Guide
How Do Cold-Drawn Bar Tolerances Affect a Turned Part's Diameter and Cost?
Cold-drawn bar tolerances are the limits a mill guarantees on a bar's diameter, roundness and straightness before any machining starts. The class matters because a turned part whose drawing diameter is looser than the bar's own tolerance can often keep the drawn surface as it is, while a tighter or smoother diameter needs a turning pass and stock to cut.
By Saola Manufacturing engineering team · Published · Updated
This guide is for engineers drawing small turned parts such as pins, shafts and bushings from round bar. It sets the ISO h9, h10 and h11 bands side by side, explains why the class a mill will sell you depends on the finish and the standard, and shows how to write the drawing and RFQ so the supplier does not have to guess which diameters are left as drawn.
A note on figures. The bands below are the ISO 286 shaft bands as tabulated by a steel stockholder, and two other stockist tables agree with them where they overlap. Which class a given bar finish is sold in varies between sources, so none of this replaces the mill's own quotation. This page gives no stock-allowance numbers, because none of the sources checked publish them.
On this page
What do h9, h10 and h11 mean on a bar diameter?
They are ISO 286 shaft tolerance bands that start at the nominal diameter and run downward, so the bar is never larger than nominal. Each step from h9 to h11 widens the band, and the band also grows with diameter.
The letter h means the upper limit is zero, so the whole band sits below nominal. The number is the width grade: a higher number is a wider, cheaper band. That matches how bright bar is sold, where the tolerance runs to the minus side only. PMPA, the precision machined products association, describes the same convention for cold-drawn steel bar, with tolerances held to the minus and nothing to the plus, which makes loading into workholding easier (PMPA, 2023).
The table puts the three bands next to each other for the diameters that small turned parts use most. Read across a row to see what a looser class costs you in diameter variation, and down a column to see how the band grows with size.
| Bar diameter range | h9 | h10 | h11 |
|---|---|---|---|
| Over 6 to 10 mm | 0 / -36 | 0 / -58 | 0 / -90 |
| Over 10 to 18 mm | 0 / -43 | 0 / -70 | 0 / -110 |
| Over 18 to 30 mm | 0 / -52 | 0 / -84 | 0 / -130 |
| Over 30 to 50 mm | 0 / -62 | 0 / -100 | 0 / -160 |
Two things stand out. At the same diameter, h11 is roughly two and a half times wider than h9, which is the difference between a diameter you can leave alone and one you have to cut. And because every band is below nominal, a drawing that asks for a plus-and-minus tolerance on a bar-finish diameter is asking for something the bar cannot center on.
Which bar finishes come in which class?
Drawn, turned and ground bar are each offered in a range of classes, and the sources disagree on which one is the usual default. Name the class on the order instead of assuming it.
The European standard for bright bar, EN 10278, covers products in the drawn, turned or ground condition delivered in straight lengths. The 2023 edition is current at BSI, and BSI describes its scope as mainly stainless steels and other special steels such as tool and bearing steels, with possible use for cold heading steel (BSI, BS EN 10278:2023). A supplier's summary of the standard gives h10 as the class for drawn round bar, with h11 for quenched and tempered grades, and lists turned round bar from h9 to h12 (Zhejiang Guanyu, accessed 2026).
A UK stockist table instead lists bright drawn bar at h9, smooth turned bar at h10 and ground bar at h8 (Right on Blackburns, accessed 2026), while the other summary says ground products are covered by h9. Those two statements cannot both be the default, which is why the class belongs on the purchase order.
US carbon and alloy steel bar follows a different system. ASTM A108 gives size tolerances for cold-finished round bar in inch tables, with separate tables for carbon and alloy grades. PMPA says the values depend on bar diameter, carbon content and any thermal treatment, run to the minus side only, and put the out-of-roundness limit at half the size tolerance (PMPA, 2023). So an ISO h-class on a drawing and an A108 table on a US mill order are two different promises; ask for the nearest equivalent in writing.
When can the bar's own diameter be the finished diameter?
When the drawing tolerance on that diameter is wider than the bar's class band with room for roundness and bow, and the surface is acceptable as drawn. Otherwise the diameter needs a turning pass.
PMPA lists the reasons mills and shops choose cold-drawn bar: the abrasive scale is gone, the bar is straighter and rounder, and the tight size and out-of-round control may reduce the processing needed. It also says the diameter and shape of the bar are often usable as they are for many mechanical components (PMPA, 2023). The decision table turns that into a drawing-review habit.
| What the drawing asks for on the diameter | Bar choice | Turn the diameter? |
|---|---|---|
| Tolerance much wider than the h11 band, no surface callout | Drawn bar, h11 or the mill's standard class | No, if bow and surface seams are acceptable |
| Tolerance close to the h10 or h9 band | Drawn bar in h10 or h9, ordered by class | Only if roundness or finish fails first-article checks |
| Tolerance tighter than the h9 band | Ground or turned-and-polished bar, or oversize bar | Yes, one finishing pass or a grinding step |
| Fine surface callout on the diameter | Any bar; drawn finish is not warranted | Yes, to hold the finish |
| Diameter is a bearing, seal or press-fit surface | Bar sized to leave stock | Yes, and add the roundness callout |
| Diameter is a non-functional blank end | Drawn bar | No |
What does a ground or tighter bar buy compared with drawn bar?
It buys a tighter diameter and a smoother, rounder surface before the part touches a lathe, at a higher bar price. It pays off only where it removes a turning or grinding operation from every part.
Surface finish is where the gap shows most. PMPA notes that bright drawn bar typically sits in the neighborhood of 125 to 32 microinches, roughly 3.2 to 0.8 micrometers Ra, and that it is not warranted for surface finish because no stock was removed (PMPA, 2023). A drawing that asks for a finer finish than that on a bar-finish diameter has already decided that the diameter will be machined, whatever the tolerance says.
The same source warns that no stock removal also means seams and other surface imperfections are still possible, and more likely in resulfurized steel. ASTM A108 limits surface discontinuities by sulfur content and bar diameter in its own table. For a part where a seam would matter, such as a sealing or sliding surface, specify the discontinuity limit on the order or plan to cut below the seam depth.
Cost is a trade of three things: the bar's price premium, the turning time saved, and the scrap risk from a surface defect found late. PMPA says cold-drawn bar costs more than hot-rolled bar of the same grade because of manufacturing cost and yield loss, and still concludes the benefits outweigh the premium for precision shops. This page does not quote prices; the useful step is to ask the supplier to price the part both ways.
How much stock should you leave when the bar cannot be used as drawn?
Enough to clean up the worst combination of out-of-roundness, bow, surface imperfection and stress movement for that bar size. None of the sources checked gives a universal allowance, so the supplier's process sets the number.
Think of the allowance as four layers that add up. Start with the roundness limit, which for ASTM A108 bar is half the size tolerance according to PMPA. Add the straightness of the length being turned, because a bowed bar removes more metal on one side. Add the depth of any seam or surface layer you need to clear. Then add margin for movement: cold drawing leaves residual stress, and the stress-relief and warping behavior of drawn bar is covered in the hot-rolled versus cold-rolled guide.
If the part is long and thin, the same bar that is straight enough for a short pin may not be for a slender shaft, and holding a tight diameter on the finished length is a process question rather than a bar-class one. Tell the supplier the finished length-to-diameter ratio so the bar can be chosen, and straightened or stress-relieved if needed, for that part. For cost effects of tolerance itself, see the guide on the cost of tight tolerances.
How should the drawing and RFQ call out the bar so the class is not guessed?
Put the bar standard and class in the material note, mark which diameters stay as drawn, and ask the quote to state the bar class it is priced on.
A workable material note names the grade, the product standard and the finish, for example a cold-finished carbon steel bar to ASTM A108, or bright drawn stainless bar to EN 10278 in a named class. If the part relies on a diameter being left as drawn, mark that diameter on the drawing with a note such as "bar finish, do not machine" and give the tolerance in the same unilateral direction as the bar. The guide on unilateral versus bilateral tolerances explains why a one-sided tolerance suits these diameters.
On the RFQ, ask the supplier three questions: which bar class and size the quote assumes, which diameters it assumes are left as drawn, and how the as-drawn diameters will be checked at inspection. Request the mill test report with the bar lot so the grade and the delivery condition are on file (how to read a mill test report). If an as-drawn diameter fails in a first article, the fix is usually a looser callout or a turned diameter, and it is cheaper to settle that before the first order than after.
Small turned parts like these are the work described on the CNC turning page. Tell the quote desk which diameters are bar-finish, and the quote states the bar assumption in writing.
Does the bar class matter at 10,000 parts a year?
Yes, because a turning pass saved on one diameter is saved on every part, so the decision repeats 10,000 times a year. At that volume it is worth pricing the part with and without the bar-finish diameter.
For a run of that size the bar class is one of the few choices that changes cycle time without changing the part. The checks that make it safe are the same ones above: confirm the diameter tolerance is wider than the bar band, confirm the surface does not need a finish the bar cannot give, and confirm the supplier has an inspection method for the as-drawn diameter. Keep the choice written down on the drawing, so a later change of mill or bar supplier does not change the part silently.
Frequently asked
Is h9 bar always better than h11 bar?
No. It is only better where the part needs the tighter diameter or the cleaner surface. A looser class that still sits inside the drawing tolerance does the same job, and the extra tightness adds nothing.
Can I put a plus-and-minus tolerance on an as-drawn bar diameter?
You can write it, but the bar cannot meet a plus side, because its tolerance runs to the minus side only. Use a unilateral tolerance on those diameters, or turn the diameter.
Does bar class replace roundness and straightness callouts?
No. The size band limits diameter, and roundness and straightness are separate limits. For ASTM A108 bar, PMPA says out-of-roundness is half the size tolerance. If the function depends on either, call it out on the drawing.
Is an ISO h class the same as an ASTM A108 tolerance?
Not automatically. They come from different systems with different tables. When a US mill supplies the bar, ask the supplier to state the tolerance in writing and compare it with the band you drew.
Who decides whether to leave a diameter as drawn?
The drawing should decide it, with the supplier's review as a check. Mark the diameters in the drawing, and ask the quote to confirm in writing.
Sources
- Cold Drawn Steel Bar Stock: How it is Manufactured, Benefits to Your Shop, Precision Machined Products Association (PMPA), June 2023
- Dimensional tolerances / Surface quality classes, WS Stahl, 2025 (ISO 286 tolerance bands for bright steel)
- BS EN 10278:2023, Dimensions and tolerances of bright steel products of stainless and other special steels, BSI, published November 2023
- EN standards for stainless steel bar, Right on Blackburns, accessed October 2026
- Tolerances to EN 10278 for bright stainless steel bars, Zhejiang Guanyu Stainless Steel Tube Co., accessed October 2026