Short answer: C2 ground ball screws hold ±5 µm lead error per 300 mm (DIN 69051 Class 2); C7 rolled screws run at roughly ±50 µm — a 10× difference that shows up directly in positioning accuracy. C7 is the right answer for transport and handling axes where cost matters more than microns; C2–C3 ground screws are the right answer anywhere the machine is bought to hold a tolerance — machining center X/Y, grinding axes, diameter-critical turning. Here is the comparison, the positioning math, and a per-axis selector based on what Songju ships (C2/C3 ground as standard, 8–100 mm diameter).
C2 vs C7: The Numbers That Matter
| Attribute | C2 (precision ground) | C7 (rolled) |
|---|---|---|
| Manufacturing | Thread ground after induction hardening | Thread rolled to net profile, no grinding |
| Lead error per 300 mm | ±5 µm (DIN 69051 Class 2) | Typically ±50 µm (0.05 mm) |
| Contribution to positioning error | ±5 µm per 300 mm of stroke | ±50 µm per 300 mm of stroke — 10× worse |
| Surface finish and friction consistency | Ground flanks, uniform friction over travel | Good but less uniform |
| Relative cost | Higher | Significantly lower |
| Best for | Positioning axes, precision X/Y, grinding machines | Transport axes, handling, cost-sensitive Z axes |
The lead-error row is the whole argument. A 300 mm axis with a C2 screw contributes ±5 µm of positioning uncertainty from the screw alone; the same axis with a C7 screw contributes ±50 µm — before backlash, thermal growth, or structural deflection enter the budget. If your machine's positioning tolerance is ±10 µm, the C7 screw has spent the entire budget before the servos start. That is why ground C2/C3 screws are standard on grinding machines and precision machining centers, while C7 rolled screws dominate material transfer, gate automation, and entry-level routers where "repeatable" matters more than "exact".
Per-Axis Selector
| Axis | Recommended grade | Reasoning |
|---|---|---|
| Machining center X / Y (contouring) | C2–C3 ground | Lead error appears directly as contour and positioning error |
| Grinding machine axes | C2 | The machine itself is finishing to microns — the screw cannot be coarser |
| Turning machine X (diameter control) | C2–C3 ground | Diameter tolerance of turned parts traces back to screw accuracy |
| Z axis (tool / ram), counterbalance | C3 acceptable | Positioning demand is lower; rigidity and cost dominate |
| Transport, pick-and-place, press feed | C7 rolled acceptable | Repeatable motion matters more than absolute lead accuracy |
A practical cost note: within Songju's ground range, stepping from C2 to C3 buys meaningful savings while staying an order of magnitude tighter than C7 — for most machining center axes C3 is the value pick, reserving C2 for grinding and metrology duties. There is no reason to run C7 on a contouring axis to "save money"; the screw is a small fraction of axis cost and the last thing to economize on.
What Songju Ships
- Series: FC, FCD, FY, FYB, and FYD — 5 series covering CNC machines, automation, and aerospace duties.
- Diameter: 8–100 mm, DIN 69051 compliant.
- Accuracy: C2 and C3 precision-ground as standard; C2 holds ±5 µm / 300 mm lead deviation.
- Support: end machining to drawing, nut and support-unit matching, 2–4 week standard delivery, inspection report with every shipment.
For the deeper manufacturing trade-offs (hardening, surface finish, preload selection), see our ground vs rolled ball screw guide. For a grade recommendation on a specific axis, send the stroke, load, and target positioning accuracy — our engineers reply within 24 hours.
Frequently Asked Questions
Is C7 good enough for a CNC machine tool?
For transport axes, Z axes, and cost-sensitive routers - yes. For the contouring X and Y axes of a machining center or any grinding application, no: at C7 the lead error alone contributes up to 50 µm per 300 mm of travel, which is larger than the positioning tolerance those machines are bought to hold.
What is the difference between C2 and C3?
Both are precision-ground classes; C2 is tighter. Songju produces C2 and C3 ground screws as standard across the FC, FCD, FY, FYB, and FYD series (8-100 mm diameter). Typical selection: C2 for grinding and metrology-grade axes, C3 for machining center axes where a small step in tolerance saves meaningful cost.
Why is C7 so much cheaper?
C7 screws are rolled, not ground: the thread is cold-rolled to net shape after hardening, skipping the thread-grinding operation that dominates ground screw cost. The trade-off is lead accuracy roughly an order of magnitude coarser and slightly less consistent friction over travel.
Can I upgrade from C7 to C2 without redesigning the axis?
Usually yes within the same diameter and lead: end journal details, nut flange, and support units are shared across grades in the FC/FCD/FY families. Send us the axis drawing or the old screw's specification and we confirm a drop-in C2/C3 equivalent.