BIZOMAT MAKEOS · Threads
Buttress Thread Wire Inspection (7°/45° Form, Wire Size, Readings, Pitch Diameter)
For this 7°/45° buttress thread, what are its diameters, what wire size and reading over wires should I use, and does my measured thread pass?
Three-wire inspection of an external 7°/45° buttress thread: the basic form and the internal and external major, pitch and minor diameters with any allowance (as the buttress thread tool), the wire size that touches at the pitch line and the reading over wires at the largest and smallest allowed pitch diameter (as the taper and buttress three-wire tool), and the pitch diameter from your measured reading against your tolerance.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Basic major diameter | mm | Basic major diameter of the thread. |
| Pitch | mm | Pitch (25.4 ÷ threads per inch). |
| Allowance on the external thread | mm | Allowance on the external thread from its class (0 for basic size). |
| Pitch diameter tolerance (below the largest) | mm | How far below the largest pitch diameter the external thread may be (your class tolerance). |
| Wires | ||
| Wire diameter (0 = pitch-line size) | mm | Diameter of the wires you have (0 uses the size that touches at the pitch line). |
| Measured reading over wires | mm | 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 nominal outside size of the 7°/45° buttress thread before any allowance, from the drawing or thread designation. Every major, pitch and minor diameter of both the internal and external thread is worked from it.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 50.8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- All diameters and both wire readings move one for one with it.
- Get it right
- Use the basic size from the designation, not a measured crest diameter of a worn or truncated thread.
What this means: Pitch
- What it is
- The axial distance from one thread form to the next along the screw. It sets the thread height, the wire size that touches at the pitch line and the wire constant used to convert the reading.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 4.233 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- For an inch thread, pitch = 25.4 ÷ threads per inch; do not type the TPI itself.
What this means: Allowance on the external thread
- What it is
- The amount the external thread is made smaller than basic so the parts assemble freely, taken from the thread class. It lowers the external major, pitch and minor diameters by the same amount.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 5 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A larger allowance lowers the largest and smallest acceptable readings over the wires by the same amount.
- Get it right
- Use 0 for a basic-size thread; the allowance applies to the external thread only in this tool.
What this means: Pitch diameter tolerance (below the largest)
- What it is
- How far the external pitch diameter may fall below its largest permitted value and still pass, from your thread class. The tool accepts a pitch diameter from the largest down to the largest minus this amount.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 5 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.12 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A wider tolerance lowers the smallest acceptable reading and accepts more parts.
- Get it right
- This is a one-sided band below the largest pitch diameter, not a plus-or-minus value; enter the full band width.
Wires
What this means: Wire diameter (0 = pitch-line size)
- What it is
- The actual diameter of the three measuring wires laid in the thread grooves, read from the wire set or measured. With 0 the tool uses the size that touches the flanks at the pitch line.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- If you enter your own wires, use their certified size; wires far from the pitch-line size may ride on the crest or root and give a false reading.
What this means: Measured reading over wires
- What it is
- The micrometer reading over the three wires sitting in the thread, one wire on one side and two on the other. The tool converts it back to the pitch diameter and compares that with the acceptance band.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 3000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 52.65 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The pitch diameter from the reading moves one for one with the reading.
- Get it right
- Measure with the same wires entered above and with even, light micrometer pressure; a wire size mismatch shifts the result directly.
Method and formulas
- h = 0.6p; internal pitch E_n = D − h; external pitch E_s = D − h − G; external minor K_s = D − 1.32542p − G (as the buttress thread tool).
- Included angle 52° with a 7° load flank: wire = p cos 7° ÷ (1 + cos 52°); M = E − [p ÷ (tan 7° + tan 45°) − W (1 + cos 19° ÷ sin 26°)] (as the taper and buttress three-wire tool).
- Pitch diameter from a reading = M + the same constant, against E_s − tolerance to E_s.
- Example: external pitch diameter 48.26 mm; read 52.699 to 52.579 mm over 2.6005 mm wires; 52.65 mm gives 48.211 mm.
Limitations
- Single-start thread; the lead-angle effect is ignored.
- Wires must touch both flanks; very small or large wires sit on the crest or root.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Inspect a buttress thread on a press screw, breech or vice
- Set the wire readings before cutting a buttress thread
- Check a worn buttress screw
Who it is for
- Inspectors
- Machinists
- Toolmakers
What you get
- Thread form and diameters
- Wire size and readings at the pitch diameter limits
- Pitch diameter from the reading and the decision
Interpreting the result
- The load is carried on the 7° flank, so wear shows mainly there; one reading over wires averages both flanks.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1945–1952, 2005–2007. Topic reference; no source tables or recommended settings are reproduced.
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