1.5 Application of Dimensions & Location of Features: Lines, Leaders, Reference Dimensions, Rectangular vs. Polar Coordinates

1.5 Application of Dimensions & Location of Features: Lines, Leaders, Reference Dimensions, Rectangular vs. Polar Coordinates

Quick Answer: Paras. 1.7 (Application of Dimensions) and 1.9 (Location of Features) govern the mechanics of getting a dimension onto a drawing and locating a feature once it is there. Key exam facts: a reference dimension is identified by enclosing it in parentheses (1.7.6); where an overall dimension is specified, one intermediate dimension is omitted or identified as reference (1.7.7); a leader terminating in a dot refers to a surface, while a leader terminating in an arrowhead refers to a line or edge (1.7.4); a dimension that disagrees with the pictorial view is identified not to scale by a straight thick underline (1.7.9); rectangular coordinate dimensioning (1.9.1) locates features from two or three mutually perpendicular planes; polar coordinate dimensioning (1.9.4) uses a linear plus an angular dimension from a fixed point; and repetitive features (1.9.5) are specified with an X preceded by a numeral, such as 6X.

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Application of Dimensions (Para. 1.7)

Dimension Lines, Extension Lines, and Leaders (1.7.1 – 1.7.4)

• Dimension lines are thin, broken for the value in the U.S. practice, and terminate in arrowheads. They are not drawn coincident with, or as an extension of, part outlines, center lines, or phantom lines.
• Extension lines carry a dimension away from the outline. They begin with a visible gap from the part outline and extend past the outermost dimension line. Where extension lines cross other extension lines or object lines without a dimension line between them, no break is used; where an extension line crosses a dimension line arrowhead, a break is used.
• Leaders (1.7.4): A leader directs a dimension, note, or symbol to the intended place. Normally a leader terminates in an arrowhead. Where a leader is intended to refer to a surface by ending within the outline of that surface, it terminates in a dot. A leader should be an inclined straight line with a short horizontal portion at the note; where a leader is directed to a circle or an arc, its direction should be radial.

Why the dot matters for GD&T: the dot-versus-arrowhead distinction reappears in the datum feature symbol and in the "all over" profile symbol. A leader ending in a dot inside a surface outline says this surface, which is exactly the semantics a surface datum feature or an area indication needs.

Reading Direction and Chain Lines (1.7.3, 1.7.5)

• Notes are placed to read from the bottom of the drawing (1.7.5.1); dimensions with dimension lines and arrowheads are placed to read from the bottom of the drawing (1.7.5.2); baseline dimensions are aligned to their extension lines and read from the bottom or right side (1.7.5.3); datum feature symbols should be placed to read from the bottom of the drawing (1.7.5.5).
• A chain line (thick, dashed, drawn parallel to and offset from the surface profile) indicates that a requirement applies to only a portion of a surface, dimensioned for length and location. If the chain line clearly indicates the location and extent, the dimensions may be omitted (1.7.3.2). Where the area is shown on a direct view, it is section lined within the chain line boundary (1.7.3.3).

Reference and Overall Dimensions (1.7.6, 1.7.7)

• Reference dimensions are identified by enclosing them in parentheses. They carry no tolerance, are for information only, and may never be used to accept or reject a part. Rule 1.4(c) says their use should be minimized.
• Overall dimensions (1.7.7): where an overall dimension is specified, one intermediate dimension is omitted or identified as a reference dimension. Where the intermediate dimensions are more important than the overall dimension, it is the overall dimension that becomes reference. This is the drawing-level defence against the double-dimensioning that rule 1.4(c) forbids.

Not-to-Scale Dimensions (1.7.9)

Agreement should exist between the pictorial presentation of a feature and its defining dimension. Where a change is made and the view cannot be updated, the defining dimension is underlined with a straight thick line; where a basic dimension symbol is used, the line is placed beneath the symbol. Where the sole authority for the product definition is a digital model, ASME Y14.41 governs instead.

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Location of Features (Para. 1.9)

   RECTANGULAR COORDINATE (1.9.1)        POLAR COORDINATE (1.9.4)
        │                                        ╱ 30°
        │◄──── 45 ────►(┼)                 (┼)╱
        │        ▲                          ╱ │
        │        │ 30                      ╱  │ R 38 (linear)
        │        ▼                        ●───┘
   ─────┴──────────────                 fixed point = intersection of
   two mutually perpendicular            the mutually perpendicular planes
   planes supply X and Y

• 1.9.1 Rectangular Coordinate Dimensioning: linear dimensions specify distances in coordinate directions from two or three mutually perpendicular planes. Coordinate dimensioning must clearly indicate which features of the part establish those planes — which is precisely the job a datum reference frame does formally.
• 1.9.2 Rectangular Coordinate Dimensioning Without Dimension Lines: dimensions may be shown on extension lines without dimension lines or arrowheads, with the base lines indicated as zero coordinates. This is ordinate dimensioning.
• 1.9.3 Tabular Dimensioning: a type of rectangular coordinate dimensioning in which dimensions from mutually perpendicular planes are listed in a table on the drawing rather than on the pictorial view. It is the standard practice for large hole-pattern plates.
• 1.9.4 Polar Coordinate Dimensioning: a linear and an angular dimension specify a distance from a fixed point at an angular direction from two or three mutually perpendicular planes. The fixed point is the intersection of those planes. Bolt circles are the classic case.
• 1.9.5 Repetitive Features or Dimensions: specified using an X in conjunction with a numeral to indicate the number of places, such as 6X ⌀8.5. Where used with a basic dimension, the X may be placed either inside or outside the basic dimension frame.

Repetitive-Feature Notation Traps

Exam framing: if a stem shows a bolt circle located by a boxed diameter and boxed equal angles, that is polar coordinate dimensioning with basic dimensions — the location tolerance comes only from the position feature control frame, and rule 1.4(j) makes the implied angles basic.