BIZOMAT MAKEOS · Fasteners
Tap Drill for Percentage of Thread
What drill size gives the required percentage of thread for a cutting or forming tap?
Theoretical tap drill or hole size for a chosen percentage of thread, for cutting taps and cold-forming (roll) taps, in metric or unified threads.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thread system | choice | Metric threads in millimetres, or unified threads in inches. |
| Basic major diameter | mm or in | Nominal size: millimetres for metric, inches for unified. |
| Pitch (metric) | mm | Metric thread pitch (ignored for unified). |
| Threads per inch (unified) | TPI | Threads per inch (ignored for metric). |
| Percentage of thread | % | About 75% is usual for cutting taps (60–65% in tough materials or deep holes); 65% is usual for forming taps. |
| Tap type | choice | Cutting taps remove metal; forming taps displace it and need a larger hole. |
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: Thread system
- What it is
- Whether the thread is metric, sized in millimetres with a pitch, or unified, sized in inches with threads per inch. It decides which pitch input is used and the unit of the result.
- Units and format
- Choose one of 2 options from the list: Metric (size in mm, pitch in mm); Unified (size in inches, threads per inch).
- Example
- The form opens with “Metric (size in mm, pitch in mm)” selected as a worked example. Choose the option that matches your case.
- Get it right
- Change the major diameter to match: millimetres for metric, inches for unified.
What this means: Basic major diameter
- What it is
- The basic major diameter of the thread: the nominal size in millimetres for metric threads or in inches for unified threads.
- Units and format
- Enter a number in mm or in: millimetres or inches, as long as every length uses the same unit. The form accepts 0.000001 mm or in to 500 mm or in. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 mm or in as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- The unit follows the thread system: 10 means 10 mm when metric but 10 inches when unified.
What this means: Pitch (metric)
- What it is
- The axial distance from one thread crest to the next for a metric thread, from the designation. Ignored for unified threads.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 20 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.
- Effect on the result
- A coarser pitch needs a smaller drill for the same percentage of thread.
- Get it right
- A plain designation such as M10 means the coarse pitch; fine threads state the pitch.
What this means: Threads per inch (unified)
- What it is
- The number of thread crests per inch of length for a unified thread: the number after the dash in the designation. Ignored for metric threads.
- Units and format
- Enter a number in TPI: threads per inch. The form accepts 0.000001 TPI to 200 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
- Do not enter the pitch in inches here; this is a count per inch.
What this means: Percentage of thread
- What it is
- How much of the full basic thread depth the hole leaves for the tap to form. Lower percentages mean a larger hole, easier tapping and slightly less thread strength.
- 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 drill size.
- Get it right
- Type the percentage itself, not a fraction. Choose it from your shop practice for the material and tap.
What this means: Tap type
- What it is
- The kind of tap: a cutting tap removes metal to form the thread; a forming (roll) tap displaces metal and needs a larger hole.
- 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
- Forming taps suit ductile materials; check suitability for brittle materials.
Method and formulas
- Cutting tap: drill = D − 0.01299 × % × P (or D − 0.01299 × % ÷ n in inches).
- Forming tap: hole = D − 0.0068 × % × P (or ÷ n in inches).
- Basic minor diameter of the internal thread = D − 1.082532P.
- Example: M10 × 1.5 at 75% with a cutting tap needs a 8.5386 mm drill.
Limitations
- Theoretical sizes; drills cut oversize, so check the actual hole and choose the nearest available drill.
- The resulting minor diameter must stay within the thread class limits.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Pick a tap drill for tough materials or deep holes
- Find the hole size for a forming tap
- Check an existing drill against thread engagement
Who it is for
- Machinists
- CNC programmers
- Manufacturing engineers
What you get
- Theoretical drill or hole size
- Drill size in both units
- Basic minor diameter for comparison
Interpreting the result
- Choose the nearest standard drill and check the minor diameter against the thread limits table.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2029–2036. Topic reference; no source tables or recommended settings are reproduced.
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