GD&T
Geometric Dimensioning & Tolerancing
GD&T is a standardized method to communicate acceptable variations in manufactured parts.
Defined by American Society of Mechanical Engineers (ASME Y14.5-M) & International Standard Organization (ISO)
GD&T goes beyond linear dimensioning and describes how features, such as hole locations, relates to other features so that the part functions as intended and can help to reduce rejected parts.
In this example, the location of the hole is 5 mm from plane A and 10 mm from plane B. The feature control frame tells us the part will function as intended when assembled if the hole is located within a diameter of 0.020" from the intended location.
Video: What is GD&T in 10 minutes
Terms
Nominal - an ideal or mathematical surface or feature such as a diameter
Tolerance - the variation in size permitted
Datum - an ideal plane of reference, indicated with a capital letter and triangle in a drawing. Example A, B, C
Basic dimension - a value describing the theoretical dimension or exact size. These are indicated with a box around it in a drawing.
Feature - is a geometric characteristic of a designed part such as an angle, or the diameter of a cylindrical part.
Feature of Size - is a feature that has two parallel side such as a cylinder. Anything that could be measured with an outside caliper is a considered a Feature of Size. An angle is a feature, but it is not a Feature of Size.
Maximum Material Condition (MMC) is when a feature of size, such as a hole, has the most amount of material remaining. i.e. the smallest hole that is acceptable. Another way to think of MMC is the object has more mass if the hole made is smaller.
Least Material Condition (LMC) is when a feature of size has the least amount of material remaining, i.e. the largest size of hole that is acceptable.
Modifiers
Feature Control Frame
A long rectangular box describing the tolerance for a feature such as a diameter or hole location.
Geometric Tolerances
Form - variation in shape
Profile - variation of a line or surface
Orientation - variation in shape relative to a fixed point of reference, or datum.
Location - variation in position relative to a datum.
Runout - variation of a feature about an axis of rotation
2 GDT Rules
Features must have perfect form when at the MMC, Maximum Material Condition.
When a feature has a tolerance, the default requirement is regardless of feature size.
Example A: a hole with a diameter of 1.00+/- 0.02" with a 0.030" position tolerance must always be within 0.030"
1.02" --> 0.030" position tolerance
1.01 --> 0.030" position tolerance
1.00 --> 0.030" position tolerance
0.99 --> 0.030" position tolerance
0.98 --> 0.030" position tolerance
However, a hole with a diameter of 1.00+/- 0.02" with a 0.030" M position tolerance. The following are acceptable:
1.02" --> 0.070" position tolerance
1.01 --> 0.060" position tolerance
1.00 --> 0.050" position tolerance
0.99 --> 0.040" position tolerance
0.98 --> 0.030" position tolerance
You can see these larger position tolerances allow more room to work with for manufactures to produce parts that will still function as intended