A bleed is needed when you produce a design where the ink goes to the edge of the paper. (On-demand printers will normally not allow type to come any closer than .375” or .5” from the edge of the trim size.)
Printers and printing presses can never print to the edge of the paper. The ink or toner will run off and build up on the back side of the paper. This will not only ruin the sheet of paper, but can also damage the printer or press. So we must print on an oversize sheet and then trim back to size.
To produce a bleed, you make everything that reaches to the edge of the page extend one-eighth inch beyond the edge and then trim the piece back to finished size after printing. That’s one-eighth inch, nine points, or a little less than four tenths of a millimeter (.375 mm to be precise). As you can see below, any place that touches the edge requires a bleed—so the example below is a four-sided or full bleed, even though three of the sides only have ink touching the edge in narrow areas.
The power cutters used in the industry are the reason a bleed is necessary. These huge guillotine cutters slide their knives through stacks of paper several inches thick. They can cut 1,000 to 3,000 sheets at a time. Those huge cuts force the paper to slide around a bit—no matter how tightly they are clamped. The result of these limitations is that cuts are only accurate to plus or minus a sixteenth of an inch or so.
Spines and crossovers
Bleeds on the inside margin of a book are a special case. They have unique problems. Normally, you will run an interior bleed to the inside edge of the page and stop there. There may be slight gaps occasionally due to variations from side to side in the folding and binding operations. However, they will be covered by the touching of the pages in the spine of the book. Basically, those slight paper-colored gaps will be unnoticeable.
The real issue is a crossover image
A crossover is when you want an image to be on both sides of the fold—flowing from the page on the left to the page on the right. In general, on a strictly practical level, you should avoid this at all costs. This is a design issue for you. Crossovers are almost never necessary. They make printing MUCH more difficult. This has to do with how books are assembled.
In a readers’ spread (the way readers see two pages in a book side by side), the page you see on the left is on a different sheet of paper than the page on the right (the sole exception being a centerspread).
Yes, crossover images are commonly this far out of alignment
Here’s an example of the nature of the problem on a very simple 8-page signature, folded out of a single sheet of paper. This simple example would be like a French-fold greeting card or an eight-page newsletter. The dashed lines indicate a folding location. Page two prints on the back of page one and so on.
As you can see, on the back of the sheet of paper the image has to be upside down. Once the page is folded and trimmed (so the two double-sided spreads with the four double-sided pages appear), pages two and three will be next to each other with page 2 on the left and page three on the right. Even in this very simple example, you can certainly see that positioning must be extremely accurate and the folding must be perfect for the images to line up.
In books, the page on the left is almost always on a different sheet of paper than the page on the right
Obviously, this makes the alignment much more difficult. I’m not saying that it cannot be done. I’m saying that it takes remarkable printing precision with absolutely accurate layouts to start with.
Folding and trimming are ±.0625″ in accuracy — at best
I already mentioned the trimming accuracy with the huge guillotine cutters above. Folders have the same tolerances. In a book, there are commonly eight or sixteen pages on each side of the sheets of paper as they come out of the printing press, before they are folded [these are called 16- or 32-page signatures]. If the crossover image is partially on the page in the furthest upper left page of the sheet on the front, and the furthest lower right page of the sheet on the back, the two halves [or whatever portions you design] of the image are separated by two to three feet of sheet position in addition to being on two completely separate pieces of paper.
If crossover images align with an eighth inch, you’re blessed
If you look carefully at crossover images in magazines (the most common place to use them), you’ll find they are rarely aligned accurately. These magazines are printed on huge multi-million dollar presses which are far beyond the printing quality possible with on-demand publishing in most cases.
Bottomline: Don’t use bleeds or crossovers without talking to the printer first
Yes, I know they are done all the time. But, every printing company and different printing machine does it differently. You have no control over the signature size (how many pages are printed on each sheet of paper). Plus, the signature size varies by printing press and book size. A single book, in a single printing company, may be printed on two or three different printing presses. This all depends on the workload at the time and press availability.
Many of the new digital printing companies ask for quarter-inch bleeds [which was unheard of, in the day]. Often, the price of the entire job will increase substantially if you add a bleed. Many companies will refuse to deal with crossover images (because clients bounce the results, in their ignorance of the difficulties involved).
Examine your reasoning carefully. You will find that crossover images are rarely a reader benefit. Plus, they are impossible in ebooks (except for PDFs or books rendered in Flash showing reader’s spreads). You can almost certainly eliminate them from your design.