Understanding Dimensions in Engineering Drawings

Engineering drawings are the universal language used to convey design information. Dimensions are critical components of these drawings, specifying the size, location, and geometric characteristics of features on a part. Each dimension provides specific information necessary for manufacturing and inspection.

Identifying Extra Information on Dimensions

Sometimes, a dimension might be included on a drawing for informational purposes only. It might be a calculated value or a dimension that is redundant because it can be derived from other dimensions. Such dimensions are included as supplementary information but are not required for manufacturing or inspecting the part based on the fundamental dimensions.

To distinguish these informational dimensions from critical manufacturing or inspection dimensions, special notation is used alongside the dimension value.

What Does 'REF' Mean on a Dimension?

In engineering drawings, the standard notation used to indicate that a dimension is for reference only is the abbreviation REF. This abbreviation is placed next to the dimension value.

• A dimension marked with REF means it is provided as extra information or for convenience.
• It is typically a dimension that is controlled by other dimensions on the drawing.
• Manufacturing and inspection should not rely on or check against a REF dimension directly; they should focus on the primary dimensions that control the part geometry.

Using REF clearly communicates to anyone reading the drawing that this particular dimension is not a driving dimension and does not need to be held to a specific tolerance for manufacturing or inspection purposes.

Analyzing the Given Options

Let's examine the provided options to determine which one correctly indicates extra information on a dimension in an engineering drawing:

• EXT: This abbreviation is not standard for indicating extra information or reference dimensions in this context. It might sometimes relate to external features or extensions depending on the specific drawing's conventions, but not to denote a reference dimension.
• PER: This abbreviation is not a standard notation for dimension modification in engineering drawings. It could potentially relate to perimeter in some calculations or notes, but not for marking a dimension as extra information.
• NR: While "NR" could conceptually mean "Not Required," it is not the standard notation used in engineering drawing standards (like ASME or ISO) to indicate a reference dimension or extra information. The universally accepted term for this is "REF".
• REF: As discussed, REF stands for "Reference" and is the standard and widely accepted notation in engineering drawings to indicate that a dimension is extra information, provided for reference only, and not required for manufacturing or inspection.

Therefore, the letters written on the dimension indicating it is extra information and NOT really required are REF.

Conclusion on Reference Dimensions

In summary, when you see the letters REF next to a dimension value on an engineering drawing, it signifies that the dimension is a reference dimension. It's supplementary information, often derived from other dimensions, and is not critical for the manufacturing or inspection processes which should instead focus on the primary, non-reference dimensions and their specified tolerances.

Revision Table: Engineering Drawing Dimensioning

Review key concepts related to dimensioning in engineering drawings:

• Dimension Line: A line used to indicate the extent of a dimension.
• Extension Line: Lines used to extend from the feature being dimensioned to the dimension line.
• Leaders: Lines pointing from a note or dimension to the feature it references.
• Dimension Value: The numerical value specifying the size or location.
• Tolerances: Permissible variations in size or location.
• Reference Dimension (REF): A dimension provided for information only, not for manufacturing or inspection verification.

Additional Information: Other Dimension Modifiers

Besides REF, engineering drawings use various symbols and notes to provide specific dimensioning instructions or information. Some common examples include:

• TYP: Typical (indicates a dimension or feature is typical for multiple locations).
• MAX: Maximum (indicates the maximum permissible size).
• MIN: Minimum (indicates the minimum permissible size).
• R: Radius (precedes a dimension value indicating a radius).
• ∅: Diameter (precedes a dimension value indicating a diameter).
• SQ: Square (precedes a dimension value for a square feature).

These modifiers and symbols are crucial for accurately interpreting engineering drawings and ensuring parts are manufactured correctly.