Also searched as aluminium extrusion tolerances, aluminum extrusion tolerance chart, EN 755-9 tolerance table, extrusion straightness tolerance, profile twist tolerance, aluminium profile dimensional tolerance mm.

The three budgets, and why mixing them causes disputes

A delivery of aluminium profiles can be non-conforming in three completely separate ways, and each one is fixed by a different party in the process. Treating them as one tolerance number is the most common reason a goods-in argument goes nowhere.

Put all three into a single line on a drawing — “tolerance ±0.5 mm” — and you have written something that cannot be inspected and cannot be enforced. Write each budget where it belongs and the delivery either conforms or it does not.

Cross-sectional tolerance: the EN 755-9 tables

EN 755-9 covers extruded profiles with a cross-section inside a circumscribing circle of up to 800 mm, which is every framing profile on the market. It sorts alloys into two groups by how difficult they are to extrude consistently, and then indexes the tolerance by two numbers: the dimension being toleranced, and the circumscribing circle.

All values in millimetres. Profiles with open ends get an addition to the tolerance across the gap between the open ends, which grows with the length of the open leg: nothing up to a 20 mm leg, then ±0.15 mm for a 20 to 30 mm leg, ±0.25 for 30 to 40, ±0.40 for 40 to 60 and ±0.60 for 80 to 100. A T-slot profile is not an open-ended section, so this normally does not apply — but a channel, a cover strip or a clip-on trim piece is, and the addition can double the tolerance you should expect.

Wall thickness: the tightest figure, and the one that surprises people

Wall thickness has its own table, and the standard distinguishes between a wall that encloses a hollow space and one that does not. On a T-slot section the wall around the centre bore and the wall behind the slot both enclose hollow spaces, so the wider figures apply to them.

Alloy group I, circumscribing circle up to 300 mm, values in millimetres. Wall A is a wall that does not enclose a hollow space; wall B is a wall that encloses one.

Read the 1.5 to 3 mm row, which is where almost every framing profile lives. A wall that encloses a hollow space in a section under 100 mm circumscribing circle carries ±0.25 mm. On a nominal 2.0 mm wall, that is a legitimate range of 1.75 to 2.25 mm — a spread of 25 per cent on the nominal.

That number has consequences that are easy to miss:

• Thread engagement. A tapped end in a thin wall derives its strength from the number of full threads, which derives from the wall. If your calculation assumed 2.0 mm and the delivery is at 1.75 mm, the engaged cross-section is smaller than designed.
• Slot lip stiffness. The material that resists a T-nut pulling out of the slot is a wall. A 12.5 per cent reduction in that wall is a reduction in pull-out capacity, and it will not show up until a joint is loaded.
• Weight and inertia. Weight is linear in wall thickness, so a section delivered at the top of the tolerance band is heavier than the catalogue figure. Inertia moves faster, because most of it comes from material at the outer face.

Straightness and twist: the tolerances that stop machines working

Here is the uncomfortable truth about incoming inspection. A profile can be dimensionally perfect, have the right wall, the right alloy and the right temper, and still be useless, because it is not straight or it is twisted. These two form tolerances are the ones that decide whether a linear rail runs true and whether a panel sits flat, and they are the two most often left off a purchase order.

What straightness actually costs you

Straightness is expressed as a deviation over a length, and the arithmetic is brutal because the deviation accumulates. A bow of 1 mm per metre sounds negligible. Over a 6 m profile it is 6 mm at the end. A linear rail bolted to that profile inherits every millimetre of it, and a carriage running along the rail will see the full 6 mm as a change in height — far more than the rail’s own straightness specification, and enough to bind a preloaded carriage.

Typical values you can contract to: general-purpose framing extrusions are commonly supplied at 0.5 mm per metre; precision sections for linear motion and for sections that slide inside one another are supplied at 0.2 to 0.3 mm per metre. Those are the figures to write on the order, and they should be accompanied by the plane in which they apply.

How to measure straightness without a CMM

The measurement is simple enough to do at goods-in with a granite surface table and a dial indicator, and it takes about two minutes per profile.

1. Support the profile at its two ends only. Do not lay it flat on the table along its whole length — you will measure the table, not the profile. Two vee blocks or two matched spacers, positioned so the profile rests on its ends.
2. Zero the indicator at mid-length. Bring the dial indicator down onto the face you care about, at the middle of the span, and set zero.
3. Read the deviation at the ends and at the quarter points. The maximum reading, positive or negative, is the deviation. Divide by the span in metres to get the figure per metre.
4. Rotate the profile 90 degrees and repeat. The two planes give different answers, and you need both. Take the worse of the two as the governing value.
5. Record the plane. A measurement without the plane it was taken in cannot be compared with the purchase order, and a dispute you cannot evidence is a dispute you lose.

Twist, and why it is worse than it sounds

Twist is rotation of the cross-section along the length. It is the tolerance that is easiest to miss, because a twisted profile looks straight when you sight along it. It also has the largest practical consequence per degree, because the width of the section multiplies the error.

Take a 40 mm wide section twisted at 0.5 degrees per metre. Over 6 m that is 3 degrees of total rotation. Across the 40 mm width, the corner height changes by 40 × tan(3°), which is about 2.1 mm. A panel bolted across two such profiles will not sit flat, and a bracket that spans the section will be pulled out of square by the same amount.

Measuring twist is as simple as measuring straightness, and it uses the same setup. Support both ends, place a dial indicator on the top face at one end and note the reading, then move the indicator to the same feature at the other end. The difference between the two readings is the twist over that length. Divide by the length in metres.

Cut length, squareness and the instruments to use

Cut length is a secondary operation and carries its own tolerance, which should never be assumed to be the same as the profile tolerance. A standard saw cut holds roughly ±1 mm on length; a CNC cut holds about ±0.1 mm. Squareness of the cut end matters more than the length for a frame, because a frame is a stack of lengths and an out-of-square end puts the whole assembly out of true.

The pin gauge row is the one buyers rarely think about and should. A T-nut grips the undercut behind the slot mouth, not the mouth itself. The mouth can measure correctly while the channel behind it is undersized, and the nut will not slide. A pin gauge or a slot-width gauge on the channel catches that in seconds, where a caliper on the mouth never will.

The sampling plan: how many to measure, and what to do with the answer

Measuring every dimension of every profile is not inspection, it is a bottleneck. The workable approach is to identify a small set of critical-to-function dimensions, then sample to a recognised plan so that the accept/reject decision is defensible rather than a matter of opinion.

For dimensional attributes, ISO 2859-1 with inspection level II is the usual basis. For a lot of 501 to 1,200 profiles the sample size is 80; for 1,201 to 3,200 it is 125; for 3,201 to 10,000 it is 200. Within that sample, the acceptance number depends on the AQL you set.

The acceptance numbers are the number of non-conforming items allowed in the sample before the lot is rejected. The classification matters as much as the level: put the dimensions that would stop the frame assembling into the AQL 1.5 class, the ones that would cause rework into AQL 2.5, and cosmetic and non-functional dimensions into AQL 4.0. A single AQL across everything is how a cosmetic scratch rejects a lot that is structurally perfect.

What to put on the drawing

The most effective single action in this whole process is to mark critical-to-function dimensions on the drawing. A tolerance that applies to every dimension equally is a tolerance that will be violated on the dimensions that matter while being met on the ones that do not.

• Box the dimensions that affect assembly: slot mouth, slot pitch, centre bore, and the envelope if the section slides inside anything.
• Mark the wall thickness behind any tapped feature, because that is where thread engagement comes from.
• State straightness and twist with the plane and the span they apply to.
• State the cut length tolerance separately, and the squareness of the cut end.
• Give the inspection level, the AQL and the sample size, so the supplier knows what will be measured.

Frequently asked questions