Bar Straightness: How It's Measured, What Each Bar Type Delivers, What to Specify

Quick answer: Bar straightness is specified as maximum deviation over a given length — commonly ≤ 1 mm per meter for general machined bars, ≤ 0.5 mm/m (or tighter) for ground and polished stock, and custom limits for long shafts. The trap: straightness is a separate purchase requirement from diameter tolerance, and h9/h11 classes control size only. If your lathe needs straight bars, write it on the PO.

How Is Bar Straightness Actually Measured?

Three shop methods, from quick to rigorous:

Note the subtlety: runout (TIR) is roughly twice the straightness deviation of a uniformly bowed bar — a bar showing 1.0 mm TIR is about 0.5 mm out of straight. Confusing the two is the most common argument with suppliers.

What Straightness Can Different Bar Types Deliver?

See how these surface/tolerance families compare in peeled vs ground vs hot-rolled bar and stock size selection.

Why Do Long Bars Warp After I Machine Them?

Straightness is not a permanent property. Machining cuts off a stressed layer asymmetrically, and the bar springs toward the side you cut — a classic on 3 m shafts turned down one side. The cures:

• Take balanced passes (rotate the part or cut both sides alternately)
• Buy stress-relieved bar if the drawing needs dead-straight finish machining
• Leave the final straightening (or grinding) to a last operation after roughing
• For long slender shafts, order oversize diameter per the machining allowance rules

What Should the PO Say?

One line does it: "Straightness ≤ 0.5 mm/m and ≤ 2 mm total over 3 m, measured mid-span deflection, certified on 3.1 MTR." Also state whether straightness is required before or after your machining — the two are different products and different prices. Packing matters for transport: bundles that sag on a 40-day voyage arrive bent, which is why finished bars ship in rigid cases per our export packing guide.

FAQ

Q: Can I straighten a bent bar myself? A: Yes for hot-rolled stock — press against a flat, check with the gauge, repeat. For ground and polished bars, no: bending marks the surface and re-introduces stress that springs back after machining.

Q: Does a bar that passes at delivery stay straight? A: If stresses were balanced (properly straightened and stress-relieved), yes. Cheap straightening (bend-and-hold) relaxes within weeks — one more reason certified straightness beats "it looked straight when it left."

Q: Is straightness or concentricity my problem? A: Concentricity (OD vs bore, on hollow bar) is a separate geometric callout. Check both on delivery inspection when the bar is pre-drilled or hollow.