Compare C7 rolled and C5 ground ball screw lead accuracy, backlash, cost, and ideal applications - with guidance on when to upgrade from rolled to ground precision for industrial automation.
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.ror.
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.nce.
- 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.les.
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 the baseline; some stages use C3 or C0 ground screws for even tighter accuracy.
Dongfeng's SFE series ground ball screws are precision-thread-ground on dedicated CNC grinders, achieving Ra 0.1-0.2 um surface finish and lead accuracy within +/-0.018 mm per 300 mm. For zero-backlash applications, the DFU series double-nut design applies axial preload through a spacer pin, eliminating backlash entirely while maintaining smooth rotation.ion.
The Cost-Benefit Decision
A C5 ground ball screw costs 2.5-4x more than the equivalent C7 rolled screw. For a 1000mm SFU2010 ball screw with nut:
- C7 rolled: approximately $45-65
- C5 ground: approximately $120-180
The upgrade is justified when one of these conditions is true:
- Your workpiece tolerance is tighter than 0.05 mm. A C7 screw's 0.05 mm lead error per 300 mm means parts machined near the end of travel will deviate by the full lead error amount.unt.
- You need zero backlash for bidirectional positioning. A preloaded C5 double nut (DFU series) achieves true zero backlash, while a C7 preloaded nut still has 0.005-0.02 mm residual backlash.
- Your machine has no linear scale (open-loop). Without encoder feedback on the linear axis, the ball screw's lead accuracy IS your machine's accuracy.ccuracy.
If none of these conditions apply, a C7 rolled ball screw with a preloaded single nut is the more economical choice without sacrificing functional performance.
Can You Upgrade from C7 to C5 Later?
Yes, if you plan ahead. Both C7 and C5 ball screws use the same standard end-machining dimensions for a given screw diameter. An SFU2010 rolled screw and an SFE2010 ground screw have identical journal diameters, bearing seat lengths, and coupling interfaces. This means you can prototype with a C7 rolled screw and upgrade to a C5 ground screw in the same machine without redesigning the bearing supports, coupling, or motor mount.
This upgrade path is particularly useful for machine builders who want to offer a standard and a precision version of the same machine. Design the axis for the C5 screw's dimensional envelope, install a C7 screw for the standard model, and offer the C5 screw as a precision upgrade option. Dongfeng supports this by providing matched end-machining specifications across rolled and ground variants in the same diameter series.ies.