Tolerance classes on a bright bar specification (h6, h7, h9, h11...) follow the ISO system of limits and fits. The "h" indicates the tolerance is applied entirely below the nominal size — meaning the bar will always measure at or under the stated diameter, never over.
What the Number Means
The number after "h" (6, 7, 8, 9, 10, 11) defines the width of the tolerance band, known as the IT grade. Lower numbers mean tighter tolerance:
- h6 – h7: Very tight tolerance, typically achieved only through centerless grinding. Used for precision sliding fits, hydraulic rods and shafts running in bushings or bearings.
- h8 – h9: Moderate tolerance, achievable through fine cold drawing or grinding. Suitable for general precision components.
- h10 – h11: Wider tolerance, typical of standard cold-drawn bright bars. Adequate for general engineering use where the bar will be machined further.
Why Tighter Tolerance Costs More
Achieving h6 or h7 requires an additional centerless grinding pass beyond cold drawing, along with more frequent in-process measurement. That's additional processing time and equipment wear — which is reflected in cost. Specifying a tighter tolerance than your application actually needs adds cost without adding value.
A Simple Way to Decide
Ask: does this surface directly form a fit, seal, or bearing surface in the final assembly?
- If yes, and it won't be machined further before assembly — specify a ground bar at h6–h8.
- If the bar will be turned, milled, or otherwise machined before final assembly — a drawn bar at h9–h11 is usually sufficient, since the critical surfaces will be finish-machined anyway.
Still unsure which tolerance class fits your part? Talk to our team — we're happy to review your drawing and recommend the most cost-effective option.