FEATURES AND FEATURES OF SIZES
A feature is basically a physical portion of the part.
It can be the surface, slot or hole.
The easiest way to remember a feature is to consider it as a surface.
Let’s take an example,
Please let us know how many features are there in the above image by posting it in the comments section.
(Hint: It’s the last digit of James Bond famous ID)
A feature of size (FOS) is a size dimension associated with 1 cylindrical or spherical surface, or a set of two opposed elements or opposed parallel surfaces.
The important point of FOS is that it should have opposed elements or surfaces.
The above figure gives us an idea about the features of size.
In each dimension mentioned above we can see that these are opposed surfaces or elements and could be used to derive an axis, median, plane or center point.
If we could just observe properly, we can notice that every feature of size has one or more features associated with it.
A cylindrical FOS has a single feature: the cylindrical surface.
A planar FOS has two parallel/unparalleled surfaces or features associated with it.
There are 2 types of feature of size.
- External FOS.
- Internal FOS.
The part surface which gives information related to external surfaces is called as External feature of size like diameter of shaft, cylindrical surface or width of a planar part.
An Internal FOS is the part surface which gives information related to the internal surfaces.
FEATURE OF SIZE DIMENSION
When a dimension is associated with a size feature it is known as feature of size dimension.
A non feature of size is a dimension which is not related with the feature of size.
ACTUAL LOCAL SIZE AND ACTUAL MATING ENVELOPE
Actual local size is any dimension at any cross section of FOS.
When two points measurement is taken with an instrument like a caliper or micrometer the measurement is known as actual local size.
There can be a number of actual local sizes associated with a FOS.
The actual mating envelope (AME) of external FOS is the smallest size of the feature that comes in contact with the surfaces at the highest points.
The actual mating envelope (AME) of an internal feature of size is the largest size of the feature that comes in contact with the surface at their highest points.
Largest cylinder of perfect form can be a perfect feature counterpart.
Till now we have understood that the geometrical tolerance can be applied on the largest, smallest or nominal (actual) size of the part.
In this section, we would understand what material conditions are and their importance in GD&T.
MAXIMUM MATERIAL CONDITION (MMC)
When the feature of size has the maximum material everywhere for example, shaft of the diameter or smallest hole diameter of a surface is called as maximum material condition.
LEAST MATERIAL CONDITION (LMC)
When the feature of size contains the minimum amount of material everywhere in the given size limit for example, smallest diameter of shaft or largest hole on a surface.
REGARDLESS OF FEATURE SIZE (RFS)
A geometric tolerance that applies at any increment of size of the feature within its size tolerance is called as Regardless of feature size (RFS).
This geometric tolerance is applied to any and every size of the part which is produced.
No symbol is associated with RFS.
MATERIAL CONDITIONS AND PART DIMENSIONS
Maximum and least material conditions is present for every feature of size.
If one has to know the material conditions they can just refer to the limit dimensions.
When there is plus minus tolerance in a drawing one has to identify the material condition from those dimensions.
MATERIAL CONDITIONS USAGE
Every material conditions are unique and have their own functional requirements.
Maximum material condition is often used when the parts are to be assembled.
When minimum distance on a part is to be obtained, geometric tolerance are always specified with a LMC.
For symmetrical relationships, RFS is applied.
MODIFIERS: GD&T SYMBOLS
Modifiers are a set of symbols used in geometrical tolerancing which communicates additional information about the tolerancing of a part.
Commonly there are 8 modifiers which will discuss for better understanding of GD&T.
MMC and LMC is discussed earlier in this article.
We can see the projected tolerance zone and the tangent plane modifiers inside the feature control frame similar to the MMC and LMC.
When the location of the tolerance zone on the part is changed, it is mainly done by the projected tolerance zone.
When the tangent plane of the tolerance surface is within the tolerance zone it is mainly done by the tangent plane modifier.
Diameter is the next modifier used in the GD&T.
The symbol is basically used for 2 purposes:
- To replace the word diameter in the drawing.
- To specify the shape of the feature in the feature control frame.
Radius and controlled radius are used outside the feature control frame.
Now, you might be thinking what feature control frame is?
Feature control frame are nothing but a block which you can see below the dimensions where the geometric symbols, tolerance, modifiers and datums are mentioned.
We’ll discuss about this in the upcoming article. So stay tuned!
RADIUS AND CONTROLLED RADIUS
A straight line which extends from the center of an arc is known as Radius.
It is denoted by R.
Typically when R symbol is used it denotes that there are 2 arcs formed which is a threshold for the part’s radius to be manufactured.
The surface need not be flat.
Controlled radius is similar to radius but the only difference is that it should have flat surface.
We can say that it is a more controlled version of the radius.