You should, hopefully, remember the image below: a 16th century depiction of the most advanced book-trimming technology of the time, the book plough.
In this previous post I discussed how it differed from the modern ploughs, mostly in that there’s no evidence in the woodcut of a rail for it to ride on or guides of any kind. It is possible, of course, that the rail was left out because it was a trade secret, or he didn’t understand what he was seeing, or it was simply a woodcut and you remove unnecessary lines when possible. The Book of Trades wasn’t a period DIY guide, after all.
Personally, I found using the plough without any guides at all to be incredibly annoying. The only thing you have to register against is the back of the book block, which is actually fine until you reach the end. Without anything to register against, keeping the plough square to the block was difficult and I tore quite a few pages
I doubt that this untidy situation lasted long, no matter what Amman depicted. I readily admit that it might simply be that I need time to get my eye in, but I still don’t think that’s the solution.
Since this much later image found in the German text Etwas für alle shows the bottom of a plough, which is still lacking evidence of a groove that would ride on a rail, I have to believe that the solution lay in the press or how the block was put into the press.
Many modern binders include a piece of hardboard at the back of the book block to protect the press from the ploughshare, and it occurs to me that if such an arrangement was present in Amman’s woodcut, we’d never know it because it would be indistinguishable from the book the man’s trimming.
We had a snow day today, so I took some time to test how this would work. I cut a scrap of pine (hardboard being a modern material, I wanted something more in keeping with the aesthetics of the thing) and it worked… beautifully.
As you can see above, the piece of pine had cutouts to ride over the screws of the press while standing proud about a half inch (13 mm), which was enough for the face of the plough to ride against. With this simple guide I was able to trim all three faces of the text block smooth enough to decorate with paint, gauffering, or gilding in very short order.

Honestly, short of us finding a clear explanation from a period craftsman (which I would dearly love to see), I think we’re going to assume this is the correct methodology and keep moving forward.
More later, as always.
~ Scott
Image sourcing
Image: Der Buchbinder, woodcut from Das Ständbuch (1568) by Jost Amman, edited, highlighted, and generally cleaned-up by yours truly; original image sourced from Wikimedia Commons
Works referenced
The Archaeology of Medieval Bookbinding by J.A. Szirmai, Aldershot, 1999 (period binding techniques)
The Thames & Hudson Manual of Bookbinding by Arthur W. Johnson, 1978, Thames & Hudson Ltd, London (modern binding techniques)