A datum feature is a real surface or feature selected to establish an exact reference for other requirements. A mounting face can establish a datum plane; a bore or shaft can establish a datum axis. The feature is physical and imperfect. The datum is the theoretically exact reference established from it.
Datum required
Depends on related control
MMC / LMC applicable
At eligible feature-of-size datum references
Control type
Reference identification
What it controls
A real feature used to establish a datum
Typical features
Mounting faces, bores, shafts and slots
INTERACTIVE APPLICATION
From a real feature to an exact reference
DATUM FEATURE
The real surface on the part
The highlighted mounting face belongs to the workpiece. Its shape is not perfectly flat. The boxed A identifies the selected physical feature; it does not turn that face into a perfect plane.
Keep the distinction
A datum feature symbol identifies a feature, not a form tolerance.
DATUM SIMULATOR
The surface used to establish the reference
A precision surface plate supports the datum feature at its high points. This physical simulator represents the ideal contact used to establish the datum. Other features may need a mandrel, chuck or other suitable simulator.
Practical setup
Use the contact and constraint rules required by the drawing, rather than an arbitrary alignment.
ESTABLISHED DATUM
The theoretically exact reference
The dashed plane represents datum A, established through simulation of the mounting face. This ideal plane is used to orient or locate other requirements. The real face, plate and datum remain three different things.
Reference framework
Further datums constrain remaining movement in the specified order. A single plane does not constrain every motion.
01 / PURPOSE
Why use this annotation?
Give design, manufacture and inspection a shared functional reference. Establish the datum from the selected feature; control that feature’s form separately when function requires it.
02 / APPLICATION
Where is it applied?
Mounting faces, locating bores, shafts and opposed faces. Choose robust, accessible features that represent assembly contact. Use datum targets when only selected contacts should establish the reference.
03 / DRAWING
Datum symbols and attachment methods
Examples show surface attachments, feature-of-size attachments and selected-contact datum targets.
Surface datum features
Connected to the surface outline
The filled marker meets the selected surface directly.
On a surface extension
Keep the marker separate from the size-dimension line.
On a face shown in area view
A filled dot identifies the face; the leader meets a short attachment line.
Feature-of-size datum features
Attached to the size dimension
Identifies a cylindrical feature or width feature establishing an axis or centre plane.
Aligned with the size line
ASME-oriented attachment variant shown in the supplied notes.
On a cylindrical end-view outline
A remote marker avoids crowding the circular outline.
On a frame attached to size
The datum marker connects to the bottom edge of the size-associated frame.
Datum targets: selected contact
Point target
Cross marks a specified point. The upper compartment is blank.
Line target
A defined line identifies the required contact.
Area target
A defined area and its size identify the contact region.
Identify a surface
Attach the datum feature marker to the surface outline, a surface extension, or a leader ending in a filled dot on the face shown in area view. The attachment location tells the reader which physical surface establishes the reference.
Identify an axis or centre plane
For a feature of size, associate the marker with the size dimension. A shaft or bore establishes an axis; opposed faces can establish a centre plane. The supplied ASME-oriented notes also show aligned size-line, cylindrical end-outline and size-associated frame variants.
For a datum axis or centre plane derived from a feature of size, associate the datum feature symbol with its size dimension. Surface attachments identify the physical surface instead.
Use targets for selected contact
A datum target uses a divided circular symbol, not the boxed-letter datum feature symbol. A1, A2 and A3 identify separate contacts that contribute to datum A. The upper half states an area size when required; the lower half contains the datum letter and target number.
Targets are useful for rough castings, large surfaces or parts supported at specific points, lines or pads. Locate them with the specified dimensions and reproduce their intended contact during inspection.
How to select a datum
Start with how the part assembles, locates and functions.
Prefer robust, stable and accessible features.
Use primary, secondary and tertiary datums in the required precedence.
For a typical planar 3–2–1 setup, the primary plane constrains three motions, the secondary two and the tertiary one. Other geometries constrain different motions.
Keep the reference separate from the control
The boxed A identifies the datum feature. A in a feature control frame references the established datum. Neither is, by itself, a tolerance on the datum feature’s form.
Separate references in primary, secondary and tertiary order. Changing the order can change the setup and result.
A common datum established from two features together. This is different from A followed by B.
Individual datum targets contributing to datum A. An area size such as ⌀8 describes the contact area, not a geometric tolerance.
A datum reference frame can constrain the part’s translations and rotations. The geometry and control determine which degrees of freedom need to be constrained. Avoid circular references: do not use a feature as its own datum for the same requirement.
05 / INSPECTION
Checking the requirement in practice
Establish the reference with a suitable fixture, surface plate or CMM datum construction. Reproduce the specified feature contact and datum precedence before assessing other requirements.
When targets are specified, use those contacts rather than replacing them with whole-surface alignment. Eligible feature-of-size datum references at material boundaries need the boundary rules specified by the governing standard.
ASME Y14.5-2018 (R2024)
ASME Y14.5 establishes datums from datum features and simulators; datum order matters.
