Trim Allowance and the Three-Knife Cut

A bound block needs allowance on three sides before it is trimmed: 3 mm on the fore-edge and 3 mm at head and tail is the common minimum for a case binding, and 5 mm is safer on a heavy coated stock that tends to break out at the cut. The allowance is not spare paper. It is the material the knife takes off when the block is trimmed square after binding, and it has to cover the creep of the folded sections, the swell left by the sewing, and the trim taken by the previous cut.

How much allowance should a bound block have?

Measure the untrimmed block, then measure the text area the job was designed around. The difference is the allowance, and it has to be spent on the three uncut sides. A 320 page book on 90 gsm paper typically swells 2 to 4 mm at the spine after sewing; the folded fore-edges of the outer sections push out further than the centre ones. A case built to the untrimmed height will be loose at the boards once the block is cut down.

For a job with a printed headband or a gilt top, the allowance at head and tail also has to cover the gilding. Gilding is done before the final trim on a hand-bound book and after it in a machine line; ask which order the bindery works in before the margin is set in the artwork.

Which side is cut first?

On a hand bench the fore-edge is cut first, so the block can be knocked up against a straight edge and the head and tail cut from a square reference. Cut the fore-edge last and the head or tail has to be squared against a folded edge that is not straight. The order is: fore-edge, head, tail, with the clamp pressure matched to the paper, because a clamp set hard on a coated stock leaves a mark 2 mm inside the cut.

A three-knife trimmer works the other way around in one cycle. The block is clamped on the fore-edge, the head and tail knives fall together, and the fore-edge knife cuts as the clamp releases. Nothing in the order changes for the artwork, but the machine has one requirement the bench does not: the block must be tight. A loose block is compressed by the clamp and springs back after the cut, and the trim is out by the amount of that movement.

Why a dull knife tears a coated sheet

A coated sheet is a sandwich: a mineral layer on each face over a fibre core. A sharp knife parts the coating and the fibre under it. A dull knife crushes the coating first, and the coating cracks away from the core along the line of the cut, which shows as a white edge on a coloured cover and as a ragged edge under a raking light.

The tell is the sound. A sharp knife on a coated stock sounds like paper being sliced. A dull knife sounds like paper being compressed, and the block gets a downward mark on the cut edge that cannot be sanded out without changing the trim. Industry guidance on the finishing of printed sheets names the blade angle and the cut quality on coated stock for this reason.

What the trim costs the page

Every trim takes paper out of the margin the designer drew. A book trimmed to a 3 mm allowance on three sides loses 6 mm of width and 6 mm of height against the untrimmed block, and the page is then centred on what is left. A reader who opens the book at a full bleed spread will see the loss on the outer edges.

The measurement to hold in a specification is the trimmed size after binding, with the allowance named separately. A printer and a binder working from two different sizes is the common cause of a margin that ends up 4 mm narrower on one side than the other.

What to write in the specification

Name the trimmed size, the allowance on each of the three sides, the order of cuts, the blade condition expected on coated stock, and who trims: the printer, the binder, or a separate trade finisher. Where the trim follows a gilt edge or a stained edge, name the order of those steps as well. A specification that names the trimmed size alone leaves the allowance to whoever holds the knife.

Sources consulted for this folio: ISO 11800 on the requirements for binding materials and methods used in the manufacture of books, together with the working ranges kept at the bench.