Bright Bar Tolerance Selector — h8, h9, h10 or h11

Two facts decide a bright bar tolerance class: the diameter, and whether anything runs or seats on the bar surface. Enter them and the selector names the class, the finishing route that can actually hold it, and the real millimetre band.

What the selector asks for

• Bar diameter in mm (4 to 180 mm)
• What the bar has to do — bearing or seal seat, sliding fit, location fit, clearance, or machined all over
• Section: round, square, hexagon or flat
• Whether the part is hardened after machining
• Optionally, the class you were going to order — it will tell you if that is tighter or looser than the duty needs

How the class is reached

All h classes are unilateral minus — the bar is never oversize, only undersize by a permitted amount that widens with diameter. On a 20 mm bar h8 allows 0 to -0.033 mm, h9 allows -0.052 mm, h10 allows -0.084 mm and h11 allows -0.130 mm. At 50 mm the same classes open to -0.039, -0.062, -0.100 and -0.160 mm, per ISO 286-2 as applied by EN 10278.

The duty sets the class. If a bearing, seal or bush seats directly on the bar, or the bar slides in a bore, it needs h8 — and h8 means centreless ground, because drawing and turning cannot hold it. A location fit needs h9. A clearance hole needs h10. If every surface is machined away, the bar is only stock and h11 is correct — paying for ground bar there is money burnt.

Two rules override the duty. Square, hexagon and flat sections cannot be centreless ground at all, so h9 cold drawn is their tightest practical class. And if the part is hardened after machining, the bar tolerance is not the finished tolerance — quenching distorts and scales the surface, so you order a class looser and grind to size afterwards.

Finishing routes and what each holds

• Cold drawn — h9 to h11, up to 80 mm, 1.6-3.2 µm Ra. The cheapest bright finish.
• Smooth turned & polished — h10 to h11, to 160 mm, and the route that removes the decarburised skin.
• Centreless ground — h8 to h9, 0.4-0.8 µm Ra. The only reliable route to h8.

Before you order

The selector covers dimensional class and finish, not grade selection or straightness guarantees on long slender bar. Send the drawing, the fit it mates with and the heat treatment route to Stancor Bright Bars and our technical team will confirm the class, finish and certification scope.

For availability, mill test certificates and a quotation, contact Stancor Bright Bars.