Rotary measuring instruments using inch or metric scale

A rotary instrument for selectively indicating measurements in English and Metric units and suitable for use with apparatus having a shaft whose rotational movement is proportional to a linear movement to be measured by the instrument, including in combination,

• a first rotary unit adapted to be secured to the shaft,
• a secondary rotary unit rotatable with respect to the first rotary unit,
• a first external annular gear member fixed to said first rotary unit,
• a second external annular gear member fixed to said second rotary unit and co-axial with said first gear member,
• an annular scale member, on which a single continuous scale is formed, carried by said second rotary unit,
• an annular stationary unit,
• a drive unit for transmitting rotary movement between said first rotary unit and said second rotary unit including a first pinion gear member meshing with said first annular gear member, a second pinion gear member meshing with the second annular gear member, and an axially shiftable stub shaft upon which the first and second pinion gear members are fixed,
• location means to anchor the stub shaft selectively to the stationary member, wherein a drive ratio which corresponds to or bears a simple relationship to the ratio between English and Metric units of length is provided between said first and said second rotary unit, or to the second rotary unit, wherein a 1:1 gear ratio is provided between said first and said second rotary unit,
• gear ratio shift means, operatively associated with said stub shaft, and
• co-operating means mounting said shift means on said stationary member such that rotation of said shift means effects axial translation of said stub shaft from a position anchored to said stationary unit to a position anchored to said second rotary unit, and
• two spaced marks formed upon said shift means against which the continuous scale can be read, the marks being so spaced that when the stub shaft is anchored to said stationary unit only one mark is visible to an operator and when the stub shaft is anchored to said second rotary unit only the other mark is visible.