BIZOMAT MAKEOS · Manufacturing
Multi-Start Thread Turning Set-up
What lead, start spacing and Z shift do I program to turn this multi-start thread?
CNC set-up for multi-start threads: lead (programmed feed) from pitch and starts, the angular spacing of the starts, the Z-axis shift between starts and number of shifts, the helix angle at the pitch diameter for tool side clearance, and the spindle speed limit from the axis feed limit.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Pitch | mm | Pitch (axial distance between adjacent threads). |
| Number of starts | Number of independent thread starts. | |
| Pitch diameter | mm | Pitch diameter of the thread. |
| Maximum Z-axis feed rate | mm/min | Highest feed rate the Z-axis can follow while threading. |
| Planned spindle speed | rpm | Spindle speed you plan to use. |
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: Pitch
- What it is
- The axial distance between adjacent thread crests. On a multi-start thread this is not the lead, which is the advance per turn.
- Units and format
- Enter a number in mm: millimetres. The form accepts 0.000001 mm to 50 mm. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 1.5875 mm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- Lead rises in proportion to pitch, so the spindle speed limit falls.
- Get it right
- Do not enter the lead: lead = starts × pitch, and the tool calculates it.
What this means: Number of starts
- What it is
- How many separate helical threads are cut side by side, each starting at an equally spaced angle round the part.
- Units and format
- Enter a plain number with no unit. The form accepts 1 to 12. 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.
- Effect on the result
- Lead is starts × pitch, so more starts increase the lead and helix angle and lower the speed the Z-axis can follow.
What this means: Pitch diameter
- What it is
- The pitch diameter of the thread, where the helix angle is worked out to check the tool’s side clearance.
- 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.
- Get it right
- Pitch diameter, not major diameter; the major diameter understates the helix angle.
What this means: Maximum Z-axis feed rate
- What it is
- The highest Z-axis feed rate the machine can follow while synchronised to the spindle in a threading cycle, from the machine or control specification.
- Units and format
- Enter a number in mm/min: millimetres per minute. The form accepts 0.000001 mm/min to 1000000 mm/min. Required: the tool does not calculate until every input is valid.
- Example
- The form opens with 5000 mm/min as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Effect on the result
- The maximum spindle speed is this feed rate divided by the lead.
- Get it right
- Use the threading feed limit, which can be lower than the rapid traverse rate.
What this means: Planned spindle speed
- What it is
- The spindle speed you intend to program for the threading passes, checked against the speed the Z-axis can follow.
- 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 800 rpm as a worked example. It is an illustration, not a recommended value: replace it with your own figure.
- Get it right
- It does not change the lead or helix angle; only the pass or fail check uses it.
Method and formulas
- Lead = number of starts × pitch (the F value for G32 or G76).
- Start spacing = 360° ÷ starts; each start is cut after shifting Z by one pitch, with starts − 1 shifts.
- Helix angle = atan[lead ÷ (π × pitch diameter)]; maximum speed = axis feed limit ÷ lead.
- Example: a 3-start thread of 1.5875 mm pitch has a 4.7625 mm lead and a 3.4701° helix angle.
Limitations
- Start the Z shift from the same synchronised position; acceleration distance must clear the part.
- Controls differ in G76 syntax; check the control manual.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Program a two- or three-start thread on a CNC lathe
- Check tool side clearance for a steep lead
- Find the spindle speed the Z-axis can follow
Who it is for
- CNC programmers
- Machinists
- Manufacturing engineers
What you get
- Lead and programmed feed
- Start spacing and Z shift
- Helix angle and speed limit
Interpreting the result
- Steep multi-start leads need extra side clearance on the leading flank of the tool.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 1304–1305. Topic reference; no source tables or recommended settings are reproduced.
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