Reference FramesASME Y14.5-2009

Dimensioning & Tolerancing

We need to know 4 things in order to understand Datum

• Datum Feature

A feature that is identified with either a datum feature symbol or a datum targetsymbol.

We need to know 4 things in order to understand Datum

• Simulated Datum

A point, axis, line, or plane (or combination thereof) coincident with or derived from

processing or inspection equipment, such as the simulators: a surface plate, a gagesurface, a mandrel, or mathematical simulation.

We need to know 4 things in order to understand Datum

• Datum Feature Simulator (Physical)

The physical boundary used to establish a simulated datum from a specified datumfeature.

• Datum Feature Simulator (theoretical)

The theoretically perfect boundary used to establish a datum from a specified datumfeature.

Datum

A theoretically exact point, axis, line, plane, or combination thereof derived

from the theoretical datum feature simulator.

The part is constrained in rotation and translation relative to the applicable datum

feature simulators in the specified order of precedence with applicable modifiers

that establish the datum reference frame. This defines the geometric relationships

that exist between the geometric tolerance zones and the datum reference frame.

Datum feature simulators shall have the following characteristics:

(a) Perfect form.

(b) Basic orientation relative to one another for all the datum references in a

feature control frame.

(c) Basic location relative to other datum feature simulators for all the datum

references in a feature control frame, unless a translation modifier or movable

datum target symbol is specified.

(d) Movable location when the translation modifier or the movable datum target

symbol is specified.

(e) Fixed at the designated size, when MMB or LMB is specified.

The datum features considered in standards are always theoretical and do not

take into account the physical datum feature inconsistencies

It is necessary to specify the geometrical tolerances of datums separately

Sufficient datums should be provided where applicable in a relationship of a set

of three mutually perpendicular planes (these are applied according to

applications. 1, 2 or 3 datums can be applied according to the requirement)

Datums features should be so specified that they take into account the

functional requirements of the toleranced feature

Temporary datums may be applied for in process parts such as castings,

forgings , or fabrication.

Datum feature symbol shall only be applied to physical features and not center

lines, axes or center planes

Datum feature symbols shall be applied in order of precedence

Planar Surface Features

• 3 Datum Planes restrict all the 6 degrees of freedom

Inclined Datum Features

• Datum must be oriented at the basic angle of the datum feature

Cylindrical Datum

Features

• Primary datum is plane

• Secondary datum is cylindrical with two theoretical mutually perpendicular planes.

Constraining holes on

circular parts

• Tertiary datum defines datum for orientation of holes

Planar Surface

Features

• 3 Datum Planes restrict all the 6 degrees of freedom

Inclined Datum

Features

• Datum must be oriented at the basic angle of the datum feature

Cylindrical Datum

Features

• Primary datum is plane

• Secondary datum is cylindrical with two theoretical mutually perpendicular planes.

Constraining holes on

circular parts

• Tertiary datum defines datum for orientation of holes

Constraining holes on

circular parts

• Tertiary datum defines datum for orientation of holes

• Least Material Condition (LMC)

The of size (e.g., maximum condition which a feature of size contains the least amount of materialwithin the stated limits hole diameter, minimum shaft Diameter)

• Maximum Material Condition (MMC)

The condition in which a feature of size contains the maximum amount of material within thestated limits of size (e.g., minimum hole diameter, maximum shaft diameter)

• Least Material Boundary (LMB)

The limit defined by a tolerance or combination of tolerances that exists on or inside thematerial of a feature(s)

• Maximum Material Boundary (MMB)

The limit defined by a tolerance or combination of tolerances that exists on or outsidethe material of a feature(s)

Where MMB is applied to a datum feature referenced in a feature control frame it

establishes the datum feature simulator of the appropriate boundary. The appropriate

boundary is determined by its collective effects of size, and any applicable geometric

tolerances relative to any higher precedence datums.

• Internal datum feature of size is the largest MMB that the datum feature(s) of size will contain

while respecting the datum feature precedence.

• External feature of size is the smallest MMB that will contain the datum feature(s) of size while

respecting the datum feature precedence.

• Internal datum feature of size is the small LMB that the datum feature(s) of size will contain while

respecting the datum feature precedence.

• External feature of size is the largest LMB that will contain the datum feature(s) of size while

respecting the datum feature precedence.

•Where datums are specified in an order of precedence, the material boundary condition at which

the datum feature applies must be determined.

•The effect of its material boundary condition and order of precedence should be considered

relative to fit and function of the part.

Where more than one datum feature is used to establish a datum feature simulator for a single

datum, the appropriate datum feature reference letters and associated modifiers, separated by a

dash, are entered in one compartment of the feature control frame.

• It is formed by coinciding datum planes that simultaneously contacts the high points of two surfaces

Single datum plane

• It is established from the axes of the datum feature simulators that constrain the two coaxial diameters simultaneously.Single axis datum

• Pattern of holes at MMB, may be used as a group in the establishment of a datum feature simulator to derive a datum reference frame.

Pattern of features at MMB

• Where RMB is applied in a feature control frame to multiple datum features of size used to establish a single datum, the datum feature simulator of each feature shall be fixed in a location relative to one another.

Pattern of features at RMB

• It is often desirable to specify only part of a surface, instead of the entire surface to serve as a datum feature. This may be indicated by means of a chain line

Partial surfaces as datums

• It is formed by coinciding datum planes that simultaneously contacts the high points of two surfaces

Single datum plane

• It is established from the axes of the datum feature simulators that constrain the two coaxial diameters simultaneously.

Single axis datum

•Pattern of holes at MMB, may be used as a group in the establishment of a datum feature simulator to derive a datum reference frame.

Pattern of features at MMB

• Where RMB is applied in a feature control frame to multiple datum features of size used to establish a single datum, the datum feature simulator of each feature shall be fixed in a location relative to one another.

Pattern of features at RMB

• It is often desirable to specify only part of a surface, instead of the entire surface to serve as a datum feature. This may be indicated by means of a chain line

Partial surfaces as datums

Datum targets are the designated points, lines, or areas that are used in establishing a

datum. Datum targets are used in establishing a datum reference frame. Because of inherent

irregularities, the entire surface of some features cannot be effectively used to establish a

datum.

•The lower half contains a letter identifying the

associated datum, followed by the target number

assigned sequentially starting with 1 for each datum.

•A radial line attached to the symbol is directed to a

target point, target line, or target area, as applicable.

•Where the datum target is an area, the size and shape

of the area (true geometric counterpart) is entered in

the upper half of the symbol; otherwise, the upper half

is left blank.

A datum target point is indicated by the target point symbol,

dimensionally located in a direct view of the surface.

A datum target line is indicated by the datum target point symbol

on an edge view of the surface, a phantom line on the direct view, or

both.

Where it is determined that an area or areas of contact is necessary to assure establishment of the

datum (that is, where spherical or pointed pins would be inadequate), a target area of the desired

shape is specified. The datum target area is indicated by section lines inside a phantom outline of

the desired shape, with controlling dimensions added.

A primary datum plane is

established by at least three

target points not on a straight

line. See Fig. 4-48. A secondary

datum plane is usually

established by two targets. A

tertiary datum plane is usually

established by one target. A

combination of target points,

lines, and areas may be used.

Two sets of three equally spaced datum targets may be used to establish a

datum axis for a primary datum feature.

Circular target lines and cylindrical target areas may be used to establish a

datum axis on round features.

For a secondary datum feature, a set of three equally spaced targets may be

used to establish a datum axis.