Tolerance Classes for Stainless Steel Bar: h7 to h11, k12, k13 and the Fits
A tolerance class is the code on a drawing or a purchase order that fixes how far the actual diameter of a bar may depart from its nominal size. For stainless steel bar the codes come from ISO 286: a letter that sets where the tolerance band sits relative to nominal, and a number that sets how wide the band is. The classes ordered on bright bar in India are h7 to h11 for drawn and ground bar, k12 and k13 for peeled and turned bar, and j6, f7 and f8 for shafting. Hot rolled bar is not ordered to an ISO class at all; its tolerance is a plus-and-minus allowance from EN 10060 or ASTM A484. This article sets out what each code means in millimetres, which finishing route produces it, and how to state it on an order.
How an ISO 286 tolerance class is built
ISO 286-1 defines twenty standard tolerance grades, IT01 to IT18, each a width in micrometres that grows with the nominal size. IT7 on a 40 mm diameter is 25 micrometres; IT11 on the same diameter is 160 micrometres; IT13 is 390. The grade is the second half of every class code. The first half, the letter, is the fundamental deviation: the position of the band relative to the nominal size. For external dimensions such as a bar diameter the letters are lower case. The letter h places the band entirely below nominal, with the upper deviation at zero: an h9 bar of 40 mm nominal measures between 40.000 and 39.938 mm. The letter k, for grades 8 and above, places the band entirely above nominal, with the lower deviation at zero: a k12 bar of 40 mm measures between 40.000 and 40.250 mm. The letter j straddles nominal by a few micrometres either side, and the letter f sits wholly below nominal with a clearance that grows with size, which is why j6, f7 and f8 are the fits called for on shafts that run in bearings and seals.
Deviations in millimetres
| Nominal diameter (mm) | h7 | h8 | h9 | h10 | h11 | k12 | k13 |
|---|---|---|---|---|---|---|---|
| over 6 to 10 | 0 / -0.015 | 0 / -0.022 | 0 / -0.036 | 0 / -0.058 | 0 / -0.090 | 0 / +0.150 | 0 / +0.220 |
| over 10 to 18 | 0 / -0.018 | 0 / -0.027 | 0 / -0.043 | 0 / -0.070 | 0 / -0.110 | 0 / +0.180 | 0 / +0.270 |
| over 18 to 30 | 0 / -0.021 | 0 / -0.033 | 0 / -0.052 | 0 / -0.084 | 0 / -0.130 | 0 / +0.210 | 0 / +0.330 |
| over 30 to 50 | 0 / -0.025 | 0 / -0.039 | 0 / -0.062 | 0 / -0.100 | 0 / -0.160 | 0 / +0.250 | 0 / +0.390 |
| over 50 to 80 | 0 / -0.030 | 0 / -0.046 | 0 / -0.074 | 0 / -0.120 | 0 / -0.190 | 0 / +0.300 | 0 / +0.460 |
| over 80 to 120 | 0 / -0.035 | 0 / -0.054 | 0 / -0.087 | 0 / -0.140 | 0 / -0.220 | 0 / +0.350 | 0 / +0.540 |
The table is read as upper deviation / lower deviation from nominal. Every h class shares the upper limit of zero, so a bar to any h class never measures over size; the classes differ only in how far under size the bar may run. The k12 and k13 classes of EN 10278 invert this: the bar never measures under size, and the plus allowance is there so that the customer, or the works, can grind the bar down to an h class afterwards without running out of material.
Which finishing route gives which class
EN 10278, the European standard for the dimensions and tolerances of bright steel products, ties the classes to the process that produces them. Cold drawing pulls the annealed and pickled hot rolled bar through a die and holds h9, h10 or h11 on diameter with a surface roughness of roughly 0.8 to 1.6 micrometres Ra. Turning and peeling remove the rolled skin with a cutting tool and are supplied to k12 or k13, or to h11 where the order requires it. Centreless grinding takes a drawn or peeled bar to h6, h7, h8 or h9 and brings the surface to 0.2 to 0.4 micrometres Ra. Polishing to K240 or K320 grit changes the surface appearance and not the tolerance class. A class tighter than the route can hold is not a specification but a request for a second operation, and it is priced as one.
| Route | Classes held | Typical surface roughness (Ra) | Ordered when |
|---|---|---|---|
| Hot rolled (black) | EN 10060 or ASTM A484 allowance, not an ISO class | Rolled scale | Every surface is machined by the customer |
| Peeled or smooth turned | k12, k13 (h11 on request) | 1.6 to 3.2 micrometres | The bar is ground later, or a skin-free surface is enough |
| Cold drawn | h9, h10, h11 | 0.8 to 1.6 micrometres | General machining stock, shafts without a fit requirement |
| Centreless ground | h6, h7, h8, h9 | 0.2 to 0.4 micrometres | Bearing seats, seal surfaces, parts used as supplied |
| Ground to a fit | j6, f7, f8 | 0.2 to 0.4 micrometres | Pump and marine shafting running in a bore |
Hot rolled bar: allowances, not classes
A hot rolled bar comes off the mill with its scale on and is sold to the dimensional tolerances of EN 10060 (metric) or ASTM A484 (inch and metric tables), which are expressed as a plus-and-minus allowance about nominal that widens with diameter, plus an ovality limit. A 50 mm hot rolled bar under EN 10060 may run about 0.8 mm either side of nominal; a bright bar of the same size to h9 runs 0.062 mm under and nothing over. The hot rolled allowance exists because the bar will be machined, and the machining allowance the customer adds, typically 1.5 to 3 mm on diameter for a rolled bar, is what makes the wider tolerance acceptable. Straightness under both standards is stated separately, usually as a maximum deviation per metre or per length, and bright bar is straightened again after finishing to 0.5 mm per metre or better.
Stating the class on an order
An order line that reads "50 mm bright bar, 304" is not complete, because bright bar is supplied to four or more different classes and the price differs between them. A complete line states the nominal size, the grade with its standard, the finishing route or the class, and the length: "50 mm diameter, AISI 304 to ASTM A276, cold drawn h9, 3 to 6 metre random lengths". Where a drawing exists, the class on the drawing governs and the order should quote it. Where the customer will grind the bar, the order states k12 or k13 and the grinding allowance is then guaranteed. Where the bar goes into a bearing, the fit class from the bearing manufacturer's table (j6 or k6 for most rolling bearings on a rotating shaft) is stated, and the works grinds to it.
Three things a class does not fix. It does not fix the surface roughness, which is stated separately in micrometres Ra when it matters. It does not fix the straightness, which is stated per metre or per length. And it does not fix the delivery condition or the mechanical properties, which come from the grade, the standard and the heat treatment named on the certificate, not from the diameter.
Classes supplied on Laxcon bar
Laxcon Steels supplies stainless steel bright bar from 5 to 115 mm to h7, h8, h9, h10, h11, k12 and k13 and to ASTM A484, with straightness to 0.5 mm per metre and surface finish to 0.2 micrometres Ra. Precision shaft quality bar from 8 to 81 mm is ground to h7 to h9 and to the j6, f7 and f8 fits, with straightness to 0.015 inch per 10 feet. Hexagon and square bright bar are drawn to h11 or supplied to k12 and k13. Hot rolled round bar from 16 to 125 mm is supplied to EN 10060 or ASTM A484. The deviation table for each product's own size range is printed on its page.
Sources
- ISO 286-1, Geometrical product specifications, ISO code system for tolerances on linear sizes, part 1: basis of tolerances, deviations and fits.
- ISO 286-2, part 2: tables of standard tolerance classes and limit deviations for holes and shafts.
- EN 10278, Dimensions and tolerances of bright steel products.
- EN 10060, Hot rolled round steel bars for general purposes, dimensions and tolerances on shape and dimensions.
- ASTM A484/A484M, Standard Specification for General Requirements for Stainless Steel Bars, Billets, and Forgings.