KEVOS® MAKEOS · Threads
Change-Gear Threading Set-up (Gear Train, Pitch Error, Thread Depth, Passes)
To cut this metric thread on a lathe with an inch leadscrew (or the other way round), which change gears do I fit, how accurate is the pitch, and how deep and in how many passes do I cut?
Set-up for single-point screw cutting on a lathe with change gears: the ratio needed from the thread lead, leadscrew and fixed headstock ratio, the best simple and compound gear trains from the gear set with their pitch errors (as the change gears tool), and the thread height, pitch diameter and number of passes for a constant-area infeed from your first-pass depth (as the metric thread tool for thread depth), checked against the pitch error you accept.
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How this tool works
Inputs, units and where to find them
| Input | Unit | Typical source |
|---|---|---|
| Thread nominal diameter | mm | Nominal diameter of the metric thread. |
| Thread pitch to cut | mm | Pitch of the thread to cut. |
| Leadscrew is given as | choice | Whether the leadscrew is inch (threads per inch) or metric (pitch). |
| Leadscrew | TPI or mm | Leadscrew threads per inch or pitch. |
| Fixed ratio in the headstock (spindle to stud) | :1 | Ratio between spindle and stud gear fixed inside the headstock. |
| Gear set | choice | Change gears you have. |
| Largest pitch error | % | Largest pitch error you accept from the gear train. |
| First pass depth (radial) | mm | Depth of the first pass; later passes get shallower to keep the chip area constant. |
Method and formulas
- Ratio = thread lead ÷ (leadscrew lead × fixed ratio); best two- and four-gear trains from the set (as the change gears tool).
- External thread height h₃ = 0.6134P; pitch diameter d₂ = d − 0.6495P (as the metric thread tool).
- Constant-area infeed: depth after pass k = a₁ √k, so passes = ⌈(h₃ ÷ a₁)²⌉.
- Example: fit drivers 20 and 75, driven 25 and 127 (error 0%); cut 0.92015 mm deep in 10 passes.
Limitations
- Gear fit on the quadrant and the idler arrangement must be checked on the machine.
- Infeed passes are a starting plan; the last passes are often spring passes.
- Example values are illustrations. Replace them with values for your actual task.
Typical uses
- Cut a metric thread on an imperial lathe
- Check a gear train’s pitch error
- Plan passes for single-point threading
Who it is for
- Machinists
- Toolroom staff
- Apprentices
What you get
- Ratio needed and best gear trains with their errors
- Thread height and pitch diameter
- Passes for the constant-area infeed
Interpreting the result
- Metric threads on an inch leadscrew need a 127-tooth gear for an exact ratio; without it, check the error over the thread length.
References
- Machinery’s Handbook, 29th edition (Industrial Press, 2012), printed pp. 2043–2049, 1867–1890. Topic reference; no source tables or recommended settings are reproduced.
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Open the Change-Gear Threading Set-up (Gear Train, Pitch Error, Thread Depth, Passes) calculator
