Don’t know your k9 from your h11? Then read on….
All steel bar is purchased to a nominal size (meaning the thickness or diameter depending on whether the bar is flat, square, round, hexagonal, etc). However, due to various types of finishing process that can be used the actual size of most bars will vary. The limits for this variation will often be stated in the specification to which the material has been purchased. For the allowed tolerances for “bright” Round Bar published by the International Standard Organisation (ISO), these tolerances are denoted by a letter followed by a one- or two-digit number.
In simple terms, tolerances beginning with the letter ‘F’ or ‘H’ are always negative and ‘K’ are either dead-on or positive but never negative. Tolerances starting with a letter ‘J’ can be either. The below table examples are for Round Bar.
Example: ISO H (tolerances are always negative)
Example ISO K (never to a negative tolerance)
Why does it matter? Well you do not want to buy more metal that you need, but if you are machining and finishing at or near the size you are buying, you will be safer buying to a K tolerance.
As we supply to many machinists we generally buy from mills with k tolerances, although if producing material by extra processing, such as cold drawing F55 for example, then we will aim to meet the tolerance given us by our customer.
As mentioned at the start, ISO have also published tables under letters F (always negative) and J (either positive or negative). If you require a copy of these in addition to the ones above, please let us know.