Buying guide
Lead Screw Pitch vs Lead: How to Read T8 and Other Specs
Why a T8x8 screw moves 8 mm per turn, what the numbers mean, and how lead sets speed and resolution.
Lead screw listings use two numbers that are often confused: pitch and lead. Getting them right decides how fast your axis moves and how fine it can position.
Pitch, lead and starts
- Pitch is the distance between two neighbouring thread crests.
- Lead is how far the nut travels in one full turn of the screw.
- Lead = pitch x number of starts. A single-start screw has lead equal to pitch. A 4-start screw with 2 mm pitch has an 8 mm lead.
So a T8x8 screw is 8 mm diameter with an 8 mm lead (typically 4 starts at 2 mm pitch), and a T8x2 has a 2 mm lead.
What lead changes
| Lead | Speed at the same RPM | Resolution and holding |
|---|---|---|
| Small (2 mm) | Slow | Fine positioning, higher thrust, holds load better |
| Large (8 mm) | Fast | Coarser steps, less thrust, may back-drive vertically |
Linear speed = lead x RPM. At 600 RPM, a 2 mm lead moves 20 mm/s and an 8 mm lead moves 80 mm/s. Step resolution of a 1.8 degree motor is lead / 200 per full step, for example 0.04 mm for an 8 mm lead.
How to choose
- Z axis of a 3D printer: a small or medium lead keeps the axis from sinking when power is off and gives fine layer positioning.
- Fast linear motion: a large lead gives speed without very high motor RPM.
- Match the nut. Screw and nut must have the same diameter and lead.
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