Thread & Gear Measuring Wires

Thread measuring wires are the most universally recognized method of measuring pitch diameter on threaded gages, threaded parts and taps. Thread Check Inc. offers a full range of thread measuring wires in inches, metric, Acme and special sizes traceable to NIST which meet or exceed ANSI/ASME B1.2 and ASME B89.1.17-2001.

Gear measuring wires are used as a quick means to check the tooth thickness of gears at the pitch diameter. It is far more accurate than using gear tooth vernier calipers. Thread Check's gear measuring wires are made by the same methods and to the same high quality as our thread measuring wires. Gear measuring wires meet or exceed former Federal specification GGG-W-366b for Grade AM.G.W. Gear measuring wires are available for both 2 and 3 wire gear measuring applications. Gear measuring wires are available in sizes from 2 200P. Thread Check can supply gear measuring wires to check enlarged pinions, external gears, internal gears and alternate series.

The pitch diameter is the most important element of a thread that can be measured because it reflects the widest spectrum of possible defects in a threaded part or gage. The pitch diameter is the theoretical diametrical plane which passes through a thread at the point where the width of the thread tooth and groove are equal. The correct pitch diameter size assures that threaded product is within the required specifications for producing interchangeability, assembly and strength. Measuring the pitch diameter of thread gages is critical to controlling and qualifying threaded product.

Thread Check Inc. offers a full range of thread measuring wires in inches, metric, Acme and special sizes traceable to NIST which meet or exceed ANSI/ASME B1.2 and ASME B89.1.17-2001.

All wire sets are supplied within .000020" of the best wire size. All are calibrated within .000005" of the size provided on the label. Commonality of the 3 wires is within .0000050". Roundness is within .000010". Thread measuring wires are supplied with a minimum hardness of Rc. 62.5. Thread Check Inc. can supply wires and technical support for many modified and special threads including trapezoidal threads, buttress threads and asymmetrical threads.

In regards to the Three Wire Thread Measuring method, the following is an excerpt from page 35 of FED-STD-H28

“The accurate measurement of pitch diameter of a thread, which may be perfect as to form and lead, presents certain difficulties which result in some uncertainty as to its true value. The adoption of a standard uniform practice in making such measurements is, therefore, desirable in order to reduce such uncertainty of measurement to a minimum. The so-called “three wire method” of measuring pitch diameter, as herein outlined, has been found to be the most generally satisfactory method when properly carried out, and is recommended for universal use in the direct measurement of thread plug and thread setting plug gages.”

The basic pitch diameter is the diameter where the thread thickness is equal to the space between the threads. If the flats at the top and bottom of the thread are the same, the pitch diameter will coincide with the middle of the sloping side of the thread.

Thread measuring wires which touch the thread at the pitch diameter are known as “Best Size” Wires. Such wires are used because the measurements of pitch diameter are least affected by errors that may be present in the angle of the thread.
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, 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 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

Please contact Thread Check’s knowledgeable sales engineering team with any questions or requirements.