What do h9, h10 and h11 mean on a bright bar?

h9, h10 and h11 are ISO tolerance classes that set how far a bright bar's diameter may fall below its nominal size. The h means the bar can never be oversize, only undersize. The number sets the width of the band: h9 is the tightest of the three and h11 the widest. On a 16 mm bar, h9 allows up to 0.043 mm undersize, h10 up to 0.070 mm and h11 up to 0.110 mm.

Tolerance is one of the lines most often left off a bright bar order, and one of the most expensive to get wrong. This guide explains how to read the class and how to choose one.

How to read a tolerance class

A class has two parts. The letter sets where the band sits relative to the nominal size. For bars, h is almost always used: the upper limit is the nominal size itself, so the bar is never larger than stated. The number is the IT grade, which sets how wide the band is. The band also grows with diameter, so an h9 bar of 40 mm has a wider band than an h9 bar of 10 mm.

Tolerance table by diameter

Maximum undersize allowed, in millimetres. The upper limit is always the nominal size.

Values per ISO 286.

Which process gives which class?

The finishing process decides which classes are practical. Cold drawn bars are commonly supplied in h9 to h11, depending on size and grade. Where a tighter class such as h8 or h7 is needed, the bar is usually ground after drawing. Asking for a class the process cannot hold economically adds cost without adding value.

How to choose the right class

Start from what happens to the bar after delivery.

If it is used as supplied, as a pin, a guide rod or a shaft that runs in a bush without further machining, the class on the bar is the class on the part. Choose the one your drawing calls for, typically h9 or tighter.

If the bar will be turned or ground anyway, the class matters much less. An h11 bar is cheaper and leaves enough stock for the final operation.

On automatic lathes, a consistent class helps bars feed and clamp reliably, even if the part is machined.

The rule of thumb: specify the tightest class you need, not the tightest class available.

Tolerance is not just diameter

A complete specification also covers straightness, ovality and length tolerance. A bar can be within its diameter class and still cause trouble if it is bowed or cut short. State these on the order if they matter to your process.

Frequently asked questions

• What does h9 mean?

  The bar may be at its nominal size or slightly under it, never over, within the IT9 band for its diameter.

  
  

• Is h9 tighter than h11?

  Yes. A smaller number means a narrower band.

  
  

• Does a tighter class cost more?

  Generally yes, because it needs closer process control, more inspection and sometimes an extra grinding step.

  
  

• What class should I put on my purchase order?

  The class your drawing requires for the finished part, or h11 if the bar will be fully machined.

  
  

• Need a specific size and class?

  WhatsApp us at +91 70653 37337 with the diameter, grade and class. SS Engineering Works has drawn bright bars in Sonipat since 1968, and we will tell you straight away whether we run it.