In the Design of any machine elements, it is necessary to follow the Geometric dimensioning and tolerancing for the Holes or shafts. The Regardless of feature size(RFS) is the feature of the size where it can be called out for a hole or a shaft when there is no need for MMC or LMC.
Regardless of feature size
Regardless of Feature size is also referred to as RFS. RFS is a material modifier used in the GD&T callout
This means that it does not matter whether the size is at the low or high limit of its noted tolerance and that the GD&T limit does not change.
Example for Regardless of feature size
If the hole on a given part was made to its smallest size (2.74 mm).
The pin on a given mating part was made to its largest size (2.46 mm).
the worst case allowable variation that could be allowed for the position would each be 0.14 mm
Total variation allowed between the two parts= 2.74 mm – 2.46 mm = 0.28 mm
It clearly shows that this condition reflects the worst-case line-to-line fit.
If, however, the size of the hole on a given part was made to its largest size (2.86 mm).
The pin on a given mating part was made to its smallest size (2.34 mm).
It would make sense that the worst case allowable positional variation could be larger than 0.14.
Evaluating this further as was done above to determine a line-to-line fit would be as follows:
The total variation allowed between the two parts= 2.86 mm – 2.34 mm = 0.52 mm
It would seem natural, due to the combined efforts of size and positional tolerance being used to determine the worst-case virtual condition boundary, that there should be some means of taking advantage of the two conditions.
How and where Regardless of the Feature of size (RFS) is used?
Regardless of the Feature of size (RFS) is used for press-fit applications.
RFS does not need any symbol representation on the Drawing GD&T. By default if there is no callout for LMC or MMC on the drawing GD&T, which means it is obvious that there is an RFS callout.
If we do not need an RFS callout for a feature, then it needs to be specified to ignore the RFS callout.
Regardless of feature size eliminates any potential bonus tolerance, allowing the GD&T tolerances to be more tightly controlled.
RFS requires the hole axis to be measured separately from the size of the hole and cannot be gauged easily. However, there is no bonus tolerance allowed in this condition so the perpendicularity would be much better controlled regardless of the size of the hole.
The Feature of Size Callouts in GD&Ts
The feature of size callouts is also known as material modifiers.
- Maximum Material Condition
- Least Material Condition
- Regardless of Feature of size
These are the 3 material modifiers.
We have mentioned that the RFS is used for press-fit applications.
Whereas the MMC is used for clearance applications. MMC clears the biggest size of bolt.
And LMC is used where the wall thickness is important, like holes on gaskets and flanges, and minimum machining is required.
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