Channel is another stolen name. Mill channel (and sometimes extruded or laser-welded box that people still call channel) is a produced section. Folded channel is a sheet blank with two bends — the custom product the old Alltrade page sold with a geometry lecture.
That lecture is still the best paragraph on the subject: the smallest internal leg they would fold was 20 mm, and the base had to be at least as wide as the legs. Measurement B ≥ A and C. A tiny U with huge ears is a wish.
They both look like a U from the mezzanine. They do not share a section modulus or a finish. If the shape is an L, you wanted angle. If it is a strap, you wanted flat bar. The rest of the custom-cut and sections map is the cut list around them.
Mill versus folded at a glance
| Question | Mill channel | Folded channel |
|---|---|---|
| How it is made | Rolled, extruded, or a produced section | Sheet blank, two bends, two radii |
| Geometry | Catalogue profile | Whatever the folder will hold |
| Smallest useful U | Catalogue minimum | Historic shop: 20 mm smallest internal leg; base ≥ each leg |
| Finish | Usually HRAP / No. 1, dull | Whatever the parent sheet was — No. 4, 2B, BA |
| Strength | A known section | A cover. Not a lintel. |
| Straightness | Mill identity | Folder plus springback |
| Typical use | Frames, tracks drawn for that profile, engineered work | Coves, cable covers, plinths, drips, light thresholds |
If the drawing says “channel” and the job is a 37 mm glass pocket, you are already in fabricated territory.
Mill channel
Delivers a consistent profile, a structural identity, and usually a dull HRAP face. You buy a catalogue size. You do not invent a 37 mm internal because a glass panel is 37 mm. That job is fabricated — or a different product, such as the slotted balustrade tube the old shop also sold.
Mill channel is cousins with mill angle and flat bar. Thickness is a bar dimension. The root is a fillet, not a fold radius.
Uses that make sense: frames an engineer has sized, sliding-door tracks drawn for that profile, ledgers, guards. Uses that do not: a kitchen plinth that must match a No. 4 bench, a cable cover that needs a 22 mm internal, a drip that wants a 1.2 mm edge. If you need bright, polish the mill section and pay for that. HRAP is grey on purpose. It is not tea stain.
Folded channel
You pick alloy, finish, thickness, internal (or external) width and two leg heights. A radius lives in each bend. Thickness comes off the sheet chart. Parent-sheet width must cover the developed blank; see sheet sizes.
Uses: coves, cable covers, light thresholds, kitchen plinths, drip details. Not a crane rail. Not a lintel.
The developed blank is three faces plus two radii. A 50 mm base with two 40 mm legs in 1.5 mm is not 130 mm of sheet. The radii consume length. The folder consumes a flange to grab.
The 20 mm rule, and why the base cannot be skinny
Historic Alltrade custom-fold notes were blunt:
- Smallest internal leg: 20 mm.
- Base (B) at least as wide as each leg (A and C). B ≥ A and B ≥ C.
- A tiny U with tall ears will not form cleanly, will spring, and will not sit.
The folder has to clamp a flange. A 12 mm internal leg on 1.5 mm sheet is a fingernail. A base narrower than the legs is a tulip, not a channel. This is a machine-and-blank rule, not a brand rule. If you need a 12 mm return, you are in angle territory (historic smallest internal around 10 mm).
Unequal legs are legal if the base still wins. A 60 mm base with a 25 mm upstand and a 40 mm downstand is a drip. A 20 mm base with two 50 mm ears is a wish.
Tolerance, springback and edge
Sheared legs show tool marks. ±2 mm on a long channel is visible at a shadow line. If two channels have to kiss, say so and pay for a dressed edge or a different process. Custom-cut is honest about this; drawings often are not.
Austenitic sheet springs. 304 and 316 work-harden at the bend. A folded channel that must run true for 2400 mm is a check-piece job, not a first-off surprise. Mill channel is straighter because the mill already had that argument.
Name internal or external. Two radii make the inside smaller than the outside by more than “two thicknesses.” Glass, gaskets and timber inserts care about the inside. Cover plates care about the outside. Write the one the adjacent trade will measure.
Grain on a seen No. 4 should run the long way. Cross-grain legs next to a long-grain base is the same lighting fight as folded angle. Edges are sheared, not extruded. Deburr if a hand will find them.
Alloy, water and the internal corner
304 indoor, 316 coastal, same as everything else in this journal. A folded 304 channel on a seaside step is a tea-stain farm in the internal corners, where water sits. Drain it or change alloy. Pair with A4 fasteners outdoors.
The U is a gutter if you install it that way. Horizontal channel holds wash water, fat, and coastal salt. Design a fall, drain holes, or accept a coastal washing programme that actually reaches the inside. Cleaning without chlorides still applies; bleach left in a channel is a pit with a lid.
Welded ends turn a channel into a pocket. Pickle and passivate the welds, or the heat tint in the corner will be the first brown line. Welding 304 and 316 is the procedure essay; a folded U still concentrates heat on thin legs.
Protecting stainless during install matters more on channel than on flat sheet. The inside face collects swarf and stays wet.
Hats, Zs and the other stolen names
If the shape is an L, you wanted angle. If the shape is a hat, you wanted a Z or a special — the old site sold folded Z with a 10 mm smallest internal leg. Draw it. Each extra bend eats a minimum flange.
A slotted channel for glass is a designed product. If the job is a 316 balustrade, buy the system drawn for the glass. A folded U with a hacksaw slot is how tea stain and a failed inspection arrive together.
How to write the line
Mill: “75 × 40 × 5 mm channel, 316, HRAP, 6.0 m, mill tolerance.”
Folded: “304, No. 4, 1.5 mm, channel 50 internal × 40 × 40 internal legs, grain long, 1800, ±2 mm, drain holes at 400 crs, filmed.”
Name alloy from 304 versus 316. Name the finish. Name internal or external. Name the 20 mm rule if someone on the job is still drawing 12 mm ears.
The fabrication and care essays take over after the fold: weld, passivate, wash. This page only has to keep the mill U and the folder U from borrowing each other’s strength.