GD&T & TYPES OF DIMENSIONING – ANSI Y14.5

An engineering drawing may communicate the following:

- Geometry of the part
- Critical functional relationship
- Tolerances
- Material, heat treat, surface coatings
- Part documentation such as part number and drawing revision

Drawing errors compound the cost of the errors *(money, time, material, and customer satisfaction)* as we move further from initial design to production. Drawing errors create Disagreements over drawing interpretation –» Difficulty communicating drawing requirements –» and Designer's intent can not be understood.

__Dimensions__: A numerical value expressed in appropriate units of measure and used to define the size, location, orientation, form or other geometric characteristics of a part. Decimal dimensioning shall be used on mechanical drawing. The follwing shall be observed where specifying millimeter dimensions used on drawings.

__Metric Dimension Notes__: Drawing must state the units used and certain other information used in the drawing: ALL DIMENSIONS IN MILLIMETRES UNLESS STATEE OTHERWISE –» DRAWING IN ACCORDANCE WITH THE STANDARD REFERENCED IN BS8888: 2004 –» COMPONENTS/PARTS TO BE STAMPED WITH THEIR PLANT IDENT NUMBER WHERE MARKED/SHOWN IN LETTERS AT LEEAST 3MM HIGH.

**Metric Dimensioning:**

- When a metric dimension is a whole number; the decimal point and zero are omitted (Ex: .27). If dimension is a whole number, neither the decimal point or a zero is shown (14).
- When a metric dimension is less than one mllimeter, a zero precedes the decimal point Ex:0.5). If dimension is less than 1 mil, a zero precedes the decimal point (0.5).
- When a metric dimension is not a whole number; a decimal point with the portion of a millimeter (10ths or 100ths) is specified (Ex: 11.5). If the dimension exceeds a whole number by a decimal fraction of 1 mil, the last digit to the right of the decimal point is not followed by a zero (ø14.2).

**Deviation** *(độ lệch)*: The difference between a size and the corresponding nominal size.

**Inch Dimensioning:** A zero is not used before the decimal point for value less than 1" (.214). Tolerance should be expressed with the same number of decimal places as its dimensioned (1.234 ±.005 or .24 ±.01 ). Where unilateral tolerancing is used and either the +/- value is nil, its dimension shall be expressed with the same number of decimal places.

Specified Dimension is the target dimension from which the limits are calculated.

**Dimensioning Limits**: All dimensioning limts are absolute. That is all dimensions are considered to have a zero after the last true digit (Ex: 1.62 means 1.620.

**Deviation** *(độ lệch)*: The difference between a size and the corresponding nominal size.

**Upper Deviation:** The difference between the maximum limiting size and the corresponding nominal size of a feature.

**Lower Deviation:** The difference between the minimum limiting size and the corresponding nominal size of a feature.

**Tolerance:** Tolerance is the total permissible variation of a size. The difference between the maximum and minimum size limits of a part.

**Actual Fit:** The actual fit between two mating parts is the raltion existing between them with respect to the amount of clearance or interference that is present when they're assembled.

**Clearance Fit:** A fit type where clearance exists between assembled parts under all tolerance conditions. A clearance fit *(khoảng hở, độ hở)* is one having limits of size so when provided or specified that a clearance always results when mating part are assembled.

**Interference Fit:** A fit type where interference exists between assembled parts under all tolerance conditions. An interference fit *(chạm vào nhau)* is one having limits of size so when provided or specified that an interference always results when mating parts are assembled.

**Transition Fit:** A fit type where clearance or interference can exist between assembled parts depending on tolerance conditions. A transition fit is one having limits of size so when provided or specified that either a clearance or an interference may result when mating parts are assembled.

TOLERANCE AND TYPES OF TOLERANCES

ANSI/ASME Standard Y14.5 . Each dimension shall have a tolerance, except those dimensions specically identified as reference, maximum, minimum, or stock. A tolerance is an acceptable amount of dimensional variation that will still allow an object to function correctly. The tolerance may be applied directly to the dimension or indicated by a general note located in the title block of the drawing. Tolerance is the total amount a specific dimension is permitted to vary from the specified dimension. The tolerance is the difference between the maximum and minimum limits.

The **The three Basic Tolerances:**

**Limit Tolerance:**When a dimension has a high and low limit stated 1.016/1.000 is a limit tolerance. Limit dimensions provide an upper limit and lower limit for the dimension. Any size between or equal to the upper limit and/or lower limit is allowed.**Bilateral Tolerance:**A tolerance that allows the dimension to vary in both the plus and minus directions (Ex: 1.00 +.01/-.015 is a bilateral tolerance). Bilateral tolerance provides an equal allowable variation, larger and smaller.- Unilateral Tolerance: Where allowable variation is only in one direction and zero in the other (Ex: .500 +.004/-.000 in a unilateral tolerance).
**Plus and Minus Tolerance:**The nominal or target value of the dimension is given first, followed by a ± expression of tolerance (Ex: .250±.005 is a plus-minus tolerance).

**Basic Tolerance:** In Geometric dimensioning and tolerancing, basic dimensions are defined as a numerical value used to describe the theoretically exact size, profile, orientation or location of a feature or datum target. Basic dimensions are currently denoted by enclosing the number of the dimension in a rectangle.

How much tolerance is a basic dimension allowed? If a basic dimension does not show a specific tolerance what tolerance should be applied? Basic dimensions by nature are a theoretically exact value; however, the feature(s) of a part they define as ideal or exact do need to have tolerances to permit acceptable levels of imperfection during manufacturing.

Coordinate Tolerancing: CT is a dimensioning system where a part feature is located by a means of rectangular dimensions with given tolerances. CT or coordinate tolerancing, does not have the completeness needed today to enable efficient and economical production of parts.

- Square or rectangular tolerance zones.
- Fixed size tolerance zones.
- Ambiguous instructions for inspection.

**Good Used** of Coordinate Dimensioning: Size, Chamfer, and Radius.

**Poor Used** of Coordinate Dimensioning: Locating Part Features, Controlling Angular Relationships, Defining the Form of Part Features.

FEATURE OF SIZE AND DIMENSION

Feature is a general term applied to a physical portion of a part such as a surface, hole or slot.

**Feature of Size**: FOS refers to any surface, or set of parallel surfaces associated with a size dimension. Ex: A hole diameter (a cylindrical surface), a plate thickness (two opposed parallel surface).

**Internal Feature of Size**: Comprised of a part surfaces, or elements, that are internal part surfaces such as hole diameter or the width of a slot

External Feature of Size: Comprised of a part surfaces, or elements that are external surfaces such as a shaft diameter or an overall width or height of a planar part

Feature of Size Dimensions: A feature of size dimension is a dimension that is associated with a feature of size.

Non-Feature of Size: Non-FOS is a dimension not associated with a feature of size.