Full form of GD&T is Geometric Dimensioning and Tolerancing. D&T is related to tolerancing in mechanical engineering. Since ideal size manufacturing is not possible so we provide tolerance. For example, if we want a simple plate of dimension 5mmX10mmX40mm then we would allow some variations, or tolerance for each dimension. Say, as a designer, you allow 0.05mm variation in thickness, 0.1 mm variation in width and 0.4 mm variation in length, respectively. So you would mention dimensions as: Length: 40.00 +/- 0.40 mm Width: 10.00 +/- 0.10 mm Thickness: 5.00 +/-0.05 mm
You provide these tolerances based on: · Standards like ISO-286 recommendations · The functionality of your design is not impacted
This is called dimensional tolerance.
Suppose you want to say the my plate has to be perfectly flat, then how would you communicate that in your design document? Suppose you have designed a shaft which has to be straight, then how you would decide how much straight is should be. Similarly how would control circularity of a circular feature?
To address such limitations of dimensional tolerances, new concept came into picture which is Geometric dimensioning and tolerancing (GD&T). It takes are of such geometric variations, to tolerances. For example then when you ask for flatness then you would say, flatness can vary wishing 0.005 mm, up or down, spike and valley, and so on.
It is a great tool which not only helps designers to communicate their requirements but also helps manufacturers to produce to meet designers functional requirement. It is greatly helping to reduce cost and rejection and increasing quality of the product.
IMPORTANT: GD&T is not replacing dimensional tolerancing. GD&T is applied on top of dimension tolerances. It is functional language.
Since GD&T is in evolving phase, so there is some scarcity of reference materials. ASME Y14.5–2009 is most followed standard across globe.
Its fairly complicated topic and therefore you would not find lots of reference material on internet. We would recommend all readers to refer Simplified GD&T book listed in Our Books section on this website. It is written to simplify the matter and bring the core of the matter to students in easiest possible manner.
I hope it helps!
Enjoy! Azuko Technical Institute