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Measuring over wire with a micrometer

“Over Wire” is a nickname for a measuring method of the pitch diameter of threads. A wire with a known precise diameter is placed between the thread and the jaw of the micrometer. The value of diameter measured “over the wire” is used to calculate the pitch diameter of the thread.

When is the Over Wire method used?

Go/Nogo gauges are the more straightforward and standard method to measure threads. It is accurate and simple. However, it has also disadvantages:

  • It is expensive.
  • It is suitable for a specific thread at a particular class of tolerance.
  • It needs frequent calibrations (More cost)
  • For non-popular or special thread standards, lead times for delivery can be lengthy.
  • It provides a yes/no answer only.

The primary use cases for choosing the Over-Wire method:

  • Small production Batch that does not justify the price of a go/nogo gauge.
  • It is not possible to wait for the delivery of a gauge.
  • You need to know the exact pitch diameter (Not just a yes/no result).

Over Wire Formulas

Threading over-wire measurment

The formulas depend on the thread’s shape and angle. Below is the formula calculating the pitch diameter resulting from the measurement over the wire for a typical 60° symmetrical thread.

\( \large d_2 = M – \left (3\,\times\,W\,-\,0.86603\,\times\,P \right )\)

Ideal Wire Diameter

Idear Wire

It is recommended to choose a wire with a diameter that touches the flanks of the thread as close as possible to the pitch diameter theoretical position. Such wires are used because the measurements are least affected by errors that may be present in the angle of the thread. This “Best Wire” diameter depends on the pitch and the thread angle.

For symmetrical thread, the formula is:

\( \large W = \LARGE \frac {\sec{(\alpha)}}{2}\large\,\times\,P \)

For the specific case of 60° thread, the formula can be simplified to:

\( \large W = .57735\,\times\,P \)

Wires are mostly sold in sets, where the diameter of the wires correlates to the standard pitches of 60° threads. 

Parameters used in the above formulas:

  • d2Pitch Diameter
  • M – Measurment over wires
  • W – Wire Diameter.
  • α – Thread half angle

Standard Inch Wires

Standard Metric Wires

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