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ANSI Buttress Thread Basic Dimensions (7°/45°)
What are the basic dimensions of this 7°/45° buttress thread?
Basic form and diameters of the American National Standard 7°/45° buttress inch screw thread (ANSI B1.9) for any pitch: sharp-V height, thread engagement, root radius and truncations, crest width, major, pitch and minor diameters of the internal and external threads with an allowance.
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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. |
| Pitch | mm | Pitch (25.4 ÷ threads per inch). |
| Allowance on the external thread | mm | Allowance (from the class of thread in the standard; 0 for basic size). |
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 basic major diameter of the buttress thread: the nominal outside diameter of the screw from which the internal and external diameters are derived.
- 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.
- Get it right
- For an inch thread, multiply the size in inches by 25.4.
What this means: Pitch
- What it is
- The axial distance from a point on one thread to the same point on the next thread.
- 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.
- Effect on the result
- Every form dimension (heights, truncations, root radius, crest width) is a fixed multiple of the pitch.
- Get it right
- For an inch thread, pitch = 25.4 ÷ threads per inch; do not type the threads per inch.
What this means: Allowance on the external thread
- What it is
- The allowance on the external thread: how much its major, pitch and minor diameters are made smaller than basic to leave clearance, from the thread class in the standard.
- 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
- Each external diameter falls by exactly this amount; the internal thread is unchanged.
- Get it right
- Enter 0 for basic size; it is a diametral amount and changes only the external thread diameters.
Method and formulas
- H = 0.89064 p; h = 0.6 p; root radius r = 0.07141 p; root truncation s = 0.0826 p; crest truncation f = 0.14532 p; crest width F = 0.16316 p; thread heights h_n = h_s = 0.66271 p.
- Internal: major D_n = D + 0.12542 p; pitch E_n = D − h; minor K_n = D − 2h.
- External: major D_s = D − G; pitch E_s = D − h − G; minor K_s = D − 1.32542 p − G.
- Example: a 50.8 mm buttress thread of 4.233 mm pitch has a basic pitch diameter of 48.26 mm.
Limitations
- Basic (maximum material) dimensions; tolerances depend on the class and length of engagement.
- Load is carried on the 7° flank; mating parts must be assembled the right way round.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Machine or inspect a buttress thread for a vise, press screw or breech
- Grind a buttress thread tool to the basic form
- Check drawing dimensions against the standard formulas
Who it is for
- Quality inspectors
- Machinists
- Gauge makers
What you get
- Thread form dimensions
- Internal thread diameters
- External thread diameters
Interpreting the result
- Buttress threads carry high axial load in one direction with low radial bursting force.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1945–1952. Topic reference; no source tables or recommended settings are reproduced.
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Open the ANSI Buttress Thread Basic Dimensions (7°/45°) calculator