What Lead Accuracy Means for Your Application

Ball screw lead accuracy is the maximum deviation of actual axial travel from theoretical travel over a defined length. If a ball screw has a 10 mm lead and you rotate it exactly 100 revolutions, the theoretical travel is 1,000 mm. In practice, the actual travel will deviate by a measurable amount - that deviation is the lead accuracy, expressed as error per 300 mm of travel.

This specification matters because it directly determines the open-loop positioning accuracy of your machine. In a closed-loop system with a linear encoder feedback, the encoder compensates for lead error in real time, and the ball screw's lead accuracy becomes less critical. But for open-loop systems driven by a stepper or servo motor with rotary encoder feedback only, the ball screw IS the positioning reference - its lead error translates directly into workpiece dimensional error.

C7 vs C5: The Numbers

Specification	C7 (Rolled)	C5 (Ground)
Lead error per 300mm	+/-0.05 mm (50 um)	+/-0.018 mm (18 um)
Lead error per 1000mm	+/-0.21 mm	+/-0.075 mm
Axial backlash (standard nut)	0.05-0.15 mm	0.003-0.008 mm
Axial backlash (preloaded nut)	0.005-0.02 mm	0 (zero backlash)
Manufacturing method	Roll-forming	Thread grinding
Typical cost ratio	1x	2.5-4x
Surface finish (Ra)	0.4-0.8 um	0.1-0.2 um

The key takeaway: C5 ground ball screws deliver roughly 3x better lead accuracy and 10-20x lower backlash than C7 rolled screws. Whether that improvement matters depends entirely on what your machine is making.

When C7 Rolled Ball Screws Are Sufficient

Rolled ball screws (C7 accuracy, +/-0.05 mm per 300 mm) are the standard choice for:

CNC wood routers: Cutting plywood and MDF at tolerances of +/-0.1 mm. The lead error is within the material's own dimensional tolerance.
3D printers and desktop CNC: Open-loop stepper-driven systems where +/-0.1 mm positioning is acceptable. A C7 SFU1605 ball screw with 0.05 mm backlash still outperforms a T8 lead screw (0.15-0.3 mm backlash) by a wide margin.
Pick-and-place gantries: Components placement at +/-0.1 mm tolerance. The vision system corrects final placement, so ball screw lead error is non-critical.
Material handling axes: Conveyor positioning, lift tables, and door openers where repeatable positioning matters but sub-0.05 mm accuracy does not.

Dongfeng's SFU series rolled ball screws (SFU1605 through SFU6310) are manufactured from GCr15 bearing steel with induction-hardened raceways (55-60 HRC) and offer excellent value for these applications. The rolled thread form has a slightly rougher surface (Ra 0.4-0.8 um) than ground screws, which means slightly higher running torque and noise, but does not affect load rating or service life in typical duty cycles.

When You Need C5 Ground Ball Screws

Ground ball screws (C5 accuracy, +/-0.018 mm per 300 mm) are specified when the workpiece tolerance demands it:

CNC milling machines: Machining aluminum and steel parts at +/-0.02 mm tolerance. The ball screw lead error must be less than the workpiece tolerance to maintain dimensional consistency across the full travel.
Surface grinders: Achieving flatness within 2-3 microns. Any lead error on the infeed axis translates directly into surface deviation.
EDM (wire and sinker): Positioning the electrode or wire guide at +/-0.005 mm. Ground ball screws with preloaded nuts provide the near-zero backlash required for spark gap consistency.
Semiconductor wafer stages: Step-and-repeat positioning at sub-micron resolution. C5 is th