KEVOS® MAKEOS · Threads
Unified Miniature Screw Thread (UNM) Limits
What are the limits of size of a unified miniature (UNM) screw thread from 0.30 to 1.40 mm, with its lead angle and the sectional area at the minor diameter?
Limits of size of unified miniature screw threads (UNM) by the ANSI B1.10 formulas the handbook gives, in millimetres: basic pitch diameter E = D − 0.64952p and minor diameter K = D − 1.04p, external design minor diameter D − 1.20p and internal design major diameter D + 0.072p, with tolerances 0.12p + 0.006 (external major), 0.08p + 0.008 (pitch, both), 0.16p + 0.008 (external minor), 0.32p + 0.012 (internal minor) and 0.168p + 0.008 (internal major), the lead angle at the basic pitch diameter and the sectional area at D − 1.28p.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal (basic major) diameter D | mm | Nominal size of the miniature thread, e.g. 1.00 for 1.00 UNM. |
| Pitch p | mm | Pitch of the thread in millimetres. |
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: Nominal (basic major) diameter D
- What it is
- Nominal size of the miniature thread in millimetres, e.g. 1.00 UNM.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.2 mm to 2 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the major diameter and all the limits.
- Get it right
- The standard series runs from 0.30 to 1.40 mm.
What this means: Pitch p
- What it is
- Axial distance between adjacent thread crests, in millimetres.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.05 mm to 0.5 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the thread depth and every tolerance.
- Get it right
- Read it from the size table of the standard, e.g. 0.250 mm for 1.00 UNM.
Method and formulas
- E = D − 0.64952p; K = D − 1.04p; threads per inch = 25.4 ÷ p.
- External: major max D, min D − (0.12p + 0.006); pitch max E, min E − (0.08p + 0.008); minor max D − 1.20p, min max − (0.16p + 0.008).
- Internal: minor min K, max K + 0.32p + 0.012; pitch min E, max E + 0.08p + 0.008; major min D + 0.072p, max min + 0.168p + 0.008.
- Lead angle = tan⁻¹ (p ÷ π E); sectional area = π ÷ 4 × (D − 1.28p)².
- Example: the 1 UNM screw has a pitch diameter of 0.83762 to 0.80962 mm.
Limitations
- The tolerances are based on lengths of engagement of 2/3 D to 1.5 D.
- The internal major maximum and external minor minimum are applied to the threading tools and are not normally gauged on the product.
- The standard series runs from 0.30 to 1.40 mm; other sizes use the same formulas at your own risk.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Set the sizes of miniature screws for instruments and watches
- Check a miniature tap or die against the design limits
- Find the stress area of a miniature screw
Who it is for
- Instrument makers
- Precision machinists
- Design engineers
What you get
- Basic pitch and minor diameters and threads per inch
- External major, pitch and minor diameter limits
- Internal minor, pitch and major diameter limits
- Lead angle and the sectional area at D − 1.28p
Interpreting the result
- Inch limits in the standard are rounded from the millimetre limits, not converted directly.
- The minor diameter of the screw is held 0.16p below the basic minor diameter for root clearance.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1858–1861. Topic reference; no source tables or recommended settings are reproduced.
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Open the Unified Miniature Screw Thread (UNM) Limits calculator