BIZOMAT 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. |
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 diameter
- What it is
- Nominal (major) diameter of the screw thread, the first number in a metric designation such as M10.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Pitch
- What it is
- Distance from one thread crest to the next along the axis, as given in the thread designation (the second number in M10 × 1.5).
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- For inch threads, pitch in millimetres = 25.4 ÷ threads per inch.
What this means: Percentage of thread
- What it is
- How much of the full basic thread height the tapped thread is to have. A lower percentage means a larger tap drill and easier tapping, but a weaker thread.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 50% 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.
- Get it right
- Type the percentage itself, not a fraction.
What this means: Tap
- What it is
- Whether the thread is cut by a fluted tap or formed by a roll (forming) tap that displaces metal instead of removing it.
- Units and format
- Choose one of 2 options from the list: Cutting tap; Forming (roll) tap.
- Example
- The form opens with “Cutting tap” selected as a worked example. Choose the option that matches your case.
- Get it right
- A forming tap needs a larger hole than a cutting tap for the same thread percentage; do not swap drill sizes between them.
What this means: Screw tensile strength
- What it is
- Minimum ultimate tensile strength of the screw or bolt, from its property class or grade. It sets the load the threads must carry.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2500 MPa. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 800 MPa as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- When the tapped material is weaker, a stronger screw needs proportionally more engagement.
- Get it right
- Use tensile strength, not proof or yield strength.
What this means: Tensile strength of the tapped material
- What it is
- Ultimate tensile strength of the part being tapped, from its material data sheet or certificate, in the condition it is supplied.
- Units and format
- Enter a number in MPa: megapascals (1 MPa = 1 N/mm²). The form accepts 0.000001 MPa to 2500 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
- When the tapped material is weaker than the screw, the engagement needed rises in inverse proportion to its strength.
- Get it right
- Use the strength of the actual material condition (cast, annealed or heat treated), not a nominal wrought value.
What this means: Tap chamfer allowance
- What it is
- Incomplete threads on the tapered lead of the tap, counted in pitches. They carry no load, so this length is added below the engagement.
- Units and format
- Enter a number in pitches: pitches. The form accepts 0 pitches to 10 pitches. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 pitches as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter pitches, not millimetres: the tool multiplies by the thread pitch.
What this means: Extra drill depth beyond the threads
- What it is
- Extra drilled depth below the last tapped thread, left as room for chips so the tap does not bottom.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 100 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Drill point angle
- What it is
- Included angle at the tip of the twist drill. It sets the length of the cone added to the drill depth.
- Units and format
- Enter a number in °: degrees of angle. The form accepts 60° to 180°. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 118° as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Included angle across the point, not the half angle.
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 10.888 mm of engagement and a 19.953 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.
Learn the Concepts Behind This Tool
These BIZOMAT Library readings explain the method this tool uses, with worked examples.
Method explained here · BIZOMAT original book
Threads, tapped holes and screw lengths
In Bolted Joints by the Numbers
What it covers: How long a thread must engage so the screw breaks before the thread strips, how to design a tapped hole, and how to choose a screw length that puts full thread where it is needed.
Method explained here · Library collection
Drilling, reaming and tapping: making holes that are the right size, in the right place
What it covers: A practical guide to holemaking for designers, buyers and workshop leaders: drill types and points, deep holes, reaming allowances, boring, countersinks, tap selection and tapped-hole design.
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 Tapped Hole Design (Drill, Engagement and Depth) calculator