BIZOMAT MAKEOS · Threads
Whitworth Thread Wire Inspection (Pitch Diameter, Wire Readings, Lead-Angle Correction)
For this Whitworth (BSW or BSF) thread, what should the reading over three wires be, how much does the helix angle change it, and does my measured thread pass?
Three-wire inspection of a 55° Whitworth external thread: thread depth and basic pitch and minor diameters (as the Whitworth thread tool), the best-size wire and the reading over wires at the basic pitch diameter (as the three-wire measurement tool for 55°), the reading corrected for the helix (lead) angle (as the three-wire lead angle tool), and the pitch diameter from your measured reading against the pitch diameter tolerance.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Major diameter | in | Major (nominal) diameter of the Whitworth thread. |
| Threads per inch | TPI | Threads per inch. |
| Pitch diameter tolerance (below basic) | in | How far below basic the pitch diameter may be (the external thread tolerance for your class). |
| 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: Major diameter
- What it is
- The major (nominal) diameter of the Whitworth thread, the size in its designation, in inches.
- 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 0.5 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 for the Whitworth size; it sets the pitch, thread depth and best wire size.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 0.000001 TPI to 100 TPI. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 12 TPI as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Pitch diameter tolerance (below basic)
- What it is
- How far below the basic pitch diameter the measured pitch diameter may fall, the external thread tolerance for your class.
- Units and format
- Enter a number in in: inches. The form accepts 0 in to 0.1 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.003 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter it as a positive amount below basic; pitch diameters above basic are rejected.
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.
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 30 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.5138 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
- Depth h = 0.6403p; pitch diameter = D − h; minor = D − 2h (as the Whitworth thread tool).
- Best wire = (p ÷ 2) ÷ cos 27.5°; M = E + W (1 + 1 ÷ sin 27.5°) − (p ÷ 2) ÷ tan 27.5° (as the three-wire measurement tool).
- Lead-angle reading from the normal-plane flank angle (as the three-wire lead angle tool); pitch diameter from a reading by the simple formula.
- Example: basic pitch diameter 0.44664 in; read 0.5153 in over 0.046974 in wires (1.4076 × 10⁻⁴ in lead-angle effect); 0.5138 in gives 0.44514 in.
Limitations
- Rounded crests and roots do not affect the wire reading when wires touch the flanks.
- Single-start threads; lead-angle effect is small for fine pitches.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Inspect a Whitworth thread on restoration or legacy machinery
- Set wire readings for a BSW or BSF thread
- See how much the lead angle matters
Who it is for
- Inspectors
- Machinists
- Restoration engineers
What you get
- Thread depth, pitch and minor diameters
- Wire size and reading at basic, with lead-angle correction
- Pitch diameter from the reading and the decision
Interpreting the result
- For single-start Whitworth threads the lead-angle effect is usually well under a tenth of a thou and can be ignored.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1980–1989, 1990–2003. Topic reference; no source tables or recommended settings are reproduced.
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