Introduction To Datums: Datums Tell Us Where To Measure From. Calling Them Out Clearly and Labelling Each

Introduction to Datums
Datums tell us where to measure from. Calling them out clearly and labelling each
Feature Control Block with the applicable Datums makes it completely clear how
things are to be measured. In the drwaing below, A and B are the Datums used to
establish the center of the hole using the Position Symbol and the Feature Control
Block for the hole center..
A simple drawing with GD&T symbols. Image Source
GD&T positions every part within a Datum Reference Frame (DRF). Some say the
DRF is the most important concept in geometric positioning and tolerancing because
it provides the skeleton or frame of reference to which all requirements are
connected. For that reason, we want to start early with a solid grounding on Datums
and Datum Reference Frames.
Definition of a Datum
A Datum is a plane, axis, or point location that GD&T dimensional tolerances are
referenced to. Typically, multiple features will be referenced by each datum, so
they're a very important part of the whole thing. Nearly every GD&T symbol except
for form tolerances (straightness, flatness, circularity, and cylindricity) can use
datums to help specify the geometric control that is needed on the part.
How Datums Are Shown

On Drawings
As we said in the definition, Datums are planes, axes, or point locations, and that's
exactly how they show up on a drawing. Let's go through each one to see how it
looks:
On a Surface (Plane)
Two examples of Datums on a Surface
We can place the datum symbol either on the surface or on one extension line from
the surface. For any surface other than a round cylinder, the datum is strictly on the
side where the symbol is shown. However, for a round cylinder, the datum is the
entire round surface.
On an Axis
Place the symbol on the dimension of a diametric tolerance for Axis control:
A Datum on an Axis
Remember: this places the Datum on the central axis through the feature, not the
surface of the feature. Placing the Datum on an axis is common with GD&T symbols
that can have axis control like runout, perpendicularity, or concentricity.
On a Point or Hole's Axis
To establish a datum axis on a feature such as a hole, there are a number of ways to
place the symbol for the datum:
The "A" Datum can appear in these three different places on a drawing...
Referring to the example above, the "A" Datum can appear in these three different
places on a drawing:
- It can be placed directly on the hole, as in the leftmost case. It is of course referring
to the axis and not the surface of the hole.
- It can be placed on the leader pointing to the hole as it is in the middle case.
- It can be placed on the Feature Control Frame for the hole.
In addition, it could be shown on a side view by denoting the axis.
Datums vs Datum
Features and Datum
Targets
At this point it is important to talk about the difference between a Datum and
a Datum Feature--they are not the same though it is tempting to refer to them as
such at first. Datums are abstract geometrical concepts. They correspond to points,
lines, and planes. When we refer to the tangible feature on the part that the Datum is
associated with, we call it a Datum Feature. The reason they're not the same is
perhaps small, but significant. It is because real features are never perfect abstract
geometrical concepts. Surfaces on parts are not perfectly flat planes. Edges on parts
are wavy and are not perfect lines.
The standard itself defines a Datum Feature as, "An actual feature of a part that is
used to establish a datum."
You may also come across the term Datum Target, which is defined as, "A specified
point, line, or area on a part used to establish a datum."
Definition of a Datum
Reference Frame
This one starts out simple:
A Datum Reference Frame is a coordinate system, and preferably it is a Cartesian
coordinate system.
Coordinate systems are valuable because they're used to locate objects. In GD&T
they are used to orient and locate tolerance zones.
Datum Reference Frames

and 6 Degrees of
Freedom
Every Datum exists within the context of some Datum Reference Frame. In practice,
we must eliminate 6 degrees of freedom before we can fully locate and orient a part
within a Datum Reference Frame:
Controlling 6 degrees of freedom...

Controlling 6 degrees of freedom means controlling 3 linear distances from Datum
planes to establish an X, Y, and Z position and controlling 3 rotary positions to orient
the part at that position. We refer to the translational degrees of freedom as X, Y, and
Z and the rotational degrees as u, v, and w.
The 3-2-1 Rule and Points

of Contact
The 3-2-1 rule defines the minimum number of points of contact required for a part
datum feature with its primary, secondary, and tertiary datum planes. It only applies
when all three plaines are used. The 3-2-1 rule says:
- The primary datum feature has at least 3 points of contact with its datum plane.
- The secondary datum feature has at least 2 points of contact with its datum plane.
- The tertiary datum feature has at least one point of contact with its datum plane.
The 3-2-1 rule only applies to planer datum features.
Inclined Datum Features

One may specify a datum feature that is at an angle other than 90 degrees relative to
other datum features. These are called Inclined Datum Features.
Datum C corresponds to an Inclined Datum Feature...
Datums in the Feature

Control Frame and Order
of Precedence
Datums are specified in the Feature Control Frame in an Order of Precedence. This
Feature Control Frame has 2 Datums specified (A and B):
A Feature Control Frame...
Datum A is the Primary Datum and B is the Secondary Datum. If there had been a
third Datum, it would be called the "Tertiary Datum." It is not necessary to specify the
Datums in alphabetical order.
The first step in dimensioning a part is always to select the Datums. When selecting
Datums, designers should consider the following characteristics:
- Functional surfaces
- Mating surfaces
- Readily accessible surfaces
- Surfaces of sufficient sizes to allow repeatable measurements
Datums are important and care must be taken when selecting them. They must be
easily identifiable on the part. When parts are symmetrical or have identical features
that make identification of Datum Features difficult, the Datum Features should be
physically identified.
Conclusion
By now you have an idea of how Datums are shown on drawings and how they're
used to establish a coordinate system. You can read them, at least a little bit, but I'll
bet they still don't feel obvious and you're wondering how to "write" them (in other
words, how to choose Datum Features on your parts).
We'll have more on exactly how to choose Datum Features shortly. First, we need to
talk about Datum Feature Simulators.

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What are datums and how are they shown on drawings?

What are datums and how are they shown on drawings?

What is the difference between datums, datum features and datum targets?

What is the difference between datums, datum features and datum targets?

Introduction to Datums

A simple drawing with GD&T symbols. Image Source

How Datums Are Shown

Two examples of Datums on a Surface

On a Point or Hole's Axis

- It can be placed on the Feature Control Frame for the hole.

In addition, it could be shown on a side view by denoting the axis.

Datum Reference Frames

Controlling 6 degrees of freedom...

The 3-2-1 Rule and Points

The 3-2-1 rule only applies to planer datum features.

Inclined Datum Features

Datums in the Feature

A Feature Control Frame...

- Readily accessible surfaces

- Surfaces of sufficient sizes to allow repeatable measurements

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