Tolerances and surface roughness in CNC turning — what really costs

Drawings of shafts and bushings regularly feature ±0.01 mm on every diameter and Ra 0.8 "to be safe". The problem is that tolerances in CNC turning translate directly into cycle time, the number of operations and the scope of inspection. Every unnecessarily tightened requirement raises the part price while changing nothing functionally — a rotational part does not work better because its free surfaces are as accurate as a bearing journal.

From this article you will learn which IT grades a CNC lathe realistically achieves, which Ra values the individual machining operations deliver, how to read a shaft–hole fit using H7/g6 as the example, and how to tolerance the drawing of a turned part so that you pay for function, not for looks.

IT grades — what CNC turning realistically achieves

The IT grade defines the width of the tolerance field for a given nominal dimension. The lower the number, the narrower the field and the harder the machining. For orientation: at a diameter of 25 mm, grade IT7 is a field of 21 µm, and IT6 — 13 µm. The difference looks unremarkable, but technologically it is often the boundary between one operation and two.

Typical, textbook capabilities of the individual machining operations for rotational parts (indicative values):

• rough turning: IT11–IT13,
• finish turning: IT7–IT9,
• precision turning under favourable conditions: IT6,
• grinding: IT5–IT6,
• lapping and honing: IT4–IT5.

The economic limit of CNC turning usually lies around IT6–IT7. Below that, abrasive operations come in — above all grinding — because holding micrometre-level tolerance fields with a cutting edge alone becomes a lottery: the influence of insert wear, part temperature and part rigidity grows.

What can be held on a lathe also depends on geometry. A slender shaft (a high length-to-diameter ratio) deflects under tool pressure, a thin-walled bushing springs in the chuck, and a part heated by roughing "walks" dimensionally after cooling down. That is why the same IT grade can be routine on a short, rigid part and a challenge on a long one.

Surface roughness Ra — typical values for turning and grinding

Roughness after turning results mainly from the feed and the insert nose radius. If you want a lower Ra, you reduce the feed — and the finishing pass takes longer. As a rough guide: going from Ra 3.2 to Ra 0.8 by feed alone can lengthen the finishing pass several times over, and below a certain limit the feed alone is no longer enough.

Typical, textbook Ra ranges for rotational parts:

Treat the values as indicative — the actual result depends on the material, the tool and the stability of the setup. The practical conclusion matters more: Ra 1.6–3.2 comes included in the price of ordinary finish turning, whereas Ra 0.4 is almost always an additional grinding operation. If such a callout sits on a surface that seals nothing and slides on nothing, you are paying for looks.

The H7/g6 fit — a practical shaft–hole example

Rotational parts rarely work alone. A shaft goes into a bushing, the bushing into a housing — and then, instead of individual tolerances, what counts is the fit. The classic example of a loose sliding fit is H7/g6.

For a nominal diameter of 25 mm it looks like this: a 25H7 hole may measure 25.000–25.021 mm, and a 25g6 shaft — 24.980–24.993 mm. The clearance between them will always be between 0.007 and 0.041 mm. The shaft can slide and rotate in the hole without perceptible play — a typical solution for sliding bushings, sliding gears or guiding elements.

Two things matter for the cost. First, g6 is grade IT6 on the shaft — that is, around the limit of turning capability; in practice, often precision turning or grinding of the journal. Second, an H7 hole in the bushing requires reaming or accurate boring plus accurate gauges. The single callout "H7/g6" on a drawing therefore sets the technology for both parts. How to describe such a set in an enquiry is shown in the post how to prepare a shaft or bushing for a turning quote.

Drawing requirement → technology → cost

The table below organises how typical requirements for turned parts translate into technology and cost. The cost scale is indicative — it shows the mechanism, not a price list.

The most expensive things are not single tight tolerances but their combinations: a tight diameter plus low Ra plus runout relative to a distant datum. The part then travels through several operations and several setups, and every re-clamping means risk and time. A fuller accounting of accuracy across machining as a whole can be found in the article how much accuracy costs in CNC machining.

Inspection is a separate line item. A dimension in grade IT9 can be checked with a calliper, IT7 already requires a micrometer, and IT5 — a dial indicator, plug gauges or a coordinate measuring machine and a temperature-stabilised part. With a batch comes measurement frequency: the narrower the tolerance field, the more often you have to measure to catch dimensional drift from a wearing insert in time. That time also sits in the part price, even though you cannot see it on the drawing.

Tolerance the function, not the looks

The rule is simple: a tolerance should follow from what the surface does, not from how it should look. In practice, a few rules work well for turned parts:

• leave non-critical dimensions to general tolerances (e.g. ISO 2768-m) — that is what they exist for,
• tighten only mating surfaces: journals, fitted holes, surfaces for seals and bearings,
• write fits as symbols (e.g. H7/g6) instead of inventing your own tolerance fields,
• specify Ra where the surface slides, seals or makes contact — not globally across the whole part,
• tie runout and coaxiality to a real assembly datum, not to a random surface,
• state in the enquiry what the part does — the manufacturer will suggest where a tolerance can be relaxed.

A good practice is a short review of the drawing before sending the enquiry: count how many dimensions have a tolerance tighter than IT8. If more than a few — check each one individually to see whether it really follows from the function. Most often, the dimensions tightened "just in case" are free diameters, non-functional lengths and chamfers — and those are exactly the items that are easiest to recover in the price with zero technical risk.

If, after this selection, requirements below IT6 or Ra 0.4 still remain on the drawing, it is a sign that the part genuinely needs abrasive operations. Exactly when it is worth planning them is described in the post precision grinding — when it is necessary.

Summary

CNC turning with no surcharges typically delivers IT7–IT9 and Ra 1.6–3.2. The economic boundary lies around IT6 and Ra 0.8 — below it, grinding begins, meaning an extra operation and a clear price jump. Write fits as symbols, tighten tolerances only on functional surfaces, and leave the rest to general tolerances. That is what a drawing you do not overpay for looks like.

Do you have a drawing of a shaft or bushing and do not know which requirements are driving up the price? Send it via the contact form — you will receive a quote within 48 hours along with information about which tolerances genuinely require grinding and which can be relaxed without risk.