BIZOMAT MAKEOS · Threads
Unified Thread Over-Wire Limits (Pitch Diameter Limits, Wire Readings, Accept)
For this Unified inch external thread and class, what readings over three wires mark the pitch diameter limits, and does my measured reading pass?
Three-wire inspection limits for a Unified (UN) external thread: basic major and pitch diameter limits for the class and length of engagement (as the Unified thread limits tool), the best-size wire and the measurement over wires at the maximum and minimum pitch diameter (as the three-wire measurement tool), and the pitch diameter worked back from your measured reading, accepted or rejected against the limits.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Basic major diameter | in | Basic major diameter of the thread. |
| Threads per inch | TPI | Threads per inch. |
| Thread class | choice | Class of the external thread. |
| Length of engagement (0 = standard) | in | Length of engagement for the tolerance (0 uses the standard length). |
| Wire diameter (0 = best size) | in | Diameter of the wires you have (0 uses the best size). |
| Measured reading over wires | in | Micrometer reading over the three wires. |
What each input means
Open an input to see what it is, its units, the worked example and the common mistakes, with a small diagram.
What this means: Basic major diameter
- What it is
- The basic major diameter of the Unified thread, the size in its designation (3/8-16 UNC has a basic major diameter of 0.375 in).
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 10 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter inches as a decimal: convert fractions first (3/8 = 0.375).
What this means: Threads per inch
- What it is
- The number of threads per inch from the thread designation; it sets the pitch, the best wire size and the tolerance.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 0.000001 TPI to 200 TPI. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 13 TPI as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Thread class
- What it is
- The tolerance class of the external thread: 1A is the loosest and 3A the tightest, and 3A has no allowance.
- Units and format
- Choose one of 3 options from the list: 1A; 2A; 3A.
- Example
- The form opens with “2A” selected as a worked example. Choose the option that matches your case.
What this means: Length of engagement (0 = standard)
- What it is
- The length of thread engagement used in the pitch diameter tolerance formula; zero uses the standard length.
- Units and format
- Enter a number in in: inches. The form accepts 0 in to 100 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter 0 to use the tool’s standard length, the smaller of nine pitches and the basic major diameter.
What this means: Wire diameter (0 = best size)
- What it is
- The measured diameter of the three wires you will use; zero uses the best-size wire for this pitch.
- Units and format
- Enter a number in in: inches. The form accepts 0 in to 1 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the size marked on your wire set, in inches; wires far from the best size are flagged.
What this means: Measured reading over wires
- What it is
- The micrometer reading over the three wires laid in the thread grooves, from which the tool works back the pitch diameter.
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 20 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5126 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure with the same wires entered above; a reading over different wires gives the wrong pitch diameter.
Method and formulas
- Pitch diameter limits from the allowance and tolerance formulas for the class (as the Unified thread limits tool).
- Best wire = (P ÷ 2) ÷ cos 30°; M = E + W (1 + 1 ÷ sin 30°) − (P ÷ 2) ÷ tan 30° (as the three-wire measurement tool).
- Pitch diameter from a reading E = M − [W (1 + 1 ÷ sin 30°) − (P ÷ 2) ÷ tan 30°].
- Example: with 0.044412 in wires the reading must lie between 0.51012 and 0.51512 in; 0.5126 in means a pitch diameter of 0.44598 in.
Limitations
- Lead and flank angle errors are assumed small; they bias three-wire readings.
- External threads only; internal threads are checked with gauges.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Inspect a UN bolt or screw with thread wires
- Write wire-reading limits on an inspection sheet
- Check a thread cut on a lathe
Who it is for
- Inspectors
- Machinists
- Quality technicians
What you get
- Pitch diameter limits for the class
- Wire size and readings at the limits
- Pitch diameter from the reading and the decision
Interpreting the result
- Use wires close to the best size: then flank angle errors hardly affect the reading.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1813–1866, 1990–2003. Topic reference; no source tables or recommended settings are reproduced.
- ASME B1.1 Unified Inch Screw Threads (UN and UNR Thread Form)
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Open the Unified Thread Over-Wire Limits (Pitch Diameter Limits, Wire Readings, Accept) calculator