"The surface of the spindle forms an arc when placed in a circle, and its radius is what we specify as curvature here. Assume the two ends of the spindle forming a line. When both the ends touch the circumference of a circle, the line forms a chord. The chord and the arc form the spindle—with the chord as the axis— when rotated a full circle. For a spindle with a curvature of 50 centimetres, the diameter at its centre is about 2.02 centimetres."
"The lower the curvature, the greater the spindle's diameter at its centre; for example, the Type 57B Spindle has a curvature of 45 centimetres. Its diameter at the centre is about 2.25 centimetres. The Type 57B2 Spindle has a curvature of 40 centimetres and its diameter at the centre is 2.54 centimetres."
