The 1/2 Size and 1/4 Size scales (see PDF below) are examples of mechanical engineering scales designed for creating or interpreting drawings at reduced proportions—specifically at one-half and one-quarter of actual size. Although their physical form resembles an architect’s scale, including the extended graduations before the zero mark, their function is quite different.
Unlike an architect’s scale—where the numbers represent feet, and each inch along the scale is subdivided to match that interpretation—these mechanical scales operate in true fractional size. For example, on the 1/2 Size scale, each division represents half the length of a real inch, so a distance of 6 on the scale represents 3 actual inches. Similarly, on the 1/4 Size scale, each unit represents one-quarter of a real inch, so 8 on the scale corresponds to 2 actual inches.
The small graduations to the left of the zero mark serve a similar purpose as they do on an architect’s scale: they allow for easy measurement of fractional increments less than a full scale unit. However, instead of measuring fractions of a foot, they measure fractions of a scaled-down inch, consistent with the mechanical drawing’s ratio.
This kind of scale is especially useful when detailing small parts in mechanical or manufacturing drawings, where maintaining proportional accuracy is important but the full-size object would be too large to fit conveniently on the page.