Ball Screw Accuracy Class: C3, C5, and C7 Explained

Accuracy class is about lead error

Ball screw accuracy class is defined by lead error, not just shaft diameter. C3, C5, and C7 are the three most common classes on the market: the smaller the number, the smaller the lead error, the higher the accuracy, and the higher the price. In practice, C5 and tighter classes are usually ground ball screws, while C7 is commonly rolled, which is why a "C5 ground ball screw" and a rolled C7 differ in both accuracy and cost.

The three classes compare as an order of magnitude (accumulated lead error over 300 mm):

Class	Lead error order over 300 mm	Usual process	Typical application
C7	About 0.05 mm	Rolled	Ordinary CNC, woodworking, general automation
C5	About 0.023 mm	Ground	Machining centers, mold equipment, precision positioning
C3	Around the 0.01 mm order	Ground, higher process requirements	Measuring equipment, high-end machining, lab instruments

Exact values come from the ISO 3408-3 / JIS B1192 grade table and depend on diameter and length. The table also carries a second figure: deviation within 2π (the allowance over one revolution), which is a different measurement from the 300 mm accumulated value and cannot be used interchangeably at acceptance. Wording matters too: the standard table gives a permissible travel variation range (peak to valley), not a ± symmetric tolerance — writing 0.05 mm as ±0.05 mm doubles the allowed band. Acceptance always follows the parameter name and value in the standard table.

Choose the class by positioning requirement

Before choosing, ask what positioning accuracy the machine actually needs. Ordinary woodworking CNC works with C7, machining centers use C5, and precision engraving or measuring equipment needs C3. Accuracy you will not use only adds cost and lead time.

One fact gets overlooked: a tighter class does not fix thermal growth or installation error. A 1 m screw rising 5 K grows on the order of 60μm — comparable to the whole gap between C3 and C7. If the error comes mainly from heat or misalignment, paying for a higher grade buys nothing.

Accuracy is not the only factor

Besides the class, confirm nut preload (to remove backlash), screw support arrangement (fixed-support or fixed-fixed), and installation and adjustment. Without enough preload, even a C5 screw can show backlash.

State acceptance in the RFQ

Accuracy class: C5 or C7, noted in the model.
Acceptance criteria: lead error, axial clearance, and runout checks.
Inspection report: whether factory measurement data travels with the goods.
Installation: on-site installation or self-assembly.
Measurement basis: whether any 300 mm section or the full travel, what one revolution means, and which end is the datum.

FAQ

How much more does C5 cost than C7? It depends on specification, length and quantity: for the same size, C5 needs grinding and a longer production run, so it costs more; the gap usually grows with diameter and length. We quote from the drawing and quantity.

Can the inspection report show accuracy? Yes. The factory report lists lead error, runout, and preload data, which you can compare with the stated class — and check whether the figure is a 300 mm value or a full-travel value.

Is C3 always better than C5? Not "better" but tighter and dearer. If the machine never uses that accuracy, the extra spend does not become a better product and stretches the lead time instead.

Next step

Turn this guide into an RFQ

When the specification direction is clear, send the details below together with quantity, lead time, and packing requirements.

Include these details

Model, diameter, lead, accuracy grade, or target application.
Load, speed, travel, mounting method, and matching rail or support-unit needs.
Quantity, lead time, packing, and whether inspection records or shipment photos are needed.

Send accuracy requirement View standard ball screws