What are Datums: Definition, Types and Uses
Precision manufacturing is a requirement of almost every industry. Moreover, GD&T is the foundation of precision manufacturing. It is a very efficient method that helps convey functional tolerances and request forms to manufacturers.
In GD&T, datum is a very important concept, and understanding it is crucial. The article below captures information about what are datums, their types, and their symbols. So let's read.
What is Datum in GD&T?
The definition of datum, as given in ASME Y14.5 (1994), states that “A theoretically exact axis, plane, or point which is derived from the geometric counterpart of a certain datum feature. The position of the features in a component establishes datum as a reference.”
Datum gives a reference for locating and measuring the different features of a component. The datum symbol offers compatibility with other symbols of GD&T.
An Overview of Datum
Understanding the datum basics helps us understand the concept better. Some of these basics are as follows:
Datum vs. Datum Feature
The measurement of features of a component is conducted by making the datum a reference point. The datum feature is the component’s physical feature, which is chosen to serve as a datum. This datum feature gives a reference to the manufacturing process.
It is crucial to understand the difference between datum and datum features. The datum feature is the point at which the datum symbol is placed on the component. At the same time, the datum is the reference point that governs measurements.
Datum Reference Frame
The datum reference frame is a term of geometric dimensioning and tolerance, which locks the degree of freedom of the part when inspection or measurement is conducted. It offers a referencing system that helps in measuring the features of the part.
There are six degrees of freedom for every feature, and all of them should be locked by a datum reference frame.
Primary and Secondary Datums
In GD&T, a single datum is not sufficient to ensure a part is complete. Hence, the need for multiple datums arises so that a comprehensive part dimensioning is done.
The datums are set based on the priority. The primary datum is a critical datum that relates to the dimensions and form of the component. The secondary datum comes after the primary datum, and in many cases, there might be a need for a tertiary datum.
Datum Target in Engineering Drawings
Datum targets are significant concepts in the drawings of engineering. The section below discusses the datum targets and how they can be used.
Datum vs. Datum Target
Datum is the accurate geometric reference point that is used in the inspection and measurement of the component.
A datum target is an area, line, or point that offers additional information to the engineers to use the datum feature for their measurement reference. It helps in specifying the contact with the component and the device used for measuring.
It is a target for specifying physical contact between the measurement device and the part.
How to Use Datum Targets?
When a component is handed over to a quality inspector, a CMM is used to perform measurements from the front surface of the component. The inspector will place the front surface of the datum on the CMM work table.
The drum targets will help the inspector choose the number of mounting pins. It also conveys where to insert these pins. The pins are meant to touch the component within the target ones so that the datum can be established as per the given specification.
How to Classify Datum Symbols?
The datum symbol is a very important concept in the GD&T system, as every dimension needs a datum. Hence, there are various ways to call a datum symbol. It is described in two different categories below.
Datum Feature Symbol on a Surface
Datum feature symbol on a surface is the most commonly adopted method of GD&T datum, which involves a surface plane axis of a circular feature working like a reference datum. There are various ways of specifying an axis or surface as the datum.
Datum Symbol on a Feature of Size
In the other category, the feature of a size is declared as a datum. It is a geometric feature of a dimension. An example of this datum can be the slots and bores as features of size. These datums are mostly used to control the engineering drawings that deal with issues like runouts.
Steps for Using Datum Targets
Datum target can be used in different steps, and these are discussed below:
Spotting Irregular Features: Consider the parts containing uneven features, such as components having contours or complex layouts.
Choose Specific Areas: Choose well-defined, small lines and points on the surface to use as a measurement reference.
Mark Datum Targets in the Drawing: Select the target on the engineering drawing with the help of proper symbols, like the target symbol for a line or a point.
Measurement Use: At the time of inspection and manufacturing, the parts must be aligned using a datum target. These reference locations ensure that the measurement of the part is carried out despite being irregular.
Conclusion
Manufacturing components for various applications requires ensuring perfection and accuracy. Components that are highly precise and have a tight tolerance need an accurate datum. We at DEK use datum references in our engineering drawings, offering highly precise components. So, for your next project, contact us to get the right components manufactured.
