Machinery's Handbook, 31st Edition
CALIBRATION OF DEPTH MICROMETERS 795 2) Mount the micrometer head vertically by the shank into a precision V-block as shown in Fig. 25, and adjust so that the zero position can be achieved making contact with the cal ibrated surface plate. If a zero position cannot be achieved with the V-block available, use an intermediary gage block of the necessary size so as to achieve contact with the spindle face and set zero on that intermediary block. This is the zero indication and will not be included in the linear measurement. Repeat the zero measurement three time to assure repeatability of the measurement (zero setting) Record the reading as zero. Verify that the reading is within the tolerance limits for the UUT. Raise the micrometer head spindle and insert the first step gage block (0.210 in or metric equivalent) lower the micrometer head until the spindle measuring face makes contact with the gage block stack. The micrometer head has no ratchet stop or friction thimble to control the force applied. It is important to not overdrive the spindle and introduce an error caused by disturbing the setup. It does not require a great amount of force to lift the setup. Care must be taken to apply the measure ment gently but firmly. Repeat the measurement three times to assure the repeatability and accuracy of the measurement. 3) Repeat Item 2) for all remaining gage block stacks throughout the full range of the UUT. Record all readings and verify that all readings are within tolerance limits. Adhere to manufacturers’ tolerances if different than the tolerance limits listed in Calibration Tolerance Tables, Table 21a and Table 21b. Calibration of Thread Micrometers Note: The Thread (Pitch) Micrometer uses a dedicated set of spindle and anvil contacts that measure the pitch diameter of a thread of a given pitch range. There are fixed anvil micrometers as well as interchangeable anvil/spindle sets. The interchangeable sets come with matched spindle/anvil sets that can be installed into the micrometer frame and then set to zero. Linearity Calibration (Fixed Pitch).— The fixed anvil micrometers usually do not have the capability to reset the zero indication other than the standard barrel adjustment. The calibration of these instruments is achieved by the measurement of master setting thread plug members that are selected to cover, as close as possible, 25, 50 and 75 percent of the range of the UUT. 1) Three calibrated Master Setting Thread Plug Gages are selected that cover 25, 50 and 75 percent of the measuring range of the UUT in the range of pitch that the UUT is designed for. (i.e., 8–13 pitch, 16–18 pitch, 18–24 pitch). 2) Mount the pitch micrometer in a micrometer stand to enable easier operation of the UUT and to provide better control of the standard. 3) Gently bring the Anvil and Spindle together to achieve a zero indication. If zero read ing is not exact, adjust the barrel so no error from zero exists. Repeat the zero indication to assure repeatability. 4) Open the micrometer anvil and spindle beyond the point of the first standard pitch diameter and seat the standard against the anvil. 5) Slowly close the micrometer until the spindle cone seats into the thread opposite the supported anvil. 6) Gently roll the standard up and down against the spindle cone to seek the highest point on the diameter. When this point is located, take a measurement of the pitch diameter. 7) Record the measurement and assure that the measurement is within the tolerance limits for the UUT. Note: UUT with the resolution of ±0.001 require a Class X master thread setting plug gage. UUT with a resolution of ±0.0001 require a class W master thread setting plug gage. 8) Remove the first standard and repeat measurement throughout the remainder of the master thread setting plug gages selected for the UUT. Interchangeable Anvil Pitch Micrometer Sets.— 1) Interchangeable Anvil Pitch Mi - crometer sets include several sets of Anvil and Spindle Cone, and V Anvils. These sets are
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