KEVOS® 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. |
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.
WORKOS 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
