A practical guide for procurement engineers and machine builders comparing ball screw accuracy classes C5 and C7. Covers lead error tolerances in real numbers, application fit for CNC, automation, and general machinery, and the cost/performance trade-off — with a clear selection framework and supplier communication tips.
BALL SCREW BUYER GUIDE
If you are sourcing ball screws for a CNC machine, a laser cutting table, or a simple pick-and-place axis, the accuracy grade stamped on the drawing is probably the first spec your supplier asks about. And the most common question we hear from buyers is: “Should I spec C5 or C7?”
The honest answer is: it depends on what your machine is actually trying to do. Choosing C5 when C7 would do the job means paying 30–60% more for precision you never use. Choosing C7 for a positioning-critical axis means reworking your machine or accepting scrap parts.
This guide breaks down the real difference between C5 and C7 — the lead error numbers, the application boundaries, and the cost logic — so you can spec correctly the first time.
- C5 and C7 differ primarily in lead error tolerance — C5 is roughly 2.5× tighter than C7 per 300 mm of travel.
- C5 is for positioning-critical axes (CNC milling, EDM, precision engraving); C7 is for transport and general motion (feeding, lifting, simple actuation).
- Cost difference is real: C5 typically costs 30–60% more than the same screw in C7, depending on diameter and length.
- Preload, backlash, and mounting are separate specs — do not conflate them with accuracy grade.
- For long travel (> 1000 mm), C5 lead error accumulates slower than C7, which matters more than the per-300 mm number.
What Does “Accuracy Grade” Actually Mean?
Ball screw accuracy grade is a standardized classification of how closely the actual lead (the linear travel per revolution) matches the theoretical lead. It is defined in ISO 3408-3 and JIS B 1192, and it is expressed as a lead error tolerance over a defined travel length.
The grade is not about surface finish, hardness, or load capacity. It is purely about positioning repeatability and cumulative error over travel.
For a given screw, the manufacturer grinds or rolls the thread to a target lead. No process is perfect, so there is always a small deviation. The accuracy grade tells you the maximum allowable deviation.
C5 vs C7: The Numbers That Matter
The most commonly referenced spec is the lead error over 300 mm of travel. This is the number you will see on datasheets and in supplier RFQs.
| Accuracy Grade | Lead Error per 300 mm | Typical Manufacturing Process | Relative Cost Index |
|---|---|---|---|
| C7 | ±0.050 mm (50 µm) | Rolled thread, no post-grinding | 1.0 (baseline) |
| C5 | ±0.018 mm (18 µm) | Rolled + precision finishing, or ground | 1.3 – 1.6 |
| C3 (reference) | ±0.008 mm (8 µm) | Ground thread | 2.0 – 3.0 |
These numbers are per 300 mm segment. For longer travel, the tolerance is not linear — it grows, but at a slower rate. For example, a C7 screw with 1000 mm travel has a typical cumulative lead error of around ±0.090 mm, while a C5 screw at the same length stays around ±0.030 mm.
This is the key insight: the longer the axis, the more the accuracy grade matters.
Application Scenarios: Where Each Grade Fits
Choosing the grade is not about “better” — it is about matching the error budget of your machine.
When C5 Is the Right Choice
You need C5 when the screw’s positioning error directly affects the quality of the finished part. Typical cases:
- CNC milling and drilling machines — tool position error shows up as dimensional deviation on the workpiece. A 0.05 mm error on a machined pocket is often out of spec.
- Wire-cut EDM and precision grinding — these machines hold tolerances in the low micron range; C7 would be the bottleneck.
- Laser cutting and waterjet — for contour accuracy on thin sheet metal, C5 keeps the kerf path true.
- PCB drilling and component insertion — hole-to-hole pitch accuracy requires repeatable positioning within ±0.02 mm.
- Measuring and inspection equipment — any axis that moves a probe or sensor needs the error to be smaller than the measurement tolerance.
In these applications, the extra cost of C5 is justified because the alternative is rejected parts, rework, or a machine that cannot hold its advertised tolerance.
When C7 Is the Right Choice
C7 is the workhorse grade for machines where the screw moves something, but the final position is not a precision-critical dimension. Typical cases:
- Feeding and transfer axes — moving a workpiece from station A to B; a 0.05 mm error is irrelevant.
- Lifting columns and scissor lifts — vertical positioning with a tolerance of 1–2 mm is common.
- Packaging and labeling machines — the product itself has dimensional variation larger than the screw error.
- Welding positioners and manipulators — the weld seam tolerance is far looser than C7 error.
- Simple actuator axes in automation — where a pneumatic cylinder would work but you need controlled speed and mid-stroke positioning.
- 3D printers and hobby-class CNC — cost-sensitive builds where the frame and stepper motor introduce more error than the screw.
For these, paying for C5 is a waste of budget. The machine’s overall accuracy is limited by other components — the frame stiffness, the guide rails, the motor encoder — not the screw.
The Cost Trade-off: What You Pay For
The price difference between C5 and C7 is not arbitrary. It comes from the manufacturing process and the scrap rate.
C7 screws are typically rolled — the thread is formed by cold rolling, which is fast and cheap. The process is consistent, but the lead error is inherently higher due to thermal and mechanical variation during rolling.
C5 screws can be rolled with tighter process control, but for smaller diameters and longer lengths, they are often ground — the thread is cut and then finished with a grinding wheel. Grinding is slower, requires skilled setup, and produces heat that must be managed to avoid distortion. Scrap rate is higher.
In practical terms, for the same diameter and length:
- C7 cost index: 1.0
- C5 cost index: 1.3 – 1.6
- C3 cost index: 2.0 – 3.0
For a typical 25 mm diameter, 800 mm long screw, the C5 premium is roughly $30–$80 per piece in small quantities. For a machine with 4 axes, that is $120–$320 extra — usually less than 1% of the machine’s total cost. But for a high-volume automation product making 500 units a year, that same premium becomes a real line-item cost.
So the cost question is really: how many screws do you buy, and what is the cost of one failed part on your machine?
Common Misconceptions (and How to Avoid Them)
Buyers often confuse accuracy grade with other specs. Here are the three most common mix-ups we see in RFQs:
1. “C5 means less backlash.”
No. Backlash is controlled by preload, which is a separate spec. You can have a C7 screw with zero backlash (preloaded nut) and a C5 screw with backlash (no preload). If you need bidirectional positioning without play, specify preload separately.
2. “C5 is always ground.”
Not true. Some manufacturers produce C5 with precision rolling and then measure and match nuts to compensate. The process matters less than the final measured lead error. Always ask for the inspection report, not the process description.
3. “Higher grade = higher load capacity.”
No. Load capacity is determined by the ball diameter, number of circuits, and nut design — not the accuracy grade. A C7 screw can carry the same load as a C5 of the same size.
How to Specify Correctly (A Simple Checklist)
When you send an RFQ, include these five items. It will save you and the supplier a round of clarification emails:
- Accuracy grade — C5 or C7 (or C3 if you truly need it).
- Lead error requirement — state the per-300 mm tolerance you expect, and the cumulative error over full travel.
- Preload / backlash spec — e.g., “zero backlash, light preload 3–5 µm” or “no preload, backlash acceptable.”
- Travel length and total screw length — including the thread length and any unmachined ends.
- Mounting configuration — fixed-fixed, fixed-supported, or fixed-free. This affects thermal growth and effective lead error in operation.
Decision Framework: C5 or C7?
Use this quick matrix to make the call in under a minute:
| Your Machine Type | Positioning Tolerance Needed | Recommended Grade |
|---|---|---|
| CNC milling / drilling / grinding | ±0.01 – 0.03 mm | C5 (or C3 for finishing machines) |
| Laser cutting / waterjet | ±0.03 – 0.05 mm | C5 |
| Pick-and-place / transfer axes | ±0.1 – 0.5 mm | C7 |
| Lifting / clamping / feeding | ±0.5 mm or looser | C7 |
| Measuring / inspection stage | ±0.005 mm or tighter | C3 or C5 with compensation |
One more consideration: thermal growth. Even a C5 screw will shift position if the machine heats up. For high-duty cycles, you may need a cooling system or a compensation table in the CNC controller, regardless of grade.
Final Word: Buy the Grade Your Machine Needs
The C5 vs C7 decision is not about quality — it is about matching the screw to the machine’s error budget. Over-specifying wastes money; under-specifying costs you rework and customer trust.
If you are unsure, send us your application details — travel length, load, duty cycle, and the positioning tolerance you are targeting. We can recommend a grade and provide the measured lead error data so you can verify it before committing.
We supply both C5 and C7 ball screws in diameters from 12 mm to 63 mm, with custom machining for the shaft ends. Inspection reports are available upon request for every batch.
Need a quote or a technical discussion? Contact our engineering team with your drawing or your target specs — we typically respond within one business day.