To measure the effective diameter of an external metric thread (included angle is $$60$$ degrees) of $$3.5$$ $$mm$$ pitch, a cylindrical standard of $$30.5$$ $$mm$$ diameter and two wires of $$2$$ $$mm$$ diameter each are used. The micrometer reading over the standard and over the wires are $$16.532$$ $$mm$$ and $$15.398$$ $$mm$$ respectively. The effective diameter (in $$mm$$) of the thread is

Observation of a slip gauge on a flatness interferometer produced fringe counts numbering $$10$$ and $$14$$ for two readings. The second reading is taken by rotating the setup by $${180^ \circ }$$ Assume that both faces of the slip gauge are flat and the wavelength of the radiation is $$0.5086$$ $$\mu m.$$ The parallelism error (in $$\mu m.$$) between the two faces of the slip gauge is