BIZOMAT MAKEOS · Manufacturing
Thread Milling Plan (Speed, Feeds, Radial Passes and Batch Time)
What spindle speed and programmed feed does this thread mill need, how many radial passes for the depth, and how long will a batch of threads take?
Plan of helical thread milling with a single-form or multi-form cutter: spindle speed from cutting speed (held to the machine maximum), feed at the cutting edge, tool-centre path diameter and the programmed centre feed corrected for the path (as the thread milling path tool), radial passes for the thread depth at your largest radial step, milling time per thread and for the batch with approach and retract time, and the cutter-size check for internal threads.
Open the calculatorRuns in your browser. No account needed.
How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thread | choice | Internal (in a hole) or external (on a boss) thread. |
| Major diameter | mm | Major diameter of the thread. |
| Pitch | mm | Pitch. |
| Thread length | mm | Length of thread to mill. |
| Cutter diameter | mm | Cutting diameter of the thread mill. |
| Cutter flutes | Number of cutting flutes. | |
| Cutting speed | m/min | Cutting speed for the cutter and material. |
| Feed per tooth (at the cutting edge) | mm | Feed per tooth at the cutting edge. |
| Largest spindle speed | rpm | Highest spindle speed. |
| Largest radial depth per pass | mm | Largest radial depth the cutter takes in one pass (your value for the cutter and material). |
| Approach, arc-in and retract per thread | s | Time for approach, arc-in, arc-out and retract per thread. |
| Threads in the batch | Number of threads in the batch. |
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
- What it is
- Whether the thread is cut inside a hole (internal) or on the outside of a boss or shaft (external). It decides whether the cutter path is inside or outside the thread and the thread depth used.
- Units and format
- Choose one of 2 options from the list: Internal; External.
- Example
- The form opens with “Internal” selected as a worked example. Choose the option that matches your case.
What this means: Major diameter
- What it is
- The major (nominal) diameter of the thread: the crest diameter of an external thread, or the root diameter of an internal thread.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 20 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the nominal size from the thread designation, not the tap drill size.
What this means: Pitch
- What it is
- The axial distance from one thread crest to the next. Each helical orbit of the cutter advances by one pitch.
- 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 2.5 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Thread depth, and so the number of radial passes, grows with pitch.
- Get it right
- For inch threads convert TPI: pitch = 25.4 ÷ TPI.
What this means: Thread length
- What it is
- The axial length of thread to be milled, along the hole or boss.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 2000 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 25 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Milling time grows in proportion to thread length.
What this means: Cutter diameter
- What it is
- The cutting diameter of the thread mill, measured over its teeth. It sets the spindle speed and the size of the tool-centre path.
- 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 14 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- Use the cutting diameter, not the shank diameter.
What this means: Cutter flutes
- What it is
- The number of cutting flutes on the thread mill, used with the feed per tooth to give the feed at the cutting edge.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 20. Use a whole number. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 3 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Cutting speed
- What it is
- The surface speed at the cutting edge of the thread mill, from the cutter maker’s data for your workpiece material.
- Units and format
- Enter a number in m/min: metres per minute. The form accepts 0.000001 m/min to 1000 m/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 120 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’s speed limit is reached.
- Get it right
- Use metres per minute, not surface feet per minute.
What this means: Feed per tooth (at the cutting edge)
- What it is
- The feed per tooth at the cutting edge, from the cutter maker’s data. The tool converts it to the slower feed programmed at the tool centre.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 1 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.08 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Feed rises in proportion, so milling time is inversely proportional to it.
- Get it right
- This is the feed at the edge; do not also correct it for the path diameter, the tool does that.
What this means: Largest spindle speed
- What it is
- The highest spindle speed the machine can run. If the cutting speed asks for more, the tool holds the spindle at this limit.
- 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 12000 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
What this means: Largest radial depth per pass
- What it is
- The largest radial depth the thread mill can take in one pass in this material, from the cutter maker or your trials. The full thread depth is split into passes of no more than this.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.05 mm to 10 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 0.8 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- A smaller step means more passes and a longer milling time.
- Get it right
- This is the radial step, not the axial length cut per pass.
What this means: Approach, arc-in and retract per thread
- What it is
- The non-cutting time for each thread: rapid approach, arc-in to the thread, arc-out and retract.
- 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 4 s as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Adds directly to the time per thread.
- Get it right
- Enter seconds per thread, not minutes.
What this means: Threads in the batch
- What it is
- The number of threads to be milled 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 50 as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Batch time is the time per thread multiplied by this number.
Method and formulas
- n = min(1000 v_c ÷ π d_c, n_max); edge feed f_e = n z f_z.
- Path diameter = D − d_c (internal) or D + d_c (external); centre feed f_c = f_e × path diameter ÷ D (as the thread milling path tool).
- Thread depth = 0.5413P (internal) or 0.6134P (external); radial passes = depth ÷ largest step (rounded up).
- Time per thread = (L ÷ P × passes) × helix length ÷ f_c + approach; batch = threads × time.
- Example: 2,728.4 rpm with a programmed feed of 196.44 mm/min mills the thread in 2 passes, 120.15 s per thread.
Limitations
- Single-form cutter making one helix per pass; multi-form cutters cover several pitches per revolution.
- Deflection of long, small cutters may need extra spring passes.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Program a thread milling cycle
- Compare thread milling with tapping for a batch
- Check cutter size for a small internal thread
Who it is for
- CNC programmers
- Process planners
- CNC machinists
What you get
- Spindle speed, edge feed and programmed centre feed
- Radial passes and helical revolutions
- Time per thread and for the batch
Interpreting the result
- Forgetting the path-diameter correction overfeeds an internal thread by the ratio D ÷ (D − d_c).
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2058–2070, 1292–1300. Topic reference; no source tables or recommended settings are reproduced.
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 Thread Milling Plan (Speed, Feeds, Radial Passes and Batch Time) calculator