KEVOS® MAKEOS · Threads

Unified Inch Cut Thread Tap Limits (Major and Pitch Diameter, ASME B94.9)

For a cut thread (not ground) tap for a Unified inch screw thread, what are the minimum and maximum major and pitch diameters by the ASME B94.9 Table 3 formulas, and is my measured tap inside them?

Thread limits of Unified inch cut thread taps by the formulas of ASME B94.9 Table 3 that the handbook gives: minimum major diameter = basic diameter + B + C, maximum major diameter = minimum major + A, minimum pitch diameter = basic pitch diameter + B, maximum pitch diameter = minimum pitch diameter + D, where C is 20 % of the theoretical truncation for 2 to 5½ threads per inch and 25 % for 6 to 80 (calculated here), and the tolerances A and D and the amount B are entered from the table for the tap diameter and pitch. A measured tap is checked against the limits, with the lead tolerance of ±0.003 in in any inch.

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How this tool works

Inputs, units and where to find them

InputUnitTypical source
Basic (nominal) diameterinBasic major diameter of the Unified thread, for example 0.25 in for 1/4-20.
Threads per inchTPIThreads per inch; the pitch is 1 ÷ n.
Table 3 values
Major diameter tolerance AinMajor diameter tolerance A from Table 3 for the tap diameter.
Amount over basic for minimum pitch diameter BinAmount over basic for the minimum pitch diameter B from Table 3 for the diameter and threads per inch.
Pitch diameter tolerance DinPitch diameter tolerance D from Table 3 for the diameter and the thread series (coarser than NF, or NF and finer).
Measured tap
Measured pitch diameter of the tapinPitch diameter of the tap measured over wires or with a thread micrometer.
Measured major diameter of the tapinMajor diameter of the tap measured over the crests.

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 (nominal) diameter
Illustration: Basic (nominal) diameterA circle with a line straight through its centre from edge to edge, marking the diameter Basic (nominal) diameter.Dmeasured acrossthe centreExample: 0.25 inBasic (nominal) diameter
What it is
Basic major diameter of the Unified thread the tap cuts.
Units and format
Enter a number in in: inches. The form accepts 0.000001 in to 10 in. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.25 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Sets the basic pitch diameter and every limit.
Get it right
For a 1/4-20 tap it is 0.25 in; numbered sizes have their own basic diameters.
What this means: Threads per inch
Illustration: Threads per inchA screw-thread profile with crests and roots, the pitch from crest to crest and the major and minor diameters, for Threads per inch.pitchmajorminornExample: 20 TPIThreads per inch
What it is
Threads per inch of the Unified thread the tap cuts.
Units and format
Enter a number in TPI: threads per inch. The form accepts 2 TPI to 80 TPI. Required: the tool does not calculate until every input is valid.
Example
The form opens with 20 TPI as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Sets the pitch, the basic pitch diameter and the constant C.
Get it right
Read it from the tap marking.

Table 3 values

What this means: Major diameter tolerance A
Illustration: Major diameter tolerance AA nominal size line with an upper and a lower limit around it; parts inside the band are acceptable. The figure illustrates Major diameter tolerance A.upperlowerAExample: 0.0025 inMajor diameter tolerance A
What it is
Major diameter tolerance A from Table 3 for the tap diameter.
Units and format
Enter a number in in: inches. The form accepts 0.000001 in to 0.1 in. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.0025 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Widens the major diameter limits.
Get it right
Read the row for the tap diameter range.
What this means: Amount over basic for minimum pitch diameter B
Illustration: Amount over basic for minimum pitch diameter BA nominal size line with an upper and a lower limit around it; parts inside the band are acceptable. The figure illustrates Amount over basic for minimum pitch diameter B.upperlowerBExample: 0.0005 inAmount over basic for minimum pitch diameter B
What it is
Amount over basic for the minimum pitch diameter, B, from Table 3.
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 0.0005 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Raises both the major and pitch diameter limits.
Get it right
Read the column for 36 or more, or 34 or fewer, threads per inch.
What this means: Pitch diameter tolerance D
Illustration: Pitch diameter tolerance DA nominal size line with an upper and a lower limit around it; parts inside the band are acceptable. The figure illustrates Pitch diameter tolerance D.upperlowerD_tExample: 0.002 inPitch diameter tolerance D
What it is
Pitch diameter tolerance D from Table 3 for the tap diameter and series.
Units and format
Enter a number in in: inches. The form accepts 0.000001 in to 0.1 in. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.002 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Widens the pitch diameter limits.
Get it right
Use the NF-and-finer column for fine threads and for taps over 1½ in with 10 or more threads per inch.

Measured tap

What this means: Measured pitch diameter of the tap
Illustration: Measured pitch diameter of the tapA circle with a line straight through its centre from edge to edge, marking the diameter Measured pitch diameter of the tap.E_mmeasured acrossthe centreExample: 0.2185 inMeasured pitch diameter of the tap
What it is
Pitch diameter of the tap as measured.
Units and format
Enter a number in in: inches. The form accepts 0.000001 in to 10 in. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.2185 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Only moves the pitch diameter check.
Get it right
Measure over three wires or with a thread micrometer, near the front of the full threads.
What this means: Measured major diameter of the tap
Illustration: Measured major diameter of the tapA circle with a line straight through its centre from edge to edge, marking the diameter Measured major diameter of the tap.D_mmeasured acrossthe centreExample: 0.2545 inMeasured major diameter of the tap
What it is
Major diameter of the tap as measured over the thread crests.
Units and format
Enter a number in in: inches. The form accepts 0.000001 in to 10 in. Required: the tool does not calculate until every input is valid.
Example
The form opens with 0.2545 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
Effect on the result
Only moves the major diameter check.
Get it right
Measure with a micrometer across the lands, behind the chamfer.

Method and formulas

  1. Basic pitch diameter = basic diameter − 0.649519 ÷ n.
  2. C = 20 % (2 to 5½ threads per inch) or 25 % (6 to 80) of the theoretical truncation 0.216506 ÷ n.
  3. Minimum major = basic diameter + B + C; maximum major = minimum major + A.
  4. Minimum pitch diameter = basic pitch diameter + B; maximum = minimum + D.
  5. Example: a 1/4-20 cut thread tap has pitch diameter limits 0.21802 to 0.22002 in and major diameter limits 0.25321 to 0.25571 in.

Limitations

  • The formulas are ASME B94.9-1999 Table 3 for cut thread taps; ground thread taps are in the Unified inch ground thread tap tool.
  • Take A, B and D from Table 3; taps over 1½ in with 10 or more threads per inch take the NF and finer tolerances.
  • Half-angle tolerances (Table 3) and the lead tolerance are checked separately on a thread comparator or lead tester.
  • The handbook’s table prints C = 0.0078 in for 6 threads per inch, where 25 % of the truncation gives 0.0090 in; the tool uses the formula.
  • Example values are illustrations. Replace them with values for your actual task.

Typical uses

  • Check a cut thread tap before tapping production holes
  • Specify a special cut thread tap
  • Compare cut and ground tap limits

Who it is for

  • Quality inspectors
  • Toolroom staff
  • Manufacturing engineers

What you get

  • Basic pitch diameter
  • Constant C for the threads per inch
  • Minimum and maximum major diameter
  • Minimum and maximum pitch diameter
  • Measured tap against the limits

Interpreting the result

  • The tap is made above basic so that it cuts a thread inside the nut’s limits after tap oversize; a tap below the minimum pitch diameter cuts tight threads.
  • Use the same Table 3 row for A and D as the tap diameter and series.

References

  • Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 910. Topic reference; no source tables or recommended settings are reproduced.

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Open the Unified Inch Cut Thread Tap Limits (Major and Pitch Diameter, ASME B94.9) calculator

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