Aluminium extrusion slot width is the number your customer actually needs, and it's the one number "2020" doesn't tell them. A buyer orders 2020 profile, orders T-nuts to match, and finds the nuts won't enter the slot — or enter it and spin. Nobody sent the wrong goods. The order just described the outside of the bar and said nothing about the groove.
Here's the part most catalogues skip: there is no published standard for T-slot framing at all. As the reference summary of the system puts it, no published standard defines it — it survives on "a series of conventional sizes" that mills happen to agree on. Conventions are not specifications. These are the five places that gap turns into a claim.
Mistake 1: Treating "2020" as a spec instead of an outside dimension
What people do: write "2020 aluminium profile, 6 m, 20 pcs" on the PO and consider the item defined.
Why it fails: 2020 means a 20 × 20 mm square cross-section. That's it. It doesn't fix the aluminium extrusion slot width, the wall thickness, the corner geometry, the internal bore, or the alloy temper. Two mills can both ship exactly what the PO said and ship two incompatible bars.
The fix: treat the series number as a family name and specify the slot separately. In the 20 series, the slot is nominally 6 mm — Wellste publishes 6.2 mm actual across 2020, 2040 and 2080. The 30 and 40 series commonly run 8 mm, the 45 series 10 mm. "Commonly" is doing real work in that sentence, which is the whole problem.
| Series | Cross-section | Typical slot width | What still varies |
|---|---|---|---|
| 10 (imperial) | 1 in | ~0.256 in | Wall, corner radius, fastener system |
| 20 | 20 mm | 6 mm (6.2 mm actual) | Wall, slot profile shape, corner |
| 30 | 30 mm | 8 mm | Wall, slot profile shape, corner |
| 40 | 40 mm | 8 mm | Wall, slot profile shape, corner |
| 45 | 45 mm | 10 mm | Wall, slot profile shape, corner |
Mistake 2: Assuming same series means same wall thickness
What people do: compare two quotes for "8080 profile" on price per metre and pick the cheaper one.
Why it fails: the two bars are not the same product. On 8080, the European-convention profile runs a 7 mm wall at 8.33 kg/m, while the Chinese national-convention profile runs a 5 mm wall at 6.46 kg/m — a documented 29 % difference in metal at identical outside dimensions. Your buyer sees the same "8080" and a 15 % lower price and assumes they found a better supplier.
The fix: quote and buy on weight per metre, not on the series number. Mass per metre is the one figure that cannot be styled, rounded or reinterpreted — it is a direct proxy for how much aluminium is in the section, and it exposes a thin-wall substitute instantly. Put kg/m on the quotation, the drawing and the packing list.
This is the same failure pattern as sheet metal gauge to mm, where a legacy count is treated as a thickness. A designation that describes a family is being read as a measurement.
Mistake 3: Ordering fasteners before the slot is confirmed
What people do: order profile from one supplier and T-nuts, brackets and end caps from another, matched by series number.
Why it fails: a T-nut cut for an 8 mm slot does not fit a 6 mm slot, and there is no workaround — you can't file it, and a smaller nut in a larger slot rotates under load instead of gripping. Worse, the two conventions use different fastener systems entirely: the national-standard profiles typically take square nuts with socket-head bolts, while European-convention profiles take T-bolts or purpose-shaped nuts.
The fix: confirm three things before releasing the hardware order, in this order:
- Slot width, in millimetres, measured — not inferred from the series
- Slot profile shape — rectangular groove versus trapezoidal T-slot; they accept different nuts even at the same nominal width
- Fastener family the profile was designed around
If a buyer can't answer all three, they haven't specified the framing system yet, only its silhouette.
Mistake 4: Ignoring the corner geometry
What people do: assume any 40 × 40 profile takes any 40-series panel gasket, corner bracket or end cap.
Why it fails: the two conventions shape the corners differently. National-standard profiles are essentially square-cornered; European-convention profiles carry a noticeably larger corner radius. That radius changes what a panel retainer seats against, how flush an end cap sits, and whether a corner bracket contacts the flat or rides on the curve.
The fix: dimension the corner radius on the drawing and state the panel groove depth. These are two lines that prevent a whole category of "the accessories don't fit" emails — and they're never in a series number.
Mistake 5: Selling from a photo that carries no scale
What people do: publish a clean render or a cut-end photo of the profile on the catalogue page and put the specs in a PDF underneath.
Why it fails: extruded profiles are the least self-scaling product category there is. A 2020 and a 4040 photographed square-on are the same picture at different zoom levels. A buyer scanning a marketplace listing has no way to judge slot width, wall thickness or corner radius from the image, so they judge on the number in the title — which, as established, isn't a spec.
The fix: put the four numbers on the cut-end view itself, dimensioned to the features they describe: outside dimension, slot width, wall thickness and corner radius, plus kg/m in the corner. The mechanism matters here — those callouts have to be measured off the real section geometry and pinned to the actual edge, then exported at the size the buyer's catalogue or marketplace takes. That's a different job from retouching a product shot, and a very different one from an image generator, which will letter a confident "8 mm" onto a slot it never measured. On a profile listing, an invented number and a verified one look identical until the T-nuts arrive.
The spec block that ends the argument
Any of the five mistakes above disappears if the profile is described by these eight fields instead of by a series number:
- Outside cross-section (mm × mm)
- Slot width (mm), measured — and how many slots, on which faces
- Slot profile shape: rectangular groove or trapezoidal T-slot
- Wall thickness (mm)
- Weight per metre (kg/m) — the anti-substitution number
- Corner radius (mm)
- Alloy and temper (e.g. 6063-T5), plus surface finish and coating thickness
- Fastener family the profile is designed for (T-bolt / square nut / proprietary)
Written as one line on a quotation: 40 × 40 mm, 8 mm trapezoidal T-slot ×4, wall 2.0 mm, 1.62 kg/m, corner R2, 6063-T5 anodised 10 µm, T-bolt system. Nothing in that sentence can be honestly delivered two ways.
Next steps
If you sell extruded profile into export markets, the sequence that removes the most rework is:
- Add kg/m to every line on the quotation. It costs nothing and it makes a thin-wall substitution visible before the order, not after. Buyers who have been burned once will notice you publish it.
- Publish a dimensioned cut-end drawing per SKU, not per family. One drawing for "40 series" is what created the problem.
- Send the drawing with the quote, not after the enquiry. Slot width is the single most repeated pre-sale question in this category; answering it in the image ends the thread.
- State the fastener family explicitly, even when it seems obvious from the region you're selling into. It isn't obvious to a buyer sourcing from three countries.
For step 2 and 3, the practical requirement is a spec image whose numbers come from the real geometry — snap the dimension to the measured edge, label what each number is measured across, export at listing size. If you want the cost side before committing the hour per SKU, the return cost calculator prices what one incompatible-hardware rework takes out of the order margin. And when the profile is structural rather than framing, the same discipline applies to the section callouts in steel profile dimensions for export.
FAQ
What is the slot width of 2020 aluminium extrusion?
Nominally 6 mm; published actual figures run around 6.2 mm. But 2020 only guarantees a 20 × 20 mm outside section — the slot width is a separate specification, and there is no published standard forcing every mill to the same value. Ask for the measured slot width in millimetres before ordering T-nuts.
Are all 40-series aluminium profiles interchangeable?
No. Two 40 × 40 profiles can differ in slot width, slot shape (rectangular groove versus trapezoidal T-slot), wall thickness, corner radius and fastener system. The series number describes the outside square only. Interchangeability is a convention between manufacturers, not a standard.
Why do two suppliers quote different prices for the same 8080 profile?
Usually because the bars contain different amounts of aluminium. The European-convention 8080 runs a 7 mm wall at 8.33 kg/m; the national-convention version runs 5 mm at 6.46 kg/m. That's a 29 % metal difference at the same outside size, which is most of the price gap. Compare on kg/m and the quotes stop looking mysterious.
What is the difference between a rectangular slot and a T-slot?
A rectangular groove has parallel walls; a trapezoidal T-slot narrows toward the opening so a shaped nut can be inserted and rotated to lock behind the lip. They take different fasteners — square nuts with socket-head bolts for the former, T-bolts or purpose-shaped nuts for the latter — even when the mouth measures the same width.
Will an 8 mm T-nut fit a 6 mm slot?
No, and there is no workaround. The nut cannot enter. The reverse — a 6 mm nut in an 8 mm slot — is worse in practice: it enters, appears to fit, then rotates under load instead of gripping the lip. Match the nut to the measured slot, not to the series number.
Sources & References
- T-slot structural framing — series designations by cross-section, and the absence of any published standard defining the system
- Wellste — 20 Series T-Slot Profile specifications (6.2 mm slot width, 0.65 kg/m for 2020, 1.1 kg/m for 2040, 1.98 kg/m for 2080)
- FONNOV Aluminium — Slot width of T-slot extrusion, and why T-nuts are slot-specific
- 澳铝 — 8080 profile compared across conventions (7 mm wall / 8.33 kg/m vs 5 mm wall / 6.46 kg/m; slot shape, corner radius and fastener system differences)