Every printed sheet ends at the guillotine

Almost nothing leaves a print shop at the size it was printed. Business cards come off the press twenty to a sheet, flyers eight-up, book pages in signatures. The last machine every one of those jobs meets is the guillotine, and its accuracy sets the final size of the product. A press can hold colour to a fraction of a percent, but if the cutting is a millimetre out, the card has a white sliver on one edge and the customer sees only that.

This article explains how a production guillotine works, where cutting tolerance comes from, and how to lay out a job so that the normal variation of the cutting process never shows. It is written for people who buy print in volume and want to understand why the specification sheet says what it says.

How a programmable guillotine cuts a job

A modern guillotine holds a stack of sheets (a lift) against a back gauge, clamps it, and brings a single blade down at an angle. The operator programs the sequence of cuts, the back gauge moves to each position automatically, and the operator turns the lift between cuts. A 20-up business card sheet needs roughly a dozen cuts: first the four edges to strip the bleed and marks, then the rows, then the columns.

Three things determine how accurate the result is:

• Back gauge positioning. A well-maintained machine positions to within 0.1 mm. This is rarely the problem.
• Clamp pressure and stack height. When the blade enters a thick lift, the lower sheets are pushed slightly forward; this is called draw. A skilled operator sets clamp pressure for the stock and cuts lifts of a sensible height (thinner for coated stock, which is slippery). Cutting too tall a lift to save time is the commonest cause of size variation between the top and bottom of a stack.
• Sheet-to-sheet registration on the press. If the image moved by 0.3 mm from sheet to sheet during printing, no amount of cutting accuracy can put it back. The guillotine cuts the paper, not the image. Our piece on what the press operator checks during the run covers this side.

Add these up and a realistic finished-size tolerance for commercial work is plus or minus 0.5 mm on a single cut, and up to 1 mm across the whole product where several cuts stack. Good shops hold tighter than this most of the time. Nobody holds zero.

What tolerance means for your design

Three layout rules follow directly from the way cutting works, and they explain the numbers printers keep asking for.

Bleed: 3 mm, all sides

Any colour, photo or rule meant to reach the edge must extend 3 mm past the trim line. If the cut lands 0.5 mm outside the intended position, the extra image fills the gap and the edge still looks solid. Without bleed, the same 0.5 mm shows as a white hairline. Bleed of 1 or 2 mm is a gamble; 3 mm covers ordinary variation with margin, and 5 mm is sensible for very thick stock or products with many cuts.

Safe area: 3 mm inside the trim

The same variation works inward. Text, logos and anything that must not be clipped should stay at least 3 mm inside the trim line. A border that runs exactly 2 mm from the edge is the classic mistake: a 0.5 mm cutting shift makes it 1.5 mm on one side and 2.5 mm on the other, and the eye sees a crooked card even though every element is straight.

Thin borders and symmetrical layouts amplify error

A design with a 1 mm rule around the edge, or two columns that must be exactly centred, exposes cutting variation that a busier design hides completely. If the design needs a border, make it at least 5 mm wide or move it well inboard.

Multi-up and gutter cutting

When many copies share a sheet, the space between them is either zero (a shared cut, one blade stroke separates two cards) or a gutter (a strip of waste between them, cut away with two strokes). A shared cut is faster and uses paper better, but it forces neighbouring pieces to share their bleed: the bleed of one card is the image of the next. That works when all pieces have the same edge colour or a plain edge. It fails when a black card sits next to a white one, because any cut shift shows black on the white card's edge.

For mixed designs on one sheet, or anything with a dark full-bleed edge, a gutter of at least 6 mm between pieces is the safe option, even though it means more cuts and slightly higher waste. Tell the printer if you are supplying your own imposed sheet, and ask which approach they intend to use.

Reading the trim marks on a proof

A print-ready PDF carries crop marks: short lines outside the corners that show the trim line. On a proof, check that the marks sit outside the bleed, that the distance from the marks to the content is what you expect, and that the marks on all four corners describe a rectangle of the right size. A file where the artboard was resized after the marks were placed will have marks that no longer match the intended trim, and the guillotine operator will cut to the marks.

Special cases

• Thick stock (350 gsm and above). The blade compresses the top of a thick lift more than the bottom, so the shop cuts smaller lifts. Expect a little more cost, not less accuracy.
• Laminated sheets. The film can pull away from the edge if the blade is dull. Lamination is usually applied to the sheet before cutting so that the film wraps nothing; if edge sealing matters, ask for the lamination to be trimmed flush and check a sample.
• Books. Bound books are trimmed on three sides in one pass on a three-knife trimmer. Trim tolerance there is typically 1 mm, and the inside margin near the spine is set by the binding, not the cut.
• Shapes. Rounded corners are a separate punching operation after guillotining; anything non-rectangular is die cutting, covered in our guide to die cutting and forme design.

Cutting accuracy at S7 Printing

Our finishing department runs programmable guillotines that are calibrated and blade-changed on a schedule, and the operators cut lift heights appropriate to the stock rather than as tall as the clamp allows. Every multi-up job is checked at the top, middle and bottom of the stack against the ordered size before it goes to packing, and products with dark full-bleed edges are imposed with gutters by default.

If you are supplying files for a large run, the notes above are all you need: 3 mm bleed, 3 mm safe area, no thin borders. If you supply your own imposed sheets, send us a note about which pieces share edges and we will confirm the cutting plan. Details of our production capabilities are on the general printing services page, and for a job with unusual sizes or tight tolerances, contact us and we will tell you what we can hold on that stock.