Technical Drawing Lettering and Dimensioning Standards

Standards for Technical Drawing Lettering

Lettering involves the use of letters, numbers, and symbols. Since a clear picture alone cannot express everything (such as dimensions and specific details), characters are used in technical drawings. These are standardized according to UNE 1 034-75 and must meet requirements for readability, consistency, and suitability for photographic reproduction.

Types of Lettering Styles

• Italics: The characters in this lettering style are inclined at 75 degrees toward the right relative to the horizontal. This may be Type B, which is commonly used in the metallurgical industry.
• Vertical: This lettering style is used in plans for the construction industry as well as in electrical and electronic schemes. It can also be Type B.

Standardized Dimensioning Symbols

Specific signs are used to record dimensions accurately:

• Diameter (Ø): The sign is positioned before the dimension number at the same height as the figures.
• Radio (R): The letter 'R' is placed before the dimension number when the center of the arc is not determined.
• Sphere: When showing a spherical form, the word Sphere takes precedence before the dimension figure. If the sphere is not complete, the diameter sign will be used.
• Square (□): A square symbol is placed ahead of the dimension number at an equal height.
• Inches: Symbolized by two strokes inclined at 75°, sized at 3/10 of the normal lettering height, and placed as a superscript.

Precautions for Technical Lettering

When applying tags and text, follow these rules:

• Font inclination must remain constant.
• Titles are not underlined.
• Capital letters must be accentuated.
• Uppercase characters are reserved for titles and headers.
• Lowercase captions are used for dimensions, notes, etc.

Elements of Dimensioning

Dimension lines serve to indicate measurements and arrange the dimensions to be indicated. They must be separated by at least 8 mm from the edges of the object and at least 5 mm between parallel dimensions.

Lines and Arrows in Drafting

• Auxiliary Lines: If auxiliary lines are located between the edges of the object, dimension lines are used perpendicular to the reference lines.
• Reference Lines: These lines are used to indicate specific characteristics of the drawing. They should be avoided as far as possible; they must be short and removed from the picture sideways.
• Arrows: Dimension lines end in two arrows. The arrowhead sides form an angle of approximately 15°, and the intermediate space is filled. They rest on the extension or ridge line.

Figures and Angular Dimensions

Figures: Their shape is determined by normalized lettering. The height should not be less than 3 mm. All drawing figures are indicated in the same unit, typically millimeters (mm).

Angular Dimensions: These are expressed by a circular arc bounded by two radii at the sides of the angle, placing a degree symbol (°) at the top right of the number.