The three wire method is the most accurate and universally recognized method of measuring the effective or pitch diameter of an external screw thread or thread plug gage. The pitch diameter is the most important measurement which can be made on a thread because it reflects the widest spectrum of potential defects in a thread gage or threaded part.
In measuring precision screw threads and thread plug gages, it is desirable to use best size wires which touch the thread at or near the pitch diameter for the reason that with such "Best Size" wires the measurement of pitch diameter is least effected by an error in the included angle of the thread. Maximum and minimum wires should be used which are safely within the limiting dimensions given by the formulas below in blue. It is possible to use alternative sizes within the limits and adjust the constant to achieve accurate measurements.
In order to achieve accurate results in the measurement of working thread plug gages, thread set plug gages and precision screw threads; it is critical that the thread measuring wires are calibrated in accordance with ASME B89.1.17-2001. This important ASME standard establishes the uniform practices for the measurement of thread measuring wires and provides the critical equations for calculating constants and “Best-Size” wires for Inch and Metric series 60 degree threads, 29 degree Acme threads, and 7/45 degree Buttress threads. The standard covers methods for direct and comparison measurement of both master and working class thread measuring wires.
To simulate the deformation that occurs when thread wires are used to measure pitch diameter, master thread wires are measured in a deformed condition. The wires are measured between a steel roll and a .375 inch diameter flat carbide contact under specified force. Working thread measuring wires can be measured in the same manner or by comparative measurement to a properly calibrated master wire.
It is critical that thread measuring wires are measured per the ASME standard. Incorrect measurements of thread measuring wires will result in incorrect measurements of thread gages and precision parts and may have a damaging impact on a company’s product quality.
Thread Check's expert sales engineers can assist our customers in selecting and calculating the correct wire sizes and constants. Thread Check Inc. also manufactures Three Wire Thread Measuring Systems that make thread measurement fast and easy.