BIZOMAT MAKEOS · Threads
Unified Tapped Hole Check (Tap Drill, Thread Percentage, Length of Engagement)
For a Unified inch screw in a tapped hole, what drill do I need, and is the thread long enough for the hole material to be as strong as the screw?
Plan and check of a tapped hole for a Unified inch thread: the basic pitch and minor diameters and tensile stress area (as the Unified thread tool), the tap drill for your percentage of thread and tap type (as the tap drill tool), and the length of engagement needed so the internal thread does not strip before the screw breaks, with the strength ratio of the screw and the tapped material (as the thread engagement tool), against the engagement you have.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal (major) diameter | in | Nominal (basic major) diameter of the thread. |
| Threads per inch | TPI | Threads per inch. |
| Percentage of thread | % | Percentage of full thread height the tapped hole is to have (often 60–75%). |
| Tap type | choice | Cutting taps need a smaller drill than forming taps for the same thread percentage. |
| Length of engagement available | in | Length of full thread the screw engages in the hole. |
| Materials | ||
| Tensile strength of the screw | MPa | Ultimate tensile strength of the screw. |
| Tensile strength of the tapped material | MPa | Ultimate tensile strength of the material being tapped. |
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: Nominal (major) diameter
- What it is
- The basic major diameter of the Unified inch screw thread, as named first in the thread designation. It sets the pitch and minor diameters, the stress area and the tap drill.
- 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 0.5 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Type the size in decimal inches; a fractional size such as 3/8 is 0.375.
What this means: Threads per inch
- What it is
- The count of thread crests in one inch of screw length, from the thread designation. The tool takes the pitch as one divided by this count.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 0.000001 TPI to 100 TPI. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 13 TPI as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Coarse (UNC) and fine (UNF) series have different counts for the same diameter; check which the drawing calls for.
What this means: Percentage of thread
- What it is
- How much of the full basic thread height the tapped hole is to have, which fixes the tap drill size. A lower percentage means a larger drill, easier tapping and less thread depth.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 30% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 75% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A higher percentage gives a smaller tap drill; it does not change the engagement length needed.
- Get it right
- Type the percentage itself, not a fraction. The tool warns when the value is outside the band it treats as usual.
What this means: Tap type
- What it is
- Whether the hole is threaded with a cutting tap, which removes metal, or a forming tap, which displaces it. The tool uses a different drill constant for each, so the drill differs for the same thread percentage.
- Units and format
- Choose one of 2 options from the list: Cutting tap; Forming tap.
- Example
- The form opens with “Cutting tap” selected as a worked example. Choose the option that matches your case.
- Get it right
- Forming taps need a larger hole than cutting taps for the same percentage; do not use a cutting-tap drill chart with a forming tap.
What this means: Length of engagement available
- What it is
- The length of full thread in the hole that the screw actually engages, from the face of the hole to the end of the screw or of the full thread, whichever comes first. It is compared with the engagement needed.
- Units and format
- Enter a number in in: inches. The form accepts 0.000001 in to 50 in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.125 in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Do not count the chamfer, the countersink or the incomplete threads at the bottom of a blind hole.
Materials
What this means: Tensile strength of the screw
- What it is
- The ultimate tensile strength of the screw or bolt material, from its property class or grade specification. It sets how strong the external thread is and feeds the strength ratio with the tapped material.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 830 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A stronger screw raises the strength ratio and so lengthens the engagement needed when the tapped material is weaker.
- Get it right
- Use ultimate tensile strength, not proof or yield strength. Above a certain strength the tool switches to a different stress area formula for the screw.
What this means: Tensile strength of the tapped material
- What it is
- The ultimate tensile strength of the part the hole is tapped into, such as a cast iron housing or an aluminium block, from the material specification or supplier data.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 3000 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 310 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A weaker tapped material raises the strength ratio and lengthens the engagement needed.
- Get it right
- Use the strength of the material in its delivered condition; cast and heat-affected materials can be much weaker than the wrought grade.
Method and formulas
- p = 1 ÷ n; E = D − 0.649519p; minor = D − 1.082532p; A_t = 0.7854 (D − 0.9743p)² (as the Unified thread tool).
- Drill = D − k × % thread × p with k 0.01299 for cutting and 0.0068 for forming taps (as the tap drill tool).
- L_e = 2A_t ÷ [πK_n(½ + 0.57735n(E_s − K_n))]; J = A_s S_ue ÷ (A_n S_un); required = J L_e when J > 1 (as the thread engagement tool, at basic diameters).
- Example: drill 0.42506 in for 75% thread; 0.56008 in of engagement needed (strength ratio 1.9127).
Limitations
- Basic diameters; the thread class limits lengthen the engagement needed slightly.
- Static tensile load; vibration and repeated assembly call for inserts or more length.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Check a tapped hole in aluminium or cast iron
- Pick a tap drill and thread percentage
- Find how deep a tapped hole must be
Who it is for
- Design engineers
- Machinists
- Draftspeople
What you get
- Pitch, diameters and tensile stress area
- Tap drill
- Engagement needed and the decision
Interpreting the result
- In a material much weaker than the screw the engagement needed passes 1.5–2 diameters; a thread insert is often the better answer.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1806–1866, 2029–2036, 1586–1590. Topic reference; no source tables or recommended settings are reproduced.
BIZOMAT tools are screening and decision-support utilities. Verify important financial, engineering, safety, legal or operational decisions using appropriate professional standards and your organisation’s approved data.
Open the Unified Tapped Hole Check (Tap Drill, Thread Percentage, Length of Engagement) calculator