SFU1204, SFU1605 and SFU2005 are common metric ball screws, but the choice is not only about “how big the machine is". You must consider diameter vs length (critical speed), lead and end support, and for larger diameters also rotational inertia.

The thinner and longer a screw is, the easier it will “whip" at high speed (like a jump rope). Critical speed depends on diameter, unsupported length and support type, but some practical rules of thumb (for rotating screws) are:

• SFU1204 (12 mm diameter)

  • Good for shorter strokes, e.g. up to about 400–600 mm at medium speed.

  • When you approach 600–700 mm or more and want high RPM (around 1000 rpm), the risk of resonance and whipping increases sharply unless you keep speed low or improve supports.

• SFU1605 (16 mm diameter)

  • Noticeably higher stiffness than 1204.

  • Commonly used for strokes around 400–1000/1200 mm at medium-to-high speeds with standard support (BK12/BF12).

• SFU2005 (20 mm diameter)

  • Chosen not only for load, but also to handle longer spans and reduce deflection and whipping.

  • For axes longer than ~1000–1200 mm, or heavier gantries, SFU2005 often becomes a safer choice if you want to keep speed and vibration under control.

Even on a light machine, a 1 m long rotating SFU1204 at high speed can whip badly. If you need long travel at high speed, move up in diameter or reduce rotational speed.

Lead determines how far the nut travels per motor revolution:

• SFU1204 – 4 mm lead

  • 1 motor rev → 4 mm travel.

  • Acts like built-in reduction:

    • Higher thrust and finer resolution for the same motor torque and microstepping.

    • But lower linear speed at the same RPM.

• SFU1605 / SFU2005 – 5 mm lead

  • 1 rev → 5 mm travel.

  • An “industry standard" lead, easy for step/mm calculations.

  • Allows higher linear speed at the same motor RPM, with slightly lower thrust and resolution than a 4 mm lead.

Each screw size is usually paired with matching support units:

• SFU1204 → BK10 / BF10

  • Smaller bearings, suitable for lighter loads and shorter screws.

  • For strong cutting or long travel, the BK10 fixed bearing can become a stiffness bottleneck before the screw itself.

• SFU1605 → BK12 / BF12

  • Very common industrial combination with larger fixed bearings and better rigidity.

  • A solid choice for many desktop and 6040-style CNC machines.

• SFU2005 → BK15 / BF15 or similar

  • Even larger bearings and housing, designed to support higher loads and longer screws.

Screw inertia grows roughly with the diameter to the fourth power. A 20 mm screw can have several times the rotational inertia of a 16 mm screw of similar length. This means:

• It needs more motor torque to accelerate and decelerate.

• If you pair SFU2005 with a small stepper (for example a modest NEMA23) and try aggressive acceleration, you may see stalling or missed steps.

Whenever you choose SFU2005 (or larger), plan on using a stronger motor and drive (larger NEMA frame or servo), or use more conservative acceleration profiles.

Putting it together:

• Choose SFU1204 when:

  • Stroke is relatively short (around ≤ 400–600 mm),

  • The axis is light and you value fine resolution and higher thrust at modest speeds,

  • You are okay with BK10/BF10-level support capacity.

• Choose SFU1605 when:

  • You have a desktop CNC or 6040-class machine with travel around 400–1000 mm,

  • You want a good balance of stiffness, speed, cost and reasonable inertia,

  • You prefer robust, standard BK12/BF12 supports.

  • This is the best starting point for most DIY and light industrial X/Y axes.

• Choose SFU2005 when:

  • Travel is ≥ 1000–1200 mm or the moving mass is clearly heavier,

  • You are concerned about deflection and whipping at your target speeds,

  • You are ready to design around bigger supports and higher motor torque to handle the increased rotational inertia.

For any important axis, it is still recommended to check the calculated critical speed and bearing load ratings instead of relying only on rules of thumb.