A sine bar is a precision metrology instrument used to measure or set angles with high accuracy. It consists of a hardened, accurately ground straight bar carrying two cylinders (rollers) of equal diameter fixed at a precisely known centre-to-centre distance, L (commonly 100 mm or 200 mm).

Working principle: One roller rests on a flat surface (a surface plate) and the other is raised by a stack of slip gauges of height H. The bar then forms the hypotenuse of a right-angled triangle whose opposite side is H and hypotenuse is L, giving:

sin θ = H / L

So θ = sin⁻¹(H/L). Because both L and the slip-gauge height H can be known to a very fine tolerance, the angle set or checked is extremely accurate. The sine bar can be used together with a dial indicator to check the angle of a component, or to set a workpiece at a required angle. Its accuracy falls off at large angles (above roughly 45°) because the sine function changes slowly there, so it is best used for smaller angles.

Why the other functions are wrong:

• Calibrating pressure gauges is unrelated to a sine bar; that is done with dead-weight testers and pressure standards.
• Checking thread pitches is done with pitch gauges, thread micrometers or the three-wire method, not a sine bar.
• Measuring linear dimensions is the job of scales, vernier callipers, micrometers and slip gauges; the sine bar uses linear inputs only to derive an angle, so its primary purpose is angular, not linear, measurement.

Hence the primary function of a sine bar is to measure angles with high accuracy.