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May 20, 2009

Datum Targets - GDT - A Better Understanding

Some manufacturing processes, such as casting, forging, welding, and heat treating, are

likely to produce uneven or irregular surfaces. Datum targets may be used to immobilize

parts with such uneven or irregular surfaces. Datum targets may also be used to support

irregular-shaped parts that are not easily mounted in a datum reference frame. Datum

targets are used only when necessary because, once they are specified, costly

manufacturing and inspection tooling is required to process them.

Datum targets are designed to contact parts at specific points, lines, and areas. These

datum targets are usually referenced from three mutually perpendicular planes to

establish a datum reference frame. A primary datum plane is established by a minimum

of three datum targets not in a straight line. Two datum targets are used to establish a

secondary datum plane. And one datum target establishes a tertiary datum plane. A

combination of datum target points, lines, and areas may be used. Datum target points

are represented on a drawing by target point symbols and identified by datum target

symbols such as datum targets B1 and B2 shown in Figure.

Actual tooling points on the fixture are not points at all but pins with hemispherical ends

contacting the part with the highest point on the hemisphere, as shown in Figure. A datum

target line is represented by a target point symbol on the edge of the part in the top view

of the drawing and by a phantom line on the front view, datum target C1. Since datum

target C1 is on the far side of the part, a dashed radial line is used to connect the datum

target symbol. Where a datum target area is required, the desired area is outlined by a

phantom line and filled with section lines, as shown for datum targets A1, A2, and A3. All

datum targets are dimensioned for location and size either by toleranced dimensions or

basic dimensions. Basic dimensions are toleranced with gage-makers’ tolerances.

GD&T & Tolerance Analysisadvanceddimensionalmanagement.com

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Note that A1, A2, A3 are datum target areas and B1, B2 are datum target points and C1 is

datum target line.

GD&T Reference Chartwww.iigdt.com

Wall chart that illustrates GD&T

symbols used in ASME Y14.5M

Early ChildhoodEducation

King Metal Spinning

► GD&T Symbols

► Datum

► Tolerance Stack Up

► Metal Targets

► Flatness Tolerance

► Process Drawing

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