BIZOMAT MAKEOS · Threads
Three-Wire Measurement of Taper and Buttress Threads
What should the measurement over three wires be for this taper or buttress thread, and what pitch diameter does a reading give?
Measurement over wires for taper screw threads (corrected for the taper, including the simplified form for American pipe threads) and for buttress threads of any flank angles, the pitch diameter equivalent to a measured value, and the wire size for pitch-line contact on buttress threads.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thread | choice | Kind of thread. |
| Pitch diameter at the measuring point | mm | Pitch diameter where the single wire is placed. |
| Pitch | mm | Pitch. |
| Wire diameter | mm | Diameter of the wires used. |
| Included thread angle (taper) | ° | Included angle of a symmetrical taper thread (60° for pipe threads). |
| Taper on diameter per foot (taper) | mm/ft | Taper on diameter per foot of length (¾ in. = 19.05 mm for American pipe threads). |
| Included angle of the buttress thread | ° | Included angle of the buttress thread groove. |
| Front (load) face angle from the perpendicular | ° | Angle of the front, load-carrying face from the perpendicular to the axis. |
| Measured value over wires (0 = none) | mm | A measured value over the wires, to convert back to pitch diameter. |
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: Thread
- What it is
- Whether the thread being measured is a symmetrical taper thread, such as a pipe thread, or a buttress thread with unequal flank angles.
- Units and format
- Choose one of 2 options from the list: Taper thread; Buttress thread.
- Example
- The form opens with “Taper thread” selected as a worked example. Choose the option that matches your case.
- Get it right
- Only the inputs marked for the chosen thread are used; the others are ignored.
What this means: Pitch diameter at the measuring point
- What it is
- The pitch diameter you want to check, at the axial position where the wires sit. On a taper thread it changes along the length, so it must be the value at that position.
- 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 86.0679 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the pitch diameter at the measuring plane, not at the small or large end, unless the wires sit there.
What this means: Pitch
- What it is
- The axial distance between corresponding points on adjacent threads of the part being measured.
- 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 3.175 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.
What this means: Wire diameter
- What it is
- The actual diameter of the three gauge wires laid in the thread grooves, from the wire set’s calibration certificate.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.8331 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 wire raises the measurement over wires by more than the change in wire size.
- Get it right
- Use the certified size of the wires in your hand, not the nominal best-wire size.
What this means: Included thread angle (taper)
- What it is
- The included angle between the two flanks of a symmetrical taper thread, used for taper threads only.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 20° to 120°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Included angle, not the half angle; the tool halves it itself.
What this means: Taper on diameter per foot (taper)
- What it is
- The change in thread diameter over one foot (304.8 mm) of length, which is how taper threads are specified; it gives the taper correction sec b.
- Units and format
- Enter a number in mm/ft: mm/ft. The form accepts 0 mm/ft to 100 mm/ft. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 19.05 mm/ft as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Taper on diameter, not on radius, in millimetres per foot: 1 in. per foot is 25.4 mm per foot.
What this means: Included angle of the buttress thread
- What it is
- The included angle of the buttress thread groove, between the front (load) face and the back face.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 20° to 90°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 52° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be larger than the front face angle.
What this means: Front (load) face angle from the perpendicular
- What it is
- The angle of the front, load-carrying flank of the buttress thread, measured from a line square to the thread axis.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 0° to 30°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 7° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Measure from the perpendicular to the axis, not from the axis itself; it must be less than the included angle.
What this means: Measured value over wires (0 = none)
- What it is
- A reading you actually took over the three wires, which the tool converts back to the pitch diameter it represents.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 2000 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
- It does not change the calculated measurement over wires, only the pitch diameter reported for your reading.
- Get it right
- Leave it at 0 if you have not measured yet; the conversion is then skipped.
Method and formulas
- Taper: M = [E − (cot a) P ÷ 2 + W (1 + csc a)] ÷ sec b, with a half the thread angle and b half the taper angle; for pipe threads M = [E − 0.866025 P + 3W] ÷ 1.00049.
- Buttress: M = E − P ÷ [tan a + tan(A − a)] + W [1 + cos(A ÷ 2 − a) csc(A ÷ 2)].
- Buttress wire for pitch-line contact: W = P cos a ÷ (1 + cos A).
- Example: the handbook 3-in. pipe thread gauge (3.3885 in. pitch diameter, 8 threads per inch, 0.07217 in. wires) measures 3.495 in. over wires.
Limitations
- The taper formula is accurate for tapers up to ¾ in. per foot.
- Wire placement on a taper thread must be at the point where the pitch diameter is wanted.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a taper pipe thread gauge or part over wires
- Measure a buttress thread’s pitch diameter
- Choose wires for buttress thread measurement
Who it is for
- Quality inspectors
- Machinists
- Gauge makers
What you get
- Measurement over wires
- Pitch diameter for a measured value
- Recommended wire size (buttress)
Interpreting the result
- Use the converted pitch diameter to judge the part against its tolerance, not the raw measurement.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2005–2007. Topic reference; no source tables or recommended settings are reproduced.
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Open the Three-Wire Measurement of Taper and Buttress Threads calculator