Aluminum coil slitting: how slit coil is made, which tolerances to ask for and how to inspect it

Few roll formers or progressive dies take the full width of a mill coil. The coil is cut lengthwise into narrower strands and each strand is rewound: that is slit coil, also sold as aluminum strip. In British spelling the search is aluminium coil slitting; in Mexico the product is called fleje, and in French-speaking Canada, bandes refendues.

How to pick the mill width and work out edge scrap is covered in aluminum coil widths. This guide is about the slitting itself: how it works, which defects it can leave, what to put in writing and how to inspect slit coil at receiving.

How a slitting line works

1. Uncoiler (payoff). Takes the master coil on its inside diameter and unwinds it against a brake that keeps the sheet under tension.
2. Slitter head. Two arbors, one above the other, carry rotary knives in pairs. Each pair cuts like scissors: the top knife overlaps the bottom one slightly, with a small side clearance between them. Spacers between the knives set the width of each strand.
3. Edge trim. Both edges of the master coil are trimmed off as scrap, so every strand has a slit edge.
4. Separators and tension stand. Before the recoiler, separator discs keep the strands apart so they do not interleave, and a tension stand pulls them evenly.
5. Recoiler. Winds all strands at once on the same mandrel. Each coil is then banded, tagged and packed.

Knife position sets the width; knife sharpness and setup set the edge.

The edge: burr and other defects

Burr is the small lip of metal left on the slit edge. It shows up when knife clearance does not match the gauge, when knives get dull or when the overlap is too deep. A study on 0.04 and 0.08 in (1 and 2 mm) 5754 aluminum found that clearance accounted for 76% of burr formation and sheet thickness for 23% (Çavuşoğlu and Gürün, Transactions of FAMENA, 2017). The study is on blanking, but the principle holds for rotary knives: setup rules.

Burr cuts hands, scores dies and roll-former rolls, and on electrical or painted parts it can be grounds for rejection. If your process is sensitive, put a maximum burr height and the acceptable side on the RFQ.

Other edge defects to check:

• Edge wave: the edge ends up longer than the center of the strand and ripples when laid flat.
• Knife marks: lengthwise scratches next to the cut.
• Slivers: thin threads of metal that peel off the edge and can get wound into the coil.

Width tolerance

The tolerance standard for aluminum mill products is ANSI H35.2, "Dimensional Tolerances for Aluminum Mill Products," from the Aluminum Association; the current edition is 2024. Its tables are not free to read. United Aluminum, a US strip producer, publishes these slit coil values from ANSI H35.2 in its technical manual:

Source: United Aluminum technical manual, page 5, accessed October 28, 2026; millimeters converted from the inches in the table. Always check against the edition of the standard cited on your purchase order.

This table is a public reference, not an Almexa commitment: the tolerance for your order is confirmed in writing on the quote. If your die needs a tighter tolerance, put it on the RFQ from the start.

Camber

Camber is how far the edge of the strand departs from a straight line: laid on the floor, the strand bows to one side. It comes from residual stress in the master coil, a thickness difference from one side to the other (crown or wedge) or misaligned knives. On a roll former, a cambered strand rides against one guide; in a progressive die, the holes walk.

It is measured with a straightedge or a taut string: the maximum gap between edge and line over a given length. The table United Aluminum publishes from ANSI H35.2 uses a 6 ft (1.83 m) length:

Narrow strands are allowed more camber. The table gives no value above 0.125 in gauge: if your process needs one, it is agreed on the quote.

Telescoping and wind quality

A telescoped coil has its wraps shifted to one side, like a spyglass or a cone. Causes: uneven rewind tension, an off-center core or uneven gauge. A coil like that gets edge damage when handled, is hard to load on the uncoiler and can shed wraps. At receiving, both coil faces should be flat and the wraps tight. How coil should be blocked and secured so it arrives that way is in how to ship aluminum coil, sheet and circles without damage.

ID, OD and weight per inch of width

Almexa coil has a 20 in (508 mm) ID and an OD of 39 to 46 in (990.6 to 1,168 mm), per its data sheet. Strands slit from the same master coil share its OD, so each strand's weight depends on its width. US buyers usually talk in PIW, pounds per inch of width.

Weight follows from coil geometry: π/4 times the difference of the squares of OD and ID, times width, times density. With a 20 in ID and 2.71 g/cm³, the density the site's coil weight calculator uses:

Our own calculation; the actual weight of each coil is on its tag. 5052 weighs about 1% less, at 2.68 g/cm³ (The Aluminum Association). If your uncoiler takes less weight or a smaller OD, say so on the RFQ.

What to specify for slit coil

1. Final width of each strand and its tolerance.
2. Gauge, alloy and temper.
3. ID, maximum OD and maximum coil weight (or PIW) your uncoiler takes.
4. Maximum burr height and side, if your process is sensitive.
5. Maximum camber, if you feed a progressive die or a long roll former.
6. Packaging: eye to sky or eye horizontal, and protection between coils.
7. Pounds or tons per month and what the part is.

With the final width and the mill width, the slitting calculator shows how many strands you get and how much edge scrap is left. The full spec, from alloy to receiving, is in how to specify and buy aluminum sheet and coil.

How to inspect slit coil at receiving

• Tag against PO: alloy, temper, gauge, width, weight and lot.
• Width: with calipers at the coil tail and at several wraps as it pays off.
• Gauge: with a micrometer at the center and near both edges.
• Burr: measure thickness at the edge and subtract the sheet gauge; check both edges of every strand.
• Camber: lay 6 ft of strand on a flat table and measure the largest gap to a straightedge or string.
• Wind: flat coil faces, no popped wraps, no dented edges.
• Mill test report: the master coil's lot MTR travels with the slit coil; how to read it is in how to read an aluminum mill test report.

If something is off, hold the coil unprocessed, photograph the tag and the defect, and notify in writing.

How Almexa works

Almexa casts, rolls and slits in Cuautitlán Izcalli, State of Mexico, with 3 slitting lines. It supplies coil 6.30 to 48 in (160 to 1,219 mm) wide in the 1000 and 3000 series and 6.30 to 36 in (160 to 914 mm) in the 5000 series, 0.0177 to 0.148 in (0.45 to 3.76 mm) thick (from 0.0197 in, 0.50 mm, in the 5000 series), on a 20 in (508 mm) ID with a 39 to 46 in OD. Tolerances follow Aluminum Association standards; the minimum strip width and the slitting tolerance depend on gauge and alloy and are confirmed in writing on the quote. Slit coil comes off coil Almexa rolls itself and ships with the lot's mill test report. See aluminum strip and aluminum coil; if your part comes off a press stroke, see also aluminum blanks.

Send your specification for a quote from the mill: final width, gauge, alloy, temper and pounds per month.

Sources

All accessed October 28, 2026.

• United Aluminum, "Aluminum Sheet & Coil Published Dimensional Tolerances," technical manual, page 5: slit coil width and camber tolerances from ANSI H35.2.
• The Aluminum Association, ANSI H35.2, "Dimensional Tolerances for Aluminum Mill Products," 2024: tolerance standard.
• Çavuşoğlu and Gürün, "The Relationship of Burr Height and Blanking Force with Clearance in the Blanking Process of AA5754 Aluminium Alloy," Transactions of FAMENA, vol. 41, no. 1, 2017: clearance and burr.
• The Aluminum Association, International Alloy Designations and Chemical Composition Limits, August 2018 revision: 5052 density.
• Almexa data sheet, October 2026: slitting lines, widths, gauges and coil diameters.