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 table

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