Developing the jointless deep-drawn body

The most significant piece of development in our range is also the least visible: there is nothing to see, because what we removed was a seam.

Starting from the failure

The work began with an observation anyone in this trade can make. Vessels that come back have almost always cracked, and the crack is almost always at the vertical seam or around the base joint.

The reason is metallurgical. Welding changes the grain structure of the metal in a narrow band beside the join. That heat-affected zone has different hardness, different ductility and often reduced corrosion resistance from the parent metal — and in service, the weld, the zone and the parent metal expand and contract at slightly different rates through every heating cycle.

A commercial kitchen delivers a few hundred cycles a year. Fatigue does the rest.

Once you see it that way, the design question is not "how do we weld better". It is "how do we not weld at all".

Why deep drawing is hard at this size

Deep drawing forms a vessel from a single circular blank. A punch presses the blank into a die and the metal flows into shape, stretching and thickening as it goes.

It is well established at small sizes. At the sizes we needed — up to a 125-litre patila — several problems compound:

• The blank is large, and controlling material flow across it is difficult.
• Draw ratio. There is a limit to how deep you can go in one operation before the wall tears. Deep vessels need several stages, each with its own tooling.
• Wall thinning. Metal stretches as it is drawn, so the wall naturally thins as it goes up. If not controlled, the wall ends up thinnest exactly where the vessel is deepest.
• Wrinkling. The blank's outer region is in compression as it draws in; too little restraint and it wrinkles, too much and it tears.
• Work hardening. Steel gets harder as it deforms. Between stages it may need annealing before it can be drawn further.
• Springback. Formed steel relaxes after the punch withdraws, so the tooling must account for the shape the metal ends at rather than the shape it is pressed to.

Each of those is a tooling and process problem, and each one is expensive to get wrong, because tooling is committed before any sellable vessel exists.

What was achieved

The result is our jointless patila in twelve sizes, 30 to 52, holding 25 to 125 litres, at a uniform 2 mm wall through the entire range — and the cook & carry range, 25 to 100 litres, on the same principle.

Uniformity across the range is the part we are most satisfied with. Wall thinning during drawing makes it easy for large sizes to end up thinner than small ones, and controlling it so that the 125-litre vessel has the same wall as the 25-litre one is where most of the process work went.

What still gets welded

Handles and lugs, and we make no apology for it. A handle sits on the side wall, away from the thermally violent base, and it carries mechanical load rather than thermal cycling. We weld them and dress the welds back flush so no crevice remains for food to lodge in.

Joints where they are harmless. None where they are not.

The guarantee follows from the process

The lifetime guarantee against cracking is not a marketing decision taken separately from the engineering. It is a statement about construction: a body with no joint in it has nothing to crack along.