The lead of a ball screw defines how far the nut travels per one revolution of the screw. It affects:

• Linear speed (mm/rev)

• Thrust and effective “mechanical reduction”

• Positioning resolution

• Back-driving / self-locking behaviour on Z-axes

• How your motor torque curve is used

You can think about it in four steps:

• Smaller lead (e.g. 2–5 mm):

  • 1 rev = fewer millimetres of travel

  • Acts like a higher gear reduction

    • More thrust for the same motor torque

    • Finer positioning resolution

  • But for the same motor RPM, linear speed is lower

• Larger lead (e.g. 10–20 mm):

  • 1 rev = more millimetres of travel

  • Acts like a lower gear reduction

    • Less thrust and lower resolution for the same motor

  • But higher maximum linear speed at the same RPM

This is the “textbook” mechanical view. In real machines, two more things matter a lot: self-locking on vertical axes and the motor torque curve.

Ball screws are generally efficient and can be back-driven, but lead still changes how easily gravity can move the axis:

• Small leads (e.g. 2–5 mm):

  • Smaller helix angle, more friction per unit of vertical force

  • With the help of nut friction and motor holding torque, a light or medium Z-axis often behaves almost self-locking – it is hard to push down by hand and less likely to drop quickly when power is off.

• Large leads (e.g. 10–20 mm):

  • Larger helix angle, easier to back-drive

  • A heavy spindle or Z-axis can slide down under its own weight as soon as power is removed if there is no brake or counterbalance.

Practical guidance:

• For vertical Z-axes, especially on machines without brake motors, it is safer to use a smaller lead (4–5 mm) so the axis is less willing to fall when power is lost.

• If you choose a large-lead screw on a heavy Z-axis, you should plan for a brake motor, counterweight or gas spring, otherwise a power cut can drop the head onto the workpiece or table.

On paper, a smaller lead always gives more thrust for a given motor torque. But in practice:

• Stepper motors lose torque rapidly at high RPM

  • At 1500–2000 rpm, a typical stepper has much less torque than at 300–600 rpm

• To reach a given linear speed with a small lead, the motor must spin much faster:

  • Example:

    • 4 mm lead at 2000 rpm → 8 m/min

    • 10 mm lead at 800 rpm → 8 m/min

• At 2000 rpm the motor torque may be very low, while at 800 rpm it is still in a stronger part of the torque curve.

The result is that for high-speed axes, a larger lead with lower motor RPM can actually deliver more usable thrust and better reliability than a small lead forced to spin very fast.

This is especially true for:

• Long axes where screw critical speed limits RPM

• Systems without high-voltage or high-current drivers to support very high motor speeds

• Precision + high thrust, moderate speed:

  • Small lead (e.g. 2–5 mm) is ideal when you want high resolution and don’t need extreme rapid speeds.

  • Good for many Z-axes, precision positioning and heavier but slower axes.

• General CNC X/Y axis (desktop to mid-size):

  • Leads around 5–10 mm are commonly used.

  • 5 mm gives a nice balance for many SFU1605 axes.

  • 10 mm can be good for light but fast gantries when paired with a strong motor.

• Vertical Z-axis without brake motor:

  • Prefer smaller leads like 2–5 mm to reduce back-driving.

  • If using 10–20 mm lead on a heavy head, plan for a brake or counterbalance.

In all cases, try to design so that the motor runs in the “plateau” region of its torque curve (not at the extreme high-RPM tail), and choose the lead accordingly instead of only looking at a simple “speed vs thrust” formula.