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Tube Definitions and Tolerances

Left perplexed by some of the phrases and technical specifications used to describe the precision tubes we produce? Then use this helpful guide which provides definitions and tolerances specific to the metal tube manufacturing industry.

  • Eccentricity +

    An eccentric tube is one which has the centre of the circle formed by the OD (outside diameter) which is at a different point from the centre of the circle formed by the ID (inside diameter), so the two circles are not concentric. Eccentricity can be checked by determining the wall thickness around the tube at any cross section to establish maximum and minimum.

    The tube’s eccentricity (in mm) at that cross section would be one half the difference between the minimum wall thickness (t2) and the maximum wall thickness (t1).

  • Wall Run-Out +

    This is derived from the tube’s eccentricity. It is the variation in wall thickness compared to the specified nominal wall (total Δ wall).

    Wall run-out can be related to eccentricity:

    Wall run-out is also known as TIR (Total Indicated Reading).

  • Light Wall +

    Any wall dimension which is less than 3% of the theoretical average OD.

  • Normal Wall +

    Any wall dimension which is 3% or more of the theoretical average wall.

  • As-Drawn Tube +

    A tube in any temper other than annealed or heat treated.

  • Heat Treated +

    A tube which has been quenched and tempered or precipitation hardened.

  • Ovality Tolerance +

    Out-of-roundness, or the difference between maximum and minimum dimensions of OD, is obtained by carefully measuring the high and low points at any one section of a tube. The ovality tolerance is considered to be a total spread, inside which both the maximum OD and minimum OD dimensions must fall. For example, on a 1" OD tube with light wall, annealed temper, the total ovality tolerance is a spread of 0.030". At any one section around the tube, the difference between the minimum OD reading and the maximum OD reading may not exceed 0.030".

  • Average OD +

    The average of a minimum of four micrometer readings, including the high and low points, equally spaced around the full circumference of the tube.

  • Tolerances +

    The table below shows standard commercial tolerances, as applicable to all round stainless steel, nickel and titanium alloy tubing. Tolerances may be specified by OD/ID or OD/wall thickness definition. Closer tolerances are achievable subject to discussion.

    OD Range Commercial OD / ID Wall % ± FT OD achievable
    mm inches mm inches mm inches
        ± 0.05 ± 0.002 10 ± 0.01 0.0004
    2.38 0.094 ± 0.08 ± 0.003 10 ± 0.01 0.0004
    4.76 0.19 ± 0.10 ± 0.004 10 ± 0.03 0.0012
    12.7 0.500 ± 0.13 ± 0.005 10 ± 0.05 0.002
    25.4 (1.00) ± 0.15 ± 0.006 10 ± 0.05 0.002
  • Straightness +

    OD mm ≤ OD inches ≤ Wall Straightness Commercial FT achievable mm
    15.9 0.625 All sizes 1 in 600 1 in 2000
    25.4 1.00 2% of OD or heavier 1 in 600 1 in 1500
    25.4 1.00 Less than 2% of OD 1 in 400 1 in 1000
  • Worked Examples +