The limits, by diameter
Each figure is how far under its nominal size the bar may be, in millimetres. The upper limit is always zero.
| Bar diameter | h6 | h7 | h8 | h9 | h10 | h11 | h12 |
|---|---|---|---|---|---|---|---|
| Up to 3 mm | 0.006 | 0.010 | 0.014 | 0.025 | 0.040 | 0.060 | 0.100 |
| Over 3 to 6 mm | 0.008 | 0.012 | 0.018 | 0.030 | 0.048 | 0.075 | 0.120 |
| Over 6 to 10 mm | 0.009 | 0.015 | 0.022 | 0.036 | 0.058 | 0.090 | 0.150 |
| Over 10 to 18 mm | 0.011 | 0.018 | 0.027 | 0.043 | 0.070 | 0.110 | 0.180 |
| Over 18 to 30 mm | 0.013 | 0.021 | 0.033 | 0.052 | 0.084 | 0.130 | 0.210 |
| Over 30 to 50 mm | 0.016 | 0.025 | 0.039 | 0.062 | 0.100 | 0.160 | 0.250 |
| Over 50 to 80 mm | 0.019 | 0.030 | 0.046 | 0.074 | 0.120 | 0.190 | 0.300 |
| Over 80 to 120 mm | 0.022 | 0.035 | 0.054 | 0.087 | 0.140 | 0.220 | 0.350 |
| Over 120 to 180 mm | 0.025 | 0.040 | 0.063 | 0.100 | 0.160 | 0.250 | 0.400 |
| Over 180 to 250 mm | 0.029 | 0.046 | 0.072 | 0.115 | 0.185 | 0.290 | 0.460 |
h6 to h8 are grinding tolerances. h9 to h11 cover most drawn and ground bar. h12 is used on larger drawn hexagon and square.
Smallest size, for common bar diameters
The largest size is the nominal diameter. These are the smallest sizes the bar may measure.
| Nominal diameter | h9 smallest (mm) | h10 smallest (mm) | h11 smallest (mm) |
|---|---|---|---|
| 6 mm | 5.970 | 5.952 | 5.925 |
| 8 mm | 7.964 | 7.942 | 7.910 |
| 10 mm | 9.964 | 9.942 | 9.910 |
| 12 mm | 11.957 | 11.930 | 11.890 |
| 16 mm | 15.957 | 15.930 | 15.890 |
| 20 mm | 19.948 | 19.916 | 19.870 |
| 25 mm | 24.948 | 24.916 | 24.870 |
| 30 mm | 29.948 | 29.916 | 29.870 |
| 40 mm | 39.938 | 39.900 | 39.840 |
| 50 mm | 49.938 | 49.900 | 49.840 |
| 60 mm | 59.926 | 59.880 | 59.810 |
| 80 mm | 79.926 | 79.880 | 79.810 |
| 100 mm | 99.913 | 99.860 | 99.780 |
Which class the bar arrives at
The classes come from BS EN 10278, the standard for the dimensions of bright steel products. These are the usual ones.
| Bar form | Usual class | Note |
|---|---|---|
| Cold drawn round | h10 | h9 and h11 are also supplied: check the quote |
| Drawn hexagon and square | h11 | h12 above 80 mm across flats |
| Drawn flat | h11 | On width and thickness |
| Precision ground round | h9 | h8, h7 and h6 to order |
| Turned round | By agreement | Ask the stockholder for the class |
| Black (hot rolled) bar | Not an h class | Looser, with mill scale: machine all over |
The class can also vary with the grade and the heat treatment condition, so confirm it on the stockholder's quote. The engineering steel bar page lists who stocks bright and ground bar.
What it means on the drawing
- If a diameter can accept the bar's own tolerance, draw it at a stock bar size and leave it as supplied. The turning operation comes off the price.
- If the diameter runs in a plain bush or carries a seal, check the class against the fit. Ground bar to h9 suits many of these as supplied.
- If the diameter needs a fit such as k6 or p6, stock bar cannot meet it. Those fits sit above nominal size, so the bar is bought oversize and turned or ground.
- If the bar is black, allow machining stock on every surface that matters.
- Put the class on the order and ask for it to be confirmed on the quote.
TrueNorth Engineering's free DFM review reads every drawing before the quote goes out, which is the point to ask whether a diameter can stay at bar size. The machining tolerances guide covers what milling and turning hold.
Four common fits, at 25 mm
The same ISO 286 system sets fits between a shaft and a hole. With an H7 hole, which is 25.000 to 25.021 mm, these are the four shaft classes a drawing most often pairs with it.
| Fit | Shaft size | Type | Result |
|---|---|---|---|
| H7 / g6 | 24.980 to 24.993 mm | Clearance | 0.007 to 0.041 mm clearance |
| H7 / h6 | 24.987 to 25.000 mm | Clearance | 0 to 0.034 mm clearance |
| H7 / k6 | 25.002 to 25.015 mm | Transition | 0.015 mm interference to 0.019 mm clearance |
| H7 / p6 | 25.022 to 25.035 mm | Interference | 0.001 to 0.035 mm interference |
Only the h shaft matches stock bar. The others are machined or ground to size.
A worked example: one line that saves an operation
A drawing shows a pin of 20 mm diameter with a tolerance of plus nothing, minus 0.05 mm. Ground bar to h9 measures 19.948 to 20.000 mm, so it meets the drawing as supplied. Drawn bar to h10 may be as small as 19.916 mm and does not.
Ordered as ground bar, the pin needs cutting to length and its end features machined. Ordered as black bar, the whole diameter has to be turned. The part is the same. The price is not.