OUT-OF-FLAT (HAIRSPRING DEFECT)

Out-of-flat (hairspring defect) is a defect in the flatness of rotation: it occurs when a wheel or a Balance does not rotate in a plane perfectly perpendicular to its axis, but “warps,” oscillating slightly up and down with each Lathe, like a warped plate. The part thus shows a flatness defect, as opposed to Out-of-round (hairspring defect), which affects concentricity (a lateral defect). On a Balance, an out-of-flat condition unbalances the component and can cause rubbing against neighboring jewels or bridges; on a wheel, it disrupts meshing. One Testing the flatness — and detects an out-of-flat condition — by rotating the part between the points of a Figure-of-eight calliper, or on a staking tool stump, while observing the axial displacement of the periphery. The defect results from imperfect fitting, a bent axis, or an impact. Its correction, through careful straightening, requires methodical work to restore flatness without introducing Out-of-round (hairspring defect).