Machinery's Handbook, 31st Edition
STANDARD DRAFTING DEFINITIONS 627 US ASME Y14.5-2018 and ISO practice use a preceding 0 before millimeter dimension values less than 1. Unlike US practice, ISO requires a comma as a decimal placeholder where US practice would use a decimal point, compare Fig. 2a and Fig. 2c. For decimal inch dimensioning, a zero is NOT used before the decimal for values less than 1 inch, Fig. 2b. 0.01
.01 Fig. 2b. US Practice, Inch Dimensions
0,01 Fig. 2c. ISO Practice
Fig. 2a. US Practice, Metric Dimensions
When dimensioning per US default practice, unless otherwise specified, dimension lines are broken and dimension text remains horizontal, hence read from the bottom of the drawing, see Fig. 3a. ISO dimensioning practices default to aligned text with dimensions placed above an unbroken dimension line, see Fig. 3b.
27.8
50 Fig. 3b. Default ISO Practice
50
Fig. 3a. Default US Practice
Standard Drafting Definitions The following terms are defined as their use applies to ASME Y14.5. Where ISO practice in actual use differs, the definitions are expanded to clarify the contrasting practices. Datum Feature: The feature of a part that is used to establish a datum, identified with either a datum feature or datum target symbols, see Fig. 4.
Leader may be appropriately directed to a feature.
Datum letter
A
A
0.25 ∅
A B C
M
Datum triangle may be filled or not filled.
A
control frame and datum identifier Combined feature
A
A
∅
Fig. 4. Datum Feature Symbol Datum Identifier: The graphic symbol on a drawing used to indicate the datum feature. Specified Datum Feature: A datum is the origin from which the location or other geo metric characteristics of features of a part are established. Datum Reference Frame: Sufficient features on a part are chosen to position the part in relationship to as many as three planes. The three planes are mutually perpendicular and together are called the datum reference frame. The planes follow an order of precedence and allow the part to be immobilized. This immobilization in turn creates measurable relationships among features. Datum Simulator: Formed by the datum feature contacting a precision surface such as a surface plate, gage surface or by a mandrel contacting the datum. Thus, the plane formed
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