KEVOS® MAKEOS · Threads
Unified Thread Limits of Size (Classes 1A–3B)
What are the limits of size for this UNC, UNF or UN thread in the chosen class?
Allowance, major, pitch and UNR minor diameter limits of an external Unified inch thread, and minor and pitch diameter limits of the mating internal thread, for classes 1A/1B, 2A/2B and 3A/3B, by the ASME B1.1 formulas.
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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 in inches. |
| Threads per inch | TPI | Threads per inch. |
| Thread class | choice | Tolerance class of the thread (2A/2B is the general-purpose class). |
| Length of engagement (0 = standard) | in | Length of engagement; 0 uses the smaller of 9 pitches and the basic diameter, which reproduces the published tables. |
Method and formulas
- P = 1/n; base pitch tolerance Td2(2A) = 0.0015 D^(1/3) + 0.0015 √LE + 0.015 P^(2/3).
- Class factors on Td2: 1A × 1.5, 2A × 1, 3A × 0.75; internal = 1.3 × external of the same class. Allowance es = 0.3 × Td2(2A) for 1A and 2A, 0 for 3A.
- dmax = D − es; major tolerance Td = 0.060 P^(2/3) (1A: 0.090); d2max = dmax − 0.64951905P; d2min = d2max − Td2; UNR minor max = dmax − 1.19078493P.
- Internal: D2min = D − 0.64951905P; D2max = D2min + TD2; D1min = D − 1.08253175P. Values are rounded to 0.0001 in as in ASME B1.1.
- Example: 0.5-13 class 2A: major 0.4985 / 0.4876 in, pitch 0.4485 / 0.4435 in.
Limitations
- Maximum internal minor diameter and class 1B/3B minor tolerances follow additional rules in ASME B1.1; take them from its tables.
- Uncoated threads; coatings must fit within the allowance.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find inspection limits for any UN thread size
- Set up thread gauges and CNC thread cutting
- Check special-diameter threads not in printed tables
Who it is for
- Quality inspectors
- Machinists
- Design engineers
What you get
- Allowance
- External major, pitch and UNR minor limits
- Internal minor (minimum) and pitch limits
- Pitch diameter tolerances
Interpreting the result
- Gauge with GO/NOT GO thread gauges; use these limits to check special sizes and set-ups.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1813–1866. 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 Limits of Size (Classes 1A–3B) calculator
