How do you choose the best datum reference frame for mechanical inspection?

2 How to choose the primary datum?

The primary datum is the most essential datum in a DRF, as it controls the degrees of freedom (DOF) of the part or feature. This datum should be chosen based on its function, stability, and accessibility. For example, the mating surface, mounting hole, or axis of rotation should be the most functionally significant feature of the part or feature. Additionally, it should be the most stable and rigid feature of the part or feature, such as the largest or flattest surface, the longest or straightest edge, or the most symmetrical or circular feature. Lastly, it should be measurable by a probe, gauge, or fixture.

3 How to choose the secondary and tertiary datums?

The secondary and tertiary datums are the second and third datums in a DRF, respectively. These datums are used to further constrain the DOF of the part or feature, depending on the geometric characteristic being controlled. When selecting secondary and tertiary datums, they should form right angles with the primary datum plane or axis, be independent of the primary datum, and be consistent with the design intent and functional requirements of the part or feature. This will ensure that they support the alignment, fit, or performance of the part or feature in its intended use.

4 How to apply GD&T symbols and rules to the DRF?

Once you have chosen the datums for your DRF, you need to utilize GD&T symbols and rules to indicate the dimensions and tolerances of the part or feature relative to the DRF. These symbols and rules are standardized by various organizations, such as ANSI, ISO, ASME, and JIS. Common GD&T symbols and rules include the feature control frame - a rectangular box containing the geometric characteristic symbol, tolerance value, datum references, and any modifiers or conditions - the geometric characteristic symbol (indicating the type of tolerance being applied such as flatness or concentricity), the tolerance value (specifying the allowable variation from its nominal shape), the datum references (identifying the datums used for the DRF), and any modifiers or conditions (like MMC or PTZ).

5 How to verify the DRF using inspection equipment?

Verifying that the DRF is correctly established and that the part or feature meets the specified dimensions and tolerances is the final step in choosing the best DRF for mechanical inspection. This can be done using various inspection equipment, like coordinate measuring machines (CMMs), optical comparators, surface plates, and gauges. CMMs use probes or sensors to measure the coordinates of points on a part or feature and compare them to the DRF. Optical comparators project an image of a part or feature onto a screen and compare it to a magnified template of the DRF. Surface plates are flat and smooth surfaces that can be used as reference planes for measuring flatness, parallelism, perpendicularity, or angularity of a part or feature. Gauges are devices that can be used to measure specific dimensions or tolerances of a part or feature, such as diameter, width, height, or position.

6 Here’s what else to consider

This is a space to share examples, stories, or insights that don’t fit into any of the previous sections. What else would you like to add?