C7 or C5: What the Tolerance Grade on Your Drawing Actually Costs
Most of the grade problems that reach us are not manufacturing problems. They were settled by accident — a C7 copied from the last drawing, a model number sent out with no grade at all, a quotation compared with one that never answered the same question.
Choosing a precision ball screw supplier usually starts with price, which is the wrong end of the decision. The grade on the drawing decides both what you pay and what the axis will hold. Get it wrong either way and the cost returns: scrap and rework on one side, or a purchase order paid for accuracy the machine never uses on the other.
The technical point, in brief
Rolling forms the thread by displacing material and normally delivers C7 or C10. Grinding cuts the thread on an already-hardened blank and normally reaches C5 and tighter, though C5 can also be reached by rolling and then grinding. Measured over 300 mm, the allowance runs about 0.05 mm at C7 against 0.018 mm at C5. Whether your axis needs the second number depends on how it closes its position loop, not on the machine type.
Three ways the grade gets lost between engineering and purchasing
1. A model number leaves the building without a grade on it. Engineering specifies a size; purchasing orders a size. The supplier quotes what the number describes — a rolled C7 or a ground C5 for the same four digits, at very different prices. The question was never asked.
2. Two quotations, two standards, one word. A drawing saying "C5" permits 18 µm over 300 mm under JIS B 1192, while a supplier quoting ISO grade 5 permits 23 µm for the same two characters — about 28% more permitted error behind one label. If neither document names the standard, both are correct and the comparison means nothing.
3. A grade bought to fix a machine problem. Tight lead accuracy does not correct thermal growth, frame deflection, coupling wind-up or nut clearance. The expensive version: a C3 screw on an axis that loses more than that to shop temperature before the first part.
What to settle before the RFQ goes out
These apply to any supplier, including us.
The grade and the standard, named. Not "precision", not "standard grade". Write both on the drawing.
The reference length, and the figure at your stroke. Grades are defined over 300 mm; your axis is longer and has its own row in the table.
The preload or clearance class, separately from the grade. Grade covers lead error only. Backlash and repeatability are set by the nut — a different line on the drawing.
Whether the loop closes on a screw or on a scale. This one answer usually decides between rolled and ground — worth telling the supplier rather than letting them infer from the machine type.
The length, and whether that grade exists in that length. Ground screws are constrained by the grinding machine; a C5 requirement on a long shaft is worth checking before the design is frozen.
How the delivered part will be verified. Agree in advance what the inspection record shows — lead deviation over which length, at what temperature — and what happens if it falls outside. Cheap now, awkward later.
The same question, answered differently by industry
- CNC machining. Long travel and high traverse rates usually make speed and stiffness the binding constraints; grade matters, but a scale-equipped axis rarely needs the tightest one.
- Semiconductor and inspection. Moderate strokes with tight repeatability and smoothness: the per-revolution behaviour of the screw matters more than its 300 mm figure.
- Medical and laboratory automation. Short fast cycles and longer gantry moves in one machine — often two different grades on two axes of the same build.
- General automation and material handling. Positioning is often loose enough that C7 is the sensible purchase, with the money better spent on the guide and frame.
None is a rule; the error budget still decides.
What we can work with you on
Dongguan Pinsi Machinery makes ball screws, linear guides, cross roller guides and linear modules, and supplies rolled and ground screws. On the commercial side:
- Grade and length confirmed in writing against your drawing before the order — including when the honest answer is that the drawing asks for more than the axis needs.
- Custom lengths and machined ends to your drawing, with support units matched where the application needs them.
- Inspection records on request — dimensional and accuracy data.
- Factory verification — we welcome customer factory visits and customer audits.
If the drawing is ambiguous, we will say so before we quote rather than price an assumption.
We are happy to answer company-verification questions directly; our registered address and contact details are on our contact page.
Questions we get asked before the RFQ goes out
Can you tell us which standard your grade figure is against? Yes. We state the grade and the standard on the quotation, and we will ask if your drawing names only one. A figure without its standard cannot be checked on delivery.
Do you supply ground screws as well as rolled? Both. The grade and length available for a given diameter are confirmed against your drawing before the order, in writing.
How is the delivery date set? It depends on the size, the grade, whether the length is stock or made to order, and whether custom end machining is required. We put the specification and the schedule in writing before production starts.
Send the drawing, or come and check
If you are choosing a grade, send the stroke, the positioning and repeatability requirements, the mounting arrangement and whether the axis runs closed loop on a scale. If the grade is already decided, send the drawing and we will confirm what we can hold.