The actual ASME standard that covers lettering on drawings is Y14.2. Section 6 governs lettering on engineering drawings.
From the standard:
6 LETTERING
For clarity within this Standard, lettering means both letters and numerals. Paragraphs 6.1 through 6.6 identify
the type and style of lettering for use on drawings.
6.1 Lettering Type
Lettering should be single stroke gothic, opaque, and well-spaced characters as shown in Figs. 6-1 and 6-2. When
additions or revisions are made to a drawing, the original style of lettering shall be maintained.
6.2 Letter Style
Either inclined or vertical lettering is permissible. Only one style of lettering shall be used throughout a drawing.
The preferred slope for the inclined characters is approximately 68 deg from the horizontal. See Fig. 6-1.
6.3 Letters — Uppercase and Lowercase
Uppercase letters shall be used for all lettering on drawings unless lowercase letters are required. See Figs. 6-1
and 6-2.
6.4 Letter Height
The minimum letter height proportions for various size drawings are shown in Table 6-1. A minimum letter
height (h) of 3 mm or .12 in. is recommended.
6.5 Letter Spacing
Letters in words should be spaced so the background areas between the letters are approximately equal, and
words are clearly separated. The space between two numerals having a decimal point between them is to be a
minimum of two-thirds the height of the lettering. The vertical space between lines of lettering shall be no more
than the height of the lettering, or no less than half the height of the lettering.
6.6 Legibility
The lettering heights, spacing, and proportions in Figs. 6-1 and 6-2, and also Table 6-1, normally provide acceptable
reproduction. If the recommended letter heights or proportions are not able to be met, the lettering is acceptable
when the minimum legibility and reproduction requirements of the accepted industry or military reproduction
specifications are met. Therefore, the basic requirement for lettering on a drawing is to produce fully legible copies.
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