KEVOS® MAKEOS · Fasteners
Tapped Hole Design (Drill, Engagement and Depth)
What drill, length of engagement and tapping depth does this tapped hole need so the thread does not strip?
Design of a blind tapped hole for a metric screw: tap drill for the chosen percentage of thread and tap type, the length of engagement at which the bolt breaks before the threads strip (handbook shear-area method, extended by the strength ratio for weaker internal threads), the full-thread depth allowing for the tap chamfer, and the drill depth including the drill point.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Nominal diameter | mm | Nominal screw diameter. |
| Pitch | mm | Thread pitch. |
| Percentage of thread | % | Percentage of full thread height (65–75% is usual for cutting taps). |
| Tap | choice | Cutting tap or thread-forming tap. |
| Screw tensile strength | MPa | Tensile strength of the screw. |
| Tensile strength of the tapped material | MPa | Tensile strength of the material being tapped. |
| Tap chamfer allowance | pitches | Incomplete threads at the tap’s chamfer, in pitches (about 2 for bottoming, 3–5 for plug taps). |
| Extra drill depth beyond the threads | mm | Clearance below the threads for chips. |
| Drill point angle | ° | Included angle of the drill point. |
Method and formulas
- Tap drill = D − k × % thread × P, with k = 0.01299 for cutting and 0.0068 for forming taps (as the tap drill tool).
- Engagement L_e = 2 A_t ÷ {π K_n [½ + 0.57735 n (E_s − K_n)]} at basic diameters; multiplied by J = (external shear area × S_ub) ÷ (internal shear area × S_un) when the internal thread is weaker (as the thread engagement tool).
- Full-thread depth = engagement + chamfer allowance; drill depth = thread depth + extra + drill point length.
- Example: M10 × 1.5 into 310 MPa material needs 16.332 mm of engagement and a 25.397 mm deep 8.5386 mm hole.
Limitations
- Basic thread diameters; tolerance class and actual percentage of thread change the shear areas.
- Static strength; vibration loosening and repeated assembly need inserts or longer engagement.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Specify drill and tap depths for a screw into aluminium or cast iron
- Check whether a tapped hole needs a thread insert
- Write a hole callout on a drawing
Who it is for
- Design engineers
- Machinists
- Manufacturing engineers
What you get
- Tap drill
- Engagement length needed
- Thread depth and drill depth
Interpreting the result
- If the engagement exceeds about 2 D, a thread insert or through-bolt usually costs less than a deep tapped hole.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1536, 2029–2031. Topic reference; no source tables or recommended settings are reproduced.
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