Bright bar is normally offered in three tolerance bands: h9, h10 and h11. The notation comes from ISO 286, and once the two halves are separated it is straightforward.
The letter sets the position
A lower-case "h" means the upper deviation is zero. The bar is never larger than the nominal size — it lies between nominal and nominal minus the tolerance. A 20 mm h9 bar is between 19.948 and 20.000 mm; it is never 20.01.
This is exactly what a machine shop needs. A bar that is never oversize will always enter a collet, a bush or a guide of nominal size. If bars could be oversize, every bar feeder setup would need clearance for the worst case.
The number sets the width
The number is the IT grade — the width of the tolerance band. A lower number is a tighter band. Critically, the band widens with size: holding 50 µm on a 6 mm bar is a far tighter proportional control than on a 60 mm bar, so ISO defines the band by size range.
- At 20 mm: h9 is 52 µm, h10 is 84 µm, h11 is 130 µm.
- At 50 mm: h9 is 62 µm, h10 is 100 µm, h11 is 160 µm.
- At 6 mm: h9 is 30 µm, h10 is 48 µm, h11 is 75 µm.
Choosing the right band
- h9 — where the bar is used with minimal machining, or where a close running or locational fit is required. Typical for hydraulic piston rods and precision shafting.
- h10 — the general purpose bright bar tolerance. Suits most turned components and bar-fed automatic work where a finishing cut follows.
- h11 — economical, for general engineering and fabricated work where the bar is machined all over or fit is not critical.
The cost question
Tighter tolerance costs more — more careful die management, more frequent verification, more material rejected at the band edges. That cost is worth paying only when your process actually uses the precision.
If your first operation turns the diameter down anyway, h9 buys you nothing over h11. If you are using the as-drawn surface as a running fit or a location, h9 may be the cheapest option overall because it removes an operation. The band should be chosen from your process, not from a general preference for precision.
Written by the technical team at Kalika Enterprises, Waluj MIDC. Guidance here is general; the values that govern your part are those on the test certificate supplied with your consignment.