KEVOS® MAKEOS · Threads
Unified Inch Ground Thread Tap Limits (Pitch and Major Diameter, H and L Limit Numbers)
For a Unified inch ground thread tap, what are its pitch and major diameter limits for the standard limits or an H or L limit number, what limit number is it, and is the tap I measured within them?
Pitch diameter and major diameter limits of an ASME B94.9 Unified inch ground thread tap by the formulas the handbook gives: thread height and basic pitch diameter from the threads per inch, the minimum and maximum tap pitch diameters for the standard limits (minimum C over basic) or for an H (over basic) or L (under basic) limit number in steps of 0.0005 in, the equivalent limit number, the maximum major diameter from the specified tap pitch diameter with the constant A = 0.130 P, and the minimum major diameter with the tolerance B by threads per inch, with your measured diameters checked against them; the table allowances C and D for the size are entered by you.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thread | ||
| Basic major (nominal) diameter | in | Basic major diameter of the Unified thread, for example 0.375 for 3/8 in. |
| Threads per inch | TPI | Threads per inch; the pitch is 1 ÷ TPI. |
| Tap limits | ||
| Pitch diameter limits | choice | Standard taps take the minimum pitch diameter C over basic from the table; H and L taps are marked with a number of 0.0005 in steps over or under basic. |
| Limit number (H or L) | Limit number marked on the tap after H or L; not used for the standard limits. | |
| Amount over basic for the minimum pitch diameter C (from the table) | in | Amount over basic for the minimum pitch diameter for the threads per inch and diameter range, from the handbook’s Table 2 (ASME B94.9); also used for the major diameter. |
| Pitch diameter tolerance D (from the table) | in | Pitch diameter tolerance for the threads per inch and diameter range, from Table 2. |
| Measured tap | ||
| Measured pitch diameter | in | Pitch diameter you measured on the tap. |
| Measured major diameter | in | Major diameter you measured on the tap. |
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.
Thread
What this means: Basic major (nominal) diameter
- What it is
- Basic major diameter of the Unified thread in inches.
- Units and format
- Enter a number in in: inches. The form accepts 0.05 in to 6 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.375 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 and major diameters.
- Get it right
- For example 0.375 for 3/8-16 or 0.190 for No. 10.
What this means: Threads per inch
- What it is
- Number of threads per inch of thread length.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 4 TPI to 80 TPI. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 16 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, thread height, A and B.
- Get it right
- The number after the dash in the size, such as 16 in 3/8-16.
Tap limits
What this means: Pitch diameter limits
- What it is
- Which pitch diameter limits the tap is made to.
- Units and format
- Choose one of 3 options from the list: Standard: minimum = basic + C; H limit number (maximum over basic); L limit number (minimum under basic).
- Example
- The form opens with “Standard: minimum = basic + C” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Decides how the minimum and maximum pitch diameters are set.
- Get it right
- Read the marking: H or L with a number, or no number for standard limits.
What this means: Limit number (H or L)
- What it is
- Limit number marked on the tap after H or L.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 40. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Moves the tap pitch diameter up (H) or down (L).
- Get it right
- Each step is 0.0005 in over or under the basic pitch diameter.
What this means: Amount over basic for the minimum pitch diamete…
- What it is
- Amount over basic for the minimum pitch diameter, from ASME B94.9 Table 2.
- Units and format
- Enter a number in in: inches. The form accepts 0 in to 0.05 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.001 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 standard minimum pitch diameter and the major diameter.
- Get it right
- Read the C column for your threads per inch and diameter range.
What this means: Pitch diameter tolerance D (from the table)
- What it is
- Pitch diameter tolerance, from Table 2.
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 0.05 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
- Sets the band between minimum and maximum pitch diameter.
- Get it right
- Read the D column for your threads per inch and diameter range.
Measured tap
What this means: Measured pitch diameter
- What it is
- Pitch diameter you measured on the tap, in inches.
- Units and format
- Enter a number in in: inches. The form accepts 0.01 in to 6 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.3357 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Checked against the pitch diameter limits.
- Get it right
- Measure over wires on the full threads behind the chamfer.
What this means: Measured major diameter
- What it is
- Major diameter you measured on the tap, in inches.
- Units and format
- Enter a number in in: inches. The form accepts 0.01 in to 6 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.381 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Checked against the major diameter limits.
- Get it right
- Measure across the crests on the full threads.
Method and formulas
- P = 1 ÷ TPI; symmetrical thread height h = 0.64951905 P; basic pitch diameter = D − h.
- Standard: minimum tap PD = basic PD + C. H limit: maximum = basic PD + 0.0005 × number. L limit: minimum = basic PD − 0.0005 × number.
- The other limit uses the tolerance D: maximum = minimum + D, or minimum = maximum − D.
- Equivalent limit number = (maximum tap PD − basic PD) ÷ 0.0005 (H), or (basic PD − minimum tap PD) ÷ 0.0005 (L) when the tap is below basic.
- Maximum major diameter = minimum tap PD − C + 0.64951904 P + A, with A = 0.130 P; minimum major = maximum − B, with B = 0.087 P (48 to 80 TPI), 0.076 P (36 to 47) or 0.065 P (4 to 35).
- Example: a 0.375 in × 16 TPI tap with standard limits has a pitch diameter from 0.33541 to 0.33591 in, which is H 3.
Limitations
- Ground thread taps with lead angles up to 5°, as the standard assumes; cut thread taps use the handbook’s Table 3.
- C and D depend on the threads per inch and the diameter range; enter them from ASME B94.9 or the handbook’s Table 2 (for intermediate pitches use the next coarser pitch).
- The handbook’s Table 2 prints A for 12 TPI as 0.1080; the formula A = 0.130 P gives 0.0108, as used here.
- Lead and angle tolerances are not checked.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Read what an H or L number on an inch tap means in diameters
- Check a measured inch tap before tapping a gauged thread
- Specify an oversize or undersize inch tap
Who it is for
- Toolroom and inspection staff
- Production engineers
- Tool buyers
What you get
- Pitch, thread height and basic pitch diameter
- Minimum and maximum tap pitch diameter
- Equivalent H or L limit number
- Constant A, tolerance B and the major diameter limits
- Measured diameters against the limits, with millimetre equivalents
Interpreting the result
- An H number gives the maximum tap pitch diameter in half-thousandths over basic; H3 for a 3/8-16 tap is 0.0015 in over.
- An oversize tap also carries a larger major diameter, because the major diameter is set from the tap pitch diameter.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 904, 906–907, 909. Topic reference; no source tables or recommended settings are reproduced.
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