KEVOS® 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. |
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)
WORKOS tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Unified Thread Over-Wire Limits (Pitch Diameter Limits, Wire Readings, Accept) calculator
