When you are calibrating thread plugs, thread setting plugs and AGD style thread ring gages it is important to know the correct size and proper constant of the thread measuring wires, as they provide the foundation for accurate thread gage calibration. Working thread plugs and thread master setting plug gages are calibrated by means of the three-wire method. The degree of accuracy in the results of your readings depends on the accuracy of the thread measuring wires. An error of one unit in the mean diameter of thread measuring wires will have a multiplying effect of three units in the pitch diameter of 60° thread plug gages.
Proper thread wire calibration starts with measuring the wires in the middle section which is about one inch in length. It is important that the wires are measured utilizing appropriate forces and anvil configurations specified in ASME B89.1.17-2001. The procedure in North America is to measure the wires with a method that reproduces the deformation that occurs when the wires are pressed against the sides of the thread flank causing an elastic deformation. There are two methods of thread wire measurement. The comparative method utilizes master thread measuring wires to which working wires are compared. The direct method involves a measuring instrument utilizing the specified standard’s measuring forces and anvil configuration. Laser micrometers are not suitable as they do not apply any measuring force to create the elastic deformation.
Hints for reducing measurement uncertainty include the following:
- Refer to the ASME B89.1.17-2001 for proper equipment, procedure and measuring forces
- Minimize temperature differences between master wires and working wires
- Eliminate handling of wires by using thread measuring holders or tweezers
- Check straightness of finer pitch wires with suitable equipment, as bent or deformed wires can create errors.
- Ensure measuring instrument is calibrated and anvil faces are flat and parallel
- Ensure vee block angle is correct
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