A formed cone looks like a simple part and is one of the more demanding things to make from sheet. Part of the difficulty is that the flat pattern is not intuitive: the blank is not a trapezium, and cutting a trapezium produces a cone with a wavy seam and a diameter that is wrong at one end.
The other part is that a cone is stiff in a way a cylinder is not. Once it is rolled and seamed it resists deformation well, which is exactly why the shape is used for transitions. But everything that makes it stiff also makes it unforgiving of a blank that is slightly off.
The shape being made is a truncated cone, a frustum: a large diameter at one end, a small diameter at the other, joined by a straight taper. Unrolled, that surface becomes a sector of an annulus, a curved band whose inner arc is the small circumference and whose outer arc is the large one.
The practical way to think about it is that the cone is a portion of a much larger cone that comes to a point. The slant height from the point to each end gives the two radii of the blank, and the sector angle is whatever wraps the outer arc around the finished circumference. That is why a shallow taper with a small difference between the bores produces a long, gently curved blank, and a steep taper produces a short, strongly curved one that is much harder to roll.
Two details are easy to get wrong. The arcs have to be concentric, or the seam will not close evenly. And the blank has to be long enough to wrap the full circumference, which on a wide cone can mean a blank longer than the sheet is wide and therefore a seam that cannot be avoided by clever nesting.
Sheet has a rolling direction, and it does not form equally in every direction. Bending across the grain and bending along it produce different amounts of springback and different amounts of resistance to deformation. On a cone the material is being curved in a direction that varies across the blank, so some parts of the blank are being bent across the grain and some along it.
The consequence is uneven springback: the rolled cone opens up more in one area than another, so the seam gap is not uniform and the finished diameter is not round. Rotating the blank so that the dominant curvature runs in the more favourable direction helps, and it is one of the reasons two shops can produce different results from the same drawing and the same material.
A slip roll bends a cylinder because both bottom rolls are parallel and the sheet passes between them at a constant radius. A cone needs a radius that changes along its length, so the sheet has to pass through the rolls at an angle, with one end of the blank working against a different part of the roll than the other.
This is why cone rolling takes more skill and more passes than cylinder rolling. The blank is fed at an angle, the rolls are adjusted asymmetrically, and the operator works from one end to the other, checking the developed shape as it goes. A cone that is rolled evenly in one pass comes out with the small end over-rolled and the large end under-rolled, and correcting it afterwards is difficult because the small end has already taken a set.
Thin sheet makes this worse. The small end of a cone has the tightest curvature, and thin material there tends to wrinkle rather than stretch. A slight crown on the rolls, or a pass with a little more pressure at the large end, is the usual way of persuading the material to follow.
A fabricated cone has a longitudinal seam, and its position should be a decision. On an exhaust transition the seam is usually placed on the side that is least visible and least exposed to the highest stress. Putting it on the top of a part that runs hot is a choice that shows up as a heat tint line and, eventually, as a crack along the seam.
The seam also has to be welded from both sides or with full penetration and a purge, because a cone in a gas path with an unpenetrated seam is a leak. That is the same discipline that applies to any thin-wall welded joint, and it is described more fully under TIG stainless steel welding.
Forming a cone stretches the material at the large end, where the circumference grows, and compresses it at the small end, where it shrinks. The compression at the small end is what produces wrinkles, and once a wrinkle has formed it cannot be rolled out; the material has buckled and the shape is set.
Thinning at the large end is the other limit. The amount of stretch is set by the taper, and a steep cone in thin sheet thins the large end enough to matter for both strength and weldability. Where that is a concern, the answer is usually a less aggressive taper, a heavier sheet, or a different process altogether.
A cone rolled from sheet has a single wall thickness all the way along. A swaged tube has a wall that thickens as the diameter reduces, because the material is pushed inward and has nowhere else to go. If a design assumes a wall that grows through the transition, a fabricated cone will not deliver it.
A fabricated cone also cannot produce a sharp internal corner at either end without extra work, because the seam and the circumferential weld both sit proud of the surface until they are dressed. In a flow path that matters, and dressing a weld inside a cone is awkward work.
Swaging wins when the change of bore is modest, the length is short, and quantity justifies a die. There is no longitudinal seam and no circumferential weld, the wall thickens through the transition, and the part comes out concentric and repeatable.
Machining wins when the transition is small, thick or has to be an accurate internal feature. A rolled cone wins when the taper is long, the diameters are far apart, or the quantity is low enough that a die is not worth making. Most fabricated exhaust transitions fall into the last group, which is why they are welded rather than formed.
A cone drawing needs to give the shop enough to derive the blank, and a surprising number do not.
These transitions are part of our stainless steel exhaust system work, alongside the tube grouped under exhaust pipe and the assemblies under titanium fabrication components. The titanium version is covered under TIG titanium welding parts, and the way a shape like this is agreed from a drawing or a sample is set out on our customization page.
