Aluminium Round Tube Dies: Types, Size Chart & Ordering Guide

What a round tube die is, and why the size you pick matters

A round tube die is a set of hardened hot-work tool steel parts, almost always H13 (1.2344) or a premium variant of it, that shapes a heated aluminium billet into a hollow circular section. Unlike a solid bar die, a tube die has two working surfaces: the outer die cap that sets the outside diameter (OD) and an inner mandrel that sets the inside diameter (ID). The gap between them is the wall thickness, and the accuracy of that gap, all the way round, decides whether the tube comes out concentric.

Because both diameter and wall are fixed by the tooling, every combination needs its own die. A Ø50 × 3 mm tube and a Ø50 × 4 mm tube are two different tools, and so are Ø50.8 mm (2 in) and Ø50 mm. Buyers who are new to extrusion often assume tube is cut from a continuous range, the way sheet is cut from coil; in reality the available sizes are a library of discrete dies built up over decades by extruders around the world.

That has a direct commercial consequence. When the OD and wall on your drawing match a die that already exists, production can start from proven tooling, with known straightness and tolerance behaviour and no tooling invoice. When they do not, a new die must be designed, machined, heat-treated, nitrided and trialled, and the first trial often needs correction before the tube is accepted. For small and medium orders, the die cost and the trial time can outweigh the metal cost, which is why it pays to check a standard size chart before freezing a design.

Tooling is only half the story. The same die can run several alloys, but not all of them equally well, and some alloys are better made as seamless tube on a different type of press. Tolerance class, temper, surface finish and length also influence which route is used. The sections below take each of these in turn.

Porthole, bridge and die-and-mandrel tooling compared

Hollow aluminium tube is extruded by two fundamentally different methods. Which one is used depends on the alloy, the size, the end use and the specification on the purchase order.

The seam welds in porthole tube are solid-state bonds formed by pressure and temperature, not fusion welds. In 6xxx alloys made with a well-designed die, they are sound and the tube meets the same tensile minimums as seamless material in most structural uses. Problems arise when the welding chamber pressure is too low, when the alloy is hard to bond (2xxx, high-strength 7xxx and high-magnesium 5xxx), or when the tube is later expanded, flared or hydroformed, which concentrates strain at the seams. That is why many specifications for pressure pipe and hydroformed parts require seamless tube, and why the purchase order should say so explicitly.

How a new round tube die is made, and what limits the size

1. DesignThe die designer sets the number of ports, the bridge shape, the welding chamber height and the bearing lengths so that metal reaches the die gap at even speed all round. Uneven flow is what causes eccentric walls, wavy tube and poor seam welds.
2. MachiningThe mandrel and cap are rough-machined from H13 bar, heat-treated to high hardness, then finish-machined and spark-eroded (EDM) to final dimensions. Bearing surfaces are polished.
3. NitridingThe working surfaces are nitrided to give a hard, wear-resistant case. Dies are re-nitrided periodically during their life.
4. TrialThe die is run on the press with production alloy. The first tube is measured for OD, wall at several points, ovality and straightness, and sectioned to check the seam welds.
5. CorrectionIf the wall is uneven or the tube bows, the die maker adjusts bearings or flow paths and the die is trialled again. Simple round tubes usually settle quickly; very thin walls and large diameters take more correction.
6. Production and maintenanceThe die is cleaned in caustic soda after each run, inspected, re-nitrided when needed and stored for repeat orders. Wear gradually enlarges the bearings, and the die is retired when it can no longer hold tolerance.

Die life varies with alloy, section and press practice, so it is quoted as an estimate rather than a fixed number. Soft alloys such as 6063 are kind to tooling; 6082, 6061 and 7xxx alloys are harder on bearings; thick walls with low extrusion ratios are easier than thin walls on large diameters.

What can be extruded is limited by the press. The OD must fit within the die diameter the press accepts, and the force needed rises with the cross-sectional reduction and the alloy's flow stress. Very large tubes, above about 300 mm OD, are made on large presses with big containers or by spreading techniques, and the range of available wall thicknesses narrows. At the other extreme, very small diameters below about 8 – 10 mm are often extruded slightly oversize and cold drawn to final dimension.

Metric aluminium round tube sizes and weights (Ø6 – 500 mm)

The table lists metric outside diameter and wall combinations that are widely produced across the extrusion industry. For each size we give the inside diameter and the theoretical weight, calculated from a density of 2.70 g/cm³ (accurate within about 1 % for 6060, 6063, 6061 and 6082). For 2024 multiply by about 1.03, for 7075 by about 1.04 and for 5083 by about 0.99. Sizes are written OD × wall in millimetres.

Whether a size runs from existing tooling depends on the alloy, the route (porthole or seamless) and the tolerance required, so confirm with your enquiry. Sizes that fall between those listed can often be reached by cold drawing or machining the next standard tube.

Inch (imperial) aluminum round tube sizes and weights

North American and many Middle Eastern drawings specify tube in fractional inches with decimal walls such as 0.035, 0.049, 0.065, 0.083 and 0.125 in, the same gauges used for steel and stainless tube. These sizes are made on inch dies, and substituting the nearest metric tube is rarely acceptable when the tube fits into a fitting, a bearing or a mating part. Weights below use 0.0975 lb/in³.

Note that aluminium pipe is a different sizing system. Pipe is ordered by nominal pipe size and schedule (for example NPS 2 Schedule 40, which has an OD of 2.375 in and a wall of 0.154 in) to ASTM B241 or B429, whereas tube is ordered by actual OD and wall. If your drawing says "2 in pipe", it does not mean a 2 in OD tube. Our aluminium pipe pressure guide covers pipe schedules and pressure ratings.

Which alloys run through round tube dies, and in which tempers

Most round tube is made from the 6xxx alloys because they flow easily, bond well in the welding chamber of a porthole die and reach good strength after press quenching and ageing. The table shows the usual route and tempers for each alloy family.

Typical tensile strengths illustrate the range: 6063-T6 is about 240 MPa, 6061-T6 about 310 MPa, 7005-T6 about 350 MPa and 7075-T6 about 570 MPa (Aluminum Association typical values). Specified minimum values for extruded tube are set by ASTM B221, ASTM B241 and EN 755-2, depend on wall thickness and are what the mill certificate reports. Design to the minimums, not the typical values.

Temper also affects tooling choice indirectly. T5 tube is quenched at the press exit by air or water mist, which suits thin-walled 6063 and 6060. Thick-walled 6061 and 6082 need an intense water quench to reach T6 through the wall, so they may be extruded on lines with water-wave quench or solution heat-treated separately. A tube that must be bent after delivery is often ordered in T4 or O and aged afterwards.

The "51" in T6511 means the tube was stretched about 1 – 3 % after quenching to relieve stress and straighten it, with minor straightening allowed afterwards. Stretched tempers move less when one end or side of the tube is machined.

Standards and tolerances for extruded and drawn round tube

Tolerance tables differ by process, so the standard on the order should match the route. In the European system, porthole and seamless tube have separate parts of EN 755, and drawn tube has its own parts of EN 754. In the American system, ASTM B221 covers extruded tube (including porthole tube), B241 covers seamless extruded tube and pipe, and B210 covers drawn seamless tube; dimensional tolerances for all of them are tabulated in ANSI H35.2.

The standards control OD (both the mean diameter and the diameter at any point, which together limit ovality), wall thickness (mean wall and wall at any point, which limit eccentricity), straightness, length and the squareness of cut ends. Tolerances widen as the diameter grows and are wider for the harder alloy group (2xxx, 5xxx with more than about 3 % Mg, and 7xxx) than for 6060, 6063 and 6082.

Drawn tube is markedly tighter than extruded tube and has a smoother surface, which is why hydraulic cylinders, telescopic tubes, precision sleeves and heat exchanger tubes are usually drawn. If a drawing gives an OD tolerance of a few hundredths of a millimetre, plan for cold drawing or machining rather than asking for it from the extrusion die.

When your size is not standard: drawing, machining or a new die

A size that is not in the charts is not necessarily a new die. There are four realistic routes, and the cheapest depends on quantity and tolerance.

Secondary processing is often what turns a tube into a usable part. Common operations include precision cut-to-length with deburring and chamfering, CNC bending (single or multi-radius, which is easiest in 6063-T5 or T4 tempers), end forming such as flaring, swaging and expansion, drilling, tapping and slotting to drawing, and MIG, TIG or brazed joints. For porthole tube that will be expanded or flared heavily, discuss the seam position and the forming strain at the enquiry stage.

Typical round tube applications and the usual specification

• Handrails and balustrades: 6063-T5 or T6 in sizes such as Ø38 × 2, Ø42.4 × 2.5 or Ø50 × 2, anodised or powder coated, made as porthole tube to EN 755-8.
• HVAC and heat exchangers: 3003 or 6063 tube around Ø19.05 (3/4 in) or Ø12.7 (1/2 in), often cold drawn to tight OD for brazed joints and expansion into fins.
• Fitness, furniture and medical equipment: 6061-T6 or 6063-T6 in Ø25 – 60 mm, with bending, drilling and powder coating.
• Bicycle and sports frames: 6061 and 7005 tube, frequently drawn or butted after extrusion to vary the wall along the length.
• Structures, scaffolding and trailers: 6082-T6 heavier-wall tube such as Ø48.3 × 4, designed to EN 1999 (Eurocode 9) or the Aluminum Design Manual.
• Pressure lines and hydroformed parts: seamless 6061, 6082 or 5083 tube to EN 755-7 or ASTM B241.
• Aerospace and defence: seamless 2024, 6061 and 7075 tube to AMS specifications, with ultrasonic inspection where required.
• Large-diameter rollers and housings: 6061 or 6082 tube above 200 mm OD for printing rollers, conveyor drums and enclosures, usually machined after extrusion.

How to order round tube from Metline

1. SpecifyAlloy and temper, OD × wall (state metric or inch), length, quantity, finish, and the governing standard (EN 755-7/-8, ASTM B221, B241, B210 or a drawing). Say whether seamless tube is required.
2. Confirm the toolingWe confirm whether the size runs from a standard die, whether drawing or machining from the nearest size is more economical, or whether a new die is justified.
3. QuotePrice, tolerance class, certification and delivery terms (EXW, FOB, CIF or DAP) for your destination.
4. Process and inspectCutting, bending or finishing as ordered; dimensional checks of OD, wall, ovality and straightness; EN 10204 3.1 certificate with chemistry and mechanical results.
5. Pack and shipEnd caps, film or paper interleaving, bundling with spacers and wooden cases or pallets for sea or air freight worldwide.

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