What is micrometer screw gauge ?

The micrometer screw gauge is an instrument designed for accurate measurement of small thickness, such as the diameter of a wire, thickness of a sheet, etc. It is based on the principle of screw. It consists of a U-Shaped frame having one end fixed to the frame and other is made adjustable by means of an accurately cut fine pitch nut and screw. The screw is worked by a drum which carries on its inner edge 100 (sometimes 50) equal division marks. This is called circular scale or head scale and is used to measure the fraction of a rotation. The Number of complete rotations can be read on the main scale or pitch scale which is graduated on the nut parallel to the axis of the screw. The range of this scale is usually 10 mm to 25 mm. The reading of circular scale is taken against a line known as the reference line. In some screw gauges the drum is provided with a ratchet arrangement. When the two studs are in contact either with each other or with some object placed in between, the ratchet slips over the screw cap without moving the screw forward. This avoids undue pressure between the studs or on the object.

What is pitch of the Screw Gauge ?

The instrument operates on the principle that when a very accurately cut screw works in a nut the linear distance through which the screw moves is proportional to the amount of rotation given to it. In other words, the linear distance p traveled by the screw for one complete rotation (through 360) is always constant. This constant is called the pitch of the screw.