GD&T - Part 2; Datums
Six Degrees of Freedom
We have all at some point in our careers; sat down at our desks to start dimensioning a 2D technical drawing, made the funny 3 axis graph from our thumb, index finger and middle finger, spun it around a few times, gotten confused… then stood up to go get a coffee. We are of course talking about Datums.
A good solid datum structure is constructed by constraining the component’s six dreeges of freedom. The six degrees of freedom consist of three directional and three rotational elements:
- Left to Right
- Up and Down
- Back and Forth
- Rotation about each directional element
Datums are created to ‘lock’ each of these degrees of freedom either individually or in a combination. Datums can be:
- A surface
- An axis
- A line (on a surface or edge)
- A point
However the most important thing to remember is that each datum should represent the functional requirements of the part. Datums will be the first symbol that manufacturing and quality look for on your technical drawings as each datum symbol indicates how we intend our part to be set-up. Therefore it is important that our datums are easy to access and should be robust enough with sufficient material to allow good stable alignments.
Primary, Secondary, Tertiary
As a minimum requirement and in most cases our component’s six degrees of freedom can be constrained by a plane (primary datum), a line (secondary datum) and a point (tertiary datum).
The order in which we structure our datums is highly important. The effect of a different orders of priority is illustrated below. You will see L1 does not equal L2!
In our previous example from GD&T Part 1, the bolted bracket, our primary function of the bracket will be to have a solid bolted connection to our baseplate. Therefore, we would call out that bolted interface surface as our primary datum ‘A’.
The secondary function of our component is to ensure that the clamping face is positioned correctly to baseplate so we call that surface out as datum ‘B’. Finally we want to control the position of our bolt holes. For this we could call out either end face of our bracket as datum ‘C’. Once the datum structure has been decided, it is best practice to dimension as many features as possible from these datum features.
In the case of rotational components such as a driveshaft or fixing, the primary datum should always be the component’s rotational axis. Our secondary datum would then become any mounting features of that component.
In the case of a uniformly swept rotational component a tertiary datum is not always necessary, this would only become applicable if the component had any features down the length of the ration axis e.g. a spline or keyway feature.
Datum Targets
Datums that cannot be defined by conventional means can be defined using datum targets. Datum targets are extremely useful on components that are of a freeform shape such as airfoils or castings. Where datum targets are used, we would recommend the following:
- Primary Datum be defined by a minimum of three targets
- Secondary Datum be defined by a minimum of two targets
- Tertiary Datum be defined by a minimum of one target
Datum targets can be defined as either a point, line or area
In the examples given above each datum target must only be located from known features i.e. a machined or controlled surface. Alternatively datum targets can be established on the component using a predefined jig.
