Hollow Bar for Machining: Turning Tubes Instead of Boring Solids

What hollow bar is

Hollow bar is thick-walled seamless tube stocked specifically as machining feedstock: bushes, rings, sleeves, cylinders, flanges with bores, gear blanks. Instead of buying solid bar and boring the middle into chips, you buy the hole already made. In Europe it is standardized by EN 10294-1 (hollow bars for machining); in North America, seamless mechanical tubing to ASTM A519 fills the same role.

The sizing logic: guaranteed machining dimensions

Hollow bar is not sold by nominal OD/ID alone. Stockists publish for each size the finished dimensions it guarantees: the largest finished OD and the smallest finished bore that can be machined cleanly from that bar. The difference between the delivered size and the guaranteed size is the machining allowance, which exists to remove:

• the outer scale and decarburized skin,
• ovality and eccentricity (the bore is never perfectly centred),
• surface defects within standard limits.

Order so that your finished OD is at or below the guaranteed OD and your finished bore is at or above the guaranteed ID — then every part cleans up, even from the worst bar in the bundle.

The savings, quantified

Consider a sleeve finished at 100 mm OD × 70 mm bore. Machined from solid Ø100 bar, the bore removes π/4 × 70² ≈ 3850 mm² of the 7850 mm² cross-section — about 49% of the purchased weight leaves as chips, along with the boring time, tool wear and chip handling. From hollow bar sized for the job, only the cleanup allowance is removed. The material saving and the cycle-time saving grow quickly with bore diameter and batch size.

Grades typically stocked

• 1045 / C45 and E355-type carbon steels for general sleeves and hydraulic cylinder tubes.
• 4140 (42CrMo4), often prehardened, for high-strength bushes, pinions and wear parts.
• 1020 / low-carbon grades where the part will be carburized or welded.
• Stainless hollow bar in 316L and duplex for corrosion-resistant rings and valve parts.

Sizes and lengths in practice

Stock ranges concentrate where machining demand concentrates: outside diameters from roughly 30 mm to 250 mm are widely stocked, with the heaviest walls (small bores) in the small and medium sizes — large thin-walled tubes belong to the pipe world, not hollow bar. Lengths are typically random mill lengths in the 3–6 m region, cut to fixed lengths on request. Because the guaranteed machining dimensions narrow the usable part of each bar, buyers planning a long production run often reserve material from a single cast or batch so the cleanup allowance behaves identically from the first bar to the last.

Watch points

• Eccentricity is the governing defect: check that the wall difference around the circumference still leaves cleanup metal at your finished bore.
• Welded tube is not hollow bar — pressure-rated or rotating machined parts call for seamless feedstock unless the specification explicitly allows welded.
• Price per tonne is higher than solid bar; the business case rests on chips avoided, machine hours saved and the value of the boring operation removed.