DATUM AXIS & DATUM CENTER PLANE

INTRODUCTION

• Here Feature of Size is used as a datum features

• When a diameter is used as a datum feature, It results in a datum axis

• When a planar is used as a datum feature, it results in a datum center plane

GOAL1.1 Describe the datum that results from a FOS datum feature

3 Ways for representing an axis as datum

• Datum identification symbol can be touching the surface of a diameter to specify axis as the datum

GOAL1.2 Describe the ways to specify an axis as a datum.

3 Ways for representing an axis as datum (Contd….)

• Datum identification symbol can be touching the beginning of a leader line of FOS to specify an datum axis

• Datum identification symbol can be touching the feature control frame to specify an axis or centre plane as datum

3 Ways for representing an axis as datum (Contd….)

• Datum identification symbol can be inline with dimension line to specify on axis or centre plane as datum

2 Ways for representing a centre plane as datum

GOAL1.3 Describe the ways to specify an centre plane as a datum.

• Datum identification symbol can replace one side of the dimension line and arrow head

2 Ways for representing a centre plane as

datum (Contd….)

Datum Terminology• Datum feature A• Datum feature

simulator / Gauge element

• Simulated datum axis A

• Simulated datum Feature A

Referencing FOS Datum Features

• When using datum features on drawings, the material condition for establishing the datum axis or center plane must be communicated.

GOAL1.4 Explain how FOS datum references communicate size condition

Referencing FOS Datum Features (contd…)

• The feature control frame indicates both the material condition (MMC, LMC, or RFS) and the datum sequence (primary, secondary, or tertiary).

Referencing FOS Datum Features (contd…)

• Whenever a FOS datum feature is referenced in a feature control frame, without showing any modifier, it is automatically referenced in the RFS condition.

FOS Datum feature applications (RFS)

• Datum selection and referencing are directly related to the function of the part

• features that orient and locate the part in its assembly became the datum features for the part

Datum axis RFS Primary

GOAL1.5 Draw the datum feature simulator for an external and internal FOS datum feature (RFS primary)

• When a FOS is used as a datum feature and referenced as a primary datum RFS

• Devices that are adjustable in size-such as a precision chuck, collets, or centering device are used as the datum feature simulator.

• The datum feature simulator holds the part securely on the datum feature.

Datum axis RFS Primary (contd…)

Datum Center plane RFS Primary

Datum Center plane RFS Primary(Contd…)

• When a FOS is used as a datum feature and referenced as a primary datum RFS, the following applies:– An adjustable gage element is needed to simulate the

true geometric counterpart of a datum feature– The part is oriented by the gage – The part is held securely in the gage

Datum Axis RFS Secondary

GOAL1.6 Draw the datum feature simulator for an FOS datum feature (RFS secondary).

Datum Axis RFS Secondary (Contd…)

• When referencing the datums with the face primary and the diameter secondary (RFS)

• The part will have a minimum of three-point contact with the primary datum plane.

• The secondary datum feature simulator will be adjustable in size.

• The datum axis is perpendicular to the primary datum plane.• A second and third datum plane will be associated with the

datum axis.

Datum Axis RFS Secondary, Datum Center plane RFS Tertiary

GOAL1.7 Draw the datum feature simulator for an FOS datum feature (RFS tertiary).

Datum Axis RFS Secondary, Datum Center plane RFS Tertiary

(Contd…)

• When referencing the datums with the face primary, diameter secondary (RFS), and slot tertiary (RFS).

• The part will have a minimum of three points of contact with the primary datum plane.

• A datum axis perpendicular to the primary datum plane will exist.

• A datum center plane that will pass through the datum axis and be perpendicular to the primary datum plane will exist

Datum Axis from Coaxial Diameters, RFS Primary

GOAL1.8 Describe coaxial datum features

Datum Axis from Coaxial Diameters, RFS Primary (Contd…)

• When coaxial diameters are used to establish a datum axis, they are called coaxial datum features

• The datum axis is simulated by contacting the high points of both datum features simultaneously.

• Coaxial diameters are often used when both datum features have an equal role in locating the part in its assembly.

• A single datum axis from two coaxial diameters is normally used as a primary datum.

FOS datum feature referenced at MMC

FOS datum feature referenced at MMC (Contd…)

• The gauging equipment that serves as the datum feature simulator is a fixed size

• The datum axis or center plane is the axis or center plane of the gage element

• The size of the true geometric counterpart of the datum feature is determined by the specified MMC limit of size or, in certain cases, its MMC virtual condition

FOS datum feature referenced at MMC (Contd…)

• Referencing a FOS datum at MMC has two effects on the part gaging :– The gage is fixed in size – The part may be loose (shift) in the gage

GOAL2.1 List two effects of referencing a FOS datum at MMC

Datum axis MMC primary

GOAL2.8 Draw the datum feature simulator for an external and internal FOS datum feature (MMC primary).

Datum axis MMC primary (Contd…)• When diameter is designated as a datum feature and

referenced in a feature control frame as primary at MMC

• A fixed-gage element may be used as the datum feature simulator

• The size of fixed-gage element is equal to the MMC (WBC) of the datum feature

• Depending upon the actual mating size of the datum feature, a datum shift may be available

Datum centre plane MMC primary

Datum centre plane MMC primary (Contd…)

• When FOS datum feature is referenced at feature control frame as primary at MMC

• A fixed gauge element is used as datum feature simulator

• Size of fixed gauge element is equal to MMC (in certain cases worst-case boundary) of the datum feature

• The datum center plane is the center plane of the datum feature simulator

• Depending upon the actual mating size of the datum feature, a datum shift may be available

Datum axis MMC secondary

GOAL2.9 Draw the datum feature simulator for an FOS datum feature (MMC secondary with virtual condition)

Datum axis secondary (MMC) ,Datum centre plane tertiary (MMC)

Datum axis secondary (MMC) ,Datum centre plane tertiary (MMC) (Contd…)

• When referencing the datums with the face primary, diameter secondary (MMC), and slot tertiary (MMC), the following conditions apply:• The part will have a minimum of three points of

contact with the primary datum plane • The datum feature simulators will be fixed size gage

elements. • The datum axis is the axis of the datum feature

simulator

Datum axis secondary (MMC) ,Datum centre plane tertiary (MMC)

(Contd…)

• The datum axis is perpendicular to the primary datum plane

• Depending upon the datum feature's actual mating size, a datum shift may be available.

• Second and third datum planes are to be associated with the datum axis

• The tertiary datum center plane is the center plane of the tertiary datum feature simulator

Datum Axis from a Pattern of Holes, MMC Secondary

GOAL2.10 Draw the datum axis when using a pattern of FOS as a datum feature (MMC secondary)

Datum Axis from a Pattern of Holes, MMC Secondary (contd…)

• When referencing the datums with the face primary and a hole pattern secondary (MMC), the following conditions apply:– The part will have a minimum of three points of

contact with the primary datum plane – The datum feature simulators will be fixed size gage

elements equal to the virtual condition of each hole diameter

– In this case, individual datum axes are established at the basic location of each hole. Datum axis B is at the theoretical center of the pattern. A second and third datum plane exist from the datum axis

Datum Axis from a Pattern of Holes, MMC Secondary (contd…)

• When the part is mounted on the primary datum surface, the pattern of holes establishes the second and third datum planes of the datum. reference frame. A tertiary datum reference is not necessary since all six degrees of freedom are controlled

Datum sequence

Panel-A

GOAL2.11 Explain how changing the datum reference sequence in a feature control frame affects the part and gauge

Datum sequence (contd…)• Panel A

• An adjustable gauge is required.• No datum shift is permissible on datum feature A• The part is oriented in the gage by datum feature

A• Datum feature B will have a minimum of one

point contact with its datum feature simulator• The orientation of the holes will be relative to

datum axis A

Panel B

Datum feature simulator for datum plane B

• Panel B• Datum feature B will have 3- point contact with its

datum plane• The part is oriented in the gauge by datum feature B• The orientation of holes will be relative to datum

plane B• An adjustable gauge is required and no datum shift

is permissible on datum feature A

Panel C

Virtual condition=Ф10.2

Panel C• When simulating datum A, fixed gauge is allowed

and the gauge allows datum shift• Datum feature B will have 3-point contact with its

datum plane• The part is oriented in the gauge by datum feature

B• The orientation of the hole will be relative to

datum plane B