Measurement of Major diameter of a screw thread

     The major diameter of a screw thread can be determined by a normal micrometer by carefully adjusting the screw in between in two anvils of micrometer. But there is a difficulty of pressure application during adjusting the screw in between the anvils. Due to this pressure the chance of errors in measurement becomes more. So, to avoid this a special instrument that is bench micrometer used.

     A bench micrometer consists of two anvils, one is connected to fudicial indicator and another is connected to micrometer head. The fudicial indicator confirms the application of uniform and light pressure while adjusting the screw in between the two anvils. The micrometer gives the desired reading. Generally for finding the major diameter of a screw, a setting cylinder is used which a standard piece equal to the major diameter. It is used to avoid pitch errors in a micrometer screw if any. First, micrometer reading is taken for setting cylinder and the reading is assumed as R1. Next, the micrometer reading is taken for required screw and the reading is assumed as R2. The final measurement is obtained by the following formula without any errors.
                        Major diameter = D±(R2-R1)
                                                                         D = Setting cylinder diameter
                                                                         R1 = Micrometer reading on setting cylinder
                                                                         R2  = Micrometer reading on screw thread