Geometric Dimensioning & Tolerancing, commonly referred to as GD&T, as the name suggests, is a language used by design engineers to define/control and represent the "geometric shape" of a part/assembly on an engineering drawing.
While, I am not very sure of the origin but I read somewhere that the initial idea was thrown around half a century ago. Today, GD&T is defined under various international standards, but the one followed the most is from American Society of Mechanical Engineers ASME Y14.5-2009.
As mentioned above, GD&T defines the shape of the product and not the size so it is not to be confused with the conventional dimensioning and tolerancing defining the size or features of a drawn part/assembly.
GD&T generally comes into picture where the shape of the part is of great importance to its application, for instance, the cylindricity of a piston inside an engine cylinder. If the piston is not cylindrical, the engine won't be efficient for the obvious reason.
Having said that, how do we use GD&T on the drawings. GD&T has its own set of symbols and modifiers used to define the geometry of a part. While the symbols define the kind of tolerance being applied, modifiers defines the material conditions.
The symbols are divided under five categories, as shown below.
Generally, the geometry is defined with respect to a defined set of datum/datums. The only exception is the "Form" category, which do not needs a datum to define the geometry of the given part. For instance, the flatness tolerance in the following example, doesn't need a datum to define it against.
While other tolerances, for instance perpendicularity in the following example, needs a datum plane A to hold the perpendicularity against. Generally the assumption with datums is that the datum is x10 times precise than the plane given the tolerance.
Runout (also called circular runout)/Total Runout comes into the picture when we plan to control the wobbling of a shaft (hollow/solid) around its center axis. Its measured against the central axis (considered as the datum) using a Full Indicator Movement (FIM) gauge. So as shown in the picture below, the shaft is rotated around its axis and the variance of 0.1mm is measured with FIM. And as the variance is within 0.1mm, the part is good to go.
Now what are modifiers? Modifiers as mentioned above defines the material condition of the part, the tolerance is applied on.
For instance, when MMC (Max Material Condition) is applied to a Ø10±.5mm hole, it will have the smallest diameter Ø9.5mm. While the same hole at LMC (Least Material Condition), would have the largest diameter Ø10.5mm.
On the same time, if the same conditions are applied on a pin/shaft of diameter Ø10±.5mm, the pin will have the largest diameter at MMC i.e. Ø10.5mm and the smallest diameter at LMC i.e. Ø9.5mm.
This is very handy when it comes to Gauges used by Quality Engineers on the floor to measure a part. A gauge set to MMC/LMC depending on the functionality of the part is all that would be required for the quality check.
Now how do we use them to define the geometry of a part on the drawings? GD&T is defined inside what is called "Feature Control Frame"
While, I am not very sure of the origin but I read somewhere that the initial idea was thrown around half a century ago. Today, GD&T is defined under various international standards, but the one followed the most is from American Society of Mechanical Engineers ASME Y14.5-2009.
As mentioned above, GD&T defines the shape of the product and not the size so it is not to be confused with the conventional dimensioning and tolerancing defining the size or features of a drawn part/assembly.
GD&T generally comes into picture where the shape of the part is of great importance to its application, for instance, the cylindricity of a piston inside an engine cylinder. If the piston is not cylindrical, the engine won't be efficient for the obvious reason.
Having said that, how do we use GD&T on the drawings. GD&T has its own set of symbols and modifiers used to define the geometry of a part. While the symbols define the kind of tolerance being applied, modifiers defines the material conditions.
The symbols are divided under five categories, as shown below.
Generally, the geometry is defined with respect to a defined set of datum/datums. The only exception is the "Form" category, which do not needs a datum to define the geometry of the given part. For instance, the flatness tolerance in the following example, doesn't need a datum to define it against.
 |
| Flatness |
 | |||||
| Perpendicularity |
Runout (also called circular runout)/Total Runout comes into the picture when we plan to control the wobbling of a shaft (hollow/solid) around its center axis. Its measured against the central axis (considered as the datum) using a Full Indicator Movement (FIM) gauge. So as shown in the picture below, the shaft is rotated around its axis and the variance of 0.1mm is measured with FIM. And as the variance is within 0.1mm, the part is good to go.
 |
| Total Runout |
For instance, when MMC (Max Material Condition) is applied to a Ø10±.5mm hole, it will have the smallest diameter Ø9.5mm. While the same hole at LMC (Least Material Condition), would have the largest diameter Ø10.5mm.
On the same time, if the same conditions are applied on a pin/shaft of diameter Ø10±.5mm, the pin will have the largest diameter at MMC i.e. Ø10.5mm and the smallest diameter at LMC i.e. Ø9.5mm.
This is very handy when it comes to Gauges used by Quality Engineers on the floor to measure a part. A gauge set to MMC/LMC depending on the functionality of the part is all that would be required for the quality check.
Now how do we use them to define the geometry of a part on the drawings? GD&T is defined inside what is called "Feature Control Frame"
In the above example, lets assume that GD&T is applied to a hole (and not a pin) with a diameter range of Ø.505-.525in with a positional tolerance of .005 at Max Material Condition, with respect to the primary datum A, the secondary datum D (again at max material condition) and then the tertiary datum B.
So this means that the positional tolerance of .005in is applied on the hole at Ø.505in (@MMC) against the given datums.
Simplifying it further, the hole will have a positional tolerance between .500 - .510mm, or in technical terms will have an envelop of .500 - .510mm.
There are a lot of opinions people have when it comes to GD&T, like it increases the cost or is difficult to go about and what not. But as per my understanding, it is not that difficult to go about once you get your head around it and as far as the cost is concerned, GD&T is all about manufacturing a product within the right functional tolerances which further leads to low rejection rate and ends up saving the cost.
In this post I have tried to give out a rough idea of what GD&T is and how its being used. I used it in one of my previous roles where I was involved in the design and development of precise plastic components with close tolerances but not in my present role, so while writing this post, I could feel a lot of it coming back to my mind. I won't probably dig any further into the subject as its gonna be then a never ending post. But will be glad if it gives you the basic intended idea.
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