KEVOS® MAKEOS · Threads
Metric Ground Thread Tap Limits (M Profile: Pitch and Major Diameter, D and DU Limit Numbers)
For a metric M-profile ground thread tap, what are its pitch and major diameter limits for the standard limits or a D or DU 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 metric (M profile) ground thread tap by the formulas the handbook gives: the symmetrical thread height and basic pitch diameter, the maximum and minimum tap pitch diameters for the standard limits or for a D (over basic) or DU (under basic) limit number in steps of 0.013 mm, the equivalent limit number, the minimum major diameter from the specified tap pitch diameter and the maximum major diameter, with your measured pitch and major diameters checked against them; the table allowances for the size and pitch 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 | mm | Basic major diameter of the metric thread, for example 6 for M6. |
| Pitch | mm | Pitch of the thread. |
| Tap limits | ||
| Pitch diameter limits | choice | Standard taps take the maximum pitch diameter Y over basic from the table; D and DU taps are marked with a number of 0.013 mm steps over or under basic. |
| Limit number (D or DU) | Limit number marked on the tap after D or DU; not used for the standard limits. | |
| Major diameter tolerance X (from the table) | mm | Major diameter tolerance for the pitch, from the handbook’s Table 4 (ASME B94.9). |
| Amount over basic for the maximum pitch diameter Y (from the table) | mm | Amount over basic for the maximum pitch diameter for the size range and pitch, from Table 4; also used for the major diameter. |
| Pitch diameter tolerance Z (from the table) | mm | Pitch diameter tolerance for the size range and pitch, from Table 4. |
| Measured tap | ||
| Measured pitch diameter | mm | Pitch diameter you measured on the tap, for example over wires. |
| Measured major diameter | mm | 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 metric thread.
- Units and format
- Enter a number in mm: millimetres. The form accepts 1 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6 mm 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
- The number after M in the thread size, such as 6 for M6 × 1.
What this means: Pitch
- What it is
- Pitch of the thread, the distance from one thread to the next.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.2 mm to 8 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the thread height, W and the table row for X, Y and Z.
- Get it right
- The number after the × in the thread size; coarse pitches are often left off the size.
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: maximum = basic + Y; D limit number (maximum over basic); DU limit number (minimum under basic).
- Example
- The form opens with “Standard: maximum = basic + Y” selected as a worked example. Choose the option that matches your case.
- Effect on the result
- Decides how the maximum and minimum pitch diameters are set.
- Get it right
- Read the marking: D or DU with a number, or no limit number for a standard tap.
What this means: Limit number (D or DU)
- What it is
- Limit number marked on the tap after D or DU.
- 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 4 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 (D) or down (DU).
- Get it right
- Each step is 0.013 mm over or under the basic pitch diameter.
What this means: Major diameter tolerance X (from the table)
- What it is
- Major diameter tolerance for the pitch, from ASME B94.9 Table 4.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.041 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Sets the maximum major diameter.
- Get it right
- Read the X column for your pitch.
What this means: Amount over basic for the maximum pitch diamete…
- What it is
- Amount over basic for the maximum pitch diameter, from Table 4.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.065 mm 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 maximum pitch diameter and the major diameter.
- Get it right
- Read the Y column for your pitch and size range.
What this means: Pitch diameter tolerance Z (from the table)
- What it is
- Pitch diameter tolerance, from Table 4.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.025 mm 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 maximum and minimum pitch diameter.
- Get it right
- Read the Z column for your pitch and size range.
Measured tap
What this means: Measured pitch diameter
- What it is
- Pitch diameter you measured on the tap.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.5 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5.4 mm 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 or with a thread micrometer on the full threads behind the chamfer.
What this means: Measured major diameter
- What it is
- Major diameter you measured on the tap.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.5 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 6.1 mm 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
- Symmetrical thread height h = 0.64951905 P; basic pitch diameter = D − h.
- Standard: maximum tap PD = basic PD + Y. D limit: maximum = basic PD + 0.013 × number. DU limit: minimum = basic PD − 0.013 × number.
- The other pitch diameter limit is found with the tolerance Z: minimum = maximum − Z, or maximum = minimum + Z.
- Equivalent limit number = (maximum tap PD − basic PD) ÷ 0.013 (D when positive, DU from the minimum when below basic).
- Minimum major diameter = maximum tap PD − Y + 0.64951904 P + W, with W = 0.08 P; maximum major diameter = minimum + X.
- Example: an M6 × 1 tap with standard limits has a pitch diameter from 5.3905 to 5.4155 mm and a major diameter from 6.08 to 6.121 mm.
Limitations
- Ground thread taps with lead angles up to 5° and the 60° M profile with a P/8 flat at the basic major diameter, as the standard assumes.
- The table allowances X, Y and Z depend on the pitch and size range; enter them from ASME B94.9 or the handbook’s Table 4 (for intermediate pitches use the next coarser pitch).
- The handbook prints the minimum pitch diameter formula with a plus sign and the D limit step as 0.0013; its worked examples use minus and 0.013 mm, as here.
- Lead and angle tolerances are not checked.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Find the pitch diameter limits marked on a metric tap
- Check a measured tap before tapping a gauged thread
- Specify an oversize or undersize tap by its limit number
Who it is for
- Toolroom and inspection staff
- Production engineers
- Tool buyers
What you get
- Thread height and basic pitch diameter
- Maximum and minimum tap pitch diameter
- Equivalent D or DU limit number
- Minimum and maximum tap major diameter
- Measured pitch and major diameters against the limits
Interpreting the result
- A D (oversize) tap cuts a larger thread, used where plating, soft material or a looser fit needs it; a DU tap cuts closer to the minimum.
- The tap major diameter follows the tap pitch diameter, so an oversize tap also has a larger major diameter.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 904, 911–912. Topic reference; no source tables or recommended settings are reproduced.
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