BIZOMAT MAKEOS · Threads
Tapping Plan (Tap Drill, Thread Percentage, Speed and Cycle Time)
What tap drill gives the thread percentage I want, what percentage will the drill I have give, and how long does a batch of tapped holes take?
Plan of a tapping operation for a metric thread: the tap drill for a target thread percentage for cut or formed taps and the basic minor diameter (as the tap drill tool), the percentage the drill you actually have gives, spindle speed from tapping speed held to the machine maximum, synchronous feed and forward time (as the tapping feed tool), reverse time at the retract speed multiple, and the batch time.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thread diameter | mm | Nominal diameter of the thread. |
| Pitch | mm | Thread pitch. |
| Thread percentage wanted | % | Thread engagement wanted (about 65–75% for cut taps; higher for forming taps). |
| Tap type | choice | Cutting or forming tap (forming taps need a larger hole). |
| Drill you have (0 = use the calculated size) | mm | Drill size available; 0 uses the calculated size. |
| Smallest thread percentage accepted | % | Lowest thread percentage you accept (strength). |
| Largest thread percentage accepted | % | Highest thread percentage you accept (tap breakage, torque). |
| Tapping speed | m/min | Tapping speed for the tap and material. |
| Largest tapping spindle speed | rpm | Highest spindle speed for tapping on the machine. |
| Thread depth | mm | Depth of full thread. |
| Retract speed ÷ forward speed | Retract speed as a multiple of the forward speed. | |
| Positioning and dwell per hole | s | Rapid positioning and dwell per hole. |
| Holes in the batch | Number of holes to tap. |
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 diameter
- What it is
- The nominal (major) diameter of the thread being tapped, the M number for metric threads.
- 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.
- Effect on the result
- Spindle speed for a given tapping speed is inversely proportional to it.
What this means: Pitch
- What it is
- The axial distance between thread crests. The tap advances one pitch per revolution, so it sets the synchronous feed.
- 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.
- Effect on the result
- Feed is proportional to pitch, so forward time falls as pitch grows.
- Get it right
- For inch threads convert TPI: pitch = 25.4 ÷ TPI.
What this means: Thread percentage wanted
- What it is
- The thread percentage you want: how much of the full basic thread depth the hole leaves for the tap to form. It sets the calculated tap drill size.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 40% 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, for example 75, not 0.75.
What this means: Tap type
- What it is
- Whether the thread is made by a cutting tap, which removes metal, or a forming tap, which displaces it. Forming taps need a larger pre-drilled 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.
What this means: Drill you have (0 = use the calculated size)
- What it is
- The drill size you actually have or intend to use. The thread percentage it gives is checked against your limits.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0 mm to 200 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 8.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter 0 to use the calculated tap drill size.
What this means: Smallest thread percentage accepted
- What it is
- The lowest thread percentage you accept, set by the thread strength you need. Take it from your design rules.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 0% to 100%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 60% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Largest thread percentage accepted
- What it is
- The highest thread percentage you accept, set by tap torque and breakage risk. Take it from your design rules or tap supplier.
- Units and format
- Enter a number in %: percent: the number is read as a percentage, so type 8 for 8 %. The form accepts 1% to 110%. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 80% as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It must be above the smallest percentage, or the tool stops.
What this means: Tapping speed
- What it is
- The surface speed at the tap’s major diameter, from the tap maker’s data for your material.
- Units and format
- Enter a number in m/min: metres per minute. The form accepts 0.000001 m/min to 100 m/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 10 m/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Spindle speed and feed rise in proportion until the machine limit, shortening forward time.
What this means: Largest tapping spindle speed
- What it is
- The highest spindle speed the machine allows for rigid or synchronous tapping.
- Units and format
- Enter a number in rpm: revolutions per minute. The form accepts 0.000001 rpm to 100000 rpm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Machines often limit tapping speed below their maximum spindle speed; use the tapping limit.
What this means: Thread depth
- What it is
- The depth of full thread to be tapped, measured from the top of the hole.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 500 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 15 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Forward and retract times are proportional to it.
What this means: Retract speed ÷ forward speed
- What it is
- How many times faster the tap retracts than it feeds in, as set in the tapping cycle.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 5. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- A ratio, not a speed: 2 means retract at twice the forward speed.
What this means: Positioning and dwell per hole
- What it is
- The non-tapping time per hole: rapid positioning to the hole, dwell at the bottom and any reversal delay.
- Units and format
- Enter a number in s: seconds. The form accepts 0 s to 600 s. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 2 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Enter seconds per hole.
What this means: Holes in the batch
- What it is
- The number of holes to tap in the batch.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 1000000. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 100 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Batch time rises in proportion.
Method and formulas
- Tap drill = D − k × percentage × P, with k = 0.01299 (cut) or 0.0068 (form) per percent; basic minor diameter = D − 1.0825P (as the tap drill tool).
- Percentage given by a drill = (D − drill) ÷ (k P).
- n = min(1000 v_c ÷ π D, n_max); feed = P × n; forward time = depth ÷ feed (as the tapping feed tool); retract time = forward time ÷ retract multiple.
- Example: 8.5386 mm gives 75%; a 8.5 mm drill gives 76.982%. At 318.31 rpm a batch of 100 takes 8.0457 min.
Limitations
- Basic 60° metric thread; hole size in tough or soft materials may need adjusting.
- Thread percentage relates to engagement, not directly to strength; 75% gives most of the full strength.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Choose a tap drill for strength or tap life
- Check a standard drill’s thread percentage
- Estimate tapping time for a batch
Who it is for
- CNC machinists
- Process planners
- CNC programmers
What you get
- Tap drill for the target and the drill’s actual percentage
- Spindle speed and synchronous feed
- Time per hole and per batch
Interpreting the result
- Raising thread percentage above about 75% adds little strength but much more tapping torque.
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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Open the Tapping Plan (Tap Drill, Thread Percentage, Speed and Cycle Time) calculator