In ASME Y14.5M-1994 there are basically two rules.
The first rule of GD&T is related to the default conditions of features of size.
The second rule is related to default material conditions for feature control frame.
RULE 1 OF GD&T
Rule 1 is also known as Individual feature of size rule.
It is a very important concept in geometric tolerancing.
Rule 1 helps us to ensure if the feature of size will assemble with each other or not.
When Rule 1 applies, the maximum boundary of an external FOS is it’s MMC.
The minimum boundary of the internal FOS is it’s MMC.
To evaluate whether the two feature of sizes can be assembled to each other, the designer can then compare the MMC’s of the feature of size.
In industry, Rule 1 is also known as Envelope rule.
There are 2 essential components to Rule 1:
1. Envelope principle.
2. Effects on the form of a FOS as it departs from MMC.
Once the rule is applied, the limit size determines the size as well as the form of an Individual FOS.
Now, let’s take an example related to Rule 1.
When a pin diameter is at MMC, the form of the pin must be perfect.
Perfect form means perfect straightness and perfect roundness.
This would help the pin to accommodate in a other boundary equal to its MMC.
If the size of the pin was less than it’s MMC, then form error (error related to roundness or straightness) would have been developed equal to the amount the pin departed from MMC.
Limits of size controls the form of a FOS, as desired below:
1. The boundary (envelope) of perfect form at MMC shall not be extended by the surfaces of feature of size.
2. When the consideration of actual local size is changed from MMC to LMC, the form is allowed to vary as shown in the figure.
3. The actual local size of an individual feature of size must be within the specified tolerance size.
4. Boundary of perfect form at LMC has no specific requirements. A feature of size produced at LMC can vary from true form by the amount allowed by MMC boundary.
An example of Rule 1 effects on planar surface is shown below.
In Rule 1, perfect form resembles perfect straightness, flatness, circularity and cylindricity.
Please note that if a part is produced at MMC then it must have a perfect form.
If the part is not produced at MMC, then a form error is allowed.
Perfect form on a planar FOS means perfect straightness and perfect flatness.
For example let’s take the example which is shown above, if the height of the part is 10.7 then the form error allowed is the difference of MMC and the height of part (10.8-10.7 equal to 0.1).
If the part was produced at LMC, a form error of 0.6 would be allowed.
HOW TO OVERRIDE RULE 1
The important point about Rule 1 is that it gets applied when a feature of size is present in the drawing.
The two ways in which Rule 1 can be overriden:
1. A note stating “PERFECT FORM AT MMC NOT REQUIRED” is used next to FOS dimension, then Rule 1 is not applicable.
2. Rule 1 is overriden when straightness control is applied to a feature of size.
LIMITATIONS RELATED TO RULE 1 OF GD&T
There are a number of feature of size for a part.
Rule 1 does not have the authority to affect the location, orientation or relationship between feature of size.
If the features of size are shown perpendicular, symmetrical or coaxial then they must be controlled by GD&T to showcase the complete specifications on the drawing. Generally angular tolerance is mentioned in the title block or by a note.
As shown in the above figure, there are 4 FOS A, B, C and D for which Rule 1 is applicable. The angles F & G are not controlled by Rule 1.
There are exemptions to Rule 1.
1. Rule 1 is not applicable to flexible part that are not constrained.
2. It is also not applicable to bar stock, tubing, sheet metal, or structural shapes.
INSPECTING A FEATURE OF SIZE
Size and form needs to be verified for inspecting a FOS that is controlled by Rule 1.
The verification of MMC size and Rule 1 envelope can be done by GO gage.
If a part having internal or external FOS fits into a GO gage then the part has a perfect form.
The verification of MMC size and Rule 1 can be done by GO gage.
The length of GO gage must atleast be similar to the feature of size so that verification of Rule 1 can be accelerated.
A NO GO gage helps us to identify minimum size (LMC) of a FOS.
A NO GO gage does not fit into or over a FOS.
It is made at the LMC limit of the FOS.
A caliper or snap gage can be used as NO GO gage.
RULE 2 OF GD&T
Rule 2 is also called as “the all applicable geometric tolerance rule”.
In Rule 2, the RFS is applied and there is no modifying symbol used such as MMC or LMC.
MMC or LMC must be specified wherever required.