How To Make A Cone With Sheet Metal
Content Menu
● Sheet Metal and Cone Geometry Basics
● Method 1: Hand Rolling and Welding
● Method 3: Spinning on a Lathe
● Advanced Routes: Hydroforming and Blast Forming
● Inspection and Final Finishing
Introduction
Making a cone from flat sheet metal is one of those jobs that shows up constantly in real shops. A transition piece in ductwork, a hopper for a grain bin, a funnel for a powder line, a reflector for a light fixture, or the nose cone on a custom motorcycle fairing — they all start as a flat blank and end up tapered. The process looks simple until you try it the first time and discover the metal has its own ideas about where it wants to bend and where it wants to wrinkle.
Over the years I’ve built hundreds of cones in sizes from 50 mm laboratory funnels to 3-meter silos. The same rules apply whether you are working by hand in a corner of the garage or running a 200-ton hydroform press. The difference is only in speed and repeatability. This article walks through every practical way I know to get a good cone, from the slowest hand method to the fastest production routes, with the exact details that actually matter on the shop floor.
Sheet Metal and Cone Geometry Basics
Why Sheet Metal Works So Well
Sheet metal is produced by rolling, so it already has uniform thickness and predictable mechanical properties in the rolling direction. Mild steel, stainless 304/316, aluminum 5052-H32, and galvanized coil all form cones without much trouble if you stay within their forming limits. Aluminum stretches more before tearing, stainless fights springback harder, and mild steel is the forgiving middle ground most shops start with.
Geometry You Actually Need to Know
Everything comes from three numbers: base diameter D, height H, and the resulting slant height L = √((D/2)² + H²). When the cone is opened flat it becomes a sector of a circle with radius L and arc length equal to πD. The included angle of the sector in degrees is θ = 360 × (D/2) / L. Write those two equations on a sticker above the bench and you will never need to look them up again.
For a 600 mm base and 400 mm height the slant is 500 mm and the sector angle is 216°. Cut that sector, bring the straight edges together, and you have a cone. Miss the angle by five degrees and the seam will either gap or overlap badly.
Materials and Shop Setup
Start with clean coil or sheet. Oil and mill scale make welding miserable and cause rollers to slip. Typical thicknesses for hand work run 1.0–2.0 mm; anything heavier usually goes straight to a powered roller or press.
Basic kit: metal shears or plasma cutter, large compass or trammel, permanent marker, leather gloves, straightedge, MIG or TIG welder, angle grinder with flap disc, and something round to form against (pipe, round bar, or a wooden mandrel). Add a slip roll or lathe only when the job justifies it.
Method 1: Hand Rolling and Welding
Full Walk-through
Lay out the sector on the sheet, cut it out, deburr the edges. Clamp one straight edge along a length of heavy pipe. Start in the middle and work outward with a rubber mallet or a short length of hardwood, bending a little at a time. Flip the sheet every few hits to keep the curve even. When the two straight edges meet, tack weld every 50 mm, then run a full bead. Grind the weld flush if appearance matters.
HVAC Transition Piece Example
A common job is an 8-inch to 6-inch round transition in 24-gauge galvanized. The blank is roughly 750 mm radius, 190° sector. Two people can hand-form it in about 25 minutes. The finished piece bolts between standard duct sections and passes a 3-inch water leak test with no sealant.
Method 2: Slip Roll Machine
Step-by-Step on the Roller
Set the top roll slightly tighter at one end to create the cone angle. Feed the wide end first, make three or four light passes, flip the sheet end-for-end between passes. When the edges meet, pull the part out, tack, and weld. A 1.2 m wide powered roller can turn out a 900 mm base cone in under five minutes once the machine is dialed in.
Custom Exhaust Cone Example
Motorcycle shops use 1.5 mm 304 stainless rolled this way for megaphone sections. The roller leaves a smooth surface that polishes to a mirror with almost no grinding.
Method 3: Spinning on a Lathe
How the Process Runs
Cut a circular blank a little larger than the slant height. Bolt it to a wooden or steel mandrel turned to the finished cone angle. Spin at 600–900 rpm and push the metal outward with a spoon or roller tool in multiple passes. The wall thins toward the small end exactly where you need strength at the base and lightness at the tip.
Reflector Cone Example
Stage lighting companies spin 0.8 mm aluminum reflectors 300 mm deep. The seamless surface gives better light distribution than a welded cone and the process runs 40–50 pieces per shift on one lathe.
Advanced Routes: Hydroforming and Blast Forming
Hydroforming presses the blank into a conical die with oil pressure up to 100 MPa. Wall thickness stays almost constant and deep cones with small apex angles become possible. Aerospace inlet ducts in titanium are routine with this method.
Explosive forming uses a shock wave in water to slam a blank onto a die in microseconds. It is reserved for very large or very hard alloys where conventional presses cannot reach the required strain in one shot.
Typical Problems and Fixes
Wrinkles on the small end → increase blank holder pressure or add draw beads. Cracks at the nose → anneal the blank first or switch to a more formable grade. Springback → over-form by 3–7° depending on material and thickness. Weld burn-through → drop voltage two points and increase travel speed.
Inspection and Final Finishing
Check diameter at both ends with a tape, check straightness with a straightedge along two generators, and pressure test if it has to hold air or liquid. Grind welds flush, bead blast or powder coat, and you are done.
Wrapping Up
Every shop has its favorite way to make cones, but the underlying math and metal behavior stay the same. Start simple with hand rolling to understand what the material is telling you, then move to rollers or spinning as volume demands. The difference between an amateur cone and a professional one is usually just patience on the first few passes and respect for the geometry. Once you have made ten cones you will never need to measure twice again — the feel becomes automatic.