Mechanical Machining | Basic Knowledge

Nov 29,2021

Parts are composed of multiple surfaces, each with specific dimensional and positional requirements relative to the others. The relative positioning of these surfaces involves two key aspects: dimensional accuracy in terms of distance between surfaces, and geometric accuracy in terms of their mutual orientation (such as coaxiality, parallelism, perpendicularity, and circular runout). Understanding the relative relationships among part surfaces is inseparable from establishing a proper datum—without a clearly defined datum, it’s impossible to determine the exact location of any surface on the part. In its most general sense, a datum refers to the reference point, line, or plane on a part that serves as the basis for defining the positions of other points, lines, or surfaces. Depending on their function, datums can be broadly categorized into two main types: design datums and manufacturing datums.

1. Design Basis

On a part drawing, the reference used to establish other points, lines, and surfaces is called the design datum. In the case of a piston, the design datums refer to the piston centerline and the pinhole centerline.

2. Process Datum

The reference points used during the machining and assembly processes are called process datums. Depending on their specific purpose, process datums are further categorized into positioning datums, measuring datums, and assembly datums.

1) Datum for Positioning: The reference used to ensure the workpiece occupies the correct position in the machine tool or fixture during machining is called the positioning datum. Depending on the type of positioning element, the two most commonly used categories are as follows:

Automatic centering and positioning: such as with a three-jaw chuck for alignment.

Locating sleeve positioning: Fabricate the locating component as a locating sleeve, such as with a stop-flange disc for positioning.

Others include positioning in V-shaped fixtures, as well as positioning in semi-circular holes, and so on.

2) Measuring Datum: During part inspection, the reference used to measure the dimensions and positions of machined surfaces is referred to as the measuring datum.

3) Assembly Datum: The datum used during assembly to determine the position of a part within an assembly or product is called the assembly datum.

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