Guide
When Does a Shaft Tolerance Force You to Grind Instead of Turn?
Shaft tolerance is the limit on a shaft’s diameter, written under ISO 286 as a letter and an IT grade such as h6 or g6. As typical shop routing, a turned diameter is held to IT7 or wider, while h6 and g6 are IT6 bands, which is where grinding usually enters the routing.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers who have written h6 or g6 on a shaft drawing and want to know whether the part can be turned or must be ground. It gives the band widths from ISO 286, the usual routing rule that shops apply to them, and the drawing and RFQ habits that keep the grinding step visible. The routing rule is general guidance and not the capability of any single shop; no process capability table and no cost percentages are given, because none applies to every part.
On this page
How wide are h6, g6 and h7 at common shaft sizes?
Per ISO 286, an h6 or g6 shaft band is 8 to 13 µm wide for sizes from 3 to 30 mm, and an h7 band is 12 to 21 µm. The letter fixes where the band sits and the number fixes how wide it is.
Table 1 gives the limits. A 10 mm shaft is the common check case: the RoyMech ISO 286-2 shaft table lists h6 in the over 6 up to 10 mm range as 0 to −9 µm, a 9 µm band. The band width is simply the upper deviation minus the lower one, which is how the last two columns are derived.
That distinction decides routing. g6 and h6 have the same width, so a g6 shaft is no harder to produce than an h6 shaft; the target simply sits lower in the band. Width is what the process has to hold, and width is what separates the turned range from the ground range.
| Nominal size (mm) | g6 limits (µm) | h6 limits (µm) | h7 limits (µm) | IT6 band (µm) | IT7 band (µm) |
|---|---|---|---|---|---|
| over 3 up to 6 | −4 to −12 | 0 to −8 | 0 to −12 | 8 | 12 |
| over 6 up to 10 | −5 to −14 | 0 to −9 | 0 to −15 | 9 | 15 |
| over 10 up to 18 | −6 to −17 | 0 to −11 | 0 to −18 | 11 | 18 |
| over 18 up to 30 | −7 to −20 | 0 to −13 | 0 to −21 | 13 | 21 |
What can CNC turning hold, and where does it stop?
As industry guidance, a well-set CNC lathe holds a diameter to about IT7 in steady production, and IT6 bands such as h6 and g6 are where cylindrical grinding usually takes over. Actual capability depends on size, material, workholding, tool condition and shop practice, so treat this as a routing default, not a limit.
The same rule of thumb runs through the guide to the cost of tight tolerances: each grade tighter takes away a large share of the band, and IT6 and finer on a diameter often points toward grinding. Table 1 shows why. Going from h7 to h6 at 10 mm takes the band from 15 to 9 µm per ISO 286-2, so the lathe loses well over a third of its working room.
Grinding is a finishing process. The Wikipedia article on grinding says normal grinding is used primarily to finish surfaces, takes very shallow cuts, and suits very hard materials, which is why it follows turning and hardening rather than replacing them.
This routing rule is general industry guidance, not a statement of what any one shop holds. At Saola the turned-feature tolerance is confirmed against your drawing in the written quote.
Is IT7 turned and IT6 ground a hard line?
No, it is a routing default for stable production, and a shop can sometimes hold an h6 on a small, short, stable part. The margin is thin and the inspection burden grows, so the safe assumption is that IT6 needs a finishing operation.
Measurement is part of the problem. Per ISO 286-1, the h6 band at 10 mm is 9 µm wide, so an ordinary gauge uncertainty takes a large share of it. A shop that claims h6 from the lathe should also be able to show how it gauges the diameter.
Other things push a shaft toward grinding even when the class looks reachable: a high length-to-diameter ratio that makes a slender shaft deflect (see the guide to runout and bent shafts), a surface finish finer than turning leaves, tight roundness, and hardening after turning, which distorts the diameter and has to be corrected by grinding afterward.
When do you actually pay for grinding?
When the function needs IT6 or finer: bearing seats, seal journals, close sliding fits in bushings and press-fit locating diameters. Shoulders, clearance diameters and threads usually stay turned.
Table 2 sorts typical callouts by route. The cheapest lever is placement: put the tight class only on the length that does the work, and leave the rest of the shaft on a general tolerance. The guide to the cost of tight tolerances and the guide to standard machining tolerances cover the general side.
At steady annual volumes, ask whether the ground feature can run on a centerless grinder. The Wikipedia article on cylindrical grinders describes through-feed centerless grinding as particularly efficient and easier to combine with automatic loading than centered grinding.
Grinding also leaves a finer, more even surface than turning, which is another reason to specify it when a surface finish is the real requirement rather than the diameter. Put the finish value on the drawing either way.
| Callout on the diameter | Usual route | Reason |
|---|---|---|
| IT8 to IT9 classes such as h8 or h9 | Turned | Inside the usual turning range |
| IT7 classes such as h7 or f7 | Turned, with first-article checks | At the edge of the usual turning range |
| IT6 classes such as h6 or g6 | Turned, then ground; or ground stock | Below the usual turning range, inside the grinding range |
| IT5 classes | Ground, sometimes honed or lapped | Needs the finest finishing processes |
| Any class, with hardening after turning | Ground after hardening | Heat treatment distorts a turned diameter |
How should the drawing and RFQ show the grinding step?
Put the tolerance class on the functional diameter only, say whether the part is ground, and ask the supplier to list the route. A drawing that implies grinding without saying so is where late surprises start.
- Callout: class or limits on the journal, with the length it applies to.
- Route: ask the quote to say whether each feature is turned only or turned and ground.
- Sequence: state whether grinding is before or after any heat treatment, and who leaves the grind stock.
- Datums: center holes allowed or not, since centered grinding uses them.
- Finish: surface texture on the ground diameter, with the method for checking it.
- Inspection: the gauge, the number of sections measured and any roundness requirement.
Saola turns shafts from bar at its lead CNC plant (CNC turning). Grinding and heat treatment are arranged through partner plants that Saola manages, and the quote shows grinding as its own routing step and names the production site and country of origin. Send the drawing through the quote form.
Can precision shafting stock replace the grinding step?
Often, for plain shafts: stock shafting is sold to h-class diameters, so only the ends and steps need machining. The limit is that the ground diameter survives only on the untouched length.
Precision shafting, such as the shafting sold for linear bearings, is commonly supplied ground to an h-class diameter. If your shaft is a plain cylinder with a few turned ends, ground stock can deliver the h6 diameter without a separate grinding operation; name the stock class on the drawing and ask the supplier to confirm it from the bar certificate.
It is a poor fit when you need steps, keyways or grooves along the length, because machining changes the stress in the bar and the ground diameter is lost wherever metal is cut. In that case the usual route is to turn with grind stock and grind the journals last.
Frequently asked
Can CNC turning hold h6?
As typical shop routing, standard turning is planned at IT7 and wider, and h6 is IT6. Some small, short parts may reach it, but the safe assumption is that h6 needs grinding or ground stock.
Does g6 need grinding more than h6?
No. g6 and h6 have the same width, so they are equally hard to hold; g6 only sits lower in the band, as Table 1 shows.
Why not call every diameter h6?
Because each tighter class adds an operation and an inspection burden, and most of a shaft is not functional. Use h6 or g6 only where the shaft fits a bearing, seal or bushing.
How much more does grinding cost?
No honest percentage applies to every part. Grinding adds a machine, a setup and an inspection step, so ask the supplier to price the part turned only and turned and ground, and compare the two.
What surface finish does grinding leave?
A finer and more even surface than turning, but the value depends on the wheel, the dressing and the material. Specify the finish you need rather than relying on the process.
Sources
- RoyMech, ISO 286-2 shaft limits, reference table, accessed October 2026
- Wikipedia, Grinding (abrasive cutting), reference article, accessed October 2026
- Wikipedia, Cylindrical grinder, reference article, accessed October 2026