How to read an extrusion cross-section drawing before you send it out to quote

Read it as an instruction to a die, not as a picture of a part

A cross-section drawing does one job: it tells a die maker what shape of aluminium has to come out of a press. Everything about it that matters follows from that. The length of the part is not on the drawing because it is not a property of the section. The colour is not on the drawing because finishing happens after extrusion. What is on the drawing is the profile of the opening the metal will be pushed through — and that opening has to be physically makeable.

That reframing is useful, because it tells you which questions belong on the drawing and which belong in the enquiry email. Designers who think of the drawing as a picture of the finished part tend to put finish notes and lengths on it and leave out the things a toolmaker actually needs.

Before anything else, check that the section is closed and unambiguous — that every wall meets another wall, that no line is left hanging, and that the drawing is a true section and not an outline traced from a photograph. We see traced sections more often than you would expect, usually when someone is trying to match a competitor's part.

The four things to check first

**Circumscribing circle.** Draw the smallest circle that fully contains the section. That diameter, more than the nominal width or height, decides which press the profile can run on and whether it is feasible at all. Our range runs to 500 mm circumscribing circle; a section that fits inside a modest circle but is quoted as "300 × 40" may still be an easy part, and a slim-looking L that spans a large diagonal may not be.

**Wall thickness — and how uniform it is.** We extrude from 0,7 mm walls upward, but the absolute minimum is less interesting than the ratio between the thinnest and thickest wall in the same section. Metal flows faster through thick sections than thin ones, and a section that pairs a heavy structural spine with a thin decorative fin is asking the die to balance two different flow rates. It can be done. It is just a design decision worth making knowingly rather than by accident.

**Hollow, semi-hollow or solid.** A closed void makes it a hollow, which needs a porthole-type die with a mandrel supported inside the flow. A deep narrow channel with a small opening makes it a semi-hollow, where the die has a thin tongue holding the gap open under extrusion pressure. Solid is the simplest case. This one line changes tooling type, lead time and cost more than almost anything else on the sheet.

**Symmetry.** A section balanced about an axis extrudes more predictably than one where all the mass sits on one side. Perfect symmetry is not required, and plenty of good profiles are asymmetric — but if you have design freedom left, moving mass toward balance is nearly free at this stage and expensive later.

The semi-hollow trap

The most common problem we find in drawings we receive is not a wall that is too thin or a section that is too large. It is a designer who created a semi-hollow without realising it.

It usually happens like this: a channel is drawn to take a screw, a gasket or a glazing bead, and then at some point in the revision cycle the opening is narrowed — for a better seal, for a cleaner look, because the gasket supplier changed. Nobody notices that the gap-to-depth relationship crossed a line. On the drawing it is a minor edit. In the die it turns a robust tongue into a fragile one that has to hold that gap open against the full pressure of the press, on every metre of every billet.

The consequences do not show up as a rejection. They show up as shorter die life, more frequent die changes, tighter limits on how fast the profile can be run, and a price that is higher than a nearly identical section would have been. That is the annoying part: the alternative was often available for free, if anyone had asked at drawing stage.

So when you look at a section with any deep narrow recess in it, mark it and ask. If the answer is that opening the mouth of that channel by a small amount moves the part into an easier category, that is a decision the design team can usually make in an afternoon — and almost never wants to make after the die is cut.

Which dimensions are functional — and why the drawing has to say so

On a typical section, only a handful of dimensions actually control whether the part works. The screw port that has to accept a specific fastener. The groove that has to hold a specific gasket. The face that mates with the adjacent profile. The rest are dimensions that exist because the geometry has to be defined, not because anything depends on them.

The drawing almost never distinguishes between the two. When it does not, the supplier has exactly one sensible response: hold everything to the standard default, quote on that basis, and inspect on that basis. Our standard reference is EN 12020, with ±0,1 mm available where a dimension needs it — but which dimensions need it is your call, not ours, and we cannot infer it from an unmarked drawing.

This matters in both directions. If you leave the mating dimensions unmarked, you may get a part that meets the drawing and still does not assemble. If you mark every dimension as critical — which happens, usually as a way of feeling safe — you are asking to pay for precision on faces nobody will ever touch, and you make the inspection report longer without making the part better.

The practical fix is a single note on the drawing identifying the two or three dimensions that are functional. What each tolerance class actually commits a supplier to is a longer subject, and we have written it up separately in our piece on extrusion tolerances.

The half of the specification that is never on the drawing

A section drawing describes a shape. A purchase needs considerably more than a shape, and the missing half is the same every time.

Alloy and temper are not on the drawing, and the choice changes both what the profile can do and how cleanly it finishes. Cut length is not on the drawing — we extrude to 12.000 mm, but what you need is a length you can actually ship and handle. Finish is not on the drawing, and it interacts with the dimensions in a way that catches people out: an anodic layer or a coating sits on the surface, so a fit that is exactly right on bare metal can be tight after finishing. If a dimension is critical and the part is being finished, the drawing should say whether the number applies before or after.

Nor is the drawing the place for quantity, packing or delivery terms — but a quotation cannot be produced without them, and a section drawing sent alone will simply come back with all of these as questions. Sending them together turns two rounds of email into one.

What to send with the drawing, and when to push back

Send the drawing in an editable CAD format if you have one — DWG or DXF. A PDF is workable, and a clear photograph of a paper drawing is workable, but every step away from CAD adds a chance that a radius or a wall thickness gets read wrong. If all you have is a sample part, say so and send the part; measuring a physical sample is a normal thing for us to do, and it is far more reliable than tracing it.

Say what the profile does. "Frame member for a sliding door, carries the glass, spans about two metres" tells us more about which alloy and which tolerances matter than three pages of dimensions do. This is also the sentence that lets us tell you when a standard section would do the job — which is sometimes the answer, and a custom die is not always the right money to spend.

Ask two questions of any supplier before the die is cut. First: is this section hollow, semi-hollow or solid, and would a small change move it to an easier category? Second: which dimensions are you planning to inspect, and to what? A supplier who answers both without hedging has actually looked at your drawing. One who only quotes a price and a lead time may not have.

And if a supplier tells you a section is straightforward without ever mentioning the circumscribing circle, the wall ratio or the tooling type, push back once. Not because they are wrong — often they are right — but because the answer to that question is what tells you whether the quote in front of you is based on your drawing or on a guess. Send us the section and we will tell you what we see in it, including the parts you would rather we did not.

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