In precision machining, accuracy doesn't end at the machine; it extends into the inspection process to ensure every part meets stringent specifications. A critical element of this process is the proper use of datums—geometric references that guide the measurement of a part's dimensions.

Engineering Drawings vs. 3D CAD Models: Bridging the Gap
In modern manufacturing, 3D CAD models are often the primary source of dimensional information. However, dimensioned drawings still play a crucial role.
In the past, dimensioned engineering drawings were the only source for dimension data. A well-done drawing conveys the design intent of the engineer or designer to the fabricator. While the transition to 3D CAD models as the primary dimensional reference has streamlined the manufacturing process, it often misses the opportunity to convey design intent.
The current trend is to support the 3d CAD models with "Reduced Dimension" drawings that focus on critical features, providing only the necessary tolerances, not easily extracted from the 3D CAD model, for inspection references. The combination of a 3D model and a well-designed drawing reduces ambiguity and ensures clarity during the inspection process.
The GD&T (Geometric Dimensioning & Tolerancing) Language

GD&T is a system or language that specifies the size, shape, and orientation of features on a part. It conveys the design intent and ensures that parts fit together properly and function as intended.
The American Society of Mechanical Engineers (ASME) Y14.5 standard is widely recognized as the primary GD&T standard in the United States and is often used in conjunction with other national and international standards. It is known for its comprehensive coverage of GD&T principles, including symbols and terms, tolerance zones, form tolerances, orientation tolerances, location tolerances, runout tolerances, datum reference frames, and applications of GD&T.
What Are Datums? Exploring Primary, Secondary, and Tertiary References
Datum features are used in engineering drawings to define a reference coordinate system for measuring and controlling the geometric features of a part. The datum features are the surfaces, points, or axes that establish the datum reference frame.
Datum features can be either primary or secondary, depending on their importance in controlling the part's geometry. Primary datum features are the most critical features, and they are used to establish the primary datum of a reference frame. Secondary datum features are used to establish the secondary and tertiary datums to finally establish a reference frame, a three-dimensional coordinate system used to control the geometry of the part.
The datum reference frame consists of three orthogonal planes defined by the primary, secondary, and tertiary datum features. It is used to establish the zero point for all measurements and tolerances on the part. Establishing datums is a critical step in creating an accurate and reliable engineering drawing.
For example, on a rectangular block:
The bottom face might be designated as the primary datum A to establish a theoretically flat plane.
Another face, or edge, might serve as the secondary datum B.
Finally, a third face, edge or other point can serve as the tertiary datum C to complete the reference frame.
In some cases, multiple datum reference frames may be required to control a part's geometry. This can occur when the part has multiple functional surfaces that require different levels of control. Additional datum features and datum planes can establish multiple datum reference frames.
How Datums Simplify Post-Machining Inspections

Using datums simplifies inspection by providing a consistent reference frame for measuring critical dimensions:
1. Establish the Datum Reference Frame (DRF): This is the coordinate system defined by the datums, against which all measurements are taken.
2. Inspect Features Relative to Datums: Features are measured in relation to the DRF, ensuring dimensional accuracy, alignment, and adherence to design intent.
Why Proper Datum Usage is Crucial in Manufacturing
In post-machining inspection, the use of datums:
Reduces ambiguity and potential tolerance stacks in measurements
Ensures conformity to design intent
Simplifies communication between engineers, machinists, and quality inspectors
DMAC Machining's Approach to Datum-Driven Accuracy
At DMAC Machining, we recognize the importance of proper datum usage. We deliver parts that meet the highest quality standards by combining advanced 5-axis machining with rigorous inspection protocols.
Whether we are working with a detailed engineering drawing or a reduced-dimension drawing combined with a 3D model, you can trust that our processes prioritize accuracy and adherence to design intent at every step.
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