Round Bar Tolerances: Hot Rolled, Peeled and Precision Ground

Why tolerance is a purchasing decision

A round bar is rarely used exactly as delivered. Tolerance decides how much metal you must remove to reach a clean, true size, how the bar behaves in a chuck or collet, and whether it can go straight into a bearing seat or a sliding fit. Three finishing routes cover most of the market.

Hot rolled (EN 10060)

Bars rolled to size at the mill, delivered with mill scale.

• Diameter tolerance is generous — on the order of ±0.8 mm on a 50 mm bar in the standard class, with out-of-roundness allowed up to roughly three quarters of the tolerance band.
• Surface carries scale and some decarburization; straightness and surface defects are mill-standard.
• Cheapest per tonne, and correct when the part will be machined all over with generous allowance.

EN 10060 also offers tighter tolerance classes on request, but they must be named in the order.

Peeled or turned (EN 10278)

The bar is machined on a peeling machine or lathe, removing the scale and decarburized skin.

• Diameter tolerance typically to h11, sometimes h10: on a 40 mm bar h11 means 0 / −0.16 mm.
• Bright, scale-free surface; the decarb layer is gone, so hardness after heat treatment is consistent.
• The standard choice for shafts that are machined further but cannot tolerate scale or deep surface defects.

Cold drawn and precision ground

Cold-drawn bars (EN 10278, typically h9–h11) are pulled through a die for a bright, closely sized finish. Centerless-ground bars go a step further:

• Ground bars are routinely supplied to h9, h8 or tighter — on a 40 mm bar, h9 is 0 / −0.062 mm and h8 is 0 / −0.039 mm (ISO 286, 30–50 mm step).
• Roundness and surface finish (often Ra 0.4 µm or better) suit bearing seats, piston rods and parts used with little or no further machining.
• Highest price per tonne, but often the lowest finished-part cost when machining time is counted.

What each step buys

Route	Typical tolerance	Surface	You pay for
Hot rolled	±0.5–1.5 mm range	Scaled	Raw material only
Peeled/turned	h11 (h10)	Bright, decarb-free	Scale removal, truer size
Cold drawn	h11–h9	Bright, smooth	Size, straightness
Ground	h9–h6	Precision finish	Fit-ready dimensions

Specifying tips

• Always order bright bars to a named standard and tolerance class (e.g. EN 10278 h11) — "bright bar" alone guarantees nothing.
• Remember ISO fits are one-sided: an h9 shaft is never oversize, so design the mating bore accordingly.
• Tolerance is diameter-step dependent: the same h9 class allows more deviation at 100 mm than at 40 mm.