The Micrometer Screw Gauge is another well known measuring instrument. It is used to measure the thickness of any object or the external diameters of circular objects.
It consists of a hollow cylinder mounted on a U frame. The hollow cylinder leads to a ratchet which is meant for fine adjustment. The U frame consists of a flat end known as stud and a screw on the other side. This screw can be moved inside the nut by fitted in the U frame by rotating the hollow cylinder called the thimble. This is called the main scale. The hollow cylinder or the thimble is graduated into 50 or 100 equal parts. This is called the circular scale.
The screw has a known pitch such as 0.5 mm. Pitch of the screw is the distance moved by the spindle per revolution. Hence in this case, for one revolution of the screw the spindle moves forward or backward 0.5 mm. This movement of the spindle is shown on an engraved linear millimeter scale on the sleeve. On the thimble there is a circular scale which is divided into 50 or 100 equal parts.
When the anvil and spindle end are brought in contact, the edge of the circular scale should be at the zero of the sleeve (linear scale) and the zero of the circular scale should be opposite to the datum line of the sleeve. If the zero is not coinciding with the datum line, there will be a positive or negative zero error as shown in figure below.
The Least count of the screw gauge can be calculated by dividing the Pitch of the screw by the Number of divisions on the circular scale. For example, if you had the pitch as 1mm and 50 divisions on the circular scale, your least count would be 1/50 = 0.02mm.
To take the reading, we first find the zero error by measuring the deviation when the circular scale is zero.Then, place the object in between the anvil and the spindle (such that the object is just held but not pressured by them) and lock them in that position. Now, note the main scale reading,circular scale reading and the pitch of the instrument.
The Length = Main Scale Reading + (coinciding Circular Scale reading x L.C.) – Zero Error (if any)
In the given picture, Length = 5.5 + (28 x 0.02) = 5.6 mm
The following is a modern version of the micrometer screw gauge with with better accuracy (and lesser effort required to measure 😉 )
That’s all for now! Hope this article helped you. If not, do let me know in the comments below how I can improve it.