I haven't had to use crop marks before – my books have been printed on sheets the same size as the InDesign page. Now however, I am about to print a small book, 4-Up, on sheets larger than the page size, so I will need to add crop marks. I want to run my understanding of how to place crop marks on the page and how they will be used by the guillotine man, past some of the experts on this forum. And before anyone says: "Crop marks are the printer's job. Just let him do it", or somesuch comment – in this case it's not the printer's job, it's mine. I've chosen to do everything: the writing, the layout, purchase of the paper (910 x 650 sheets), the cutting of the paper, the imposition, the printing, arranging the cutting of the sheets, and then the binding. So, I need to know about crop marks. This is what I have discovered, or guessed (please correct me if I'm wrong): Assume a page size in InDesign of 100 x 200 mm. When you export to PDF and tick Crop Marks and set Offset to a certain dimension, two things happen: The page size is increased to (100 + 2 (Offset + 5.3)) in width, and (200 + 2 (Offset + 5.3)) in height. In the case of an offset of 5 mm, the page dimensions of the PDF become 120.6 x 220.6 mm. Crop marks of 5.3mm in length (5/12"), are placed at the four corners of the new page pointing inwards to the original page size. Thus, in this case, there is a gap of 5mm, the chosen offset, between the end of the crop marks and the original page. This offset allows for movement of the sheets during printing, so that when the crops are made the crop marks won't appear on any of the pages. i.e. the offset is greater than the expected sheet movement. The guillotine man, when he comes to cut the sheets, lines up the narrow beam of light from the guillotine with one of the crop marks – and cuts that edge. Now there are only two crop marks left because two have been cut off in this first cut. The guillotine man then rotates the sheets 90º and cuts again. Now there is only one crop mark left. The sheets are again rotated, the beam of light lined up with the remaining crop mark, and the third cut is made. There are no more crop marks left. For the final cut, the sheets are rotated 90º, but as there are no crop marks left, the appropriate dimension has to be entered into the guillotine (or marked out on the top sheet), and the cut is made. I have yet to confirm with my guillotine man that steps 3-6 describe how he will do it. That's simply how I imagine it would happen. And in the case of 4-Up, a couple of extra cuts will be needed in the centre, depending on how the imposed pages are positioned. Is the above an accurate description of how crop marks are inserted and how they are used?
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