taking this simple part below, it's easy to me recognize the hole as a feature and a feature of size because it has a cilindrical surface (so it's a feature) and a directly dimension associated (so it's a feture of size). Up to now everything is fine. But when it comes to the thickness of the plate my mind blows up!! I can recognize the two parallel oposed surface and the directly dimension associated with this althouh I can not see the feature itself... Reading a bunch of articles and watching another bunch of videos when people identify something as a feature of size they point to a dimension not a feature...

So my question is: "Is feature of size a dimension or a physical portion of a piece?"

Taking the hole as example: What R E A L L Y is the feature of size: the hole itself (the physical thing which is made by drilling process) or the Ø8 mm dimension?

2. In general a FOS is a feature that has surface elements that are directly opposite of one another and is specified with dimensions and is toleranced. Such as a cylindrical surface (hole or shaft), spherical surface, a circular element, or a set of two parallel line elements or opposed parallel surfaces (thickness) associated with a single directly toleranced dimension.

A feature is any surface element, plane, half circular surface like a radius, etc...

Note that there are actually two types of Feature of Size (FOS) called "Regular Feature of Size" and "Irregular Feature of Size".

3. Originally Posted by Kelly Bramble
In general a FOS is a feature that has surface elements that are directly opposite of one another and is specified with dimensions and is toleranced. Such as a cylindrical surface (hole or shaft), spherical surface, a circular element, or a set of two parallel line elements or opposed parallel surfaces (thickness) associated with a single directly toleranced dimension.

A feature is any surface element, plane, half circular surface like a radius, etc...

Note that there are actually two types of Feature of Size (FOS) called "Regular Feature of Size" and "Irregular Feature of Size".
Alright! That’s what reading from multiples sources although when comes time to point where are the feature of size they point to a dimension not to a feature… Specifically talking about the FOS associated with thickness of the plate which is the FOS? The bottom surface, the top surface, both are considered feature of size, or both forms one feature of size?

4. Originally Posted by ivanginato
Alright! That’s what reading from multiples sources although when comes time to point where are the feature of size they point to a dimension not to a feature… Specifically talking about the FOS associated with thickness of the plate which is the FOS? The bottom surface, the top surface, both are considered feature of size, or both forms one feature of size?
For a plate, the top surface would be a feature and the bottom surface would be a feature as well. These two separately are plane features. The thickness will and should have a dimension and tolerance specified somewhere thus referencing the thickness and that associated dimension and tolerance it is a FOS.

5. Ohhh yeah!! Now I feel like bender’s meme from futurama: “Now I see it, now I got it, now I all the piece are fall into place” 😂😂

Thank you, Kelly!

6. Originally Posted by ivanginato
Ohhh yeah!! Now I feel like bender’s meme from futurama: “Now I see it, now I got it, now I all the piece are fall into place”

Thank you, Kelly!
Just a side note, the top and bottom surfaces as referenced above are known as "Features without size".

7. What is the best way to Datum a Trapezoidal shape? Should I just choose one of the angled sides to be the tertiary datum? What if I have features (such as holes) on both of the angled sides - is proper convention to create a fourth datum on the other side, so that the first datum in the feature control frame is the plane on which the hole is positioned? I have attached an example of my best guess at the correct approach to this. Any suggestions would be appreciated. Thanks!
Screenshot 2023-01-04 140223.png

8. Originally Posted by ben_knechel
What is the best way to Datum a Trapezoidal shape? Should I just choose one of the angled sides to be the tertiary datum? What if I have features (such as holes) on both of the angled sides - is proper convention to create a fourth datum on the other side, so that the first datum in the feature control frame is the plane on which the hole is positioned? I have attached an example of my best guess at the correct approach to this. Any suggestions would be appreciated. Thanks!
Screenshot 2023-01-04 140223.png
Without knowing the exact application / mating surfaces I believe the scheme shown is logical.

9. Ben,
First, if the intention is for the holes to be centered in the plate I would use the overall length of the plate as Datum B rather than just the bottom edge as shown in the top view.
Second, I would use the overall width of the part as the tertiary datum regardless of the fact that the width dimension is referencing two sharp edges.
Third, if the design intent of this part and assembly requires that the angled holes be exactly perpendicular to the angled surfaces than yes you can use those angled surfaces as the primary datum for orienting those holes. However, if it is not that critical then you can still use the original A | B | C datum structure. You'd need to dimension the angled surfaces as basic and how high up on the angled surfaces from Datum A the centerpoint of those angled holes are and dimension that as basic as well. To give a tolerance to the angled surfaces you could use a profile control.

10. Now lets really confuse hime and tell him about "RFS".