We all know that GD & T is an integral part of engineering drawings. There are two basic GD and T rules that apply to all drawings. This article is all about discussing those GD & T rules with examples. At the end of this article, you will get a link to download GD and T rules PDF for your future reference.
What is GD And T ( GD & T)?
GD and T is a language, a medium, a group of symbols through which you can communicate your design intent to the cross-functional teams. GD & T ( Geometrical dimensions and tolerances) helps the engineer to communicate to the manufacturing team about what is acceptable in a physical part. The acceptable variation could be form, feature size, or any other variations.
Before going ahead with this article, you should read below two articles where I discussed MMC, LMC, and RFS material conditions. Knowing this term is essential to understand GD & T rules
Must Read : What are MMC and LMC
Must Read : What is RFS ( Regardless of feature size)
Which is the Present GD & T governing standard?
The latest GD & T standard is ASME Y 14.5 2018. It is maintained and regulated by American Society Of Mechanical Engineers (ASME)
GD And T Rules #1
When tolerances of size are specified for a feature, the limit of the size prescribed the extend to which a feature can vary considering the variation of form and size.
In simple words, when a feature is at MMC it has the perfect form. When a size tolerance is specified, it controls both size tolerance and forms tolerance of that feature.
GD & T Rule #1 Examples
To explain the GD & T rule #1 for Shafts, let us take this below example
In this example, the nominal size of the shaft is 50.000 inches and the size tolerance is .050 inches So as per GD & T rule #1, the maximum boundary condition for that feature should be 50.000+.050= 50.050 inches.
What that means is that at Dia. 50.050 inches, it will have perfect straightness, circularity, and cylindricity unless there is no flatness of straightness tolerances.
Let us take the example of a hole which has a nominal diameter of 50.000 inches and the size tolerance is .050 inches, then the maximum boundary condition for that hole would be 50.000-.050=49.950 inches.
So at dia. 49.950 inches, the hole will have perfect straightness, circularity, and cylindricity unless there are no flatness or straightness tolerances.
For planer feature
In this below example, the slot width is 50.000 inches and tolerance is .050 inches. So the maximum boundary condition or worst-case boundary for this slot would be 50.000-.050= 49.950 inches
But now consider that the slot is produced at 49.850. In that case, the straightness or flatness can have a tolerance of up to (49.950-49.850)=.100 inches. What that means. that the minimum size of the slot can be 49.950 inches. In case the slot is produced at MMC, no straightness of flatness tolerance is allowable unless it is mentioned.
Where GD & T rule #1 does not apply?
GD & T rule #1 does apply only for a feature of size ( FOS) dimension and not for any positional dimension.
In this example, the dimension 50.000 inches is a feature of size but the dimension 30.000 inches is a positional dimension. So GD & T rule #1 does not apply to the 30.000 dimensions.
Here are a few scenarios where GD &T rule#1 does not apply.
- For positional dimension
- When a feature has both form tolerance and size tolerance.
GD And T rules #2
GD & T rule#2 states that RFS ( Regardless of feature size) applies to all tolerances in a feature control frame where there is no MMC or LMC defined.
In this feature control frame, you notice that there is no MMC or LMC defined. So as per GD & T rule#2, RFS automatically applies to that. What RFS means is that the feature won’t have any bonus tolerance if the dimension shift towards LMC or MMC. RFS increases part cost as RFS offers tight tolerances to manufacturers.
We should always try to give MMC or LMC features to reduce part cost unless it is a very critical part and any deviation may affect the form, fit, and function.
Conclusion: GD and T rules
So I hope that you all got a fair idea about the fundamental rules of GD & T. While creating engineering drawings, we all should follow these rules to communicate the design intent to other stack holders. If you have any questions or doubts, do write to me in the comment section and I will be happy to help.
Frequently Asked Questions (FAQ)
What are GD and T Rules?
Rule #1 : When only size tolerance is specified for feature, then it controls both the size tolerance and form tolerance.
Rule #2 : When no MMC or LMC is specified , then RFS applies by default
What is the exception to GD and T rule #1
When any other tolerance is specified apart from size tolerance , the rule #1 does not apply.
Video Credit : Mahesh Engineering Academy