The precise ratio required to convert a lathe with an Imperial (inch) leadscrew to metric (millimeter) threading is 100 / 127 = 0.7874... . The best approximation with the fewest total teeth is very often 37 / 47 = 0.7872... . This transposition gives a constant -0.020 percent error over all customary and model-maker's metric pitches (0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.60, 0.70, 0.75, 0.80, 1.00, 1.25, 1.50, 1.75, 2.00, 2.50, 3.00, 3.50, 4.00, 4.50, 5.00, 5.50 and 6.00 mm).